Restarting process when receiving a signal with sigaction

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.

How are you waiting the signal? I mean, how do you keep alive the program? — David Ranieri, Oct 09 '20 at 05:22
@David Ranieri I've updated the code in the OP to be a [mcve] — ikegami, Oct 09 '20 at 05:32
There's a while loop in the main function that asks for user input — leosole, Oct 09 '20 at 05:42
The edit was made by ikegami. In my code, it's also a while loop, but it has printf's and fgets to ask for user input. — leosole, Oct 09 '20 at 05:51

David Ranieri · Accepted Answer · 2020-10-09T14:50:53.417

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);
    }
}

Thank you! It's working for SIGINT, but when i try it with SIGUSR1, it terminates the process. Do you know why this might be happening? I'm sending the SIGUSR1 doing kill -SIGUSR1 pid — leosole, Oct 09 '20 at 13:05
You are welcome, this is very odd, are you calling `sigaction` with `SIGUSR1`? I'm use `ps -ef | grep demo` to get the PID and then `kill -SIGUSR1 14282` where 12482 is the PID and it works fine for SIGINT and also for SIGUSR1, what OS are you using? — David Ranieri, Oct 09 '20 at 13:08
Yes, I'm replacing both SIGINT in the code to SIGUSR1. I don't know if this can be OS specific. I'm on MacOS — leosole, Oct 09 '20 at 13:13

Restarting process when receiving a signal with sigaction

1 Answers1