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Old 07-30-2004, 03:40 AM   #1
Registered: Feb 2004
Location: Athens, Greece
Distribution: Gentoo,FreeBSD, Debian
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How to use return values from system functions?

I 'd like to write a program with C, which uses system commands. But I also want to know the values they return to my program, in order to understand if everything went right. How do I get these values?
Old 07-30-2004, 03:54 AM   #2
Registered: Sep 2003
Location: Ankara/Turkey
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From the
man system
(Please refer them first ;-)):

The value returned is -1 on error (e.g. fork failed), and the return
status of the command otherwise.


[baris@rhinox]$ cat return.c
#include <stdlib.h>

int main(int argc, char **argv)
int i = system("test 4 -gt 2");

if ( i == -1 )
printf("$?: %d\n", i);

return 0;

[baris@rhinox]$ ./a.out
$?: 0
[baris@rhinox]$ test 4 -gt 2
[baris@rhinox]$ echo $?
Old 07-30-2004, 04:09 AM   #3
Registered: Aug 2003
Location: 63123
Distribution: OpenSuSE/Ubuntu
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Are you talking about the linux system calls, the ones that reside in file:/usr/include/asm/unistd.h ? Most system calls have C wrappers
that return the values using normal C procedure. You can issue a
man nameofsystemcall to see how each is set up. If you are
calling them from assembly, they return their value in EAX with the
arguments pushed onto the stack in reverse order.


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