Table of Contents for

Postfix: The Definitive Guide

Compiler options are set in the CCARGS variable. C source code files require header files that define certain functions and variables. The standard location for header files is the /usr/include directory. If your header files are located somewhere else, you have to tell the compiler where to look for them. The -I compiler option is used to specify additional directories where the compiler might find header files. If you are linking with libraries from external packages, the header files might be located where the package is installed rather than in the standard location. A common convention for external packages is to install header files in /usr/local/include. If you want to tell the compiler to look in that directory as well as the standard location when building Postfix, specify the options and directory with CCARGS:

CCARGS='-I/usr/local/include/'

Use additional -I options for each additional directory the compiler should search.

Postfix uses conditional compilation during its build, depending on which libraries or other resources are available on your system. It defines certain macros based on what it discovers about your system or based on options you have selected. The -D option provides a way to define macros at the time you compile Postfix. Add-on packages for Postfix require that you define a particular macro to tell Postfix to include it when building. For example, if you want to include support for MySQL, you define the HAS_MYSQL macro:

CCARGS='-DHAS_MYSQL'

Linker options are set in the AUXLIBS variable. After Postfix has compiled the object files, it links them together with required libraries into executable files. The standard location for system libraries is /usr/lib. To tell the linker to search additional directories for libraries, use the -L option:

AUXLIBS='-L/usr/local/lib'

You must also tell the linker which specific libraries to link in. The -l option is used to name specific libraries. The library files must be in a standard location or a directory indicated with the -L option. Library archive files are named starting with lib, followed by their name, followed by the extension, which is normally .a for static libraries and .so or .sl for shared objects or shared libraries. When you use the -l option, you leave off the initial lib and the extension of the library file. To link with the MySQL client library for example, where the library file is called libmysqlclient.a, the -l option is specified as follows:

AUXLIBS='-L/usr/local/lib -lmysqlclient'

Most linkers choose runtime or dynamic libraries over the static versions. Runtime libraries are linked when a program is running rather than during compilation. At compile time, the linker adds information so the program can find the libraries when it is executed. If you always install all of your dynamic libraries in a standard location such as /usr/lib, your system won’t have any trouble finding the libraries at runtime. However, some external packages install libraries in nonstandard directories such that they cannot be found at runtime. Different systems use different conventions for locating dynamic libraries using fixed path information and environment variables. Be sure to configure your system to be able to find your dynamic libraries or make sure that the libraries are installed in your system’s standard directories. Another option is to provide the actual path to specific libraries when you build your programs.

The linker uses an argument to include directories in a runtime search path for dynamic libraries. The argument differs depending on your linker and platform. The GNU linker (Linux, FreeBSD) uses -rpath, as does IRIX. Solaris, on the other hand uses -R, and HP-UX uses +b. Consult the manpage for your linker, ld(1), to see which argument you should use to set the runtime library search path.

Using the SSL library as an example, if your libssl.so file is located in /usr/local/lib and you are building Postfix on FreeBSD or another system that uses rpath, define AUXLIBS as follows:

AUXLIBS='-L/usr/local/lib -rpath/usr/local/lib -lssl'

When linking Postfix with external libraries, if you have multiple versions of the libraries installed, it is very important to make sure that you link Postfix with the version you need. Also, make sure that the library version you link to corresponds to the correct version of the header files you include. Version mismatch problems are often the source of compiler errors. Sometimes the compiler does not complain, in which case your build may succeed, but you’re likely to find unusual errors from Postfix at runtime that can be tricky to track down.

AUXLIBS='-L/usr/local/lib -Wl,-rpath,/usr/local/lib -lssl'