Cover image
Mikael Olsson
Modern C Quick Syntax ReferenceA Pocket Guide to the Language, APIs, and Library2nd ed.
Mikael Olsson
Hammarland, Länsi-Suomi, Finland
ISBN 978-1-4842-4287-2e-ISBN 978-1-4842-4288-9
https://doi.org/10.1007/978-1-4842-4288-9
Library of Congress Control Number: 2018967003
© Mikael Olsson 2019
This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed.
Trademarked names, logos, and images may appear in this book. Rather than use a trademark symbol with every occurrence of a trademarked name, logo, or image we use the names, logos, and images only in an editorial fashion and to the benefit of the trademark owner, with no intention of infringement of the trademark. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.
While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.
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Introduction

The C programming language is a general-purpose, middle-level language originally developed by Dennis M. Ritchie at Bell Labs. It was created over the period 1969 through 1973 for the development of the UNIX operating system, which had previously been written in assembly language. The name C was chosen because many of its features derived from an earlier language called B. Whereas the B language is no longer in common use, C became and still remains one of the most popular and influential programming languages in use today.

Although C is a general-purpose language, it is most often used for systems programming. This includes software that controls the computer hardware directly, such as drivers, operating systems, and software for embedded microprocessors. C can also be used for writing applications, which run on top of system software. However, it has largely been superseded in that domain by higher-level languages, such as C++, Objective-C, C#, and Java. The features of these and many other languages are heavily influenced by C, as can be seen in some of their names.

The development of C was a major milestone in computer science, as it was the first widely successful middle-level language for system development. The foremost reasons for its success were that the language was concise, fast, and powerful. It offered comparable speed to assembly with far improved usability. The high-level constructs of the language allowed programmers to focus mainly on the software’s design, while its low-level capabilities still provided direct access to the hardware when needed, as assembly had done. Furthermore, the language is relatively simple to understand, with few keywords and what many consider to be an elegant syntax.

Another major reason for the success of C was its portability. Unlike assembly, the C language is platform independent. A standards-compliant C program can therefore be compiled for a wide variety of computer systems with few changes to its source code. Moreover, the C compiler was small and easy to port to different CPU architectures, which together with the language’s popularity has made C compilers available on most computer systems.

C Versions

In 1978, Brian Kernighan and Dennis Ritchie produced the first publicly available description of C, now known as K&R C. This description was succeeded in 1989 when the American National Standards Institute (ANSI) provided a comprehensive definition of C known as ANSI C or C89. In the following year, the same specification was adopted as an international standard by the International Organization for Standardization and became known as ISO C90 or just C90. C has since undergone four more revisions by ISO (successively adopted by ANSI) with further language extensions, including C95, C99, C11, and most recently C18, which is the latest ANSI standard for the C programming language. The current C18 standard provided only clarifications to C11 and introduced no new language features.

Table of Contents

Chapter 1:​ Hello World 1
Creating a Project 1
Adding a Source File 2
Hello World 2
IntelliSense 3
Chapter 2:​ Compile and Run 5
Visual Studio Compilation 5
Console Compilation 6
Comments 7
Chapter 3:​ Variables 9
Data Types 9
Declaring Variables 10
Assigning Variables 10
Printing Variables 11
Integer Types 12
Signed and Unsigned 13
Sized Integers 15
Floating-Point Types 16
Literal Suffixes 16
Char Type 17
Bool Type 18
Variable Scope 18
Chapter 4:​ Operators 21
Arithmetic Operators 21
Assignment Operators 22
Increment and Decrement Operators 22
Comparison Operators 23
Logical Operators 23
Bitwise Operators 24
Operator Precedence 25
Chapter 5:​ Pointers 27
Creating Pointers 27
Dereferencing Pointers 27
Pointing to a Pointer 28
Null Pointer 29
Chapter 6:​ Arrays 31
Array Declaration and Allocation 31
Array Assignment 31
Multi-Dimensional Arrays 32
Arrays and Pointers 32
Array Size 33
Chapter 7:​ Strings 35
Escape Characters 36
String Functions 37
Chapter 8:​ Conditionals 39
If Statement 39
Switch Statement 40
Ternary Operator 41
Chapter 9:​ Loops 43
While Loop 43
Do-While Loop 43
For Loop 44
Break and Continue 45
Goto Statement 45
Chapter 10:​ Functions 47
Defining Functions 47
Calling Functions 47
Function Parameters 48
Void Parameter 48
Return Statement 49
Forward Declaration 50
Variable Parameter Lists 51
Pass by Value 52
Pass by Address 53
Return by Value or Address 53
Inline Functions 54
Chapter 11:​ Typedefs 57
Chapter 12:​ Enums 59
Enum Example 60
Enum Constant Values 60
Enum Conversions 61
Enum Scope 61
Chapter 13:​ Structs 63
Struct Objects 63
Member Access 64
Struct Pointers 65
Bit Fields 66
Chapter 14:​ Unions 69
Chapter 15:​ Type Conversions 71
Implicit Conversions 71
Explicit Conversions 72
Chapter 16:​ Storage Classes 73
Auto 73
Register 73
External 74
Static 75
Volatile 76
Chapter 17:​ Constants 79
Constant Variables 79
Constant Pointers 79
Constant Parameters 80
Constant Guideline 81
Chapter 18:​ Preprocessor 83
Including Source Files 84
Define 84
Undefine 85
Predefined Macros 85
Macro Functions 86
Conditional Compilation 87
Compile if Defined 88
Error and Warning 89
Line 89
Pragma 89
Chapter 19:​ Memory Management 91
Malloc 91
Free 92
Realloc 93
Calloc 94
Void Pointers 95
Function Pointers 96
Chapter 20:​ Input Handling 99
Keyboard Input 100
File Input and Output 102
Error Handling 103
Chapter 21:​ Headers 105
Why Use Headers 105
What to Include in Headers 106
Include Guards 108
Chapter 22:​ Strings and Numbers 109
String Conversion 109
Number Conversion 112
Index 113

About the Author and About the Technical Reviewer

About the Author

Mikael Olsson

is a professional web entrepreneur, programmer, and author. He works for an R&D company in Finland where he specializes in software development. In his spare time, he writes books and creates websites that summarize various fields of interest. The books he writes are focused on teaching their subject in the most efficient way possible, by explaining only what is relevant and practical without any unnecessary repetition or theory. The portal to his online businesses and other websites is Siforia.com .

 

About the Technical Reviewer

Michael Thomas
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has worked in software development for more than 20 years as an individual contributor, team lead, program manager, and vice president of engineering. Michael has more than 10 years of experience working with mobile devices. His current focus is in the medical sector, using mobile devices to accelerate information transfer between patients and health care providers.

 
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_1

1. Hello World

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

To begin programming in C, you need a text editor and a C compiler. You can get both at the same time by installing an Integrated Development Environment (IDE) that includes support for C. A good choice is Microsoft’s Visual Studio Community Edition, which is a free version of Visual Studio available from Microsoft’s website.1 This IDE has built-in support for the C89 standard and includes most features up to C99 as of the 2017 version.

Some other popular cross-platform IDEs include Eclipse CDT, Visual Studio Code, Code::Blocks, and CodeLite. Alternatively, you can develop using a simple text editor—such as Notepad—although this is less convenient than using an IDE. If you choose to do so, just create an empty document with a .c file extension and open it in the editor of your choice. By convention, the .c extension is used for files that contain source code for C programs.

Creating a Project

After installing Visual Studio, with the C++ component selected during installation, go ahead and launch the program. You then need to create a project, which will manage the C source files and other resources. Go to File ➤ New ➤ Project to display the New Project window. From there, select the Visual C++ template type in the left frame. Then select the Empty Project template in the right frame. At the bottom of the window you can configure the name and location of the project. When you are finished, click the OK button and the wizard will create your empty project.

Adding a Source File

You have now created a C/C++ project. In the Solution Explorer panel (View ➤ Solution Explorer), you can see that the project consists of four empty folders: External Dependencies, Header Files, Resource Files, and Source Files. Right-click on the Source Files folder and select Add ➤ New Item. From the Add New Item dialog box, choose the C++ File (.cpp) template. Give this source file the name myapp.c. The .c file extension will make the file compile in C instead of C++. Click the Add button, and the empty C file will be added to your project and opened for you.

Hello World

The first thing to add to the source file is the main function. This is the entry point of the program, and the code inside of the curly brackets is what will be executed when the program runs. The brackets, along with their content, are collectively referred to as a code block, or just a block.
int main(void) {}
Your first application will simply output the text "Hello World" to the screen. Before this can be done, the stdio.h header needs to be included. This header provides input and output functionality for the program, and is one of the standard libraries that comes with all C/C++ compilers. What the #include directive does is effectively replace the line with everything in the specified header before the file is compiled.
#include <stdio.h>
int main(void) {}
With stdio.h included you gain access to several new functions, including the printf function that is used for printing text—in this case to a console window. To call this function, you type its name followed by a set of parentheses that includes the text string that will be displayed. The string is delimited by double quotes, and the whole statement is followed by a semicolon. The semicolon is used in C to mark the end of a code statement.
#include <stdio.h>
int main(void) {
  printf("Hello World");
  return 0;
}

The main function here ends with a return statement , which returns a status code as the program exits. This can be useful if the intent is for your program to be executed by another program. The status code can then signal to the caller the success or failure of your program to complete its function. By convention, the return code 0 is used to indicate that a program or function has executed successfully.

The C89 standard requires the return statement to be present, but following C90 the statement became optional. As of C90 the compiler will automatically include the return statement if it is omitted. For brevity the statement will be left out from future code examples.

IntelliSense

When writing code in Visual Studio, a window called IntelliSense will pop up wherever there are multiple predetermined alternatives from which to choose. This window can also be brought up manually at any time by pressing Ctrl+Space to provide quick access to any code entities you are able to use within your program. This is a very powerful feature that you should learn to make good use of.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_2

2. Compile and Run

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Before a program can run, the source code has to be translated into an executable format by a compiler. This step transforms the human-readable source code into binary machine code, which is a sequence of instructions that can be executed by a computer.

Visual Studio Compilation

Continuing from the last chapter, the Hello World program is now complete and ready to be compiled and run. You can do this by going to the Build menu and choosing Debug ➤ Start Without Debugging (Ctrl+F5). Visual Studio then compiles and runs the application that displays the text in a console window.

Depending on your version of Visual Studio, the console window displaying Hello World may close as soon as the main function has finished executing. To prevent this, you need to explicitly specify that this is a console application. First right-click the Project node in the Solution Explorer and then click on Properties to bring up the project’s properties window. From there, navigate to Configuration Properties ➤ Linker ➤ System and set the SubSystem option to Console using the dropdown list. Click OK and the console window will now no longer close automatically.

Another way to prevent the console window from closing is to add a call to the getchar function at the end of main. This function, included with the stdio.h header, will read a character from the keyboard and thereby prevent the program from exiting until the return key is pressed.
#include <stdio.h>
int main(void) {
  printf("Hello World");
  getchar();
}

Console Compilation

As an alternative to using an IDE, you can also compile source files from a terminal window as long as you have a C compiler. For example, on a Linux machine you can use the GNU C compiler, which is available on virtually all UNIX systems—including Linux and the BSD family—as part of the GNU Compiler Collection (GCC). This compiler can also be installed on Windows by downloading MinGW1 or on Mac as part of the Xcode development environment.2

To use the GNU compiler, you type its name gcc in a terminal window and give it the input and output filenames as arguments. It then produces an executable file, which when run gives the same result as one compiled under Windows in Visual Studio.
gcc myapp.c -o myapp.exe
./myapp.exe
Hello World

Comments

Comments are used to insert notes into the source code. They have no effect on the end program and are meant only to enhance the readability of the code, both for you and for other developers. The C89 standard featured only one comment notation, a multiline comment delimited by /* and */.
/* multi-line
   comment */
The C99 standard added the single-line comment, which starts with // and extends to the end of the line. This comment was standardized since it was a convenient feature found in many other programming languages, such as C++. Many C compilers also started to support the single-line comment long before the C99 standard was formalized.
// single-line comment

Keep in mind that whitespace characters—such as comments, spaces, and tabs—are generally ignored by the compiler. This gives you a lot of freedom in how you format your code.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_3

3. Variables

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Variables are used for storing data during program execution.

Data Types

Depending on what data you need to store, there are several different kinds of built-in data types in C. The so-called simple or primitive types are listed in the following table. They consist of four integer types, three floating-point types, as well as the char type .

Data Type

Size (Byte)

Description

char

1

Integer or character

short

int

long

long long

2

4

4 or 8

8

Integer

float

double

long double

4

8

8 or 16

Floating-point number

In C, the exact sizes of the data types are not fixed. The sizes shown in the previous table are those commonly found on 32-bit and 64-bit systems. The C standard only specifies the minimum range that is guaranteed to be supported. The minimum size for char is 8 bits, for short and int it is 16 bits, for long it is 32 bits, and long long must contain at least 64 bits. Most modern compilers make int 32 bits, which nearly universally means 4 bytes. Each integer type in the table must also be at least as large as the one preceding it. The same applies to the floating-point types where each type must provide at least as much precision as the preceding one.

Declaring Variables

Before a variable can be used, it first has to be declared (created). To declare a variable, you start with the data type you want the variable to hold followed by an identifier, which is the name of the variable.
int myInt;
The identifier can consist of letters, numbers, and underscores, but it cannot start with a number. It also cannot contain spaces or special characters and must not be a reserved keyword.
int _myInt32; /* allowed */
int 32Int;    /* incorrect (starts with number) */
int my Int;   /* incorrect (contains space) */
int Int@32;   /* incorrect (contains special character) */
int int;      /* incorrect (reserved keyword) */

Note that C is a case-sensitive programming language, so uppercase and lowercase letters have different meanings.

Assigning Variables

To assign a value to a declared variable, you use the equals sign, which is known as the assignment operator (=). This is called assigning or initializing the variable.
myInt = 50;
The declaration and assignment can be combined into a single statement. When a variable is assigned a value it then becomes defined.
int myInt = 50;
If you need to create more than one variable of the same type, there is a shorthand way of doing this using the comma operator (,).
int x = 1, y = 2, z;
Once a variable has been defined (declared and assigned), you can use it by simply referencing the variable’s name. For example, to copy the value to another variable, you can use the following.
int a = x;

Printing Variables

In addition to strings, the printf function can be used to print values and variables to the standard output stream. This is done by embedding format specifiers into the string where the value is to be printed. Each specifier must be matched by a corresponding argument to printf of the correct type, as seen in the following example.
#include <stdio.h>
int main() {
  int x = 5;
  printf("x is %d and 2+3 is %d", x, 2+3);
}
The %d specifier displays an integer of the char, short, or int type. Other commonly used format specifiers are seen in the following table.

Specifier

Output

%d or %i

char, short, or int

%c

Character

%s

String of characters

%f

float or double

%Lf

long double

%ld

long int

%lld

long long int

%u

Unsigned char, short or int

%lu

Unsigned long int

%llu

Unsigned long long int

%p

Pointer address

For more information on printf and other standard library functions, visit the C library reference on cplusplus.com .1

Integer Types

There are four native integer (whole number) types you can use depending on how large a number you need the variable to hold. Typical ranges on 32-bit and 64-bit systems are given here.
char  myChar  = 0; /* -128   to +127 */
short myShort = 0; /* -32768 to +32767 */
int   myInt   = 0; /* -2^31  to +2^31-1 */
long  myLong  = 0; /* -2^31  to +2^31-1 */
C99 added support for the long long data type, which is guaranteed to be at least 64 bits in size.
long long myLL = 0; /* -2^63 to +2^63-1 */
To determine the exact size of a data type, you can use the sizeof operator. This operator returns the number of bytes that a type occupies in the system you are compiling for. The type returned is size_t, which is an alias for an integer type. The specifier %zu was introduced in C99 as a portable way to format this type with printf. Visual Studio does not support this specifier and uses %Iu instead.
#include <stdio.h>
int main(void) {
  size_t s = sizeof(int);
  printf("%zu", s); /* "4" (C99) */
  printf("%Iu", s); /* "4" (Visual Studio) */
}
In addition to standard decimal notation, integers can also be assigned by using octal or hexadecimal notation. The following values all represent the same number, which in decimal notation is 50.
int myDec = 50    /* decimal notation */
int myOct = 062;  /* octal notation (0) */
int myHex = 0x32; /* hexadecimal notation (0x) */

Signed and Unsigned

By default, all integer types in C are signed and may therefore contain both positive and negative values. This can be explicitly specified using the signed keyword.
signed char  myChar;   /* -128   to +127 */
signed short myShort;  /* -32768 to +32767 */
signed int   myInt;    /* -2^31  to +2^31-1 */
signed long  myLong;   /* -2^31  to +2^31-1 */
signed long long myLL; /* -2^63  to +2^63-1 */
If only positive values need to be stored, the integer types can be declared as unsigned to double their upper range.
unsigned char  uChar;   /* 0 to 255 */
unsigned short uShort;  /* 0 to 65535 */
unsigned int   uInt;    /* 0 to 2^32-1 */
unsigned long  uLong;   /* 0 to 2^32-1 */
unsigned long long uLL; /* 0 to 2^64-1 */
When an unsigned value is printed, the specifier %u is used for the unsigned char, short, and int types. The unsigned long type is specified with %lu and unsigned long long is specified with %llu.
unsigned int uInt = 0;
printf("%u", uInt); /* "0" */
The signed and unsigned keywords may be used as types on their own, in which case the int type is assumed by the compiler.
unsigned uInt; /* unsigned int */
signed sInt;   /* signed int */
In the same way, the short and long data types are abbreviations of short int and long int.
short myShort; /* short int */
long myLong;   /* long int */
 

Sized Integers

As mentioned, the actual sizes of the integer types are implementation dependent. For more precise specification of size, the C99 standard introduced a number of exact-width integer types. They can be enabled by including the stdint.h standard header.
#include <stdint.h>
/* Signed exact-width integers */
int8_t  iSmall;  /* 8 bits */
int16_t iMedium; /* 16 bits */
int32_t iLarge;  /* 32 bits */
int64_t iHuge;   /* 64 bits */
Unsigned versions of these types are available as well. Like the signed versions, these exact-width integer types are guaranteed to have the same number of bits across all implementations.
/* Unsigned exact-width integers */
uint8_t  uSmall;  /* 8 bits */
uint16_t uMedium; /* 16 bits */
uint32_t uLarge;  /* 32 bits */
uint64_t uHuge;   /* 64 bits */
It is recommended to use sized integers when available, to more easily keep track of the range of your integer variables and to enhance the portability of your programs. Compilers that comply with standards prior to C99 may provide sized integers with different type names. Visual Studio, for example, has built-in support for the following signed exact-width integers.
/* Visual Studio signed exact-width integers */
__int8  iSmall;  /* 8 bits */
__int16 iMedium; /* 16 bits */
__int32 iLarge;  /* 32 bits */
__int64 iHuge;   /* 64 bits */
 

Floating-Point Types

The floating-point types can store real numbers with different levels of precision.
float myFloat;    /* ~7 digits */
double myDouble;  /* ~15 digits */
long double myLD; /* typically same as double */
The precision shown here refers to the total number of digits. A float can accurately represent about 7 digits, whereas a double can handle around 15 digits.
float myFloat = 12345.678;
printf("%f", myFloat); /* "12345.677734" */
When printing a floating-point number you can limit the decimal places to, for instance, two in the following way.
printf("%.2f", myFloat); /* "12345.68" */
Floating-point numbers can be expressed using decimal, exponential, or hexadecimal notation. Exponential (scientific) notation is used by adding E or e followed by the decimal exponent, while the hexadecimal floating-point notation uses P or p to specify the binary exponent. Support for the hexadecimal notation was not standardized until C99.
double fDec = 1.23;
double fExp = 3e2;   /* 3*10^2 = 300 */
double fHex = 0xAp2; /* 10*2^2 = 40 */

Literal Suffixes

An integer literal (constant) is normally treated as an int by the compiler, or a larger type if needed to fit the value. Suffixes can be added to the literal to change this evaluation. With integers the suffix can be a combination of U and L, for unsigned and long, respectively. C99 also added the LL suffix for the long long type. The order and casing of these letters do not matter.
int i = 10;
long l = 10L;
unsigned long ul = 10UL;
A floating-point literal is treated as a double. The F or f suffix can be used to specify that a literal is of the float type instead. Likewise, the L or l suffix specifies the long double type.
float f = 1.23F;
double d = 1.23;
long double ld = 1.23L;

The compiler implicitly converts literals to whichever type is necessary, so this type distinction for literals is usually not necessary. If the F suffix is left out when assigning to a float variable, the compiler may give a warning since the conversion from double to float involves a loss of precision.

Char Type

The char type is commonly used to represent ASCII characters. Such character constants are enclosed in single quotes and can be stored in a variable of char type.
char c = 'x'; /* assigns 120 (ASCII for x) */
When the char is printed with the %c format specifier the ASCII character is displayed.
printf("%c", c); /* "x" */
Use the %d specifier to instead display the numerical value.
printf("%d", c); /* "120" */

Bool Type

C99 introduced a _Bool type to increase compatibility with C++. Variables of this type can store a Boolean value, which is a value that can only be either 1 (true) or 0 (false).
_Bool b = 0; /* false value */
The type _Bool is usually accessed via its alias name bool defined by the standard header stdbool.h. This header also defines the macros true and false as aliases for 1 and 0.
#include <stdbool.h>
bool b = true; /* true value */

Variable Scope

The scope of a variable refers to the region of code within which it is possible to use that variable. Variables in C may be declared both globally and locally. A global variable is declared outside of any code blocks and is accessible from anywhere after it has been declared. A local variable, on the other hand, is declared inside of a function and will only be accessible within the function in which it was declared. The lifetime of a local variable is also limited. A global variable will remain allocated for the duration of the program, while a local variable will be destroyed when its function has finished executing.
int globalVar; /* global variable */
int main(void) {
  int localVar; /* local variable */
}
The default values for these variables are also different. Global variables are automatically initialized to zero by the compiler, whereas local variables are not initialized at all. Uninitialized local variables will therefore contain whatever garbage is already present in that memory location.
int globalVar; /* initialized to 0 */
int main(void) {
  int localVar; /* uninitialized */
}
Using uninitialized variables is a common programming mistake that can produce unexpected results. It is therefore a good idea to always give your local variables initial values when they are declared.
int main(void) {
  int localVar = 0; /* initialized to 0 */
}
In C89, local variables must be declared before any other statements within their scope. The later C99 standard changed this to allow variables to be declared anywhere within a function’s scope, which can be more intuitive.
int main(void) {
  int var1;
  /* Other statements */
  int var2; /* C99 only */
}
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_4

4. Operators

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

A numerical operator is a symbol that makes the program perform a specific mathematical or logical manipulation. The numerical operators in C can be grouped into five types: arithmetic, assignment, comparison, logical, and bitwise operators.

Arithmetic Operators

There are the four basic arithmetic operators, as well as the modulus operator (%), which is used to obtain the division remainder.
int x = 3 + 2; /* 5 - addition */
    x = 3 - 2; /* 1 - subtraction */
    x = 3 * 2; /* 6 - multiplication */
    x = 3 / 2; /* 1 - division */
    x = 3 % 2; /* 1 - modulus (division remainder) */
Notice that the division sign gives an incorrect result. This is because it operates on two integer values and will therefore truncate the result and return an integer. To get the correct value, one of the numbers must be explicitly converted to a floating-point number.
float f = 3 / (float)2; /* 1.5 */

Assignment Operators

The second group is the assignment operators. Most important, it is the assignment operator (=) itself, which assigns a value to a variable.
int x = 0; /* assignment */
A common use of the assignment and arithmetic operators is to operate on a variable and then to save the result back into that same variable. These operations can be shortened with the combined assignment operators.
x += 5; /* x = x+5; */
x -= 5; /* x = x-5; */
x *= 5; /* x = x*5; */
x /= 5; /* x = x/5; */
x %= 5; /* x = x%5; */

Increment and Decrement Operators

Another common operation is to increment or decrement a variable by one. This can be simplified with the increment (++) and decrement (--) operators.
x++; /* x = x+1; */
x--; /* x = x-1; */
Both of these can be used either before or after a variable.
x++; /* post-increment */
x--; /* post-decrement */
++x; /* pre-increment */
--x; /* pre-decrement */
The result on the variable is the same whichever is used. The difference is that the post-operator returns the original value before it changes the variable, while the pre-operator changes the variable first and then returns the value.
int x, y;
x = 5; y = x++; /* y=5, x=6 */
x = 5; y = ++x; /* y=6, x=6 */

Comparison Operators

The comparison operators compare two values and return either 1 or 0, representing true or false. They are mainly used to specify conditions, which are expressions that evaluate to either true or false.
int x = (2 == 3); /* 0 - equal to */
    x = (2 != 3); /* 1 - not equal to */
    x = (2 > 3);  /* 0 - greater than */
    x = (2 < 3);  /* 1 - less than */
    x = (2 >= 3); /* 0 - greater than or equal to */
    x = (2 <= 3); /* 1 - less than or equal to */

Logical Operators

The logical operators are often used together with the comparison operators. Logical and (&&) evaluates to true if both the left and right sides are true, and logical or (||) is true if either the left or right side is true. For inverting a Boolean result, there is the logical not (!) operator. Note that for both “logical and” and “logical or” the right side will not be evaluated if the result is already determined by the left side.
int x = (1 && 0); /* 0 - logical and */
    x = (1 || 0); /* 1 - logical or  */
    x = !(1);     /* 0 - logical not */
Recall that as of C99, the stdbool.h header can be included to use the bool type to store Boolean values. The header also defines the constants true and false to represent 1 and 0, which allows the previous example to be rewritten as seen here.
#include <stdbool.h>
/* ... */
bool b = (true && false); /* false - logical and */
     b = (true || false); /* true  - logical or  */
     b = !(true);         /* false - logical not */

Bitwise Operators

The bitwise operators can manipulate individual bits inside an integer. For example, the bitwise left shift operator (<<) moves all bits to the left with the specified number of steps.
int x = 5 & 4;  /* 101 & 100 = 100 (4) - and */
    x = 5 | 4;  /* 101 | 100 = 101 (5) - or */
    x = 5 ^ 4;  /* 101 ^ 100 = 001 (1) - xor */
    x = 4 << 1; /* 100 << 1  =1000 (8) - left shift */
    x = 4 >> 1; /* 100 >> 1  =  10 (2) - right shift */
    x = ~4;     /* ~00000100 = 11111011 (-5) - invert */
The bitwise operators also have combined assignment operators.
int x=5; x &= 4;  /* 101 & 100 = 100 (4) - and */
    x=5; x |= 4;  /* 101 | 100 = 101 (5) - or */
    x=5; x ^= 4;  /* 101 ^ 100 = 001 (1) - xor */
    x=4; x <<= 1; /* 100 << 1  =1000 (8) - left shift */
    x=4; x >>= 1; /* 100 >> 1  =  10 (2) - right shift */

Operator Precedence

In C, expressions are normally evaluated from left to right. However, when an expression contains multiple operators, the precedence of those operators decides the order in which they are evaluated. The order of precedence can be seen in the following table, where the operator with the lowest precedence will be evaluated first. This same order also applies to many other languages, such as C++ and C#.

Pre

Operator

Pre

Operator

1

() [] . -> x++ x--

8

&

2

! ~ ++x --x (type) sizeof * &

9

^

3

* / %

10

|

4

+ -

11

&&

5

<< >>

12

||

6

< <= > >=

13

= op=

7

== !=

14

,

To give an example, multiplication binds harder than addition and will therefore be evaluated first in the following line of code.
int x = 4 + 3 * 2; /* 10 */
This can be clarified by enclosing the part of the expression that will be evaluated first in parentheses. As seen in the table, parentheses have the highest precedence of all operators.
int x = 4 + (3 * 2); /* 10 */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_5

5. Pointers

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

A pointer is a variable that contains the memory address of another variable, called the pointee.

Creating Pointers

Pointers are declared as any other variable, except that an asterisk (*) is placed between the data type and the pointer’s name. The data type determines what type of memory it will point to.
int* p; /* pointer to an integer */
int *q; /* alternative syntax */
A pointer can point to a variable of the same type by prefixing that variable with an ampersand, in order to retrieve its address and assign it to the pointer. The ampersand is known as the address-of operator (&).
int i = 10;
p = &i; /* address of i assigned to p */

Dereferencing Pointers

The pointer now contains the memory address to the integer variable. Referencing the pointer will retrieve this address. To obtain the actual value stored in that address, the pointer must be prefixed with an asterisk, known as the dereference operator (*).
printf("Address of i: %p \n", p); /* ex. 0017FF1C */
printf("Value of i: %d",  *p); /* 10 */
When writing to the pointer the same method is used. Without the asterisk the pointer is assigned a new memory address, and with the asterisk the actual value of the variable pointed to will be updated.
p = &i;  /* address of i assigned to p */
*p = 20; /* value of i changed through p */
If a second pointer is created and assigned the value of the first pointer, it will then get a copy of the first pointer’s memory address.
int* p2 = p; /* copy address stored in p */

Pointing to a Pointer

Sometimes it can be useful to have a pointer that can point to another pointer. This is done by declaring a pointer with two asterisks and then assigning it the address of the pointer that it will reference. This way, when the address stored in the first pointer changes, the second pointer can follow that change.
int** r = &p; /* pointer to pointer */
Referencing the second pointer now gives the address of the first pointer. Dereferencing the second pointer gives the address of the variable, and dereferencing it again gives the value of the variable.
printf("Address of p: %p \n",  r); /* ex. 0017FF28 */
printf("Address of i: %p \n", *r); /* ex. 0017FF1C */
printf("Value of i: %d",  **r); /* 20 */

Null Pointer

A pointer should be set to zero when it is not assigned to a valid address. Such a pointer is called a null pointer . Doing this allows you to check whether the pointer can be safely dereferenced, because a valid pointer will never be zero.
int* p = 0; /* null pointer */
The constant NULL can also be used to signify a null pointer. NULL is typically defined as zero in C, making the choice of which to use a matter of preference. The constant is defined by several standard library files, including stdio.h and stddef.h.
#include <stdio.h>
/* ... */
int* p = NULL; /* null pointer */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_6

6. Arrays

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

An array is a data structure used to store a collection of values that all have the same data type.

Array Declaration and Allocation

To declare an array, you start as you would a normal variable declaration, but in addition, you append a set of square brackets following the array’s name. The brackets contain the number of elements in the array. The default values for these elements are the same as for variables—elements in global arrays are initialized to their default values, and elements in local arrays remain uninitialized.
int myArray[3]; /* integer array with 3 elements */

Array Assignment

To assign values to the elements, you can reference them one at a time by placing the element’s index inside the square brackets, starting with zero.
myArray[0] = 1;
myArray[1] = 2;
myArray[2] = 3;
Alternatively, you can assign values at the same time as the array is declared by enclosing them in curly brackets. The specified array length may optionally be left out to let the array size be decided by the number of values assigned.
int myArray[3] = { 1, 2, 3 };
int myArray[] = { 1, 2, 3 }; /* alternative */
Once the array elements are initialized, they can be accessed by referencing the index of the element you want.
printf("%d", myArray[0]); /* 1 */

Multi-Dimensional Arrays

Arrays can be made multi-dimensional by adding more sets of square brackets. As with single-dimensional arrays, they can either be filled in one at a time or all at once during the declaration.
int mArray[2][2] = { { 0, 1 }, { 2, 3 } };
mArray[0][0] = 0;
mArray[0][1] = 1;
The extra curly brackets are optional, but including them is good practice since it makes the code easier to understand.
int mArray[2][2] = { 0, 1, 2, 3 }; /* alternative */

Arrays and Pointers

An array in C can be treated as a constant pointer that points to the first element in the array. As such, array elements can referenced just as well with pointer arithmetic. By incrementing the pointer by one, you move to the next element in the array, because changes to a pointer’s address are implicitly multiplied by the size of the pointer’s data type.
*(myArray+1) = 10; /* myArray[1] = 10; */
Pointer arithmetic is an advanced feature that should be used with care. The four arithmetic operators that can be used with pointers include: +, -, ++, and --.
int* ptr = &myArray;
printf("Address of myArray[0]: %p \n", ptr); /* ex. 0028FF14 */
ptr++;
printf("Address of myArray[1]: %p", ptr); /* ex. 0028FF18 */

Array Size

Just as with any other pointer it is possible to exceed the valid range of an array and thereby rewrite some adjacent memory. This should always be avoided since it can lead to unexpected results or crash the program.
int myArray[2] = { 1, 2 };
myArray[2] = 3; /* out of bounds */
To determine the length of a regular (statically allocated) array, the sizeof operator can be used.
int length = sizeof(myArray) / sizeof(int); /* 2 */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_7

7. Strings

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
A string consists of an array of characters and is delimited by double quotes. There is no string type for storing strings in C. Instead, strings are commonly assigned to a character array, as shown here.
char myString[] = "Hi";
Strings in C are terminated with a null character \0, which is used to know where the string ends. The null character is added automatically by the compiler for quoted strings, as in the previous example. The same statement can also be written using regular array initialization syntax, in which case the null character needs to be explicitly included.
char myString[3] = { 'H', 'i', '\0' };
Both statements produce the same result: a char array with three elements. Note that individual characters are delimited by single quotes and not double quotes. To print a string the format specifier %s is used with the printf function, which outputs the literals of the string until the null character is encountered.
printf("%s", myString); /* "Hi" */
As an alternative to the character array, a char pointer may be set to point to a string. The string is then automatically stored in the compiled file, giving the pointer a location to point to. In most compilers this location is a read-only block, so unlike with the char array, the characters in this string cannot be changed.
char* ptr = "Hi";
printf("%s", ptr); /* "Hi" */

Escape Characters

To add new lines to a string, the escape character \n is used to represent a line break.
printf("First line\nSecond line");
This backslash notation is used to write special characters that are difficult or impossible to type on a regular keyboard. In addition to the newline and null characters, there are several other such characters, as seen in the following table.

Character

Meaning

Character

Meaning

\n

Newline

\f

Form feed

\t

Horizontal tab

\a

Alert sound

\v

Vertical tab

\'

Single quote

\b

Backspace

\"

Double quote

\r

Carriage return

\\

Backslash

\0

Null character

\?

Question mark

\000

Octal number (1-3 digits)

\xhh

Hexadecimal number

Any one of the 128 ASCII characters can be expressed by writing a backslash followed by the ASCII code for that character, represented as either an octal or hexadecimal number. This is illustrated here, where the newline character is represented in three different ways:
char line = '\n';   /* escape code */
     line = '\012'; /* octal notation */
     line = '\x0A'; /* hexadecimal notation */

String Functions

Because strings in C are arrays, the only way to make changes to them is to change each element in the array. To simplify common string operations, the standard header string.h includes a collection of functions for manipulating null-terminated strings. Consider the following code, which will be used as the template for the following string examples.
#include <stdio.h>
#include <string.h>
int main(void) {
  char s1[11] = "Hello";
  char s2[11] = "World";
  int result;
}
The string function strcat (string concatenation) appends the second string onto the first string. For this to work, it is important that the destination is large enough to hold the entire string.
/* Append s2 to s1 */
strcat(s1, s2); /* s1 = "HelloWorld" */
Another string function is strcpy , which copies the characters in the second string into the first string. The function stops when the terminating null character for the second argument is reached.
/* Copy s1 into s2 */
strcpy(s2, s1); /* s2 = "HelloWorld" */
A string can be compared with another string using the strcmp function. If all characters match, the function returns zero.
/* Compare s1 and s2 */
result = strcmp(s1, s2); /* 0 (equal) */
When manipulating strings, it is important to take their lengths into account to avoid overwriting adjacent memory. The length of a string stored in a char array can be found with the strlen function . With regular (statically allocated) strings, the allocated size can also be retrieved using the sizeof operator.
/* Length of s1 (excluding null char) */
result = strlen(s1); /* 10 */
/* Allocated size for s1 */
result = sizeof(s1); /* 11 */
The next example shows how the sizeof operator can be used to allocate a character array large enough to hold two other strings. Note that the resulting string will only have one null character, so the size of its allocated array can be one character less than the sizes of the two other strings.
char a[] = "Hello"; /* sizeof(a) = 6 */
char b[] = "World"; /* sizeof(b) = 6 */
char c[sizeof(a) + sizeof(b) -1]; /* sizeof(c) = 11 */
strcpy( c, a );
strcat( c, b ); /* "HelloWorld" */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_8

8. Conditionals

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Conditional statements are used to execute different code blocks based on different conditions.

If Statement

The if statement will execute only if the expression inside the parentheses is evaluated to true. In C, this does not have to be a Boolean expression. It can be any expression that evaluates to a number, in which case zero is false and all other numbers are true.
if (x < 1) {
  printf("x < 1");
}
To test for other conditions, the if statement can be extended by any number of else/if clauses.
else if (x > 1) {
  printf("x > 1");
}
The if statement can have one else clause at the end, which will execute if all the previous conditions are false.
else {
  printf("x == 1");
}
As for the curly brackets, they can be left out if only a single statement needs to be executed conditionally. However, it is considered good practice to always include them since they improve readability.
if (x < 1)
  printf("x < 1");
else if (x > 1)
  printf("x > 1");
else
  printf("x == 1");
Any numeric value except for 0 is considered true when evaluated in a Boolean context. This allows a true or false check to be simplified in the following manner.
int done = 0;
/* ... */
/* if not done – same as if(done == 0) */
if(!done) {}
/* if done – same as if(done != 0) */
if(done) {}

Switch Statement

The switch statement checks for equality between an integer and a series of case labels, and then passes execution to the matching case. It may contain any number of case clauses, and it can end with a default label for handling all other cases.
switch (x) {
  case 0: printf("x is 0"); break;
  case 1: printf("x is 1"); break;
  default: printf("x is not 0 or 1"); break;
}

Note that the statements after each case label end with the break keyword to skip the rest of the switch. If the break is left out, execution will fall through to the next case, which can be useful if several cases need to be evaluated in the same way.

Ternary Operator

In addition to the if and switch statements, there is the ternary operator (?:) that can replace a single if/else clause. This operator takes three expressions. If the first one is true then the second expression is evaluated and returned; and if it is false, the third one is evaluated and returned.
x = (x < 0.5) ? 0 : 1; /* ternary operator (?:) */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_9

9. Loops

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

There are three looping structures in C, all of which are used to execute a specific code block multiple times. Just as with the conditional if statement, the curly brackets for the loops can be left out if there is only one statement in the code block.

While Loop

The while loop runs through the code block only if its condition is true, and it will continue looping for as long as the condition remains true. Bear in mind that the condition is only checked at the start of each iteration (each loop).
int i = 0;
while (i < 10) {
  printf("%d", i++); /* 0-9 */
}

Do-While Loop

The do-while loop works in the same way as the while loop, except that it checks the condition after the code block. It will therefore always run through the code block at least once. Notice that this loop ends with a semicolon.
int j = 0;
do {
  printf("%d", j++); /* 0-9 */
} while (j < 10);

For Loop

The for loop is used to run through a code block a specific number of times. It uses three parameters. The first one initializes a counter and is always executed once before the loop. The second parameter holds the condition for the loop and is checked before each iteration. The third parameter contains the increment of the counter and is executed at the end of each iteration.
int k;
for (k = 0; k < 10; k++) {
  printf("%d", k); /* 0-9 */
}
Since the C99 standard the first parameter may contain a declaration, typically a counter variable. The scope of this variable is limited to the for loop.
for (int k = 0; k < 10; k++) {
  printf("%d", k); /* 0-9 */
}
The for loop has several variations. One such variation is to split the first and third parameters into several statements by using the comma operator.
int k, m;
for (k = 0, m = 0; k < 10; k++, m--) {
  printf("%d", k+m); /* 000... (10x) */
}
Another option is to leave out any one of the parameters. If all parameters are left out, it becomes a never-ending loop, unless there is another exit condition defined .
for (;;) { /* infinite loop */ }

Break and Continue

There are two jump statements that can be used inside loops: break and continue . The break keyword ends the loop structure, and continue skips the rest of the current iteration and continues at the beginning of the next iteration.
int i;
for (i = 0; i < 10; i++)
{
  if (i == 2) continue; /* start next iteration */
  else if (i == 5) break; /* end loop */
  printf("%d", i); /* "0134" */
}

Goto Statement

A third jump statement that may be useful to know of is goto, which performs an unconditional jump to a specified label within the same function. This instruction is generally never used since it tends to make the flow of execution difficult to follow.
goto myLabel; /* jump to label */
/* ... */
myLabel: /* label declaration */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_10

10. Functions

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Functions are reusable code blocks that will only execute when called. They allow developers to divide their programs into smaller parts that are easier to understand and reuse.

Defining Functions

A function can be created by typing void followed by the function’s name, a set of parentheses containing another void, and a code block. The first use of the void keyword specifies that this function will not return a value. The second void inside the parentheses means that the function does not accept any arguments.
void myFunction(void) {
  printf("Hello World");
}

Calling Functions

The previous function will print out a text message when it is called. To invoke it, the function’s name is specified followed by an empty set of parentheses.
int main(void) {
  myFunction(); /* "Hello World" */
}

Function Parameters

The parentheses that follow the function’s name are used for passing arguments to the function. To do this, you must first add the corresponding parameters to the function’s parameter list.
void sum(int a, int b) {
  int sum = a + b;
  printf("%d", sum);
}
A function can be defined to take any number of arguments, and they can have any data types. Just ensure the function is called with the same types and number of arguments. In this example, the function accepts two integer arguments and displays their sum.
sum(2, 3); /* "5" */

To be precise, parameters appear in function definitions, while arguments appear in function calls. However, the two terms are sometimes used interchangeably.

Void Parameter

In C, functions that leave out the void keyword from their parameter list are allowed to accept an unknown number of arguments. This is different from C++, where leaving out void means the same as including it: that the function takes no arguments. Therefore, to have the compiler ensure that no arguments are mistakenly passed to a parameterless function, it is necessary in C to include void in the parameter list.
/* Accepts no arguments */
void foo(void) {}
/* Accepts an unknown number of arguments */
void bar() {}

As of C99, the use of an empty parameter list has been deprecated and results in a warning from the compiler.

Return Statement

A function can return a value. The void keyword before the function’s name is then replaced with the data type the function will return, and the return keyword is added to the function’s body followed by an argument of the specified return type.
int getSum(int a, int b) {
  return a + b;
}
Return is a jump statement that causes the function to exit and return the specified value to the place where the function was called. To illustrate, the previous function can be passed as an argument to the printf function since it evaluates to an integer.
printf("%d", getSum(5, 10)); /* "15" */
The return statement can also be used in a void function as a way to exit the function before the end block is reached.
void dummy(void) { return; }
The main function must be set to return an int type, but including an explicit return value is only required in the C89 standard. As of C90 the compiler will automatically add a return statement to the end of the main function if no such statement is present, and with the C99 standard, this implicit return value is guaranteed to be zero.
int main(void) {
  return 0; /* optional */
}

Forward Declaration

An important thing to keep in mind in C is that a function must be declared before it can be called. This can either be achieved by placing the function’s implementation before any references to it, or by adding a declaration of the function before it is called. This kind of forward declaration is known as a prototype and provides the compiler with the information needed to allow the function to be used before it has been defined.
void myFunction(int a); /* prototype */
int main(void) {
  myFunction(0);
}
void myFunction(int a) {}
The parameter names do not need to be included in the prototype; only the data types are required.
void myFunction(int);
In early versions of C, an undeclared function that is referenced is implicitly declared as a function that returns an int and takes an unspecified number of parameters. Relying on this behavior is not recommended and usually results in a warning from the compiler. As of C99 this feature has been removed and will instead result in an error.
int main(void) {
  foo();
}
/* Warning: implicit declaration of foo */
int foo() { return 0; }

Variable Parameter Lists

A function can be defined to accept a variable number of arguments, similar to the printf function. The parameter list of such a function must end with an ellipsis (…) and there must be at least one additional parameter. An int parameter is typically included to let the function know the number of extra arguments that are passed to it.

In the following example, the function accepts a variable number of arguments that are summed up and returned to the caller. To access these arguments the stdarg.h header file is included. This header defines a new type, called va_list, and three functions that operate on variables of this type: va_start, va_arg, and va_end.
#include <stdio.h>
#include <stdarg.h>
int sum(int num, ...) {
  va_list args; /* variable argument list */
  int sum = 0, i = 0;
  va_start(args, num); /* initialize argument list */
  for (i = 0; i < num; i++) /* loop through arguments */
    sum += va_arg(args, int); /* get next argument */
  va_end(args); /* free memory */
  return sum;
}
int main(void) {
  printf("Sum of 1+2+3 = %d", sum(3,1,2,3)); /* 6 */
}

In contrast to C++, C does not allow function overloading or default parameter values. However, variable parameter lists can be used to implement functions that behave in similar ways.

Pass by Value

Variables are by default passed by value. This means that only a copy of the value is passed to the function. Therefore, changing the parameter in any way will not affect the original variable, and passing large variables back and forth can have a negative impact on performance.
#include <stdio.h>
void set(int i) { i = 1; }
int main(void) {
  int x = 0;
  set(x);
  printf("%d", x); /* "0" */
}

Pass by Address

The alternative to passing by value is to use pointer syntax to instead pass the variable by address. When an argument is passed by address, the parameter can be changed or replaced, and the change will affect the original variable.
void set(int* i) { *i = 1; }
int main(void) {
  int x = 0;
  set(&x);
  printf("%d", x); /* "1" */
}
Recall that arrays can be treated as pointers. As such they will automatically be passed by address, as shown in the following example.
void set(int a[]) { a[0] = 1; }
int main(void) {
  int x[] = { 0 };
  set(x);
  printf("%d", x[0]); /* "1" */
}

Return by Value or Address

In addition to passing variables by value or address, a variable may also be returned in one of these two ways. By default a function returns by value, in which case a copy of the value is returned to the caller.
int byVal(int i) { return i + 1; }
int main(void) {
  int a = 10;
  printf("%d", byVal(a)); /* "11" */
}
To instead return by address, the dereference operator is appended to the function’s return type. The function must then return a variable and not an expression or literal, as is allowed when returning by value. The variable returned should never be a local variable since the memory to these variables is released when the function ends. Instead, return by address is commonly used to return an argument that has also been passed to the function by address.
int* byAdr(int* i) { (*i)++; return i; }
int main(void) {
  int a = 10;
  int *p = byAdr(&a);
  printf("%d", *p); /* "11" */
}

Inline Functions

When calling a function it is important to keep in mind that a certain performance overhead occurs. To potentially remove this overhead, the programmer can recommend that the compiler inlines the calls to a specific function by using the inline function modifier. This keyword was added in the C99 standard. It is most suited for use with small functions that are called inside loops, as shown in the following example. Larger functions should not be inlined since this can significantly increase the size of the code, which may instead decrease performance.
inline int increment(int a) { return ++a; }
int main(void) {
  int i;
  for(i = 0; i < 100;) {
    i = increment(i);
  }
}

Note that the inline keyword is only a recommendation. The compiler may—in its attempts to optimize the code—choose to ignore this recommendation, and it may also inline functions that do not have the inline modifier .

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_11

11. Typedefs

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
An alias for a type can be created using the typedef keyword followed by the type and alias name. By convention, uppercase letters are commonly used for these definitions.
typedef unsigned char BYTE;
Once defined, the alias can be used as a synonym for its specified type.
BYTE b; /* unsigned char */
typedef does not only work for existing types, but can also include a definition of a user-defined type—such as a struct, union, or enum. This can make a complex type easier to understand.
typedef struct { int points; } score;
score a, b, c;
a.points = 10;

If used properly, a type alias can simplify a long or confusing type name, making the code easier to understand. Another benefit they provide is the ability to change the definition of a type from a single location, which can help make a program more portable.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_12

12. Enums

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
An enum is a user-defined type consisting of a fixed list of named constants. In the following example, the enumeration type is called color and contains three constants: RED, GREEN and BLUE.
enum color { RED, GREEN, BLUE };
The color type can be used to create variables that may hold one of these constant values. In C, the variable declaration must be preceded by enum, whereas this is optional in C++.
int main(void) {
  enum color c = RED;
}
Enum variables may also be declared when the enum is defined, by placing the variable names before the final semicolon. This position is known as the declarator list.
enum color { RED, GREEN, BLUE } c, d;

Enum Example

The switch statement provides a good example of when enumerations can be useful. Compared to using ordinary constants, the enumeration has the advantage that it lets the programmer clearly specify what values a variable should be allowed to contain.
switch(c) {
  case RED:   break;
  case GREEN: break;
  case BLUE:  break;
}
Bear in mind that enums in C are not typesafe, unlike their C++ equivalent. It is up to the programmer to ensure that enum types and constants are used correctly, as most compilers will not enforce this. Enums in C simply provide a way to group a set of integer constants and have them be automatically numbered. For this purpose, the enum identifier is not strictly necessary and may optionally be omitted.
enum { RED, GREEN, BLUE } c;

Enum Constant Values

Enumerated constants are of the int type. Usually there is no need to know the underlying values that these constants represent, but in some cases it can be useful. By default, the first constant in the enum list has the value zero and each successive constant is one value higher.
enum color {
  RED   /* 0 */
  GREEN /* 1 */
  BLUE  /* 2 */
};
These default values can be overridden by assigning values to the constants. The values can be computed and do not have to be unique.
enum color {
  RED = 5,       /* 5 */
  SCARLET = RED, /* 5 */
  BLUE = RED+2,  /* 7 */
};

Enum Conversions

The compiler can implicitly convert an enumerated constant to an integer. An integer can also be converted back into an enum variable.
int i = RED;
enum color c = i;
Some compilers warn when an integer is assigned to an enum variable since this makes it possible to assign a value that is not one of its specified constants. To suppress this warning, an explicit type cast can be used.
enum color c = (enum color)i;

Enum Scope

An enum does not have to be declared globally. It can also be placed locally within a function, in which case it will only be usable within the function in which it was defined.
/* Global enum */
enum speed { SLOW, NORMAL, FAST };
int main(void) {
  /* Local enum */
  enum color { RED, GREEN, BLUE };
}
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_13

13. Structs

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
A struct or structure is a user-defined type used for grouping a collection of related variables under a single name. To define a structure you use the struct keyword, followed by an optional identifier and a code block containing variable declarations. The definition of this new type ends with a semicolon.
struct point {
  int x, y;
};

Unlike arrays, structs allow data items of different kinds to be combined. Structs may contain variables, pointers, arrays, or other user-defined types. In contrast to C++, structs in C may not contain functions.

Struct Objects

To declare a variable of a struct type, the struct keyword is followed by the type name and the variable identifier. Variables of struct type are commonly referred to as objects or instances.
int main(void) {
  struct point p; /* object declaration */
}
Objects may also be created when the struct is defined, by placing the object names before the final semicolon. This position is called the declarator list. If the optional struct identifier is left out, this becomes the only way to create objects of the struct type. Such a struct without an identifier is called an unnamed struct and provides a way for programmers to prevent any more instances of the type from being created.
struct /* unnamed struct */
{
  int x, y;
} a, b; /* object declarations */
It is common in C to use typedef when defining structures. This aliasing removes the need to include the struct keyword when declaring objects of the struct type, resulting in the shorter syntax used in C++.
typedef struct point point;
struct point {
  int x, y;
};
int main(void) {
  point p; /* struct omitted */
}

Member Access

Variables of a struct type are called fields or members. These fields are accessed using the member of operator (.) prefixed by the object name. Fields of an object are by default undefined, so it is important to assign them a value before they are read.
int main(void) {
  point p;
  p.x = 1;
  p.y = 2;
}
Similar to an array, struct objects may also be initialized when they are declared by enclosing the values in curly brackets. The values are then assigned in order based on the corresponding members of the struct. This way of assigning values to a composite type is known as aggregate initialization.
struct point {
  int x, y;
} r = { 1, 2 }; /* assigns x and y */
int main(void) {
  point p = { 1, 2 };
}
C99 introduced designated initializers, which allow structures to be initialized in any order by specifying the names of the fields. Any omitted fields will be automatically initialized to 0.
int main(void) {
  point p = { .y = 2, .x = 1 };
}

Struct Pointers

A struct is a value type, not a reference type like an array. As such, any assignment or argument passing for objects will copy the field values and not the object reference. This is different from many modern languages where composite types are automatically assigned and passed by reference.
int main(void) {
  point p = { 1, 2 };
  point r = p; /* copies field values */
}
For large structures the performance cost of this copy operation may be significant. Therefore it is common to use pointers when passing objects to functions, to avoid having to copy and return the whole object.
void init_struct(point* a) {
  (*a).x = 1;
  (*a).y = 2;
}
int main(void) {
  point p;
  init_struct(&p);
}
As shown in this example, the pointer must be dereferenced before the member of the operator can be used to access the fields. Since this operation is so common, there is a syntactical shortcut available, known as the infix operator (->), which automatically dereferences the pointer.
point p;
point* r = &p;
r->x = 1; /* same as (*r).x = 1; */
 

Bit Fields

The C programming language offers a way to optimize memory use within a struct type by allowing the bit length of integer fields to be specified. Such a field is called a bit field , and its length is set by placing a colon after the field name followed by the number of bits. The length must be less than or equal to the bit length of the specified type.
struct my_bits
{
  unsigned short f1 : 1;
  unsigned short f2 : 1;
  unsigned short id : 10;
} a;
Bit fields are packed as compactly as possible, while keeping in mind that the size of an object needs to be a multiple of the size of the types it contains. In this case the needed 12 bits will require 16 bits (two bytes) to be reserved for the object, as that is the size of the short type. Had bit fields not been used, the three shorts and consequently the struct would occupy 48 bits instead.
int main(void) {
  printf("%d bytes", sizeof(a)); /* "2 bytes" */
}

This feature is useful when programming for embedded systems, where hardware resources may be very constrained.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_14

14. Unions

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
The union type is identical to the struct type, except that all fields share the same memory position. Therefore, the size of a union is the size of the largest field it contains. In the following code, this is the integer field, which is four bytes.
union mix {
  char c;  /* 1 byte */
  short s; /* 2 bytes */
  int i;   /* 4 bytes */
};
Given this memory sharing, the union type can only be used to store one value at a time, because changing one field will overwrite the value of the others.
int main(void) {
  union mix m;
  m.c = 0xFF; /* set first 8 bits */
  m.s = 0;    /* reset first 16 bits */
}
The benefit of a union, in addition to efficient memory usage, is that it provides multiple ways of using the same memory location. For example, the following union has three data members that allow access to the same group of four bytes in different ways.
union mix {
  char c[4];                  /* 4 bytes */
  struct { short hi, lo; } s; /* 4 bytes */
  int i;                      /* 4 bytes */
} m;
The int field accesses all four bytes at once, the struct two bytes at a time, and the char array allows each byte to be referenced individually. The bit pattern for this is illustrated in the next example. Keep in mind that the internal order of bytes for primitive data types is not defined in C. Because of this, the order of the four bytes that make up the int may be reversed on some platforms.
m.i=0xFF00F00F; /* 11111111 00000000 11110000 00001111 */
m.s.lo;         /* 11111111 00000000                   */
m.s.hi;         /*                   11110000 00001111 */
m.c[3];         /* 11111111                            */
m.c[2];         /*          00000000                   */
m.c[1];         /*                   11110000          */
m.c[0];         /*                            00001111 */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_15

15. Type Conversions

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Converting an expression from one type to another is known as type casting or type conversion. This can be done either implicitly by the compiler or explicitly with code.

Implicit Conversions

An implicit conversion is performed automatically by the compiler when an expression needs to be converted into one of its compatible types. For example, any conversions between the primitive data types can be done implicitly.
long l = 5;   /* int -> long */
double d = l; /* long -> double */
These implicit conversions can also take place within an expression, allowing you to mix different primitive types together. When types of different sizes are involved, the result will be of the larger type, so an int and double will produce a double value.
double d = 5 + 2.5; /* int -> double */
Implicit conversions of primitive types can be further grouped into two kinds: promotion and demotion. Promotion occurs when an expression gets implicitly converted into a larger type, and demotion occurs when converting an expression to a smaller type.
/* Promotion */
long l = 5;   /* int promoted to long */
double d = l; /* long promoted to double */
/* Demotion */
int i = 10.5; /* warning: possible loss of data */
char c = i;   /* warning: possible loss of data */

Because a demotion can result in the loss of information, these conversions generate a warning on many compilers. If the potential information loss is intentional, the warning can be suppressed by using an explicit cast.

Explicit Conversions

An explicit cast is performed by placing the desired data type in parentheses to the left of the expression that needs to be converted.
int i = (int)10.5; /* double demoted to int */
char c = (char)i;  /* int demoted to char */

Keep in mind that casting a variable only makes it temporarily evaluate as a different type; it does not change the variable’s underlying type.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_16

16. Storage Classes

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Every variable has a storage class that determines its scope and lifetime. These storage classes include the following: auto, register, extern, and static. Each of these classes is also a keyword that can be placed before the data type to determine to which storage class a variable belongs.

Auto

The default storage class for local variables is auto, which can be explicitly specified with the auto keyword. Memory for automatic variables is allocated when the code block is entered and freed upon exit. The scope of these variables is local to the block in which they are declared, as well as any nested blocks.
int main(void) {
  auto int localVar; /* auto variable */
}

Register

The register storage class hints to the compiler that a local variable will be heavily used and should therefore be kept in a CPU register instead of RAM memory to provide quicker access. Variables of the register storage class cannot use the address-of operator (&), since registers do not have memory addresses. They also cannot be larger than the register size, which is usually the same as the processor’s word size.
int main(void) {
  register int counter; /* register variable */
}

Use of the register keyword has become deprecated since modern compilers are automatically able to optimize which variables should be stored in registers.

External

The external storage class, specified with the extern keyword, is used to reference a variable or function defined in another compilation unit. A compilation unit consists of a source file plus any included header files. Functions default to the external storage class, so marking function prototypes with extern is optional.
/* app.c */
extern void foo(void); /* declared function */
int main(void) {
  foo(); /* external function call */
}
/* func.c */
void foo(void) {} /* defined function */
When extern is used with a global variable it becomes declared but not defined, so no memory is allocated for it. This tells the compiler that the variable is defined elsewhere. As with functions, it is necessary to declare global variables before they can be used in a compilation unit outside the one containing the definition.
/* app.c */
int globalVar; /* defined variable */
int main(void) {
  globalVar = 1;
}
/* func.c */
extern int globalVar; /* declared variable */
int foo(void) {
  globalVar++;
}

Keep in mind that a global variable or function may be declared externally multiple times in a program, but they may only be defined once.

Static

The static storage class restricts the scope of a global variable or function to the compilation unit that defines it. The lifetime of static entities is the whole program duration, which is the same as entities belonging to the external storage class.
/* Only visible within this compilation unit */
static int myInt;
static void myFunc(void) {}
Local variables may be declared as static to make the function preserve the variable for the duration of the program. A static local variable is only initialized once—when execution first reaches the declaration—and that declaration is then ignored every subsequent time the execution passes through.
/* Store number of calls to this function */
void myFunc(void) {
  static int count = 0;
  count++;
}

Knowing that a code entity can only be accessed and altered within a limited scope simplifies debugging, as it reduces potential dependencies between compilation units. Therefore, it is a good idea to declare all global variables and functions as static, unless they have an actual need to be exposed outside of their own compilation unit.

Volatile

Another type modifier in C is volatile. This modifier tells the compiler that a variable’s value may be changed by something external to the program and that the value must therefore be reread from memory every time it is accessed. Like const, the volatile modifier can appear either before or after the type, and it can be used together with a storage class modifier.
volatile int var; /* recommended order */
int volatile var; /* alternative order */
In the following example, the function waits for a variable to be set by some external event. Without the volatile modifier, the compiler may decide to optimize this loop condition by replacing it with an infinite loop, as it assumes the variable is never changed.
volatile int ext = 0;
void poll(void) {
  while(ext == 0) {}
}

Global variables should be declared volatile if their value is shared and can be changed externally. This can occur because an interrupt service routine modifies the variable, or because it is changed by another thread in a multi-threaded application. A third use case for volatile is with memory-mapped peripheral devices, which can change hardware registers outside of the program’s control.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_17

17. Constants

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

A constant is a variable with a value that cannot be changed once it has been assigned. This allows the compiler to enforce that a variable’s value is not changed anywhere in the code by mistake.

Constant Variables

A variable can be made into a constant by adding the const keyword either before or after the data type. This modifier makes the variable read-only, and it must therefore be assigned a value at the same time as it is declared. Attempting to change the constant anywhere else results in a compile-time error.
const int var = 5;   /* recommended order */
int const var2 = 10; /* alternative order */

Constant Pointers

When it comes to pointers, const can be used in two ways. First, the pointer can be made constant, which means that it cannot be changed to point to another location.
int myPointee;
int* const p = &myPointee; /* constant pointer */
Second, the pointee can be declared constant. This means that the variable pointed to cannot be modified through this pointer.
const int* q = &var; /* constant pointee */
It is possible to declare both the pointer and the pointee as constant to make them both read-only.
const int* const r = &var; /* constant pointer & pointee */
Referencing a constant from a non-constant pointer will produce a warning or error on most compilers. This is because such an assignment makes it possible to accidentally rewrite the constant’s value.
int* s = &var; /* error: const to non-const assignment */

Constant Parameters

Function parameters can be made constant to prevent them from being altered within the function. The main benefit of this is to let programmers know that the function leaves its pointer arguments untouched. When used consistently, it can also provide information about which functions can be expected to modify their pointer arguments.
#include <stdio.h>
void foo(const int* x) {
  if (x != NULL) {
    int i = *x; /* allowed */
    *x = 1;     /* compile-time error */
  }
}

Constant Guideline

In general, it is a good idea to always declare variables as constants if they do not need to be modified. This ensures that the variables are not changed anywhere in the program by mistake, which in turn can help prevent bugs.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_18

18. Preprocessor

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
The preprocessor is a text substitution tool that modifies the source code before it is compiled. This modification is done according to the preprocessor directives that are included in the source files. The directives are easily distinguished from normal programming code in that they all start with a hash sign (#). They must always appear as the first non-whitespace character on a line and do not need to end with a semicolon. The following table shows the preprocessor directives available in C along with their functions.

Directive

Description

#include

File include

#define

#undef

Define macro

Undefine macro

#ifdef

#ifndef

If macro defined

If macro not defined

#if

#elif

#else

#endif

If

Else if

Else

End if

#line

#error

#pragma

Set line number

Abort compilation

Set compiler option

Including Source Files

The #include directive inserts the contents of a file into the current source file. Its most common use is to include header files (.h), both user-defined and library ones. Library header files are enclosed between angle brackets (<>). This tells the preprocessor to search for the header in the default directory where it is configured to look for standard header files.
#include <stdio.h> /* search library directory */
Header files that you create for your own program are enclosed within double quotes (""). The preprocessor will then search for the file in the same directory as the current file. In case the header is not found there, the preprocessor will then search among the standard header files.
#include "myfile.h" /* search current, then default */
The double quoted form can also be used to specify an absolute or relative path to the file.
#include "c:\myfile.h" /* absolute path */
#include "..\myfile.h" /* relative path */

Define

Another important directive is #define , which is used to create compile-time constants, also called macros. After the directive, the name of the constant is specified followed by what it will be replaced by.
#define PI 3.14 /* macro definition */
The preprocessor will go through the code and change any occurrences of this constant with whatever comes after it in its definition until the end of the line.
double d = PI; /* d = 3.14 */

By convention, constants should be named in uppercase letters with each word separated by an underscore. That way, they are easy to spot when reading the source code.

Undefine

A #define directive should not be used to directly override a previously defined macro. Doing so will give a compiler warning, unless the macro definitions are the same. In order to redefine an existing macro, it first needs to be undefined using the #undef directive. Attempting to undefine a macro that is not currently defined will not generate a warning.
#undef PI /* undefine */
#undef PI /* allowed */

Predefined Macros

There are a number of macros that are predefined by the compiler. To distinguish them from other macros, their names begin and end with two underscores. The standard macros that all ANSI C-compliant compilers include are listed in the following table.

Directive

Description

__FILE__

The name and path of the current file.

__LINE__

The current line number.

__DATE__

The compilation date in MM DD YYYY format.

__TIME__

The compilation time in HH:MM:SS format.

__func__

The name of the current function. Added in C99.

__STDC__

Defined as 1 if the compiler complies with the ANSI C standard.

A common use for predefined macros is to provide debugging information. To give an example, the following error message includes the file name and line number where the message occurs.
printf("Error in %s at line %d", __FILE__, __LINE__);

Macro Functions

A macro can be made to take arguments. This allows them to define compile-time functions. For example, the following macro function gives the square of its argument.
#define SQUARE(x) ((x)*(x))
The macro function is called just as if it were a regular C function. Keep in mind that for this kind of function to work, the arguments must be known at compile time.
int x = SQUARE(2); /* 4 */
Note the extra parentheses in the macro definition that are used to avoid problems with operator precedence. Without the parentheses the following example would give an incorrect result, as the multiplication would then be carried out before the addition.
#define SQUARE(x) x*x
int main(void) {
  int x = SQUARE(1+1); /* 1+1*1+1 = 3 */
}
To break a macro function across several lines, you can use the backslash character. This will escape the newline character that marks the end of a preprocessor directive. For this to work there must not be any whitespace after the backslash.
#define MAX(a,b)  \
            a>b ? \
            a:b

Although macros can be powerful, they tend to make the code more difficult to read and debug. Macros should therefore only be used when they are necessary and should always be kept short. C code such as constant variables, enums, and inline functions can often accomplish the same goal more efficiently and safely than #define directives can.

Conditional Compilation

The directives used for conditional compilation can include or exclude part of the source code if a certain condition is met. First, there are the #if and #endif directives, which specify a section of code that will be included only if the condition after the #if directive is true. Note that this condition must evaluate to a constant expression.
#define DEBUG_LEVEL 3
#if DEBUG_LEVEL > 2
 /* ... */
#endif
Just as with the C if statement, any number of #elif (else if) directives and one final #else directive can be included.
#if DEBUG_LEVEL > 2
 /* ... */
#elif DEBUG_LEVEL == 2
 /* ... */
#else
 /* ... */
#endif
Conditional compilation also provides a useful means of temporarily commenting out large blocks of code for testing purposes. This often cannot be done with the regular multi-line comment since they cannot be nested.
#if 0
 /* Removed from compilation */
#endif

Compile if Defined

Sometimes, a section of code should only be compiled if a certain macro has been defined, irrespective of its value. For this purpose, two special operators can be used: defined and !defined (not defined).
#define DEBUG
#if defined DEBUG
 /* ... */
#elif !defined DEBUG
 /* ... */
#endif
The same effect can also be achieved using the directives #ifdef and #ifndef, respectively. The #ifdef section is compiled only if the specified macro has been previously defined. Note that a macro is considered defined even if it has not been given a value.
#ifdef DEBUG
 /* ... */
#endif
#ifndef DEBUG
 /* ... */
#endif

Error and Warning

When the #error directive is encountered, the compilation is aborted. This directive can be useful, for example, to determine whether or not a certain line of code is being compiled. It can optionally take a parameter that specifies the description of the generated compilation error.
#error "Compilation aborted"
Many C compilers also include the non-standard directive #warning. This directive displays a warning message without halting the compilation.
#warning "Function X is deprecated, use Y instead"

Line

A less commonly used directive is #line, which can change the line number that is displayed when an error occurs during compilation. Following this directive, the line number will as usual be increased by one for each successive line. The directive can take an optional string parameter that sets the file name that will be shown when an error occurs.
#line 5 "myapp.c"

Pragma

The last standard directive is #pragma, or pragmatic information. This directive is used to specify options to the compiler; and as such, they are vendor specific. To give an example, #pragma message can be used with many compilers to output a string to the build window.
/* Show compiler message */
#pragma message "Compiling " __FILE__ "..."
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_19

19. Memory Management

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

In the examples so far, the programs have only had as much memory available as has been declared for the variables at compile time. This is referred to as static allocation. If any additional memory is needed at runtime, it becomes necessary to use dynamic allocation. The C standard library provides several functions for managing dynamically allocated memory, including malloc, free, realloc, and calloc. These functions are found in the stdlib.h header file.

Malloc

The malloc function takes a size in bytes and returns a pointer to a block of free memory of that size. This dynamically allocated memory is uninitialized and can only be accessed through pointers.
#include <stdio.h>
#include <stdlib.h>
int main(void) {
  /* Dynamic memory allocation (1*5 = 5 bytes) */
  char* ptr = malloc(sizeof(char) * 5);
}
The sizeof operator is used to get the number of bytes for the given data type on the current system. This number is multiplied by five to allocate a block large enough to contain exactly five chars, provided that the system has that much free memory available. If it does not, malloc returns null to signal that it has failed to allocate the memory, in which case the function may need to signal to its caller that it too has failed.
char* ptr = malloc(sizeof(char) * 5);
if (ptr == NULL) {
  /* No memory allocated, exit function */
  return -1;
}

By convention, functions that return a pointer, such as malloc, use the value NULL to indicate failure. This is different from functions that return a value, which traditionally use 0 to indicate success and -1 to signal failure. Additional return values can be used to give the user of the function more detailed information about the return state.

Free

An important thing to remember about dynamic allocation is that this memory will not be released when the pointer goes out of scope, as with local variables. Instead, the memory has to be manually released with the function free , which releases the memory block at the specified address.
free(ptr); /* release allocated memory */

This allows you to control the lifetime of a dynamically allocated object, but it also means that you are responsible for freeing that memory once it is no longer needed. Forgetting to free dynamic memory will give the program unwanted memory leaks, because that memory will stay allocated until the program shuts down.

A pointer to released memory should be set to NULL immediately to show that it is no longer set to a valid reference. This is especially important for pointers that are repeatedly allocated and freed within a program, as trying to free an already freed memory block leads to undefined behavior. Likewise, a NULL check should be carried out prior to using a pointer to make sure it is valid.
if (ptr) { /* same as: if (ptr != NULL) */
  free(ptr);
  ptr = NULL; /* null pointer */
}

It is interesting to note that the function free only accepts one argument, the starting address for the memory block. The actual size of the block does not need to be provided. This is because the implementation of malloc and free keeps track of the size of each block as it is allocated, typically by storing the size next to the block.

Realloc

An allocated memory block can be resized with the realloc function . This function takes two arguments: the pointer to a previously allocated memory block and the new total size requested. If the pointer passed to realloc is NULL then the function behaves as malloc.
/* Allocate space for 5 chars */
char* p = malloc(sizeof(char) * 5);
/* Increase size to 10 chars */
char* new_p = realloc(p, sizeof(char) * 10);
This return value is stored in a new pointer, in case realloc fails to allocate the extra memory and returns NULL. This prevents the only reference to the previously allocated memory block from being lost, which would lead to a memory leak.
/* On failure, free memory and exit */
if (!new_p) { /* if (new_p == NULL) */
  if (p) {
    free(p);
    p = NULL;
  }
  return -1;
}
/* On success, update pointer */
else {
  p = new_p;
}

Calloc

The memory block returned by malloc is uninitialized, so it will contain whatever data happens to be in that memory region. If it is important to fill the memory block with zeroes then the calloc function can be used instead. In addition to initializing the memory to zero, this function takes an extra argument in the first position, specifying the number of blocks to be allocated.
int *a;
/* Allocate memory for 3 integers, all set to 0 */
a = (int*)calloc(3, sizeof(int));
printf("Sum is: %d\n", a[0]+a[1]+a[2]); /* "Sum is 0" */
free(a); /* deallocate memory */
Another way to initialize a memory block is with the memset function. This function takes three arguments: a pointer to the starting address, the value to be filled, and the number of bytes to fill with this value.
int *arr;
int size = 3 * sizeof(int);
/* Allocate memory for 3 integers */
arr = (int*)malloc(size);
/* Fill entire memory block with zeros */
memset(arr, 0, size);
free(arr); /* deallocate memory */

Void Pointers

It is sometimes necessary to use pointers without regard to the type they reference. This is achieved by specifying the pointer type as void*, known as a void pointer. A void pointer can store the address of any type of variable and can be cast to any pointer type, making them useful as a universal pointer. This type is what allows the free function to accept any pointer argument, and allows malloc to return a pointer that can be cast to any pointer type. The following example illustrates how the void pointer can be used to change the values of two variables of different types.
int i;
char c;
/* Change i through void pointer */
void *vptr = &i;
*((int*)vptr) = 1;
/* Change c through void pointer */
vptr = &c;
*((char*)vptr) = 'a';

Note that a void pointer may not be dereferenced without first casting it to the appropriate pointer type. The compiler is unable to check that this type cast is valid, which is why void pointers should be used with care.

Function Pointers

Another useful application of pointers is to point to functions. This allows functions to be passed to other functions or stored in arrays. Consider the following function.
void func(int x) {
  printf("Value is %d\n", x);
}
A function pointer that can reference this function needs to have a matching return type and list of argument types. Note the parentheses wrapped around the pointer identifier in the following function pointer declaration. They are important so that the compiler does not view this as a function declaration.
void (*p)(int);
Now a reference to the function can be assigned to this function pointer. Since the compiler knows this is a function the address-of operator (&) is not necessary. Also, since this pointer points to code and not data, this memory must not be deallocated with free.
p = func;
p = &func; /* alternative */
The function can be called like an ordinary function. The compiler will automatically dereference the function pointer if necessary when it is called as a function.
func(5); /* "Value is 5" */
(*func)(5); /* alternative */
Function pointers can be passed to other functions, allowing functionality to be plugged into existing code. Keep in mind that the return type and list of argument types of the passed function pointer need to match the parameter of the function.
void mycaller(void (*a)()) {
  a();
}
void myfunc() {
  printf("Hello World\n");
}
int main(void) {
  mycaller(&myfunc); /* & is optional */
}
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_20

20. Input Handling

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
When a C program is executed, it can accept arguments that are passed to it from the command line. These command-line arguments are useful for controlling the behavior of the program from outside the code. The arguments are passed to the main function, which is then set to accept two parameters as shown here.
int main(int argc, char* argv[]) {}
The integer argc is the number of arguments passed, and argv is a pointer array to the supplied command-line arguments. The first argument argv[0] is the name of the program, so the argument count in argc begins with 1 when no arguments are passed. If the program name is not available on the host environment, the first element will be an empty string instead. A simple program that prints all arguments is given here.
#include <stdio.h>
int main(int argc, char* argv[]) {
  int i;
  for(i=0; i<argc; i++) {
    printf("Argument %d is: %s\n", i, argv[i]);
  }
}
When a program is executed from a terminal window, the string arguments are listed after the file name, separated by spaces. If the argument itself contains a space it can be delimited by double quotes. In the following example, the previous program is compiled and executed with two arguments.
gcc myapp.c -o myapp.exe
./myapp.exe test "Hello World"
Argument 0 is myapp.exe
Argument 1 is test
Argument 2 is Hello World

Keyboard Input

Input from the command line can also be passed to a program while it is running, using for instance the scanf function. The first argument to this function is a formatting string that uses the same placeholders as printf, such as %d for accepting an int type. The second argument is the address to a variable that will hold the value of the expected input type.
#define _CRT_SECURE_NO_DEPRECATE
#include <stdio.h> /* scanf */
int main(void)
{
  int a, b, c;
  printf("Enter first number: ");
  scanf("%d", &a);
  printf("Enter second number: ");
  scanf("%d", &b);
  c = a + b;
  printf("%d + %d = %d\n", a,b,c);
}
Visual Studio considers scanf to be unsafe, so to use it with that compiler, you need the define directive seen at the start of this code. Given the proper input (followed by pressing Return), the function performs as expected.
gcc myapp.c -o myapp.exe
./myapp.exe
Enter first number: 2
Enter second number: 3
2 + 3 = 5
The scanf function does not perform bounds checking, so it is susceptible to buffer overflows when reading more than one character at a time. A more robust function for handling user input is fgets, which reads a string of text until the newline character appears. This function takes three arguments: the buffer, the maximum length of characters to read (including the null character), and the stream from which to read. The stream in this case is the standard input stream, designated by stdin .
#include <stdio.h> /* fgets */
int main(void)
{
  char name[10];
  printf("Enter your name: ");
  fgets(name, 10, stdin);
  printf("Hi %s", name);
}
Keep in mind that the newline character that marks the end of the input will be included in the string. If the string provided is too long, only the first nine characters plus the NULL character will be stored in this char array.
gcc myapp.c -o myapp.exe
./myapp.exe
Enter your name: Tom
Hi Tom

File Input and Output

Writing a file to the hard drive allows a program to store data that persists even after the program has shut down. A file can be opened for reading or writing using the fopen function. This function returns a pointer to a file handler and it takes two arguments: the file to open (including the path if needed) and the access mode for the file. Specifying the access mode as "w+" will create an empty file and return a handler that allows both reading and writing. Once it’s opened, a variant of printf called fprintf can be used to write content to the file. Note that the file handler is passed as the first argument to this function. After completing this task the file handler should be closed to free up resources by passing it to the fclose function.
#define _CRT_SECURE_NO_DEPRECATE /* for Visual Studio */
#include <stdio.h> /* fopen, fprintf, fclose */
int main(void)
{
  /* Create and open a file for reading/writing */
  FILE *fp = fopen ("file.txt", "w+");
  /* Write text to the file */
  fprintf(fp, "%s\n%s", "Hello", "World");
  /* Close the file */
  fclose(fp);
}
A text file has now been created in the same folder as the program. We will now read its content with another program. First the file is opened for reading by passing the r argument to fopen. Using the fgets function, the file is read one line at a time in a loop until the end of the file is reached, indicated by NULL being returned. Once all content has been read, the file is closed.
#define _CRT_SECURE_NO_DEPRECATE /* for Visual Studio */
#include <stdio.h> /* fopen, fgets, fclose */
int main(void)
{
  char buf[100];
  FILE *fp = fopen("file.txt", "r");
  while(fgets(buf,100,fp)!=NULL) {
    printf("Line contains: %s\n", i, buf);
  }
  fclose(fp);
}

Error Handling

The return value of file-handling functions should be checked to make sure each operation is successful. For instance, in this case if the file to be read is missing, then fopen will fail. In addition to returning NULL, the function will set the errno integer. This variable is used by many standard C library functions to indicate what kind of error has occurred. The variable is provided by the errno.h header file and is zero at program startup, indicating no error. Prior to calling a library function, the variable should be reset to zero since library functions will never do this in the absence of errors.
#define _CRT_SECURE_NO_DEPRECATE /* for Visual Studio */
#include <stdio.h> /* fopen, fclose */
#include <errno.h> /* errno */
int main(void)
{
  errno = 0;
  FILE *fp = fopen("missing.txt", "r");
  if(fp == NULL) {
    perror("File read failed");
    return 1; /* failed */
  }
  fclose(fp);
  return 0; /* success */
}
The perror function seen here prints a descriptive error message based on the current errno value.
gcc myapp.c -o myapp.exe
./myapp.exe
File read failed: No such file or directory
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_21

21. Headers

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

As a project grows it is common to split the code up into different source files. When this happens, the interface and implementation are generally separated. The interface is placed in a header file, which commonly has the same name as the source file and a .h file extension. This header file contains forward declarations for the source file entities that need to be accessible to other compilation units in the project.

Why Use Headers

C requires everything to be declared before it can be used. It is not enough to just compile the necessary source files in the same project. For example, if a function is placed in func.c, and a second file named app.c in the same project tries to call it, the compiler will report that it cannot find the function (or, prior to C99, that it has implicitly declared it).
/* func.c */
void myFunc(void) {
  /* ... */
}
/* app.c */
int main(void) {
  myFunc(); /* error: myFunc identifier not found */
}
To make this work as intended, the function's prototype has to be included in app.c.
/* app.c */
void myFunc(void); /* prototype */
int main(void) {
  myFunc(); /* ok */
}
This can be made more convenient if the prototype is placed in a header file named func.h and this header is included in app.c through the use of the #include directive. This way, when changes are made to func.c, there is no need to update the prototypes in app.c. Furthermore, any source file that wants to use the shared code in func.c can just include this one header.
/* func.h */
void myFunc(void); /* prototype */
/* app.c */
#include "func.h"

What to Include in Headers

As far as the compiler is concerned there is no difference between a header file and a source file. The distinction is only conceptual. The key idea is that the header should contain the interface of the implementation file—that is, the code that other source files will need to use. This may include shared macros, constants, and type definitions, as those shown here.
/* app.h - Interface */
#define DEBUG 0
const double PI = 3.14;
typedef unsigned long ulong;
The header can also contain prototypes of the shared functions defined in the source file. Internal functions used only within the source file should be left out of the header, to keep them private from the rest of the program.
void myFunc(void); /* prototype */
Additionally, shared global variables are typically declared as extern in the header, while their definitions lay in the source file.
/* app.h */
extern int myGlobal;
/* app.c */
int myGlobal = 0;

It should be noted that the use of shared global variables is discouraged. This is because the larger a program becomes, the more difficult it is to keep track of which functions access and modify these variables. The preferred method is to instead pass variables to functions only as needed, in order to minimize the scope of those variables.

The header should not include any executable statements, with one exception. A shared function that is declared as inline needs to be defined in the header. Otherwise, the compiler will not have the definition necessary for inlining the function available to it.
/* app.h */
inline void inlineFunc(void) {}
If a header requires other headers, it is common to include those files as well, to make the header stand alone. This ensures that everything needed is included in the correct order, solving potential dependency problems for every source file that needs the header.
/* app.h */
#include <stddef.h>
void mySize(size_t);

Note that since headers mainly contain declarations, any extra headers included should not affect the size of the program, although they may slow down the compilation.

Include Guards

An important thing to bear in mind when using header files is that a code entity, such as a constant, typedef, or enum, may only be defined once in every project. Consequently, including the same header file more than once will often result in compilation errors. The standard way to prevent this is to use a so-called include guard. An include guard is created by enclosing the body of the header in a #ifndef section that checks for a macro specific to that header file. Only when the macro is not defined is the file included. The macro is then defined, which effectively prevents the file from being included again.
/* app.h */
#ifndef APP_H
#define APP_H
/* ... */
#endif
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_22

22. Strings and Numbers

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

The standard C library contains several functions for making conversions between strings and numbers.

String Conversion

One built-in conversion function is atoi , which converts a string into an integer type. The following example illustrates the use of this function by converting all command-line string arguments into numbers and adding them together.
#include <stdio.h> /* printf */
#include <stdlib.h> /* atoi */
int main(int argc, char* argv[]) {
  int i, sum=0;
  for(i=0; i < argc; i++) {
    sum += atoi(argv[i]);
  }
  printf("Sum is: %d\n", sum);
}
Passing a non-numeric argument to atoi will typically evaluate as zero; however, the C standard does not define this behavior so it is not guaranteed across all implementations. Also, the function does not distinguish between a failed conversion and the number zero. Here is an example output from running the program with both correct and incorrect arguments.
gcc myapp.c -o myapp.exe
./myapp.exe 2 3
Sum is 5
./myapp.exe Hello 0
Sum is 0
Since no error-handling capabilities are provided by atoi, use of this function is discouraged. The preferred alternative is to instead convert strings with the safer strtol function . As seen in the following code, this function takes as many characters as it can to form a valid integer value. If a valid integer cannot be formed, then zero is guaranteed to be returned.
#include <stdio.h> /* printf */
#include <stdlib.h> /* strtol */
int main(void) {
  char str[] = "-123test";
  char *end;
  long r;
  r = strtol(str, &end, 10);
  printf("Number part %ld\n", r); /* -123 */
  printf("String part %s\n", end); /* "test" */
}
In addition to the string to be converted, the strtol function takes two more arguments, a char pointer and an integer. The char pointer will point to the last parsed character in the string, so for a successful conversion of the entire string this will be the null character found at the end of the string. The integer argument is the expected base value (between 2 and 36) of the integer, so for a hexadecimal number this would be set to 16 instead of 10, as seen in the next example. Note that the function skips any whitespace characters at the beginning of the string and stops if it encounters a non-numeric character.
#include <stdio.h> /* printf */
#include <stdlib.h> /* strtol */
int main(void) {
  char str[] = "5 F";
  char *end;
  long r1, r2;
  r1 = strtol(str,&end,10); /* 5 */
  r2 = strtol(end,&end,16); /* 15 */
  if (*end == '\0') { /* entire string parsed */
    printf("%ld+%ld is %ld",r1,r2,r1+r2); /* 5+15 is 20 */
  }
}
Recall that the errno variable is changed by some C library functions to indicate certain errors. In the case of the strtol function , the errno variable is used to signal overflow, which is caused when the number is too large or small to fit in the long data type. When this occurs errno is set to ERANGE, which is a macro defined in errno.h to represent a range error. In the next example, overflow occurs when trying to convert a number larger than what the long type can hold. The long type will then be assigned the largest value it can hold instead, a shortcut for which is provided by the LONG_MAX macro.
#include <stdio.h> /* printf */
#include <stdlib.h> /* strtol */
#include <errno.h> /* errno */
int main(void) {
  const char* str = "9223372036854775808"; /* LONG_MAX+1 */
  char *end;
  long res;
  errno = 0;
  res = strtol(str,&end,10); /* LONG_MAX */
  if (errno == ERANGE) {
    printf("Overflow"); /* "Overflow" */
  }
}

Number Conversion

When printf is called with a number as an argument, there is a string conversion happening. The standard library provides a variant of printf called snprintf that can be used to store the resulting string instead of printing it, as seen in the following code.
int main(void) {
  int num = 123;
  char buffer[4];
  snprintf(buffer, sizeof(buffer), "%d", num);
}

In addition to the typical arguments supplied to printf, the snprintf function takes two more arguments at the start: the output buffer and the maximum number of characters (including the null character) that it will store in the buffer.

Index

A

Aggregate initialization
Arithmetic operators
Arrays
assignment
declaration and allocation
multi-dimensional
and pointers
sizeof operator
Assignment operators
atoi function

B

Bit field
Bitwise operators
Break and continue statements

C

Calloc function
Char type variable
Command line arguments
Comparison operators
Compile and run
comments
console compilation
visual studio compilation
Conditional statements
if statement
switch statement
Constants
guideline
parameters
pointers
variable

D

Declarator list
Dereference operator
Dereference pointers
Do-while loop

E

Enum
color
constant values
conversions
scope
switch statement
variables
Escape characters
Exact-width integer types
Explicit conversions

F

Floating-point literal suffix
Floating-point type variable
For loop
Free function
Functions
calling
definition
forward declaration
inline
parameters
pass by address
pass by value
return by value/address
return statement
variable parameter lists
void parameter

G

GNU Compiler Collection (GCC)
Goto statement

H

Headers
function’s prototype
include guards
interface and implementation
shared global variables
Hello world
integrated development environment (IDE)
IntelliSense
printf function
project
return statement
source file

I, J, K

If statement
Implicit conversions, types of
Increment and decrement operators
Infix operator
Input Handling
error
file
keyboard
Integer type variable
Integrated development environment (IDE)
IntelliSense

L

Logical operators
Loops
break and continue
do-while
for
goto statement
while

M

Macros
Malloc function
Memory management
calloc function
description
free function
malloc function
realloc function
void pointer
Multi-dimensional arrays

N

Null pointer
Number conversion

O

Operator precedence

P, Q

Pointers
address-of operator
data type
dereferencing
null
pointing to pointer
Preprocessor
ANSI C compliant compilers
conditional compilation
define macro (#define)
description
error and warning
file include (#include)
if macro defined (#ifdef)
line
macro functions
pragma
undefine macro (#undef)
printf function

R

realloc function

S

Single-dimensional arrays
Static allocation
Storage classes
auto
external
register
static
volatile
String
character array
conversion
escape character
functions
null character
strcat (string concatenation)
strcpy (string copy)
strlen function
strtol function
Struct/structure
bit field
member access
objects
pointers
user-defined type
Switch statement

T

Ternary operator
Type conversions
explicit
implicit
Typedef

U

Union
Unnamed struct

V

Variables
assigning
bool type
char type
data types
declaration
floating-point types
global
integer size
integer types
literal suffixes
local
printf function
signed and unsigned
Void pointer
Volatile modifier

W, X, Y, Z

While loop
Cover image
Mikael Olsson
Modern C Quick Syntax ReferenceA Pocket Guide to the Language, APIs, and Library2nd ed.
Mikael Olsson
Hammarland, Länsi-Suomi, Finland
ISBN 978-1-4842-4287-2e-ISBN 978-1-4842-4288-9
https://doi.org/10.1007/978-1-4842-4288-9
Library of Congress Control Number: 2018967003
© Mikael Olsson 2019
This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed.
Trademarked names, logos, and images may appear in this book. Rather than use a trademark symbol with every occurrence of a trademarked name, logo, or image we use the names, logos, and images only in an editorial fashion and to the benefit of the trademark owner, with no intention of infringement of the trademark. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.
While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.
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Introduction

The C programming language is a general-purpose, middle-level language originally developed by Dennis M. Ritchie at Bell Labs. It was created over the period 1969 through 1973 for the development of the UNIX operating system, which had previously been written in assembly language. The name C was chosen because many of its features derived from an earlier language called B. Whereas the B language is no longer in common use, C became and still remains one of the most popular and influential programming languages in use today.

Although C is a general-purpose language, it is most often used for systems programming. This includes software that controls the computer hardware directly, such as drivers, operating systems, and software for embedded microprocessors. C can also be used for writing applications, which run on top of system software. However, it has largely been superseded in that domain by higher-level languages, such as C++, Objective-C, C#, and Java. The features of these and many other languages are heavily influenced by C, as can be seen in some of their names.

The development of C was a major milestone in computer science, as it was the first widely successful middle-level language for system development. The foremost reasons for its success were that the language was concise, fast, and powerful. It offered comparable speed to assembly with far improved usability. The high-level constructs of the language allowed programmers to focus mainly on the software’s design, while its low-level capabilities still provided direct access to the hardware when needed, as assembly had done. Furthermore, the language is relatively simple to understand, with few keywords and what many consider to be an elegant syntax.

Another major reason for the success of C was its portability. Unlike assembly, the C language is platform independent. A standards-compliant C program can therefore be compiled for a wide variety of computer systems with few changes to its source code. Moreover, the C compiler was small and easy to port to different CPU architectures, which together with the language’s popularity has made C compilers available on most computer systems.

C Versions

In 1978, Brian Kernighan and Dennis Ritchie produced the first publicly available description of C, now known as K&R C. This description was succeeded in 1989 when the American National Standards Institute (ANSI) provided a comprehensive definition of C known as ANSI C or C89. In the following year, the same specification was adopted as an international standard by the International Organization for Standardization and became known as ISO C90 or just C90. C has since undergone four more revisions by ISO (successively adopted by ANSI) with further language extensions, including C95, C99, C11, and most recently C18, which is the latest ANSI standard for the C programming language. The current C18 standard provided only clarifications to C11 and introduced no new language features.

Table of Contents

Chapter 1:​ Hello World 1
Creating a Project 1
Adding a Source File 2
Hello World 2
IntelliSense 3
Chapter 2:​ Compile and Run 5
Visual Studio Compilation 5
Console Compilation 6
Comments 7
Chapter 3:​ Variables 9
Data Types 9
Declaring Variables 10
Assigning Variables 10
Printing Variables 11
Integer Types 12
Signed and Unsigned 13
Sized Integers 15
Floating-Point Types 16
Literal Suffixes 16
Char Type 17
Bool Type 18
Variable Scope 18
Chapter 4:​ Operators 21
Arithmetic Operators 21
Assignment Operators 22
Increment and Decrement Operators 22
Comparison Operators 23
Logical Operators 23
Bitwise Operators 24
Operator Precedence 25
Chapter 5:​ Pointers 27
Creating Pointers 27
Dereferencing Pointers 27
Pointing to a Pointer 28
Null Pointer 29
Chapter 6:​ Arrays 31
Array Declaration and Allocation 31
Array Assignment 31
Multi-Dimensional Arrays 32
Arrays and Pointers 32
Array Size 33
Chapter 7:​ Strings 35
Escape Characters 36
String Functions 37
Chapter 8:​ Conditionals 39
If Statement 39
Switch Statement 40
Ternary Operator 41
Chapter 9:​ Loops 43
While Loop 43
Do-While Loop 43
For Loop 44
Break and Continue 45
Goto Statement 45
Chapter 10:​ Functions 47
Defining Functions 47
Calling Functions 47
Function Parameters 48
Void Parameter 48
Return Statement 49
Forward Declaration 50
Variable Parameter Lists 51
Pass by Value 52
Pass by Address 53
Return by Value or Address 53
Inline Functions 54
Chapter 11:​ Typedefs 57
Chapter 12:​ Enums 59
Enum Example 60
Enum Constant Values 60
Enum Conversions 61
Enum Scope 61
Chapter 13:​ Structs 63
Struct Objects 63
Member Access 64
Struct Pointers 65
Bit Fields 66
Chapter 14:​ Unions 69
Chapter 15:​ Type Conversions 71
Implicit Conversions 71
Explicit Conversions 72
Chapter 16:​ Storage Classes 73
Auto 73
Register 73
External 74
Static 75
Volatile 76
Chapter 17:​ Constants 79
Constant Variables 79
Constant Pointers 79
Constant Parameters 80
Constant Guideline 81
Chapter 18:​ Preprocessor 83
Including Source Files 84
Define 84
Undefine 85
Predefined Macros 85
Macro Functions 86
Conditional Compilation 87
Compile if Defined 88
Error and Warning 89
Line 89
Pragma 89
Chapter 19:​ Memory Management 91
Malloc 91
Free 92
Realloc 93
Calloc 94
Void Pointers 95
Function Pointers 96
Chapter 20:​ Input Handling 99
Keyboard Input 100
File Input and Output 102
Error Handling 103
Chapter 21:​ Headers 105
Why Use Headers 105
What to Include in Headers 106
Include Guards 108
Chapter 22:​ Strings and Numbers 109
String Conversion 109
Number Conversion 112
Index 113

About the Author and About the Technical Reviewer

About the Author

Mikael Olsson

is a professional web entrepreneur, programmer, and author. He works for an R&D company in Finland where he specializes in software development. In his spare time, he writes books and creates websites that summarize various fields of interest. The books he writes are focused on teaching their subject in the most efficient way possible, by explaining only what is relevant and practical without any unnecessary repetition or theory. The portal to his online businesses and other websites is Siforia.com .

 

About the Technical Reviewer

Michael Thomas
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has worked in software development for more than 20 years as an individual contributor, team lead, program manager, and vice president of engineering. Michael has more than 10 years of experience working with mobile devices. His current focus is in the medical sector, using mobile devices to accelerate information transfer between patients and health care providers.

 
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_1

1. Hello World

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

To begin programming in C, you need a text editor and a C compiler. You can get both at the same time by installing an Integrated Development Environment (IDE) that includes support for C. A good choice is Microsoft’s Visual Studio Community Edition, which is a free version of Visual Studio available from Microsoft’s website.1 This IDE has built-in support for the C89 standard and includes most features up to C99 as of the 2017 version.

Some other popular cross-platform IDEs include Eclipse CDT, Visual Studio Code, Code::Blocks, and CodeLite. Alternatively, you can develop using a simple text editor—such as Notepad—although this is less convenient than using an IDE. If you choose to do so, just create an empty document with a .c file extension and open it in the editor of your choice. By convention, the .c extension is used for files that contain source code for C programs.

Creating a Project

After installing Visual Studio, with the C++ component selected during installation, go ahead and launch the program. You then need to create a project, which will manage the C source files and other resources. Go to File ➤ New ➤ Project to display the New Project window. From there, select the Visual C++ template type in the left frame. Then select the Empty Project template in the right frame. At the bottom of the window you can configure the name and location of the project. When you are finished, click the OK button and the wizard will create your empty project.

Adding a Source File

You have now created a C/C++ project. In the Solution Explorer panel (View ➤ Solution Explorer), you can see that the project consists of four empty folders: External Dependencies, Header Files, Resource Files, and Source Files. Right-click on the Source Files folder and select Add ➤ New Item. From the Add New Item dialog box, choose the C++ File (.cpp) template. Give this source file the name myapp.c. The .c file extension will make the file compile in C instead of C++. Click the Add button, and the empty C file will be added to your project and opened for you.

Hello World

The first thing to add to the source file is the main function. This is the entry point of the program, and the code inside of the curly brackets is what will be executed when the program runs. The brackets, along with their content, are collectively referred to as a code block, or just a block.
int main(void) {}
Your first application will simply output the text "Hello World" to the screen. Before this can be done, the stdio.h header needs to be included. This header provides input and output functionality for the program, and is one of the standard libraries that comes with all C/C++ compilers. What the #include directive does is effectively replace the line with everything in the specified header before the file is compiled.
#include <stdio.h>
int main(void) {}
With stdio.h included you gain access to several new functions, including the printf function that is used for printing text—in this case to a console window. To call this function, you type its name followed by a set of parentheses that includes the text string that will be displayed. The string is delimited by double quotes, and the whole statement is followed by a semicolon. The semicolon is used in C to mark the end of a code statement.
#include <stdio.h>
int main(void) {
  printf("Hello World");
  return 0;
}

The main function here ends with a return statement , which returns a status code as the program exits. This can be useful if the intent is for your program to be executed by another program. The status code can then signal to the caller the success or failure of your program to complete its function. By convention, the return code 0 is used to indicate that a program or function has executed successfully.

The C89 standard requires the return statement to be present, but following C90 the statement became optional. As of C90 the compiler will automatically include the return statement if it is omitted. For brevity the statement will be left out from future code examples.

IntelliSense

When writing code in Visual Studio, a window called IntelliSense will pop up wherever there are multiple predetermined alternatives from which to choose. This window can also be brought up manually at any time by pressing Ctrl+Space to provide quick access to any code entities you are able to use within your program. This is a very powerful feature that you should learn to make good use of.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_2

2. Compile and Run

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Before a program can run, the source code has to be translated into an executable format by a compiler. This step transforms the human-readable source code into binary machine code, which is a sequence of instructions that can be executed by a computer.

Visual Studio Compilation

Continuing from the last chapter, the Hello World program is now complete and ready to be compiled and run. You can do this by going to the Build menu and choosing Debug ➤ Start Without Debugging (Ctrl+F5). Visual Studio then compiles and runs the application that displays the text in a console window.

Depending on your version of Visual Studio, the console window displaying Hello World may close as soon as the main function has finished executing. To prevent this, you need to explicitly specify that this is a console application. First right-click the Project node in the Solution Explorer and then click on Properties to bring up the project’s properties window. From there, navigate to Configuration Properties ➤ Linker ➤ System and set the SubSystem option to Console using the dropdown list. Click OK and the console window will now no longer close automatically.

Another way to prevent the console window from closing is to add a call to the getchar function at the end of main. This function, included with the stdio.h header, will read a character from the keyboard and thereby prevent the program from exiting until the return key is pressed.
#include <stdio.h>
int main(void) {
  printf("Hello World");
  getchar();
}

Console Compilation

As an alternative to using an IDE, you can also compile source files from a terminal window as long as you have a C compiler. For example, on a Linux machine you can use the GNU C compiler, which is available on virtually all UNIX systems—including Linux and the BSD family—as part of the GNU Compiler Collection (GCC). This compiler can also be installed on Windows by downloading MinGW1 or on Mac as part of the Xcode development environment.2

To use the GNU compiler, you type its name gcc in a terminal window and give it the input and output filenames as arguments. It then produces an executable file, which when run gives the same result as one compiled under Windows in Visual Studio.
gcc myapp.c -o myapp.exe
./myapp.exe
Hello World

Comments

Comments are used to insert notes into the source code. They have no effect on the end program and are meant only to enhance the readability of the code, both for you and for other developers. The C89 standard featured only one comment notation, a multiline comment delimited by /* and */.
/* multi-line
   comment */
The C99 standard added the single-line comment, which starts with // and extends to the end of the line. This comment was standardized since it was a convenient feature found in many other programming languages, such as C++. Many C compilers also started to support the single-line comment long before the C99 standard was formalized.
// single-line comment

Keep in mind that whitespace characters—such as comments, spaces, and tabs—are generally ignored by the compiler. This gives you a lot of freedom in how you format your code.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_3

3. Variables

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Variables are used for storing data during program execution.

Data Types

Depending on what data you need to store, there are several different kinds of built-in data types in C. The so-called simple or primitive types are listed in the following table. They consist of four integer types, three floating-point types, as well as the char type .

Data Type

Size (Byte)

Description

char

1

Integer or character

short

int

long

long long

2

4

4 or 8

8

Integer

float

double

long double

4

8

8 or 16

Floating-point number

In C, the exact sizes of the data types are not fixed. The sizes shown in the previous table are those commonly found on 32-bit and 64-bit systems. The C standard only specifies the minimum range that is guaranteed to be supported. The minimum size for char is 8 bits, for short and int it is 16 bits, for long it is 32 bits, and long long must contain at least 64 bits. Most modern compilers make int 32 bits, which nearly universally means 4 bytes. Each integer type in the table must also be at least as large as the one preceding it. The same applies to the floating-point types where each type must provide at least as much precision as the preceding one.

Declaring Variables

Before a variable can be used, it first has to be declared (created). To declare a variable, you start with the data type you want the variable to hold followed by an identifier, which is the name of the variable.
int myInt;
The identifier can consist of letters, numbers, and underscores, but it cannot start with a number. It also cannot contain spaces or special characters and must not be a reserved keyword.
int _myInt32; /* allowed */
int 32Int;    /* incorrect (starts with number) */
int my Int;   /* incorrect (contains space) */
int Int@32;   /* incorrect (contains special character) */
int int;      /* incorrect (reserved keyword) */

Note that C is a case-sensitive programming language, so uppercase and lowercase letters have different meanings.

Assigning Variables

To assign a value to a declared variable, you use the equals sign, which is known as the assignment operator (=). This is called assigning or initializing the variable.
myInt = 50;
The declaration and assignment can be combined into a single statement. When a variable is assigned a value it then becomes defined.
int myInt = 50;
If you need to create more than one variable of the same type, there is a shorthand way of doing this using the comma operator (,).
int x = 1, y = 2, z;
Once a variable has been defined (declared and assigned), you can use it by simply referencing the variable’s name. For example, to copy the value to another variable, you can use the following.
int a = x;

Printing Variables

In addition to strings, the printf function can be used to print values and variables to the standard output stream. This is done by embedding format specifiers into the string where the value is to be printed. Each specifier must be matched by a corresponding argument to printf of the correct type, as seen in the following example.
#include <stdio.h>
int main() {
  int x = 5;
  printf("x is %d and 2+3 is %d", x, 2+3);
}
The %d specifier displays an integer of the char, short, or int type. Other commonly used format specifiers are seen in the following table.

Specifier

Output

%d or %i

char, short, or int

%c

Character

%s

String of characters

%f

float or double

%Lf

long double

%ld

long int

%lld

long long int

%u

Unsigned char, short or int

%lu

Unsigned long int

%llu

Unsigned long long int

%p

Pointer address

For more information on printf and other standard library functions, visit the C library reference on cplusplus.com .1

Integer Types

There are four native integer (whole number) types you can use depending on how large a number you need the variable to hold. Typical ranges on 32-bit and 64-bit systems are given here.
char  myChar  = 0; /* -128   to +127 */
short myShort = 0; /* -32768 to +32767 */
int   myInt   = 0; /* -2^31  to +2^31-1 */
long  myLong  = 0; /* -2^31  to +2^31-1 */
C99 added support for the long long data type, which is guaranteed to be at least 64 bits in size.
long long myLL = 0; /* -2^63 to +2^63-1 */
To determine the exact size of a data type, you can use the sizeof operator. This operator returns the number of bytes that a type occupies in the system you are compiling for. The type returned is size_t, which is an alias for an integer type. The specifier %zu was introduced in C99 as a portable way to format this type with printf. Visual Studio does not support this specifier and uses %Iu instead.
#include <stdio.h>
int main(void) {
  size_t s = sizeof(int);
  printf("%zu", s); /* "4" (C99) */
  printf("%Iu", s); /* "4" (Visual Studio) */
}
In addition to standard decimal notation, integers can also be assigned by using octal or hexadecimal notation. The following values all represent the same number, which in decimal notation is 50.
int myDec = 50    /* decimal notation */
int myOct = 062;  /* octal notation (0) */
int myHex = 0x32; /* hexadecimal notation (0x) */

Signed and Unsigned

By default, all integer types in C are signed and may therefore contain both positive and negative values. This can be explicitly specified using the signed keyword.
signed char  myChar;   /* -128   to +127 */
signed short myShort;  /* -32768 to +32767 */
signed int   myInt;    /* -2^31  to +2^31-1 */
signed long  myLong;   /* -2^31  to +2^31-1 */
signed long long myLL; /* -2^63  to +2^63-1 */
If only positive values need to be stored, the integer types can be declared as unsigned to double their upper range.
unsigned char  uChar;   /* 0 to 255 */
unsigned short uShort;  /* 0 to 65535 */
unsigned int   uInt;    /* 0 to 2^32-1 */
unsigned long  uLong;   /* 0 to 2^32-1 */
unsigned long long uLL; /* 0 to 2^64-1 */
When an unsigned value is printed, the specifier %u is used for the unsigned char, short, and int types. The unsigned long type is specified with %lu and unsigned long long is specified with %llu.
unsigned int uInt = 0;
printf("%u", uInt); /* "0" */
The signed and unsigned keywords may be used as types on their own, in which case the int type is assumed by the compiler.
unsigned uInt; /* unsigned int */
signed sInt;   /* signed int */
In the same way, the short and long data types are abbreviations of short int and long int.
short myShort; /* short int */
long myLong;   /* long int */
 

Sized Integers

As mentioned, the actual sizes of the integer types are implementation dependent. For more precise specification of size, the C99 standard introduced a number of exact-width integer types. They can be enabled by including the stdint.h standard header.
#include <stdint.h>
/* Signed exact-width integers */
int8_t  iSmall;  /* 8 bits */
int16_t iMedium; /* 16 bits */
int32_t iLarge;  /* 32 bits */
int64_t iHuge;   /* 64 bits */
Unsigned versions of these types are available as well. Like the signed versions, these exact-width integer types are guaranteed to have the same number of bits across all implementations.
/* Unsigned exact-width integers */
uint8_t  uSmall;  /* 8 bits */
uint16_t uMedium; /* 16 bits */
uint32_t uLarge;  /* 32 bits */
uint64_t uHuge;   /* 64 bits */
It is recommended to use sized integers when available, to more easily keep track of the range of your integer variables and to enhance the portability of your programs. Compilers that comply with standards prior to C99 may provide sized integers with different type names. Visual Studio, for example, has built-in support for the following signed exact-width integers.
/* Visual Studio signed exact-width integers */
__int8  iSmall;  /* 8 bits */
__int16 iMedium; /* 16 bits */
__int32 iLarge;  /* 32 bits */
__int64 iHuge;   /* 64 bits */
 

Floating-Point Types

The floating-point types can store real numbers with different levels of precision.
float myFloat;    /* ~7 digits */
double myDouble;  /* ~15 digits */
long double myLD; /* typically same as double */
The precision shown here refers to the total number of digits. A float can accurately represent about 7 digits, whereas a double can handle around 15 digits.
float myFloat = 12345.678;
printf("%f", myFloat); /* "12345.677734" */
When printing a floating-point number you can limit the decimal places to, for instance, two in the following way.
printf("%.2f", myFloat); /* "12345.68" */
Floating-point numbers can be expressed using decimal, exponential, or hexadecimal notation. Exponential (scientific) notation is used by adding E or e followed by the decimal exponent, while the hexadecimal floating-point notation uses P or p to specify the binary exponent. Support for the hexadecimal notation was not standardized until C99.
double fDec = 1.23;
double fExp = 3e2;   /* 3*10^2 = 300 */
double fHex = 0xAp2; /* 10*2^2 = 40 */

Literal Suffixes

An integer literal (constant) is normally treated as an int by the compiler, or a larger type if needed to fit the value. Suffixes can be added to the literal to change this evaluation. With integers the suffix can be a combination of U and L, for unsigned and long, respectively. C99 also added the LL suffix for the long long type. The order and casing of these letters do not matter.
int i = 10;
long l = 10L;
unsigned long ul = 10UL;
A floating-point literal is treated as a double. The F or f suffix can be used to specify that a literal is of the float type instead. Likewise, the L or l suffix specifies the long double type.
float f = 1.23F;
double d = 1.23;
long double ld = 1.23L;

The compiler implicitly converts literals to whichever type is necessary, so this type distinction for literals is usually not necessary. If the F suffix is left out when assigning to a float variable, the compiler may give a warning since the conversion from double to float involves a loss of precision.

Char Type

The char type is commonly used to represent ASCII characters. Such character constants are enclosed in single quotes and can be stored in a variable of char type.
char c = 'x'; /* assigns 120 (ASCII for x) */
When the char is printed with the %c format specifier the ASCII character is displayed.
printf("%c", c); /* "x" */
Use the %d specifier to instead display the numerical value.
printf("%d", c); /* "120" */

Bool Type

C99 introduced a _Bool type to increase compatibility with C++. Variables of this type can store a Boolean value, which is a value that can only be either 1 (true) or 0 (false).
_Bool b = 0; /* false value */
The type _Bool is usually accessed via its alias name bool defined by the standard header stdbool.h. This header also defines the macros true and false as aliases for 1 and 0.
#include <stdbool.h>
bool b = true; /* true value */

Variable Scope

The scope of a variable refers to the region of code within which it is possible to use that variable. Variables in C may be declared both globally and locally. A global variable is declared outside of any code blocks and is accessible from anywhere after it has been declared. A local variable, on the other hand, is declared inside of a function and will only be accessible within the function in which it was declared. The lifetime of a local variable is also limited. A global variable will remain allocated for the duration of the program, while a local variable will be destroyed when its function has finished executing.
int globalVar; /* global variable */
int main(void) {
  int localVar; /* local variable */
}
The default values for these variables are also different. Global variables are automatically initialized to zero by the compiler, whereas local variables are not initialized at all. Uninitialized local variables will therefore contain whatever garbage is already present in that memory location.
int globalVar; /* initialized to 0 */
int main(void) {
  int localVar; /* uninitialized */
}
Using uninitialized variables is a common programming mistake that can produce unexpected results. It is therefore a good idea to always give your local variables initial values when they are declared.
int main(void) {
  int localVar = 0; /* initialized to 0 */
}
In C89, local variables must be declared before any other statements within their scope. The later C99 standard changed this to allow variables to be declared anywhere within a function’s scope, which can be more intuitive.
int main(void) {
  int var1;
  /* Other statements */
  int var2; /* C99 only */
}
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_4

4. Operators

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

A numerical operator is a symbol that makes the program perform a specific mathematical or logical manipulation. The numerical operators in C can be grouped into five types: arithmetic, assignment, comparison, logical, and bitwise operators.

Arithmetic Operators

There are the four basic arithmetic operators, as well as the modulus operator (%), which is used to obtain the division remainder.
int x = 3 + 2; /* 5 - addition */
    x = 3 - 2; /* 1 - subtraction */
    x = 3 * 2; /* 6 - multiplication */
    x = 3 / 2; /* 1 - division */
    x = 3 % 2; /* 1 - modulus (division remainder) */
Notice that the division sign gives an incorrect result. This is because it operates on two integer values and will therefore truncate the result and return an integer. To get the correct value, one of the numbers must be explicitly converted to a floating-point number.
float f = 3 / (float)2; /* 1.5 */

Assignment Operators

The second group is the assignment operators. Most important, it is the assignment operator (=) itself, which assigns a value to a variable.
int x = 0; /* assignment */
A common use of the assignment and arithmetic operators is to operate on a variable and then to save the result back into that same variable. These operations can be shortened with the combined assignment operators.
x += 5; /* x = x+5; */
x -= 5; /* x = x-5; */
x *= 5; /* x = x*5; */
x /= 5; /* x = x/5; */
x %= 5; /* x = x%5; */

Increment and Decrement Operators

Another common operation is to increment or decrement a variable by one. This can be simplified with the increment (++) and decrement (--) operators.
x++; /* x = x+1; */
x--; /* x = x-1; */
Both of these can be used either before or after a variable.
x++; /* post-increment */
x--; /* post-decrement */
++x; /* pre-increment */
--x; /* pre-decrement */
The result on the variable is the same whichever is used. The difference is that the post-operator returns the original value before it changes the variable, while the pre-operator changes the variable first and then returns the value.
int x, y;
x = 5; y = x++; /* y=5, x=6 */
x = 5; y = ++x; /* y=6, x=6 */

Comparison Operators

The comparison operators compare two values and return either 1 or 0, representing true or false. They are mainly used to specify conditions, which are expressions that evaluate to either true or false.
int x = (2 == 3); /* 0 - equal to */
    x = (2 != 3); /* 1 - not equal to */
    x = (2 > 3);  /* 0 - greater than */
    x = (2 < 3);  /* 1 - less than */
    x = (2 >= 3); /* 0 - greater than or equal to */
    x = (2 <= 3); /* 1 - less than or equal to */

Logical Operators

The logical operators are often used together with the comparison operators. Logical and (&&) evaluates to true if both the left and right sides are true, and logical or (||) is true if either the left or right side is true. For inverting a Boolean result, there is the logical not (!) operator. Note that for both “logical and” and “logical or” the right side will not be evaluated if the result is already determined by the left side.
int x = (1 && 0); /* 0 - logical and */
    x = (1 || 0); /* 1 - logical or  */
    x = !(1);     /* 0 - logical not */
Recall that as of C99, the stdbool.h header can be included to use the bool type to store Boolean values. The header also defines the constants true and false to represent 1 and 0, which allows the previous example to be rewritten as seen here.
#include <stdbool.h>
/* ... */
bool b = (true && false); /* false - logical and */
     b = (true || false); /* true  - logical or  */
     b = !(true);         /* false - logical not */

Bitwise Operators

The bitwise operators can manipulate individual bits inside an integer. For example, the bitwise left shift operator (<<) moves all bits to the left with the specified number of steps.
int x = 5 & 4;  /* 101 & 100 = 100 (4) - and */
    x = 5 | 4;  /* 101 | 100 = 101 (5) - or */
    x = 5 ^ 4;  /* 101 ^ 100 = 001 (1) - xor */
    x = 4 << 1; /* 100 << 1  =1000 (8) - left shift */
    x = 4 >> 1; /* 100 >> 1  =  10 (2) - right shift */
    x = ~4;     /* ~00000100 = 11111011 (-5) - invert */
The bitwise operators also have combined assignment operators.
int x=5; x &= 4;  /* 101 & 100 = 100 (4) - and */
    x=5; x |= 4;  /* 101 | 100 = 101 (5) - or */
    x=5; x ^= 4;  /* 101 ^ 100 = 001 (1) - xor */
    x=4; x <<= 1; /* 100 << 1  =1000 (8) - left shift */
    x=4; x >>= 1; /* 100 >> 1  =  10 (2) - right shift */

Operator Precedence

In C, expressions are normally evaluated from left to right. However, when an expression contains multiple operators, the precedence of those operators decides the order in which they are evaluated. The order of precedence can be seen in the following table, where the operator with the lowest precedence will be evaluated first. This same order also applies to many other languages, such as C++ and C#.

Pre

Operator

Pre

Operator

1

() [] . -> x++ x--

8

&

2

! ~ ++x --x (type) sizeof * &

9

^

3

* / %

10

|

4

+ -

11

&&

5

<< >>

12

||

6

< <= > >=

13

= op=

7

== !=

14

,

To give an example, multiplication binds harder than addition and will therefore be evaluated first in the following line of code.
int x = 4 + 3 * 2; /* 10 */
This can be clarified by enclosing the part of the expression that will be evaluated first in parentheses. As seen in the table, parentheses have the highest precedence of all operators.
int x = 4 + (3 * 2); /* 10 */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_5

5. Pointers

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

A pointer is a variable that contains the memory address of another variable, called the pointee.

Creating Pointers

Pointers are declared as any other variable, except that an asterisk (*) is placed between the data type and the pointer’s name. The data type determines what type of memory it will point to.
int* p; /* pointer to an integer */
int *q; /* alternative syntax */
A pointer can point to a variable of the same type by prefixing that variable with an ampersand, in order to retrieve its address and assign it to the pointer. The ampersand is known as the address-of operator (&).
int i = 10;
p = &i; /* address of i assigned to p */

Dereferencing Pointers

The pointer now contains the memory address to the integer variable. Referencing the pointer will retrieve this address. To obtain the actual value stored in that address, the pointer must be prefixed with an asterisk, known as the dereference operator (*).
printf("Address of i: %p \n", p); /* ex. 0017FF1C */
printf("Value of i: %d",  *p); /* 10 */
When writing to the pointer the same method is used. Without the asterisk the pointer is assigned a new memory address, and with the asterisk the actual value of the variable pointed to will be updated.
p = &i;  /* address of i assigned to p */
*p = 20; /* value of i changed through p */
If a second pointer is created and assigned the value of the first pointer, it will then get a copy of the first pointer’s memory address.
int* p2 = p; /* copy address stored in p */

Pointing to a Pointer

Sometimes it can be useful to have a pointer that can point to another pointer. This is done by declaring a pointer with two asterisks and then assigning it the address of the pointer that it will reference. This way, when the address stored in the first pointer changes, the second pointer can follow that change.
int** r = &p; /* pointer to pointer */
Referencing the second pointer now gives the address of the first pointer. Dereferencing the second pointer gives the address of the variable, and dereferencing it again gives the value of the variable.
printf("Address of p: %p \n",  r); /* ex. 0017FF28 */
printf("Address of i: %p \n", *r); /* ex. 0017FF1C */
printf("Value of i: %d",  **r); /* 20 */

Null Pointer

A pointer should be set to zero when it is not assigned to a valid address. Such a pointer is called a null pointer . Doing this allows you to check whether the pointer can be safely dereferenced, because a valid pointer will never be zero.
int* p = 0; /* null pointer */
The constant NULL can also be used to signify a null pointer. NULL is typically defined as zero in C, making the choice of which to use a matter of preference. The constant is defined by several standard library files, including stdio.h and stddef.h.
#include <stdio.h>
/* ... */
int* p = NULL; /* null pointer */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_6

6. Arrays

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

An array is a data structure used to store a collection of values that all have the same data type.

Array Declaration and Allocation

To declare an array, you start as you would a normal variable declaration, but in addition, you append a set of square brackets following the array’s name. The brackets contain the number of elements in the array. The default values for these elements are the same as for variables—elements in global arrays are initialized to their default values, and elements in local arrays remain uninitialized.
int myArray[3]; /* integer array with 3 elements */

Array Assignment

To assign values to the elements, you can reference them one at a time by placing the element’s index inside the square brackets, starting with zero.
myArray[0] = 1;
myArray[1] = 2;
myArray[2] = 3;
Alternatively, you can assign values at the same time as the array is declared by enclosing them in curly brackets. The specified array length may optionally be left out to let the array size be decided by the number of values assigned.
int myArray[3] = { 1, 2, 3 };
int myArray[] = { 1, 2, 3 }; /* alternative */
Once the array elements are initialized, they can be accessed by referencing the index of the element you want.
printf("%d", myArray[0]); /* 1 */

Multi-Dimensional Arrays

Arrays can be made multi-dimensional by adding more sets of square brackets. As with single-dimensional arrays, they can either be filled in one at a time or all at once during the declaration.
int mArray[2][2] = { { 0, 1 }, { 2, 3 } };
mArray[0][0] = 0;
mArray[0][1] = 1;
The extra curly brackets are optional, but including them is good practice since it makes the code easier to understand.
int mArray[2][2] = { 0, 1, 2, 3 }; /* alternative */

Arrays and Pointers

An array in C can be treated as a constant pointer that points to the first element in the array. As such, array elements can referenced just as well with pointer arithmetic. By incrementing the pointer by one, you move to the next element in the array, because changes to a pointer’s address are implicitly multiplied by the size of the pointer’s data type.
*(myArray+1) = 10; /* myArray[1] = 10; */
Pointer arithmetic is an advanced feature that should be used with care. The four arithmetic operators that can be used with pointers include: +, -, ++, and --.
int* ptr = &myArray;
printf("Address of myArray[0]: %p \n", ptr); /* ex. 0028FF14 */
ptr++;
printf("Address of myArray[1]: %p", ptr); /* ex. 0028FF18 */

Array Size

Just as with any other pointer it is possible to exceed the valid range of an array and thereby rewrite some adjacent memory. This should always be avoided since it can lead to unexpected results or crash the program.
int myArray[2] = { 1, 2 };
myArray[2] = 3; /* out of bounds */
To determine the length of a regular (statically allocated) array, the sizeof operator can be used.
int length = sizeof(myArray) / sizeof(int); /* 2 */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_7

7. Strings

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
A string consists of an array of characters and is delimited by double quotes. There is no string type for storing strings in C. Instead, strings are commonly assigned to a character array, as shown here.
char myString[] = "Hi";
Strings in C are terminated with a null character \0, which is used to know where the string ends. The null character is added automatically by the compiler for quoted strings, as in the previous example. The same statement can also be written using regular array initialization syntax, in which case the null character needs to be explicitly included.
char myString[3] = { 'H', 'i', '\0' };
Both statements produce the same result: a char array with three elements. Note that individual characters are delimited by single quotes and not double quotes. To print a string the format specifier %s is used with the printf function, which outputs the literals of the string until the null character is encountered.
printf("%s", myString); /* "Hi" */
As an alternative to the character array, a char pointer may be set to point to a string. The string is then automatically stored in the compiled file, giving the pointer a location to point to. In most compilers this location is a read-only block, so unlike with the char array, the characters in this string cannot be changed.
char* ptr = "Hi";
printf("%s", ptr); /* "Hi" */

Escape Characters

To add new lines to a string, the escape character \n is used to represent a line break.
printf("First line\nSecond line");
This backslash notation is used to write special characters that are difficult or impossible to type on a regular keyboard. In addition to the newline and null characters, there are several other such characters, as seen in the following table.

Character

Meaning

Character

Meaning

\n

Newline

\f

Form feed

\t

Horizontal tab

\a

Alert sound

\v

Vertical tab

\'

Single quote

\b

Backspace

\"

Double quote

\r

Carriage return

\\

Backslash

\0

Null character

\?

Question mark

\000

Octal number (1-3 digits)

\xhh

Hexadecimal number

Any one of the 128 ASCII characters can be expressed by writing a backslash followed by the ASCII code for that character, represented as either an octal or hexadecimal number. This is illustrated here, where the newline character is represented in three different ways:
char line = '\n';   /* escape code */
     line = '\012'; /* octal notation */
     line = '\x0A'; /* hexadecimal notation */

String Functions

Because strings in C are arrays, the only way to make changes to them is to change each element in the array. To simplify common string operations, the standard header string.h includes a collection of functions for manipulating null-terminated strings. Consider the following code, which will be used as the template for the following string examples.
#include <stdio.h>
#include <string.h>
int main(void) {
  char s1[11] = "Hello";
  char s2[11] = "World";
  int result;
}
The string function strcat (string concatenation) appends the second string onto the first string. For this to work, it is important that the destination is large enough to hold the entire string.
/* Append s2 to s1 */
strcat(s1, s2); /* s1 = "HelloWorld" */
Another string function is strcpy , which copies the characters in the second string into the first string. The function stops when the terminating null character for the second argument is reached.
/* Copy s1 into s2 */
strcpy(s2, s1); /* s2 = "HelloWorld" */
A string can be compared with another string using the strcmp function. If all characters match, the function returns zero.
/* Compare s1 and s2 */
result = strcmp(s1, s2); /* 0 (equal) */
When manipulating strings, it is important to take their lengths into account to avoid overwriting adjacent memory. The length of a string stored in a char array can be found with the strlen function . With regular (statically allocated) strings, the allocated size can also be retrieved using the sizeof operator.
/* Length of s1 (excluding null char) */
result = strlen(s1); /* 10 */
/* Allocated size for s1 */
result = sizeof(s1); /* 11 */
The next example shows how the sizeof operator can be used to allocate a character array large enough to hold two other strings. Note that the resulting string will only have one null character, so the size of its allocated array can be one character less than the sizes of the two other strings.
char a[] = "Hello"; /* sizeof(a) = 6 */
char b[] = "World"; /* sizeof(b) = 6 */
char c[sizeof(a) + sizeof(b) -1]; /* sizeof(c) = 11 */
strcpy( c, a );
strcat( c, b ); /* "HelloWorld" */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_8

8. Conditionals

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Conditional statements are used to execute different code blocks based on different conditions.

If Statement

The if statement will execute only if the expression inside the parentheses is evaluated to true. In C, this does not have to be a Boolean expression. It can be any expression that evaluates to a number, in which case zero is false and all other numbers are true.
if (x < 1) {
  printf("x < 1");
}
To test for other conditions, the if statement can be extended by any number of else/if clauses.
else if (x > 1) {
  printf("x > 1");
}
The if statement can have one else clause at the end, which will execute if all the previous conditions are false.
else {
  printf("x == 1");
}
As for the curly brackets, they can be left out if only a single statement needs to be executed conditionally. However, it is considered good practice to always include them since they improve readability.
if (x < 1)
  printf("x < 1");
else if (x > 1)
  printf("x > 1");
else
  printf("x == 1");
Any numeric value except for 0 is considered true when evaluated in a Boolean context. This allows a true or false check to be simplified in the following manner.
int done = 0;
/* ... */
/* if not done – same as if(done == 0) */
if(!done) {}
/* if done – same as if(done != 0) */
if(done) {}

Switch Statement

The switch statement checks for equality between an integer and a series of case labels, and then passes execution to the matching case. It may contain any number of case clauses, and it can end with a default label for handling all other cases.
switch (x) {
  case 0: printf("x is 0"); break;
  case 1: printf("x is 1"); break;
  default: printf("x is not 0 or 1"); break;
}

Note that the statements after each case label end with the break keyword to skip the rest of the switch. If the break is left out, execution will fall through to the next case, which can be useful if several cases need to be evaluated in the same way.

Ternary Operator

In addition to the if and switch statements, there is the ternary operator (?:) that can replace a single if/else clause. This operator takes three expressions. If the first one is true then the second expression is evaluated and returned; and if it is false, the third one is evaluated and returned.
x = (x < 0.5) ? 0 : 1; /* ternary operator (?:) */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_9

9. Loops

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

There are three looping structures in C, all of which are used to execute a specific code block multiple times. Just as with the conditional if statement, the curly brackets for the loops can be left out if there is only one statement in the code block.

While Loop

The while loop runs through the code block only if its condition is true, and it will continue looping for as long as the condition remains true. Bear in mind that the condition is only checked at the start of each iteration (each loop).
int i = 0;
while (i < 10) {
  printf("%d", i++); /* 0-9 */
}

Do-While Loop

The do-while loop works in the same way as the while loop, except that it checks the condition after the code block. It will therefore always run through the code block at least once. Notice that this loop ends with a semicolon.
int j = 0;
do {
  printf("%d", j++); /* 0-9 */
} while (j < 10);

For Loop

The for loop is used to run through a code block a specific number of times. It uses three parameters. The first one initializes a counter and is always executed once before the loop. The second parameter holds the condition for the loop and is checked before each iteration. The third parameter contains the increment of the counter and is executed at the end of each iteration.
int k;
for (k = 0; k < 10; k++) {
  printf("%d", k); /* 0-9 */
}
Since the C99 standard the first parameter may contain a declaration, typically a counter variable. The scope of this variable is limited to the for loop.
for (int k = 0; k < 10; k++) {
  printf("%d", k); /* 0-9 */
}
The for loop has several variations. One such variation is to split the first and third parameters into several statements by using the comma operator.
int k, m;
for (k = 0, m = 0; k < 10; k++, m--) {
  printf("%d", k+m); /* 000... (10x) */
}
Another option is to leave out any one of the parameters. If all parameters are left out, it becomes a never-ending loop, unless there is another exit condition defined .
for (;;) { /* infinite loop */ }

Break and Continue

There are two jump statements that can be used inside loops: break and continue . The break keyword ends the loop structure, and continue skips the rest of the current iteration and continues at the beginning of the next iteration.
int i;
for (i = 0; i < 10; i++)
{
  if (i == 2) continue; /* start next iteration */
  else if (i == 5) break; /* end loop */
  printf("%d", i); /* "0134" */
}

Goto Statement

A third jump statement that may be useful to know of is goto, which performs an unconditional jump to a specified label within the same function. This instruction is generally never used since it tends to make the flow of execution difficult to follow.
goto myLabel; /* jump to label */
/* ... */
myLabel: /* label declaration */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_10

10. Functions

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Functions are reusable code blocks that will only execute when called. They allow developers to divide their programs into smaller parts that are easier to understand and reuse.

Defining Functions

A function can be created by typing void followed by the function’s name, a set of parentheses containing another void, and a code block. The first use of the void keyword specifies that this function will not return a value. The second void inside the parentheses means that the function does not accept any arguments.
void myFunction(void) {
  printf("Hello World");
}

Calling Functions

The previous function will print out a text message when it is called. To invoke it, the function’s name is specified followed by an empty set of parentheses.
int main(void) {
  myFunction(); /* "Hello World" */
}

Function Parameters

The parentheses that follow the function’s name are used for passing arguments to the function. To do this, you must first add the corresponding parameters to the function’s parameter list.
void sum(int a, int b) {
  int sum = a + b;
  printf("%d", sum);
}
A function can be defined to take any number of arguments, and they can have any data types. Just ensure the function is called with the same types and number of arguments. In this example, the function accepts two integer arguments and displays their sum.
sum(2, 3); /* "5" */

To be precise, parameters appear in function definitions, while arguments appear in function calls. However, the two terms are sometimes used interchangeably.

Void Parameter

In C, functions that leave out the void keyword from their parameter list are allowed to accept an unknown number of arguments. This is different from C++, where leaving out void means the same as including it: that the function takes no arguments. Therefore, to have the compiler ensure that no arguments are mistakenly passed to a parameterless function, it is necessary in C to include void in the parameter list.
/* Accepts no arguments */
void foo(void) {}
/* Accepts an unknown number of arguments */
void bar() {}

As of C99, the use of an empty parameter list has been deprecated and results in a warning from the compiler.

Return Statement

A function can return a value. The void keyword before the function’s name is then replaced with the data type the function will return, and the return keyword is added to the function’s body followed by an argument of the specified return type.
int getSum(int a, int b) {
  return a + b;
}
Return is a jump statement that causes the function to exit and return the specified value to the place where the function was called. To illustrate, the previous function can be passed as an argument to the printf function since it evaluates to an integer.
printf("%d", getSum(5, 10)); /* "15" */
The return statement can also be used in a void function as a way to exit the function before the end block is reached.
void dummy(void) { return; }
The main function must be set to return an int type, but including an explicit return value is only required in the C89 standard. As of C90 the compiler will automatically add a return statement to the end of the main function if no such statement is present, and with the C99 standard, this implicit return value is guaranteed to be zero.
int main(void) {
  return 0; /* optional */
}

Forward Declaration

An important thing to keep in mind in C is that a function must be declared before it can be called. This can either be achieved by placing the function’s implementation before any references to it, or by adding a declaration of the function before it is called. This kind of forward declaration is known as a prototype and provides the compiler with the information needed to allow the function to be used before it has been defined.
void myFunction(int a); /* prototype */
int main(void) {
  myFunction(0);
}
void myFunction(int a) {}
The parameter names do not need to be included in the prototype; only the data types are required.
void myFunction(int);
In early versions of C, an undeclared function that is referenced is implicitly declared as a function that returns an int and takes an unspecified number of parameters. Relying on this behavior is not recommended and usually results in a warning from the compiler. As of C99 this feature has been removed and will instead result in an error.
int main(void) {
  foo();
}
/* Warning: implicit declaration of foo */
int foo() { return 0; }

Variable Parameter Lists

A function can be defined to accept a variable number of arguments, similar to the printf function. The parameter list of such a function must end with an ellipsis (…) and there must be at least one additional parameter. An int parameter is typically included to let the function know the number of extra arguments that are passed to it.

In the following example, the function accepts a variable number of arguments that are summed up and returned to the caller. To access these arguments the stdarg.h header file is included. This header defines a new type, called va_list, and three functions that operate on variables of this type: va_start, va_arg, and va_end.
#include <stdio.h>
#include <stdarg.h>
int sum(int num, ...) {
  va_list args; /* variable argument list */
  int sum = 0, i = 0;
  va_start(args, num); /* initialize argument list */
  for (i = 0; i < num; i++) /* loop through arguments */
    sum += va_arg(args, int); /* get next argument */
  va_end(args); /* free memory */
  return sum;
}
int main(void) {
  printf("Sum of 1+2+3 = %d", sum(3,1,2,3)); /* 6 */
}

In contrast to C++, C does not allow function overloading or default parameter values. However, variable parameter lists can be used to implement functions that behave in similar ways.

Pass by Value

Variables are by default passed by value. This means that only a copy of the value is passed to the function. Therefore, changing the parameter in any way will not affect the original variable, and passing large variables back and forth can have a negative impact on performance.
#include <stdio.h>
void set(int i) { i = 1; }
int main(void) {
  int x = 0;
  set(x);
  printf("%d", x); /* "0" */
}

Pass by Address

The alternative to passing by value is to use pointer syntax to instead pass the variable by address. When an argument is passed by address, the parameter can be changed or replaced, and the change will affect the original variable.
void set(int* i) { *i = 1; }
int main(void) {
  int x = 0;
  set(&x);
  printf("%d", x); /* "1" */
}
Recall that arrays can be treated as pointers. As such they will automatically be passed by address, as shown in the following example.
void set(int a[]) { a[0] = 1; }
int main(void) {
  int x[] = { 0 };
  set(x);
  printf("%d", x[0]); /* "1" */
}

Return by Value or Address

In addition to passing variables by value or address, a variable may also be returned in one of these two ways. By default a function returns by value, in which case a copy of the value is returned to the caller.
int byVal(int i) { return i + 1; }
int main(void) {
  int a = 10;
  printf("%d", byVal(a)); /* "11" */
}
To instead return by address, the dereference operator is appended to the function’s return type. The function must then return a variable and not an expression or literal, as is allowed when returning by value. The variable returned should never be a local variable since the memory to these variables is released when the function ends. Instead, return by address is commonly used to return an argument that has also been passed to the function by address.
int* byAdr(int* i) { (*i)++; return i; }
int main(void) {
  int a = 10;
  int *p = byAdr(&a);
  printf("%d", *p); /* "11" */
}

Inline Functions

When calling a function it is important to keep in mind that a certain performance overhead occurs. To potentially remove this overhead, the programmer can recommend that the compiler inlines the calls to a specific function by using the inline function modifier. This keyword was added in the C99 standard. It is most suited for use with small functions that are called inside loops, as shown in the following example. Larger functions should not be inlined since this can significantly increase the size of the code, which may instead decrease performance.
inline int increment(int a) { return ++a; }
int main(void) {
  int i;
  for(i = 0; i < 100;) {
    i = increment(i);
  }
}

Note that the inline keyword is only a recommendation. The compiler may—in its attempts to optimize the code—choose to ignore this recommendation, and it may also inline functions that do not have the inline modifier .

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_11

11. Typedefs

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
An alias for a type can be created using the typedef keyword followed by the type and alias name. By convention, uppercase letters are commonly used for these definitions.
typedef unsigned char BYTE;
Once defined, the alias can be used as a synonym for its specified type.
BYTE b; /* unsigned char */
typedef does not only work for existing types, but can also include a definition of a user-defined type—such as a struct, union, or enum. This can make a complex type easier to understand.
typedef struct { int points; } score;
score a, b, c;
a.points = 10;

If used properly, a type alias can simplify a long or confusing type name, making the code easier to understand. Another benefit they provide is the ability to change the definition of a type from a single location, which can help make a program more portable.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_12

12. Enums

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
An enum is a user-defined type consisting of a fixed list of named constants. In the following example, the enumeration type is called color and contains three constants: RED, GREEN and BLUE.
enum color { RED, GREEN, BLUE };
The color type can be used to create variables that may hold one of these constant values. In C, the variable declaration must be preceded by enum, whereas this is optional in C++.
int main(void) {
  enum color c = RED;
}
Enum variables may also be declared when the enum is defined, by placing the variable names before the final semicolon. This position is known as the declarator list.
enum color { RED, GREEN, BLUE } c, d;

Enum Example

The switch statement provides a good example of when enumerations can be useful. Compared to using ordinary constants, the enumeration has the advantage that it lets the programmer clearly specify what values a variable should be allowed to contain.
switch(c) {
  case RED:   break;
  case GREEN: break;
  case BLUE:  break;
}
Bear in mind that enums in C are not typesafe, unlike their C++ equivalent. It is up to the programmer to ensure that enum types and constants are used correctly, as most compilers will not enforce this. Enums in C simply provide a way to group a set of integer constants and have them be automatically numbered. For this purpose, the enum identifier is not strictly necessary and may optionally be omitted.
enum { RED, GREEN, BLUE } c;

Enum Constant Values

Enumerated constants are of the int type. Usually there is no need to know the underlying values that these constants represent, but in some cases it can be useful. By default, the first constant in the enum list has the value zero and each successive constant is one value higher.
enum color {
  RED   /* 0 */
  GREEN /* 1 */
  BLUE  /* 2 */
};
These default values can be overridden by assigning values to the constants. The values can be computed and do not have to be unique.
enum color {
  RED = 5,       /* 5 */
  SCARLET = RED, /* 5 */
  BLUE = RED+2,  /* 7 */
};

Enum Conversions

The compiler can implicitly convert an enumerated constant to an integer. An integer can also be converted back into an enum variable.
int i = RED;
enum color c = i;
Some compilers warn when an integer is assigned to an enum variable since this makes it possible to assign a value that is not one of its specified constants. To suppress this warning, an explicit type cast can be used.
enum color c = (enum color)i;

Enum Scope

An enum does not have to be declared globally. It can also be placed locally within a function, in which case it will only be usable within the function in which it was defined.
/* Global enum */
enum speed { SLOW, NORMAL, FAST };
int main(void) {
  /* Local enum */
  enum color { RED, GREEN, BLUE };
}
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_13

13. Structs

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
A struct or structure is a user-defined type used for grouping a collection of related variables under a single name. To define a structure you use the struct keyword, followed by an optional identifier and a code block containing variable declarations. The definition of this new type ends with a semicolon.
struct point {
  int x, y;
};

Unlike arrays, structs allow data items of different kinds to be combined. Structs may contain variables, pointers, arrays, or other user-defined types. In contrast to C++, structs in C may not contain functions.

Struct Objects

To declare a variable of a struct type, the struct keyword is followed by the type name and the variable identifier. Variables of struct type are commonly referred to as objects or instances.
int main(void) {
  struct point p; /* object declaration */
}
Objects may also be created when the struct is defined, by placing the object names before the final semicolon. This position is called the declarator list. If the optional struct identifier is left out, this becomes the only way to create objects of the struct type. Such a struct without an identifier is called an unnamed struct and provides a way for programmers to prevent any more instances of the type from being created.
struct /* unnamed struct */
{
  int x, y;
} a, b; /* object declarations */
It is common in C to use typedef when defining structures. This aliasing removes the need to include the struct keyword when declaring objects of the struct type, resulting in the shorter syntax used in C++.
typedef struct point point;
struct point {
  int x, y;
};
int main(void) {
  point p; /* struct omitted */
}

Member Access

Variables of a struct type are called fields or members. These fields are accessed using the member of operator (.) prefixed by the object name. Fields of an object are by default undefined, so it is important to assign them a value before they are read.
int main(void) {
  point p;
  p.x = 1;
  p.y = 2;
}
Similar to an array, struct objects may also be initialized when they are declared by enclosing the values in curly brackets. The values are then assigned in order based on the corresponding members of the struct. This way of assigning values to a composite type is known as aggregate initialization.
struct point {
  int x, y;
} r = { 1, 2 }; /* assigns x and y */
int main(void) {
  point p = { 1, 2 };
}
C99 introduced designated initializers, which allow structures to be initialized in any order by specifying the names of the fields. Any omitted fields will be automatically initialized to 0.
int main(void) {
  point p = { .y = 2, .x = 1 };
}

Struct Pointers

A struct is a value type, not a reference type like an array. As such, any assignment or argument passing for objects will copy the field values and not the object reference. This is different from many modern languages where composite types are automatically assigned and passed by reference.
int main(void) {
  point p = { 1, 2 };
  point r = p; /* copies field values */
}
For large structures the performance cost of this copy operation may be significant. Therefore it is common to use pointers when passing objects to functions, to avoid having to copy and return the whole object.
void init_struct(point* a) {
  (*a).x = 1;
  (*a).y = 2;
}
int main(void) {
  point p;
  init_struct(&p);
}
As shown in this example, the pointer must be dereferenced before the member of the operator can be used to access the fields. Since this operation is so common, there is a syntactical shortcut available, known as the infix operator (->), which automatically dereferences the pointer.
point p;
point* r = &p;
r->x = 1; /* same as (*r).x = 1; */
 

Bit Fields

The C programming language offers a way to optimize memory use within a struct type by allowing the bit length of integer fields to be specified. Such a field is called a bit field , and its length is set by placing a colon after the field name followed by the number of bits. The length must be less than or equal to the bit length of the specified type.
struct my_bits
{
  unsigned short f1 : 1;
  unsigned short f2 : 1;
  unsigned short id : 10;
} a;
Bit fields are packed as compactly as possible, while keeping in mind that the size of an object needs to be a multiple of the size of the types it contains. In this case the needed 12 bits will require 16 bits (two bytes) to be reserved for the object, as that is the size of the short type. Had bit fields not been used, the three shorts and consequently the struct would occupy 48 bits instead.
int main(void) {
  printf("%d bytes", sizeof(a)); /* "2 bytes" */
}

This feature is useful when programming for embedded systems, where hardware resources may be very constrained.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_14

14. Unions

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
The union type is identical to the struct type, except that all fields share the same memory position. Therefore, the size of a union is the size of the largest field it contains. In the following code, this is the integer field, which is four bytes.
union mix {
  char c;  /* 1 byte */
  short s; /* 2 bytes */
  int i;   /* 4 bytes */
};
Given this memory sharing, the union type can only be used to store one value at a time, because changing one field will overwrite the value of the others.
int main(void) {
  union mix m;
  m.c = 0xFF; /* set first 8 bits */
  m.s = 0;    /* reset first 16 bits */
}
The benefit of a union, in addition to efficient memory usage, is that it provides multiple ways of using the same memory location. For example, the following union has three data members that allow access to the same group of four bytes in different ways.
union mix {
  char c[4];                  /* 4 bytes */
  struct { short hi, lo; } s; /* 4 bytes */
  int i;                      /* 4 bytes */
} m;
The int field accesses all four bytes at once, the struct two bytes at a time, and the char array allows each byte to be referenced individually. The bit pattern for this is illustrated in the next example. Keep in mind that the internal order of bytes for primitive data types is not defined in C. Because of this, the order of the four bytes that make up the int may be reversed on some platforms.
m.i=0xFF00F00F; /* 11111111 00000000 11110000 00001111 */
m.s.lo;         /* 11111111 00000000                   */
m.s.hi;         /*                   11110000 00001111 */
m.c[3];         /* 11111111                            */
m.c[2];         /*          00000000                   */
m.c[1];         /*                   11110000          */
m.c[0];         /*                            00001111 */
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_15

15. Type Conversions

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Converting an expression from one type to another is known as type casting or type conversion. This can be done either implicitly by the compiler or explicitly with code.

Implicit Conversions

An implicit conversion is performed automatically by the compiler when an expression needs to be converted into one of its compatible types. For example, any conversions between the primitive data types can be done implicitly.
long l = 5;   /* int -> long */
double d = l; /* long -> double */
These implicit conversions can also take place within an expression, allowing you to mix different primitive types together. When types of different sizes are involved, the result will be of the larger type, so an int and double will produce a double value.
double d = 5 + 2.5; /* int -> double */
Implicit conversions of primitive types can be further grouped into two kinds: promotion and demotion. Promotion occurs when an expression gets implicitly converted into a larger type, and demotion occurs when converting an expression to a smaller type.
/* Promotion */
long l = 5;   /* int promoted to long */
double d = l; /* long promoted to double */
/* Demotion */
int i = 10.5; /* warning: possible loss of data */
char c = i;   /* warning: possible loss of data */

Because a demotion can result in the loss of information, these conversions generate a warning on many compilers. If the potential information loss is intentional, the warning can be suppressed by using an explicit cast.

Explicit Conversions

An explicit cast is performed by placing the desired data type in parentheses to the left of the expression that needs to be converted.
int i = (int)10.5; /* double demoted to int */
char c = (char)i;  /* int demoted to char */

Keep in mind that casting a variable only makes it temporarily evaluate as a different type; it does not change the variable’s underlying type.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_16

16. Storage Classes

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

Every variable has a storage class that determines its scope and lifetime. These storage classes include the following: auto, register, extern, and static. Each of these classes is also a keyword that can be placed before the data type to determine to which storage class a variable belongs.

Auto

The default storage class for local variables is auto, which can be explicitly specified with the auto keyword. Memory for automatic variables is allocated when the code block is entered and freed upon exit. The scope of these variables is local to the block in which they are declared, as well as any nested blocks.
int main(void) {
  auto int localVar; /* auto variable */
}

Register

The register storage class hints to the compiler that a local variable will be heavily used and should therefore be kept in a CPU register instead of RAM memory to provide quicker access. Variables of the register storage class cannot use the address-of operator (&), since registers do not have memory addresses. They also cannot be larger than the register size, which is usually the same as the processor’s word size.
int main(void) {
  register int counter; /* register variable */
}

Use of the register keyword has become deprecated since modern compilers are automatically able to optimize which variables should be stored in registers.

External

The external storage class, specified with the extern keyword, is used to reference a variable or function defined in another compilation unit. A compilation unit consists of a source file plus any included header files. Functions default to the external storage class, so marking function prototypes with extern is optional.
/* app.c */
extern void foo(void); /* declared function */
int main(void) {
  foo(); /* external function call */
}
/* func.c */
void foo(void) {} /* defined function */
When extern is used with a global variable it becomes declared but not defined, so no memory is allocated for it. This tells the compiler that the variable is defined elsewhere. As with functions, it is necessary to declare global variables before they can be used in a compilation unit outside the one containing the definition.
/* app.c */
int globalVar; /* defined variable */
int main(void) {
  globalVar = 1;
}
/* func.c */
extern int globalVar; /* declared variable */
int foo(void) {
  globalVar++;
}

Keep in mind that a global variable or function may be declared externally multiple times in a program, but they may only be defined once.

Static

The static storage class restricts the scope of a global variable or function to the compilation unit that defines it. The lifetime of static entities is the whole program duration, which is the same as entities belonging to the external storage class.
/* Only visible within this compilation unit */
static int myInt;
static void myFunc(void) {}
Local variables may be declared as static to make the function preserve the variable for the duration of the program. A static local variable is only initialized once—when execution first reaches the declaration—and that declaration is then ignored every subsequent time the execution passes through.
/* Store number of calls to this function */
void myFunc(void) {
  static int count = 0;
  count++;
}

Knowing that a code entity can only be accessed and altered within a limited scope simplifies debugging, as it reduces potential dependencies between compilation units. Therefore, it is a good idea to declare all global variables and functions as static, unless they have an actual need to be exposed outside of their own compilation unit.

Volatile

Another type modifier in C is volatile. This modifier tells the compiler that a variable’s value may be changed by something external to the program and that the value must therefore be reread from memory every time it is accessed. Like const, the volatile modifier can appear either before or after the type, and it can be used together with a storage class modifier.
volatile int var; /* recommended order */
int volatile var; /* alternative order */
In the following example, the function waits for a variable to be set by some external event. Without the volatile modifier, the compiler may decide to optimize this loop condition by replacing it with an infinite loop, as it assumes the variable is never changed.
volatile int ext = 0;
void poll(void) {
  while(ext == 0) {}
}

Global variables should be declared volatile if their value is shared and can be changed externally. This can occur because an interrupt service routine modifies the variable, or because it is changed by another thread in a multi-threaded application. A third use case for volatile is with memory-mapped peripheral devices, which can change hardware registers outside of the program’s control.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_17

17. Constants

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

A constant is a variable with a value that cannot be changed once it has been assigned. This allows the compiler to enforce that a variable’s value is not changed anywhere in the code by mistake.

Constant Variables

A variable can be made into a constant by adding the const keyword either before or after the data type. This modifier makes the variable read-only, and it must therefore be assigned a value at the same time as it is declared. Attempting to change the constant anywhere else results in a compile-time error.
const int var = 5;   /* recommended order */
int const var2 = 10; /* alternative order */

Constant Pointers

When it comes to pointers, const can be used in two ways. First, the pointer can be made constant, which means that it cannot be changed to point to another location.
int myPointee;
int* const p = &myPointee; /* constant pointer */
Second, the pointee can be declared constant. This means that the variable pointed to cannot be modified through this pointer.
const int* q = &var; /* constant pointee */
It is possible to declare both the pointer and the pointee as constant to make them both read-only.
const int* const r = &var; /* constant pointer & pointee */
Referencing a constant from a non-constant pointer will produce a warning or error on most compilers. This is because such an assignment makes it possible to accidentally rewrite the constant’s value.
int* s = &var; /* error: const to non-const assignment */

Constant Parameters

Function parameters can be made constant to prevent them from being altered within the function. The main benefit of this is to let programmers know that the function leaves its pointer arguments untouched. When used consistently, it can also provide information about which functions can be expected to modify their pointer arguments.
#include <stdio.h>
void foo(const int* x) {
  if (x != NULL) {
    int i = *x; /* allowed */
    *x = 1;     /* compile-time error */
  }
}

Constant Guideline

In general, it is a good idea to always declare variables as constants if they do not need to be modified. This ensures that the variables are not changed anywhere in the program by mistake, which in turn can help prevent bugs.

© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_18

18. Preprocessor

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
The preprocessor is a text substitution tool that modifies the source code before it is compiled. This modification is done according to the preprocessor directives that are included in the source files. The directives are easily distinguished from normal programming code in that they all start with a hash sign (#). They must always appear as the first non-whitespace character on a line and do not need to end with a semicolon. The following table shows the preprocessor directives available in C along with their functions.

Directive

Description

#include

File include

#define

#undef

Define macro

Undefine macro

#ifdef

#ifndef

If macro defined

If macro not defined

#if

#elif

#else

#endif

If

Else if

Else

End if

#line

#error

#pragma

Set line number

Abort compilation

Set compiler option

Including Source Files

The #include directive inserts the contents of a file into the current source file. Its most common use is to include header files (.h), both user-defined and library ones. Library header files are enclosed between angle brackets (<>). This tells the preprocessor to search for the header in the default directory where it is configured to look for standard header files.
#include <stdio.h> /* search library directory */
Header files that you create for your own program are enclosed within double quotes (""). The preprocessor will then search for the file in the same directory as the current file. In case the header is not found there, the preprocessor will then search among the standard header files.
#include "myfile.h" /* search current, then default */
The double quoted form can also be used to specify an absolute or relative path to the file.
#include "c:\myfile.h" /* absolute path */
#include "..\myfile.h" /* relative path */

Define

Another important directive is #define , which is used to create compile-time constants, also called macros. After the directive, the name of the constant is specified followed by what it will be replaced by.
#define PI 3.14 /* macro definition */
The preprocessor will go through the code and change any occurrences of this constant with whatever comes after it in its definition until the end of the line.
double d = PI; /* d = 3.14 */

By convention, constants should be named in uppercase letters with each word separated by an underscore. That way, they are easy to spot when reading the source code.

Undefine

A #define directive should not be used to directly override a previously defined macro. Doing so will give a compiler warning, unless the macro definitions are the same. In order to redefine an existing macro, it first needs to be undefined using the #undef directive. Attempting to undefine a macro that is not currently defined will not generate a warning.
#undef PI /* undefine */
#undef PI /* allowed */

Predefined Macros

There are a number of macros that are predefined by the compiler. To distinguish them from other macros, their names begin and end with two underscores. The standard macros that all ANSI C-compliant compilers include are listed in the following table.

Directive

Description

__FILE__

The name and path of the current file.

__LINE__

The current line number.

__DATE__

The compilation date in MM DD YYYY format.

__TIME__

The compilation time in HH:MM:SS format.

__func__

The name of the current function. Added in C99.

__STDC__

Defined as 1 if the compiler complies with the ANSI C standard.

A common use for predefined macros is to provide debugging information. To give an example, the following error message includes the file name and line number where the message occurs.
printf("Error in %s at line %d", __FILE__, __LINE__);

Macro Functions

A macro can be made to take arguments. This allows them to define compile-time functions. For example, the following macro function gives the square of its argument.
#define SQUARE(x) ((x)*(x))
The macro function is called just as if it were a regular C function. Keep in mind that for this kind of function to work, the arguments must be known at compile time.
int x = SQUARE(2); /* 4 */
Note the extra parentheses in the macro definition that are used to avoid problems with operator precedence. Without the parentheses the following example would give an incorrect result, as the multiplication would then be carried out before the addition.
#define SQUARE(x) x*x
int main(void) {
  int x = SQUARE(1+1); /* 1+1*1+1 = 3 */
}
To break a macro function across several lines, you can use the backslash character. This will escape the newline character that marks the end of a preprocessor directive. For this to work there must not be any whitespace after the backslash.
#define MAX(a,b)  \
            a>b ? \
            a:b

Although macros can be powerful, they tend to make the code more difficult to read and debug. Macros should therefore only be used when they are necessary and should always be kept short. C code such as constant variables, enums, and inline functions can often accomplish the same goal more efficiently and safely than #define directives can.

Conditional Compilation

The directives used for conditional compilation can include or exclude part of the source code if a certain condition is met. First, there are the #if and #endif directives, which specify a section of code that will be included only if the condition after the #if directive is true. Note that this condition must evaluate to a constant expression.
#define DEBUG_LEVEL 3
#if DEBUG_LEVEL > 2
 /* ... */
#endif
Just as with the C if statement, any number of #elif (else if) directives and one final #else directive can be included.
#if DEBUG_LEVEL > 2
 /* ... */
#elif DEBUG_LEVEL == 2
 /* ... */
#else
 /* ... */
#endif
Conditional compilation also provides a useful means of temporarily commenting out large blocks of code for testing purposes. This often cannot be done with the regular multi-line comment since they cannot be nested.
#if 0
 /* Removed from compilation */
#endif

Compile if Defined

Sometimes, a section of code should only be compiled if a certain macro has been defined, irrespective of its value. For this purpose, two special operators can be used: defined and !defined (not defined).
#define DEBUG
#if defined DEBUG
 /* ... */
#elif !defined DEBUG
 /* ... */
#endif
The same effect can also be achieved using the directives #ifdef and #ifndef, respectively. The #ifdef section is compiled only if the specified macro has been previously defined. Note that a macro is considered defined even if it has not been given a value.
#ifdef DEBUG
 /* ... */
#endif
#ifndef DEBUG
 /* ... */
#endif

Error and Warning

When the #error directive is encountered, the compilation is aborted. This directive can be useful, for example, to determine whether or not a certain line of code is being compiled. It can optionally take a parameter that specifies the description of the generated compilation error.
#error "Compilation aborted"
Many C compilers also include the non-standard directive #warning. This directive displays a warning message without halting the compilation.
#warning "Function X is deprecated, use Y instead"

Line

A less commonly used directive is #line, which can change the line number that is displayed when an error occurs during compilation. Following this directive, the line number will as usual be increased by one for each successive line. The directive can take an optional string parameter that sets the file name that will be shown when an error occurs.
#line 5 "myapp.c"

Pragma

The last standard directive is #pragma, or pragmatic information. This directive is used to specify options to the compiler; and as such, they are vendor specific. To give an example, #pragma message can be used with many compilers to output a string to the build window.
/* Show compiler message */
#pragma message "Compiling " __FILE__ "..."
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_19

19. Memory Management

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

In the examples so far, the programs have only had as much memory available as has been declared for the variables at compile time. This is referred to as static allocation. If any additional memory is needed at runtime, it becomes necessary to use dynamic allocation. The C standard library provides several functions for managing dynamically allocated memory, including malloc, free, realloc, and calloc. These functions are found in the stdlib.h header file.

Malloc

The malloc function takes a size in bytes and returns a pointer to a block of free memory of that size. This dynamically allocated memory is uninitialized and can only be accessed through pointers.
#include <stdio.h>
#include <stdlib.h>
int main(void) {
  /* Dynamic memory allocation (1*5 = 5 bytes) */
  char* ptr = malloc(sizeof(char) * 5);
}
The sizeof operator is used to get the number of bytes for the given data type on the current system. This number is multiplied by five to allocate a block large enough to contain exactly five chars, provided that the system has that much free memory available. If it does not, malloc returns null to signal that it has failed to allocate the memory, in which case the function may need to signal to its caller that it too has failed.
char* ptr = malloc(sizeof(char) * 5);
if (ptr == NULL) {
  /* No memory allocated, exit function */
  return -1;
}

By convention, functions that return a pointer, such as malloc, use the value NULL to indicate failure. This is different from functions that return a value, which traditionally use 0 to indicate success and -1 to signal failure. Additional return values can be used to give the user of the function more detailed information about the return state.

Free

An important thing to remember about dynamic allocation is that this memory will not be released when the pointer goes out of scope, as with local variables. Instead, the memory has to be manually released with the function free , which releases the memory block at the specified address.
free(ptr); /* release allocated memory */

This allows you to control the lifetime of a dynamically allocated object, but it also means that you are responsible for freeing that memory once it is no longer needed. Forgetting to free dynamic memory will give the program unwanted memory leaks, because that memory will stay allocated until the program shuts down.

A pointer to released memory should be set to NULL immediately to show that it is no longer set to a valid reference. This is especially important for pointers that are repeatedly allocated and freed within a program, as trying to free an already freed memory block leads to undefined behavior. Likewise, a NULL check should be carried out prior to using a pointer to make sure it is valid.
if (ptr) { /* same as: if (ptr != NULL) */
  free(ptr);
  ptr = NULL; /* null pointer */
}

It is interesting to note that the function free only accepts one argument, the starting address for the memory block. The actual size of the block does not need to be provided. This is because the implementation of malloc and free keeps track of the size of each block as it is allocated, typically by storing the size next to the block.

Realloc

An allocated memory block can be resized with the realloc function . This function takes two arguments: the pointer to a previously allocated memory block and the new total size requested. If the pointer passed to realloc is NULL then the function behaves as malloc.
/* Allocate space for 5 chars */
char* p = malloc(sizeof(char) * 5);
/* Increase size to 10 chars */
char* new_p = realloc(p, sizeof(char) * 10);
This return value is stored in a new pointer, in case realloc fails to allocate the extra memory and returns NULL. This prevents the only reference to the previously allocated memory block from being lost, which would lead to a memory leak.
/* On failure, free memory and exit */
if (!new_p) { /* if (new_p == NULL) */
  if (p) {
    free(p);
    p = NULL;
  }
  return -1;
}
/* On success, update pointer */
else {
  p = new_p;
}

Calloc

The memory block returned by malloc is uninitialized, so it will contain whatever data happens to be in that memory region. If it is important to fill the memory block with zeroes then the calloc function can be used instead. In addition to initializing the memory to zero, this function takes an extra argument in the first position, specifying the number of blocks to be allocated.
int *a;
/* Allocate memory for 3 integers, all set to 0 */
a = (int*)calloc(3, sizeof(int));
printf("Sum is: %d\n", a[0]+a[1]+a[2]); /* "Sum is 0" */
free(a); /* deallocate memory */
Another way to initialize a memory block is with the memset function. This function takes three arguments: a pointer to the starting address, the value to be filled, and the number of bytes to fill with this value.
int *arr;
int size = 3 * sizeof(int);
/* Allocate memory for 3 integers */
arr = (int*)malloc(size);
/* Fill entire memory block with zeros */
memset(arr, 0, size);
free(arr); /* deallocate memory */

Void Pointers

It is sometimes necessary to use pointers without regard to the type they reference. This is achieved by specifying the pointer type as void*, known as a void pointer. A void pointer can store the address of any type of variable and can be cast to any pointer type, making them useful as a universal pointer. This type is what allows the free function to accept any pointer argument, and allows malloc to return a pointer that can be cast to any pointer type. The following example illustrates how the void pointer can be used to change the values of two variables of different types.
int i;
char c;
/* Change i through void pointer */
void *vptr = &i;
*((int*)vptr) = 1;
/* Change c through void pointer */
vptr = &c;
*((char*)vptr) = 'a';

Note that a void pointer may not be dereferenced without first casting it to the appropriate pointer type. The compiler is unable to check that this type cast is valid, which is why void pointers should be used with care.

Function Pointers

Another useful application of pointers is to point to functions. This allows functions to be passed to other functions or stored in arrays. Consider the following function.
void func(int x) {
  printf("Value is %d\n", x);
}
A function pointer that can reference this function needs to have a matching return type and list of argument types. Note the parentheses wrapped around the pointer identifier in the following function pointer declaration. They are important so that the compiler does not view this as a function declaration.
void (*p)(int);
Now a reference to the function can be assigned to this function pointer. Since the compiler knows this is a function the address-of operator (&) is not necessary. Also, since this pointer points to code and not data, this memory must not be deallocated with free.
p = func;
p = &func; /* alternative */
The function can be called like an ordinary function. The compiler will automatically dereference the function pointer if necessary when it is called as a function.
func(5); /* "Value is 5" */
(*func)(5); /* alternative */
Function pointers can be passed to other functions, allowing functionality to be plugged into existing code. Keep in mind that the return type and list of argument types of the passed function pointer need to match the parameter of the function.
void mycaller(void (*a)()) {
  a();
}
void myfunc() {
  printf("Hello World\n");
}
int main(void) {
  mycaller(&myfunc); /* & is optional */
}
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_20

20. Input Handling

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 
When a C program is executed, it can accept arguments that are passed to it from the command line. These command-line arguments are useful for controlling the behavior of the program from outside the code. The arguments are passed to the main function, which is then set to accept two parameters as shown here.
int main(int argc, char* argv[]) {}
The integer argc is the number of arguments passed, and argv is a pointer array to the supplied command-line arguments. The first argument argv[0] is the name of the program, so the argument count in argc begins with 1 when no arguments are passed. If the program name is not available on the host environment, the first element will be an empty string instead. A simple program that prints all arguments is given here.
#include <stdio.h>
int main(int argc, char* argv[]) {
  int i;
  for(i=0; i<argc; i++) {
    printf("Argument %d is: %s\n", i, argv[i]);
  }
}
When a program is executed from a terminal window, the string arguments are listed after the file name, separated by spaces. If the argument itself contains a space it can be delimited by double quotes. In the following example, the previous program is compiled and executed with two arguments.
gcc myapp.c -o myapp.exe
./myapp.exe test "Hello World"
Argument 0 is myapp.exe
Argument 1 is test
Argument 2 is Hello World

Keyboard Input

Input from the command line can also be passed to a program while it is running, using for instance the scanf function. The first argument to this function is a formatting string that uses the same placeholders as printf, such as %d for accepting an int type. The second argument is the address to a variable that will hold the value of the expected input type.
#define _CRT_SECURE_NO_DEPRECATE
#include <stdio.h> /* scanf */
int main(void)
{
  int a, b, c;
  printf("Enter first number: ");
  scanf("%d", &a);
  printf("Enter second number: ");
  scanf("%d", &b);
  c = a + b;
  printf("%d + %d = %d\n", a,b,c);
}
Visual Studio considers scanf to be unsafe, so to use it with that compiler, you need the define directive seen at the start of this code. Given the proper input (followed by pressing Return), the function performs as expected.
gcc myapp.c -o myapp.exe
./myapp.exe
Enter first number: 2
Enter second number: 3
2 + 3 = 5
The scanf function does not perform bounds checking, so it is susceptible to buffer overflows when reading more than one character at a time. A more robust function for handling user input is fgets, which reads a string of text until the newline character appears. This function takes three arguments: the buffer, the maximum length of characters to read (including the null character), and the stream from which to read. The stream in this case is the standard input stream, designated by stdin .
#include <stdio.h> /* fgets */
int main(void)
{
  char name[10];
  printf("Enter your name: ");
  fgets(name, 10, stdin);
  printf("Hi %s", name);
}
Keep in mind that the newline character that marks the end of the input will be included in the string. If the string provided is too long, only the first nine characters plus the NULL character will be stored in this char array.
gcc myapp.c -o myapp.exe
./myapp.exe
Enter your name: Tom
Hi Tom

File Input and Output

Writing a file to the hard drive allows a program to store data that persists even after the program has shut down. A file can be opened for reading or writing using the fopen function. This function returns a pointer to a file handler and it takes two arguments: the file to open (including the path if needed) and the access mode for the file. Specifying the access mode as "w+" will create an empty file and return a handler that allows both reading and writing. Once it’s opened, a variant of printf called fprintf can be used to write content to the file. Note that the file handler is passed as the first argument to this function. After completing this task the file handler should be closed to free up resources by passing it to the fclose function.
#define _CRT_SECURE_NO_DEPRECATE /* for Visual Studio */
#include <stdio.h> /* fopen, fprintf, fclose */
int main(void)
{
  /* Create and open a file for reading/writing */
  FILE *fp = fopen ("file.txt", "w+");
  /* Write text to the file */
  fprintf(fp, "%s\n%s", "Hello", "World");
  /* Close the file */
  fclose(fp);
}
A text file has now been created in the same folder as the program. We will now read its content with another program. First the file is opened for reading by passing the r argument to fopen. Using the fgets function, the file is read one line at a time in a loop until the end of the file is reached, indicated by NULL being returned. Once all content has been read, the file is closed.
#define _CRT_SECURE_NO_DEPRECATE /* for Visual Studio */
#include <stdio.h> /* fopen, fgets, fclose */
int main(void)
{
  char buf[100];
  FILE *fp = fopen("file.txt", "r");
  while(fgets(buf,100,fp)!=NULL) {
    printf("Line contains: %s\n", i, buf);
  }
  fclose(fp);
}

Error Handling

The return value of file-handling functions should be checked to make sure each operation is successful. For instance, in this case if the file to be read is missing, then fopen will fail. In addition to returning NULL, the function will set the errno integer. This variable is used by many standard C library functions to indicate what kind of error has occurred. The variable is provided by the errno.h header file and is zero at program startup, indicating no error. Prior to calling a library function, the variable should be reset to zero since library functions will never do this in the absence of errors.
#define _CRT_SECURE_NO_DEPRECATE /* for Visual Studio */
#include <stdio.h> /* fopen, fclose */
#include <errno.h> /* errno */
int main(void)
{
  errno = 0;
  FILE *fp = fopen("missing.txt", "r");
  if(fp == NULL) {
    perror("File read failed");
    return 1; /* failed */
  }
  fclose(fp);
  return 0; /* success */
}
The perror function seen here prints a descriptive error message based on the current errno value.
gcc myapp.c -o myapp.exe
./myapp.exe
File read failed: No such file or directory
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_21

21. Headers

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

As a project grows it is common to split the code up into different source files. When this happens, the interface and implementation are generally separated. The interface is placed in a header file, which commonly has the same name as the source file and a .h file extension. This header file contains forward declarations for the source file entities that need to be accessible to other compilation units in the project.

Why Use Headers

C requires everything to be declared before it can be used. It is not enough to just compile the necessary source files in the same project. For example, if a function is placed in func.c, and a second file named app.c in the same project tries to call it, the compiler will report that it cannot find the function (or, prior to C99, that it has implicitly declared it).
/* func.c */
void myFunc(void) {
  /* ... */
}
/* app.c */
int main(void) {
  myFunc(); /* error: myFunc identifier not found */
}
To make this work as intended, the function's prototype has to be included in app.c.
/* app.c */
void myFunc(void); /* prototype */
int main(void) {
  myFunc(); /* ok */
}
This can be made more convenient if the prototype is placed in a header file named func.h and this header is included in app.c through the use of the #include directive. This way, when changes are made to func.c, there is no need to update the prototypes in app.c. Furthermore, any source file that wants to use the shared code in func.c can just include this one header.
/* func.h */
void myFunc(void); /* prototype */
/* app.c */
#include "func.h"

What to Include in Headers

As far as the compiler is concerned there is no difference between a header file and a source file. The distinction is only conceptual. The key idea is that the header should contain the interface of the implementation file—that is, the code that other source files will need to use. This may include shared macros, constants, and type definitions, as those shown here.
/* app.h - Interface */
#define DEBUG 0
const double PI = 3.14;
typedef unsigned long ulong;
The header can also contain prototypes of the shared functions defined in the source file. Internal functions used only within the source file should be left out of the header, to keep them private from the rest of the program.
void myFunc(void); /* prototype */
Additionally, shared global variables are typically declared as extern in the header, while their definitions lay in the source file.
/* app.h */
extern int myGlobal;
/* app.c */
int myGlobal = 0;

It should be noted that the use of shared global variables is discouraged. This is because the larger a program becomes, the more difficult it is to keep track of which functions access and modify these variables. The preferred method is to instead pass variables to functions only as needed, in order to minimize the scope of those variables.

The header should not include any executable statements, with one exception. A shared function that is declared as inline needs to be defined in the header. Otherwise, the compiler will not have the definition necessary for inlining the function available to it.
/* app.h */
inline void inlineFunc(void) {}
If a header requires other headers, it is common to include those files as well, to make the header stand alone. This ensures that everything needed is included in the correct order, solving potential dependency problems for every source file that needs the header.
/* app.h */
#include <stddef.h>
void mySize(size_t);

Note that since headers mainly contain declarations, any extra headers included should not affect the size of the program, although they may slow down the compilation.

Include Guards

An important thing to bear in mind when using header files is that a code entity, such as a constant, typedef, or enum, may only be defined once in every project. Consequently, including the same header file more than once will often result in compilation errors. The standard way to prevent this is to use a so-called include guard. An include guard is created by enclosing the body of the header in a #ifndef section that checks for a macro specific to that header file. Only when the macro is not defined is the file included. The macro is then defined, which effectively prevents the file from being included again.
/* app.h */
#ifndef APP_H
#define APP_H
/* ... */
#endif
© Mikael Olsson 2019
Mikael OlssonModern C Quick Syntax Referencehttps://doi.org/10.1007/978-1-4842-4288-9_22

22. Strings and Numbers

Mikael Olsson1 
(1)
Hammarland, Länsi-Suomi, Finland
 

The standard C library contains several functions for making conversions between strings and numbers.

String Conversion

One built-in conversion function is atoi , which converts a string into an integer type. The following example illustrates the use of this function by converting all command-line string arguments into numbers and adding them together.
#include <stdio.h> /* printf */
#include <stdlib.h> /* atoi */
int main(int argc, char* argv[]) {
  int i, sum=0;
  for(i=0; i < argc; i++) {
    sum += atoi(argv[i]);
  }
  printf("Sum is: %d\n", sum);
}
Passing a non-numeric argument to atoi will typically evaluate as zero; however, the C standard does not define this behavior so it is not guaranteed across all implementations. Also, the function does not distinguish between a failed conversion and the number zero. Here is an example output from running the program with both correct and incorrect arguments.
gcc myapp.c -o myapp.exe
./myapp.exe 2 3
Sum is 5
./myapp.exe Hello 0
Sum is 0
Since no error-handling capabilities are provided by atoi, use of this function is discouraged. The preferred alternative is to instead convert strings with the safer strtol function . As seen in the following code, this function takes as many characters as it can to form a valid integer value. If a valid integer cannot be formed, then zero is guaranteed to be returned.
#include <stdio.h> /* printf */
#include <stdlib.h> /* strtol */
int main(void) {
  char str[] = "-123test";
  char *end;
  long r;
  r = strtol(str, &end, 10);
  printf("Number part %ld\n", r); /* -123 */
  printf("String part %s\n", end); /* "test" */
}
In addition to the string to be converted, the strtol function takes two more arguments, a char pointer and an integer. The char pointer will point to the last parsed character in the string, so for a successful conversion of the entire string this will be the null character found at the end of the string. The integer argument is the expected base value (between 2 and 36) of the integer, so for a hexadecimal number this would be set to 16 instead of 10, as seen in the next example. Note that the function skips any whitespace characters at the beginning of the string and stops if it encounters a non-numeric character.
#include <stdio.h> /* printf */
#include <stdlib.h> /* strtol */
int main(void) {
  char str[] = "5 F";
  char *end;
  long r1, r2;
  r1 = strtol(str,&end,10); /* 5 */
  r2 = strtol(end,&end,16); /* 15 */
  if (*end == '\0') { /* entire string parsed */
    printf("%ld+%ld is %ld",r1,r2,r1+r2); /* 5+15 is 20 */
  }
}
Recall that the errno variable is changed by some C library functions to indicate certain errors. In the case of the strtol function , the errno variable is used to signal overflow, which is caused when the number is too large or small to fit in the long data type. When this occurs errno is set to ERANGE, which is a macro defined in errno.h to represent a range error. In the next example, overflow occurs when trying to convert a number larger than what the long type can hold. The long type will then be assigned the largest value it can hold instead, a shortcut for which is provided by the LONG_MAX macro.
#include <stdio.h> /* printf */
#include <stdlib.h> /* strtol */
#include <errno.h> /* errno */
int main(void) {
  const char* str = "9223372036854775808"; /* LONG_MAX+1 */
  char *end;
  long res;
  errno = 0;
  res = strtol(str,&end,10); /* LONG_MAX */
  if (errno == ERANGE) {
    printf("Overflow"); /* "Overflow" */
  }
}

Number Conversion

When printf is called with a number as an argument, there is a string conversion happening. The standard library provides a variant of printf called snprintf that can be used to store the resulting string instead of printing it, as seen in the following code.
int main(void) {
  int num = 123;
  char buffer[4];
  snprintf(buffer, sizeof(buffer), "%d", num);
}

In addition to the typical arguments supplied to printf, the snprintf function takes two more arguments at the start: the output buffer and the maximum number of characters (including the null character) that it will store in the buffer.

Index

A

Aggregate initialization
Arithmetic operators
Arrays
assignment
declaration and allocation
multi-dimensional
and pointers
sizeof operator
Assignment operators
atoi function

B

Bit field
Bitwise operators
Break and continue statements

C

Calloc function
Char type variable
Command line arguments
Comparison operators
Compile and run
comments
console compilation
visual studio compilation
Conditional statements
if statement
switch statement
Constants
guideline
parameters
pointers
variable

D

Declarator list
Dereference operator
Dereference pointers
Do-while loop

E

Enum
color
constant values
conversions
scope
switch statement
variables
Escape characters
Exact-width integer types
Explicit conversions

F

Floating-point literal suffix
Floating-point type variable
For loop
Free function
Functions
calling
definition
forward declaration
inline
parameters
pass by address
pass by value
return by value/address
return statement
variable parameter lists
void parameter

G

GNU Compiler Collection (GCC)
Goto statement

H

Headers
function’s prototype
include guards
interface and implementation
shared global variables
Hello world
integrated development environment (IDE)
IntelliSense
printf function
project
return statement
source file

I, J, K

If statement
Implicit conversions, types of
Increment and decrement operators
Infix operator
Input Handling
error
file
keyboard
Integer type variable
Integrated development environment (IDE)
IntelliSense

L

Logical operators
Loops
break and continue
do-while
for
goto statement
while

M

Macros
Malloc function
Memory management
calloc function
description
free function
malloc function
realloc function
void pointer
Multi-dimensional arrays

N

Null pointer
Number conversion

O

Operator precedence

P, Q

Pointers
address-of operator
data type
dereferencing
null
pointing to pointer
Preprocessor
ANSI C compliant compilers
conditional compilation
define macro (#define)
description
error and warning
file include (#include)
if macro defined (#ifdef)
line
macro functions
pragma
undefine macro (#undef)
printf function

R

realloc function

S

Single-dimensional arrays
Static allocation
Storage classes
auto
external
register
static
volatile
String
character array
conversion
escape character
functions
null character
strcat (string concatenation)
strcpy (string copy)
strlen function
strtol function
Struct/structure
bit field
member access
objects
pointers
user-defined type
Switch statement

T

Ternary operator
Type conversions
explicit
implicit
Typedef

U

Union
Unnamed struct

V

Variables
assigning
bool type
char type
data types
declaration
floating-point types
global
integer size
integer types
literal suffixes
local
printf function
signed and unsigned
Void pointer
Volatile modifier

W, X, Y, Z

While loop