Conditional function by target O.S. or architecture

target-example.c

#include <stdio.h>
#include "target.h"

OS_LINUX(char *os, void)
{
  return "Linux";
}

OS_WINDOWS(char *os, void)
{
  return "Windows";
}

ARCH_AMD64(char *arch, void)
{
  return "x86-64";
}

ARCH_I386(char *arch, void)
{
  return "IA-32";
}

int main(void)
{
  printf("%s | %s\n", os(), arch());
  return 0;
}

target.h

#ifndef _TARGET_H
#define _TARGET_H

#ifdef __x86_64__

#define target_arch "AMD64"
#define target_amd64 1

#define ARCH_AMD64(func_sign, ...) func_sign(__VA_ARGS__)
#define ARCH_I386(func_sign, ...) static func_sign ## _discard_me()

#endif /* __x86_64__ */

#ifdef __i386__

#define target_arch "i386"
#define target_i386 1

#define ARCH_AMD64(func_sign, ...) static func_sign ## _discard_me()
#define ARCH_I386(func_sign, ...) func_sign(__VA_ARGS__)

#endif /* __i386__ */

#ifdef __unix__

#define target_os "UNIX"
#define target_unix 1

#define OS_LINUX(func_sign, ...) static func_sign ## _discard_me()
#define OS_WINDOWS(func_sign, ...) static func_sign ## _discard_me()
#define OS_UNIX(func_sign, ...) func_sign(__VA_ARGS__)

#endif /* __unix__ */

#ifdef __linux__

#undef target_os
#undef OS_LINUX

#define target_os "Linux"
#define target_linux 1

#define OS_LINUX(func_sign, ...) func_sign(__VA_ARGS__)

#endif /* __linux__ */

#if defined(__WINNT__) || defined(_WIN32)

#define target_os "Windows"
#define target_windows 1

#define OS_LINUX(func_sign, ...) static func_sign ## _discard_me()
#define OS_WINDOWS(func_sign, ...) func_sign(__VA_ARGS__)

#endif /* __WINNT__ */

#endif /* _TARGET_H */