How can I determine if the operating system is POSIX in C?

Question

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Answer 1

It is possible to solve your problem using autoconf tool to discover the presence of unistd.h and then would add a #define HAVE_UNISTD_H 1 line in a generated config.h if unistd.h was found, but I find that autoconf is a little hard to use, and is very out-dated.

If by any chance you are using cmake, you can solve it the same way. You could create a config.h.in containing something like this:

#ifndef CONFIG_H
#define CONFIG_H

#cmakedefine HAVE_UNISTD_H 1

#endif

And your project's CMakeLists.txt would look like this:

project(MyApp)

include(CheckIncludeFiles)
check_include_file("unistd.h" HAVE_UNISTD_H)

configure_file(config.h.in config.h @ONLY)

add_executable(${PROJECT_NAME} main.c)

and then to generate from the command line:

cmake . -G"Unix Makefiles"

or generate Xcode project (OSX only):

cmake . -G"Xcode"

or generate a visual studio 2013 solution project (Windows only):

cmake . -G"Visual Studio 12 2013 Win64"

cmake generators = epic win

If your operating system is POSIX, then your generated config.h should look like this:

#ifndef CONFIG_H
#define CONFIG_H

#define HAVE_UNISTD_H 1

#endif

Otherwise, it will look like that:

#ifndef CONFIG_H
#define CONFIG_H

/* #undef HAVE_UNISTD_H */

#endif

And then you are free to your your trusted generated config.h :

#include "config.h"

#if HAVE_UNISTD_H
#   include 
#endif

int main (int argc, const char * argv[])
{
#if defined(_POSIX_VERSION)
     /* POSIX code here */
#else
     /* non-POSIX code here */
#endif
    return 0;
}