Monday, January 9, 2017

The Importance of C/C++ Program Exit Status in Unix/Linux

The return value from main() in C/C++ programs a.k.a exit status is often overlooked by less advanced Unix/Linux programmers. Nevertheless, it's important to keep in mind the exit status of your C/C++ code because it will help in the long run. There are at least 2 scenarios where exit status is important:

  1. When you're using shell script to automate processing by using several programs to perform "sub-tasks". In this case, the shell script--very possibly--need to make logical decision based on your program exit status. 
  2. When your C/C++ program is part of a multiprocess program in which your C/C++ program is called/executed (a.k.a fork-ed and exec-ed) by the parent process. In many cases, the parent process need to know whether your program executes successfully or not.

Now, let's be more concrete. Let's say you anticipated that your C/C++ program will be invoked by bash-compatible shell. In that case, your code's exit status must make sense to bash. Therefore, you should:

Following the rules above doesn't necessarily mean your C/C++ program will be bug free because some of the exit status are ambiguous. Nevertheless, it should make it more palatable to be combined into larger system and ease debugging.

As closing, let's look at a very simple code that uses sysexits.h below.
#include <stdio.h>
#include <sysexits.h>
 * Usage: 
 *  test_code  -x param1 -z param2
int main(int argc, char *argv[])

 if (argc != 5) {
  printf("Usage: %s -x param1 -z param2\n", argv[0]);
  return EX_USAGE;

      //... irrelevant code omitted

       return EX_OK;

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