Unix C - Portable WEXITSTATUS

Question

I'm trying to get the exit code of a subprocess. On Linux and FreeBSD I can go like so:

[0] [ishpeck@kiyoshi /tmp]$ uname
FreeBSD
[0] [ishpeck@kiyoshi /tmp]$ cat tinker.c 
#include <stdio.h>
#include <sys/wait.h>

int main(void)
{
    FILE *proc = popen("ls", "r");
    printf("Exit code: %d\n", WEXITSTATUS(pclose(proc)));
    return 0;
}
[0] [ishpeck@kiyoshi /tmp]$ gcc tinker.c -o tinker
[0] [ishpeck@kiyoshi /tmp]$ ./tinker
Exit code: 0
[0] [ishpeck@kiyoshi /tmp]$ grep WEXITSTATUS /usr/include/sys/wait.h 
#define WEXITSTATUS(x)  (_W_INT(x) >> 8)

However, on OpenBSD, I get complaints from the compiler...

[0] [ishpeck@ishberk-00 /tmp]$ uname   
OpenBSD
[0] [ishpeck@ishberk-00 /tmp]$ cat tinker.c                                    
#include <stdio.h>
#include <sys/wait.h>

int main(void)
{
    FILE *proc = popen("ls", "r");
    printf("Exit code: %d\n", WEXITSTATUS(pclose(proc)));
    return 0;
}
[0] [ishpeck@ishberk-00 /tmp]$ gcc tinker.c -o tinker                          
tinker.c: In function 'main':
tinker.c:7: error: lvalue required as unary '&' operand
[1] [ishpeck@ishberk-00 /tmp]$ grep WEXITSTATUS /usr/include/sys/wait.h        
#define WEXITSTATUS(x)  (int)(((unsigned)_W_INT(x) >> 8) & 0xff)

I don't really care how it's done, I just need the exit code.

This leads me to believe that I would also have this problem on Mac: http://web.archiveorange.com/archive/v/8XiUWJBLMIKYSCRJnZK5#F4GgyRGRAgSCEG1

Is there a more portable way to use the WEXITSTATUS macro? Or is there a more portable alternative?

Answer 1

OpenBSD's implementation of WEXITSTATUS uses the address-of operator (unary &) on its argument, effectively requiring that its argument have storage. You are calling it with the return value of a function, which doesn't have storage, so the compiler complains.

It is unclear whether OpenBSD's WEXITSTATUS is POSIX-compliant, but the problem can be easily worked around by assigning the return value of pclose() to a variable:

    int status = pclose(proc);
    printf("Exit code: %d\n", WEXITSTATUS(status));