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 #include int main(void) { FILE *proc = popen("ls", "r"); printf("Exit code: %d ", 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 #include int main(void) { FILE *proc = popen("ls", "r"); printf("Exit code: %d ", 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] [ish…

Accepted Answer

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
", WEXITSTATUS(status));
```

Unix C - Portable WEXITSTATUS

Solution

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