Linux system() call returns -1 but the command executes successfully

Asked Oct 31 '21 at 15:59

Active Nov 01 '21 at 00:43

Viewed 84 times

I have some bash command which I want to execute from C file:

root@/some# /sbin/iptables -t raw -I PREROUTING 5 -m set --match-set list src -j DROP
Chain PREROUTING (policy ACCEPT)
num  target     prot opt source               destination         
root@/some# echo $?
0

This works fine. When I do it from regular C file like:

int main(void) { 
     int ret;
     ret = system("/sbin/iptables -t raw -I PREROUTING 5 -m set --match-set list src -j DROP");
     printf("%d\n", ret);
}

It prints 0, so it's okay.
But when I call this command from daemonized program (after usual daemonization process) inside pthread it returns -1 but the rule is inserted, so it works fine but for some reason system() returns -1. What could be a reason?

UPD: From daemon the result of a system() call I'm writing to syslog, and it has -1 and errno - No child processes.

edited Oct 31 '21 at 16:15

asked Oct 31 '21 at 15:59

narotello

Your demon won't be able to communicate the return type of the iptables command. – Captain Giraffe Oct 31 '21 at 16:02
What error is reported by `system`? (Try logging the result of `strerror(errno)` to a file.) – rici Oct 31 '21 at 16:03
How do you know what the daemon gets for ret? – cliff2310 Oct 31 '21 at 16:06
@rici The error is `No child processes` – narotello Oct 31 '21 at 16:07
@cliff2310 I'm writing to syslog – narotello Oct 31 '21 at 16:07
@narotello: please edit your question rather than adding clarifications in comments. – rici Oct 31 '21 at 16:12
Are you ignoring `SIGCHLD`? – rici Oct 31 '21 at 16:24
@rici Yep.. I do `signal(SIGCHLD, SIG_IGN);` `signal(SIGHUP, SIG_IGN);` while daemonizing. – narotello Oct 31 '21 at 16:25
@rici So it there some common practice to get real result of `system()` in daemons which ignore `SIGCHLD` signal? – narotello Oct 31 '21 at 16:27
1

@narotello: `system` is a convenience function; if you need more precise behaviour, you can use `fork` and `exec`. There's example code in the `system` manpage IIRC. – rici Oct 31 '21 at 16:45
https://man7.org/linux/man-pages/man3/system.3p.html#RATIONALE – rici Oct 31 '21 at 16:48

Linux system() call returns -1 but the command executes successfully

0 Answers0