What would WIFEXITED(status) be when a process just ran the command 'true'?

Question

If I run the following code segment

pid_t p;
int status = 0;
p = fork();
if (p < 0)
    report_error();
if (p == 0) // child
{
    execlp("true", "true", 0);
    _exit(127); // we should not get here
}
else
{
    waitpid(p, &status, 0);
    if(WIFEXITED(status))
        printf("Exited with code %d", WEXITSTATUS(status));
}

I don't get anything printed because it seems like WIFEXITED evaluates to false. I suspect this is because "true" isn't a command per se and doesn't "exit" the child process?

Can I still rely on WEXITSTATUS(status) even if it didn't "exit?" If I were to execlp("false", "false", 0); instead, is it guaranteed that WEXITSTATUS(status) is 1? It seems to be true so far, but I'd just like to confirm that this isn't just a coincidence.

Check if [`execlp`](http://man7.org/linux/man-pages/man3/execlp.3.html) returns. It will return if there's an error, in which case it will set `errno`. — Some programmer dude, Jan 17 '15 at 10:06
@SouravGhosh I did; I've changed it now. Thanks! (That issue wasn't in my actual code; I just retyped it from memory here) — gautam, Jan 17 '15 at 10:22
@JoachimPileborg I did so; execlp does not seem to be returning. — gautam, Jan 17 '15 at 10:31

abligh · Accepted Answer · 2015-01-17T11:20:46.130

This (a slight clean up):

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main (int argc, char **argv)
{
  pid_t p;
  int status = 0;
  p = fork ();
  if (p < 0)
    {
      perror ("fork failed");
    }
  else if (p == 0)              // child
    {
      execlp ("true", "true", NULL);
      _exit (127);              // we should not get here
    }
  else
    {
      waitpid (p, &status, 0);
      if (WIFEXITED (status))
        printf ("Exited with code %d\n", WEXITSTATUS (status));
    }
}

prints

Exited with code 0

If I change both instances of true to false, it prints

Exited with code 1

I suspect the problem is thus in the code you haven't shown (e.g. any main) or because your system for some reason doesn't have /bin/true:

$ ls -la /bin/true
-rwxr-xr-x 1 root root 27168 Mar 24  2014 /bin/true

(why it takes 27168 bytes to return exit code 0 I don't know)

I tested this on the Mac on OS-X 10.9.5 too:

nimrod:~ amb$ cc --version
Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin13.4.0
Thread model: posix
nimrod:~ amb$ uname -a
Darwin nimrod.local 13.4.0 Darwin Kernel Version 13.4.0: Sun Aug 17 19:50:11 PDT 2014; root:xnu-2422.115.4~1/RELEASE_X86_64 x86_64
nimrod:~ amb$ which true
/usr/bin/true
nimrod:~ amb$ ls -la /usr/bin/true
-rwxr-xr-x  1 root  wheel  13808 18 Feb  2014 /usr/bin/true

Appreciate the cleanup and suggestion. I was able to get it to run correctly on Linux (as opposed to Mac, where I was doing my original testing.) — gautam, Jan 17 '15 at 10:46

What would WIFEXITED(status) be when a process just ran the command 'true'?

1 Answers1