Unix C - Portable WEXITSTATUS

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));