Restarting process when receiving a signal with sigaction - c

Question

I'm trying to make my process restart when it receives SIGUSR1.
Since SIGINT is easier to test, I'm using it instead.
Here's the code:
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
void sig_handler(int signo){
if (signo == SIGINT){
char *args[] = { "./a", NULL };
write(1, "Restarting...\n", 14);
execv(args[0], args);
}
}
int main(void) {
printf("Starting...\n");
struct sigaction saStruct;
sigemptyset(&saStruct.sa_mask);
sigaddset(&saStruct.sa_mask, SIGINT);
saStruct.sa_flags = SA_NODEFER;
saStruct.sa_handler = sig_handler;
sigaction(SIGINT, &saStruct, NULL);
while (1)
sleep(1);
}
Unfortunately, this only works for the first time the signal is received. After that, it does nothing. I thought that the SA_NODEFER flag should make this work the way I wanted to, but it doesn't.
Also, when I try with SIGUSR1, it simply terminates the process.

Answer 1

The problem is here:
sigaddset(&saStruct.sa_mask, SIGINT);
The way NODEFER affects signals is:
If NODEFER is set, other signals in sa_mask are still blocked.
If NODEFER is set and the signal is in sa_mask, then the signal is
still blocked.
On the other hand (from Signals don't re-enable properly across execv()):
When using signal() to register a signal handler, that signal number
is blocked until the signal handler returns - in effect the kernel /
libc blocks that signal number when the signal handler is invoked, and
unblocks it after the signal handler returns. As you never return from
the signal handler (instead you execl a new binary), SIGUSR1 stays
blocked and so isn't caught the 2nd time.
Just remove the line:
sigaddset(&saStruct.sa_mask, SIGINT);
and you are done.
#define _XOPEN_SOURCE 700
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
void sighandler(int signo)
{
if (signo == SIGUSR1)
{
char *args[] = {"./demo", NULL};
char str[] = "Restarting...\n";
write(1, str, sizeof(str) - 1);
execv(args[0], args);
}
}
int main(void)
{
printf("Starting...\n");
struct sigaction act;
act.sa_handler = sighandler;
sigemptyset(&act.sa_mask);
act.sa_flags = SA_NODEFER;
sigaction(SIGUSR1, &act, 0);
while (1)
{
sleep(1);
}
}