assembly-mac-os.c

#include <stdio.h>

int main() {
  int id;
  
   __asm__(
     "mov $0x02000004, %%eax;"
      "syscall;\n"
      : "=a"(id)
   );
  
  // application gets user id
  // you can find system call http://shell-storm.org/shellcode/files/syscalls.html   
  
  return 0
}