Child process receives parent's SIGINT

If you are generating the SIGINT with Ctrl+C on a Unix system, then the signal is being sent to the entire process group.

You need to use setpgid or setsid to put the child process into a different process group so that it will not receive the signals generated by the controlling terminal.

---

[Edit:]

Be sure to read the RATIONALE section of the setpgid page carefully. It is a little tricky to plug all of the potential race conditions here.

To guarantee 100% that no SIGINT will be delivered to your child process, you need to do something like this:

#define CHECK(x) if(!(x)) { perror(#x " failed"); abort(); /* or whatever */ }
/* Block SIGINT. */
sigset_t mask, omask;
sigemptyset(&mask);
sigaddset(&mask, SIGINT);
CHECK(sigprocmask(SIG_BLOCK, &mask, &omask) == 0);

/* Spawn child. */
pid_t child_pid = fork();
CHECK(child_pid >= 0);
if (child_pid == 0) {
    /* Child */
    CHECK(setpgid(0, 0) == 0);
    execl(...);
    abort();
}
/* Parent */
if (setpgid(child_pid, child_pid) < 0 && errno != EACCES)
    abort(); /* or whatever */
/* Unblock SIGINT */
CHECK(sigprocmask(SIG_SETMASK, &omask, NULL) == 0);

Strictly speaking, every one of these steps is necessary. You have to block the signal in case the user hits Ctrl+C right after the call to fork. You have to call setpgid in the child in case the execl happens before the parent has time to do anything. You have to call setpgid in the parent in case the parent runs and someone hits Ctrl+C before the child has time to do anything.

The sequence above is clumsy, but it does handle 100% of the race conditions.