Calling linux command cmp with C program

Question

I'm trying to make a program which is getting 2 pathes for files to main, and calling linux' cmp command in order to compare them.

If they equal, I want to return 2, and if they're different, 1.

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(int argc, const char* argv[])
{
pid_t pid;
int stat;

//child process
if ((pid=fork())==0)
{
    execl("/usr/bin/cmp", "/usr/bin/cmp", "-s",argv[1], argv[2], NULL);
}
//parent process
else
{
    WEXITSTATUS(stat);
    if(stat==0)
        return 2;
    else if(stat==1) 
        return 1; //never reach here
}
printf("%d\n",stat);
return 0;
}

For some reason if the files are the same, I do succeed in returning 2, but if they're different, it won't go into if(stat==1), but to return 0. Why is this happening? I checked that cmp on the files through terminal does truly return 1 if they're different, so why this doesnt work?

Answer 1

Do it like this:

//parent process
else
{
  // get the wait status value, which possibly contains the exit status value (if WIFEXITED)
  wait(&status);
  // if the process exited normally (i.e. not by signal)
  if (WIFEXITED(status))
    // retrieve the exit status
    status = WEXITSTATUS(status);
  // ...

Answer 2

In your code:

WEXITSTATUS(&stat);

Tries to extract a status from a pointer, but WEXITSTATUS() takes int as parameter.

Must be:

WEXITSTATUS(stat);