Intermediate C Programming

Chapter 1

Program Execution

1.1 Compile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Redirect Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.1 Compile

This chapter explains how to write, compile, and execute programs in Linux. We use

a Linux terminal and explain the commands you need to type. Why do you learn how

to use the terminal? First, the terminal is a ﬂexible and convenient interface for working

with a computer. It may take some experience to realize this, but learning how to use the

terminal may improve your productivity. Second, many cloud computing or web services

oﬀer terminal access. This is a natural method of providing computing resources, especially

when working with many computers (like in a data center). A graphical user interface (GUI)

is nice when working with one computer. However, when dealing with many computers, GUI

can become a distraction. Also, using the terminal helps you understand how UNIX systems

work. After becoming familiar with terminal commands, you may understand integrated

development environments (IDEs), and what they can do for you. The Eclipse IDE is

explained later in this book.

Start a terminal in Linux and type

$ cd

$ pwd

$ mkdir cprogram

$ cd cprogram

In this book, $ is used as the terminal prompt.

The ﬁrst command cd means “change directory”. If no argument is added after cd, as

in the ﬁrst command, then it will return to your home directory (also called “folder”).

The second command pwd means “print the current working directory”. It will be some-

thing like /home/yourname/.

The third command mkdir means “make a directory”. The command mkdir cprogram

means “make a directory whose name is cprogram”.

You should not create a directory or a ﬁle whose name includes spaces. The reason is very

simple: The international standard for directory names and ﬁle names (called International

Standard Organization or ISO 9660) disallows spaces. If a directory’s or a ﬁle’s name has

spaces, then some programs may not work.

The last command cd cprogram means “change directory to (i.e., enter) cprogram”.

This is the directory that was just created.

In the terminal, type

$ which emacs

If nothing appears in the terminal or it says “Command not found”, then please install

4 Intermediate C Programming

Emacs ﬁrst. If you do not know how to install software in Linux, please read Section A.5.

In the terminal, type

$ emacs prog1.c &

This command starts Emacs to edit a ﬁle called prog1.c. Adding & allows you to use the

terminal as well as the Emacs editor at the same time. Without the trailing &, the terminal

will force you to wait until Emacs quits. Inside Emacs, type the following code:

// prog1 .c1

#in clude < stdio .h >2

#in clude < stdlib .h >3

int main ( i n t argc , char * * argv )4

int a = 5;6

int b = 17;7

printf (" main : a = %d , b = %d , argc = %d \n" , a , b, argc );8

return EXIT _SUCCES S ;9

}10

Save the ﬁle. You can probably guess that this program prints something like

main: a = 5, b = 17, argc =

This is the ﬁrst complete program shown in this book and requires some explanation. This

program prints something by calling printf. This is a function provided by the C language

but you need to include stdio.h before you can use this function. This is a header ﬁle for

standard input and output functions. In a C program, the starting point is the main function.

The program returns EXIT SUCCESS after it successfully prints the addresses. As you can

guess, if a program can return EXIT SUCCESS, another program can return EXIT FAILURE.

Why should a program return either EXIT SUCCESS or EXIT FAILURE? In today’s complex

computer systems, many programs are called by other computer programs. Thus, it is

important that your programs inform the calling programs whether your programs have

successfully accomplished what they are designed to do. This information allows the calling

programs to decide what actions to take next. EXIT SUCCESS and EXIT FAILURE are symbols

deﬁned in stdlib.h so it is included at the second line.

In this book, source code is listed with line numbers, starting from 1. Sometimes, the

code refers to a previously mentioned example and the line number corrsponds to the value

in the earlier example.

The main function is the starting point of a C program but this is not always true for

a C++ program. If a C++ program has a static object, the object’s constructor will be

called before the main function is called. Since this book is about C programming, it is safe

to assume that the main function is the starting point of all the programs.

What is argc? It is easier to answer this question by running the program. First, we

need to explain how to convert this program from a human-readable format to a computer-

readable format.

What is typed into Emacs is a “C source ﬁle”. It is vaguely similar to English and

consists of Latin alphabet letters. However, since the computer does not understand this

format, the “source ﬁle” needs to be converted into a computer-readable format called an

executable. A compiler is the tool needed for this conversion and gcc is a popular compiler

on Linux. In the terminal, type

$ gcc prog1.c -o prog

Program Execution 5

This command means the following:

• Execute the gcc command installed in Linux.

• Use prog1.c as the input for the gcc command.

• Set the output ﬁle name to be prog (-o speciﬁes the name of the output ﬁle). The

output ﬁle is an executable ﬁle, meaning that the computer can run it.

Do not do this:

$ gcc prog1.c -o prog1.c

This command erases the ﬁle prog1.c.

The gcc compiler has many options. Please read the documentation to learn more. To

ﬁnd the documentation, type “linux gcc” in an Internet search engine and you will see

something like the following:

GCC(1) GNU GCC(1)

NAME

gcc - GNU project C and C++ compiler

SYNOPSIS

gcc [-c|-S|-E] [-std=standard]

[-g] [-pg] [-Olevel]

[-Wwarn...] [-pedantic]

[-Idir...] [-Ldir...]

[-Dmacro[=defn]...] [-Umacro]

[-foption...] [-mmachine-option...]

[-o outfile] [@file] infile...

Only the most useful options are listed here; see below for the

remainder. g++ accepts mostly the same options as gcc.

DESCRIPTION

When you invoke GCC, it normally does preprocessing,

compilation, assembly and linking. The "overall options" allow you

to stop this process at an intermediate stage. For example, the -c

option says not to run the linker. Then the output consists of

object files output by the assembler.

Other options are passed on to one stage of processing. Some

options control the preprocessor and others the compiler itself. Yet

other options control the assembler and linker; most of these are not

documented here, since you rarely need to use any of them.

Most of the command line options that you can use with GCC are

useful for C programs; when an option is only useful with another

language

The document is also called the “man page”, where “man” means manual. These manual

pages are typically well written but terse. Early computers were very expensive and the

designers tried to keep everything as short as possible. We have seen a few Linux commands

already:

6 Intermediate C Programming

• cd: change directory

• gcc: convert a human-readable ﬁle to a computer-readable ﬁle

• man: display a manual page

• mkdir: make a new directory

The output of the gcc command is an executable. In Linux, it is possible to ﬁnd some

information about a ﬁle by using the file command. Please type this in the terminal:

$ ﬁle prog

The output should be similar to the following, but the speciﬁcs will vary depending on

the computer the program is compiled on:

ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked

(uses shared libs), for GNU/Linux 2.6.24,

BuildID[sha1]=0x65dfd5517523d920d9fbaf4ededa84792a9e9c61, not

stripped

The most important thing to pay attention to is the word “executable”. This word

means that the ﬁle “prog” is a program. By convention, executable ﬁles in Linux have no

extension, unlike “.exe” used in Windows. How do you execute the program? Type this

command:

$ ./prog

Here, prog is the name of the program; ./ means the current directory. Why is it

necessary to add ./ in front of the program? It is necessary because it is possible to have

ﬁles of the same name in diﬀerent directories. By adding ./, the terminal knows that the

desired program is in this directory. Some people like to call their programs “test”. This is

a bad name for your program because “test” is also a built-in command in Linux. If you

type

$ test

then the Linux command is run. If you type

$ ./test

then your program in the directory is run.

What exactly is argc? It stores the number of arguments given to the program. Let me

demonstrate what I mean by running the program a few times.

$ ./prog

main: a = 5, b = 17, argc = 1

$ ./prog abc

main: a = 5, b = 17, argc = 2

$ ./prog abc 123

main: a = 5, b = 17, argc = 3

$ ./prog abc 123 C Programs

main: a = 5, b = 17, argc = 5

Program Execution 7

Do you notice the changes in argc? When the program is executed without anything

else, argc is 1. If some words are added after the program, then argc becomes larger.

The more words (i.e., arguments) that are added, the larger argc becomes. This illustrates

that arguments can be given to programs when they are run. Consecutive arguments are

separated by one or more spaces. The terminal tells your program (speciﬁcally, the main

function) the number of arguments. As can be seen in the examples below, adding extra

spaces between words makes no diﬀerence. One space has the same eﬀect as several spaces.

$ ./prog abc 123 C Programs

main: a = 5, b = 17, argc = 5

$ ./prog abc 123 C Programs

main: a = 5, b = 17, argc = 5

$ ./prog abc 123 C Programs

main: a = 5, b = 17, argc = 5

The program itself is always the ﬁrst argument. Since you must type the program’s name

to run the program, the value of argc is always at least one. What is the value of argc

when using this gcc command?

$ gcc prog1.c -o prog

The answer is 4.

1. gcc

2. prog1.c

3. -o

4. prog

The arguments themselves are strings, and are stored in argv. This will be covered when

explaining strings in Chapter 6.

1.2 Redirect Output

The printf function is probably one of the ﬁrst things that people learn about writing

C programs. The famous “Hello World!” program is often used as an example for beginners.

In this computer program, the text is printed to the terminal. In some cases, however, it

is useful to redirect the information from the program and save it to a ﬁle. Here are some

scenarios when this may be useful:

• A program prints too much too fast and the computer screen cannot display everything

printed.

• You do not want to wait while the program runs, but instead want to see the infor-

mation later.

• Sometimes it is useful to check whether the program produces the same information

when it is run again. If the program produces more than several lines of output,

checking the output line-by-line is too much work.

• The program may need to be run on many computers simultaneously. It may be

impossible to watch many screens at once.

Previous Chapter

I. Computer Storage: Memory and File

Next Chapter

1. Program Execution (2/2)

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Intermediate C Programming