How to use return values from system functions?

kpachopoulos · 07-30-2004, 03:40 AM

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?

barisdemiray · 07-30-2004, 03:54 AM

From the

Code:

man system

(Please refer them first ;-)):

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

Sample:

[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("error\n");
else
printf("$?: %d\n", i);

return 0;
}

[baris@rhinox]$ ./a.out
$?: 0
[baris@rhinox]$ test 4 -gt 2
[baris@rhinox]$ echo $?
0

jinksys · 07-30-2004, 04:09 AM

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.