Question

I am on ubuntu linux 10.04
I have the following

#simplec.c  
#include "stdio.h"

int main()
{

    printf("Hello World\n");
    system("ps -H");
    return 12;


}

AND

#callsimplec.c
#include "stdio.h"

int main()
{

    int ret =0;
    ret = system("./simplec");
    printf("In callsimplec ret is %d\n", ret);

}

When I do

gcc callsimplec.c -o callsimplec
gcc simplec.c -o simplec
./callsimplec

I get:

Hello World
  PID TTY          TIME CMD
27238 pts/2    00:00:00 bash
28066 pts/2    00:00:00   callsimplec
28067 pts/2    00:00:00     simplec
28068 pts/2    00:00:00       ps
In callsimplec ret is 3072

So I figured out that 3072 is printed because 256 times 12 is 3072. Whatever return value I use in simplec.c I get that value multiplied by 256 as the output in print. Why is that? I am just trying to make sense of it.

Answer 1

The value returned by system should be used with the macros: WEXITED, WIFEXITSTATUS etc.

The value returned by system (and by the wait family) is, acording to case:

• Normal termination:

  15 ............. 8 ............. 0
      exit status         0x00
  

• Killed by signal:

  15 ............. 8 7............ 0
        unused        killer signal
  

• Stopped by signal

  15 ............. 8 ............. 0
      stop signal         0x7f
  

• Continued by signal

  15 ............................. 0
                 0xFFF
  

So in your case the process exited normally and system returned 12 shifted 8 times.