CMake:How To Write Platform Checks
If you want to write software which compiles and runs on different operating systems, you have to take care for the special properties of the different platforms. On different operating systems there are subtle differences, e.g. on FreeBSD you should not use malloc.h, while it is perfectly ok to use it on Linux. These differences are usually handled by providing a header file which contains a bunch of define-statements according to the platform properties, usually named config.h:
#define HAVE_MALLOC_H 1 /* #undef HAVE_SYS_MNTTAB_H 1 */ /* #undef HAVE_SYS_MNTENT_H 1 */ #define HAVE_SYS_MOUNT_H 1
This header file is then included in the source files and the handled appropriately:
foo.c:
#include "config.h"
#ifdef HAVE_MALLOC_H
#include <malloc.h>
#else
#include <stdlib.h>
#endif
void bar() {
void *buf=malloc(1024);
...
}
The contents of config.h depend on the platform where the sources are compiled, so there needs to be a way to generate this header file before the actual compilation process starts. If you are using autotools-based software, you probably know the ./configure step, which has to be executed before starting make. The ./configure script does some system introspection and generates from the gathered information the config.h header file. CMake is able to do the same, and I'll show you how to do it.
Additionally to the builtin commands (see http://www.cmake.org/HTML/Documentation.html ), cmake offers more commands implemented by cmake script files, called modules. These files are located in the cmake module directory, on UNIX systems is this by default /usr/local/share/CMake/Modules .
To use commands from these modules, they have to be included in the CMakeLists.txt. CMake comes with several modules for checking the system, they all follow the same style, as an example here CHECK_INCLUDE_FILES:
INCLUDE (CheckIncludeFiles)
# usage: CHECK_INCLUDE_FILES (<header> <RESULT_VARIABLE> )
CHECK_INCLUDE_FILES (malloc.h HAVE_MALLOC_H)
CHECK_INCLUDE_FILES ("sys/param.h;sys/mount.h" HAVE_SYS_MOUNT_H)
The CMake module CheckIncludeFiles offers the command, well, CHECK_INCLUDE_FILES(). The first argument to this command is the header you want to check for. The second argument is the variable which will contain the result. If the given header was found, it is set to 1, otherwise it is empty. If another header is required to use the header you are looking for, you have to list the header files separated by semicolons, as you can see above. To see what CHECK_INCLUDE_FILES() exactly does, have a look at the implementation: /usr/local/share/CMake/Modules/CheckIncludeFiles.cmake . There you'll see that it tries to compile a simple source file which includes the specified header files. The results of the tests are stored in the file CMakeCache.txt, so if you want to check later whether the test succeeded or not look in the CMakeCache.txt file :
//Have include HAVE_MALLOC_H HAVE_MALLOC_H:INTERNAL=1
As long as the result is in the cache, the test won't be executed again. If you want it to be executed again, either delete the file CMakeCache.txt, then all tests will be executed again, or just remove the entries for the variables you want to have tested again. This can save some time. If the test failed, and you want so find out why, open CMakeFiles/CMakeError.log and search for the header name (or function, etc.) you were testing for. There you should see the code which failed to compile or link, the compile command and the error message. Together with the implementation of the test in CheckIncludeFiles.cmake you should be able to figure out what went wrong.
The other commands coming with cmake to do system checks follow this style, so we can handle them much shorter now.
Module: INCLUDE (CheckIncludeFiles) Usage: CHECK_INCLUDE_FILES(header variable) Example: CHECK_INCLUDE_FILES(strings.h HAVE_STRINGS_H)
As just discussed lengthy, this can be used to check for the existence of a header.
Module: INCLUDE (CheckFunctionExists) Usage: CHECK_FUNCTION_EXISTS(function variable) Example: CHECK_FUNCTION_EXISTS(madvise HAVE_MADVISE)
Checks whether the given function exists. This is done by linking a small program, which may not result in undefined references.
INCLUDE (CheckSymbolExists) INCLUDE (CheckLibraryExists) INCLUDE (CheckTypeSize) INCLUDE (CheckPrototypeExists) INCLUDE (CheckCXXSourceCompiles) INCLUDE (CheckCSourceCompiles)
