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@aspyct /signal.c
Last active Jun 26, 2017

What would you like to do?
Unix signal handling example in C, SIGINT, SIGALRM, SIGHUP...
/**
* More info?
* a.dotreppe@aspyct.org
* http://aspyct.org
* @aspyct (twitter)
*
* Hope it helps :)
*/
#include <stdio.h>
#include <stdlib.h>
#include <signal.h> // sigaction(), sigsuspend(), sig*()
#include <unistd.h> // alarm()
void handle_signal(int signal);
void handle_sigalrm(int signal);
void do_sleep(int seconds);
/* Usage example
*
* First, compile and run this program:
* $ gcc signal.c
* $ ./a.out
*
* It will print out its pid. Use it from another terminal to send signals
* $ kill -HUP <pid>
* $ kill -USR1 <pid>
* $ kill -ALRM <pid>
*
* Exit the process with ^C ( = SIGINT) or SIGKILL, SIGTERM
*/
int main() {
struct sigaction sa;
// Print pid, so that we can send signals from other shells
printf("My pid is: %d\n", getpid());
// Setup the sighub handler
sa.sa_handler = &handle_signal;
// Restart the system call, if at all possible
sa.sa_flags = SA_RESTART;
// Block every signal during the handler
sigfillset(&sa.sa_mask);
// Intercept SIGHUP and SIGINT
if (sigaction(SIGHUP, &sa, NULL) == -1) {
perror("Error: cannot handle SIGHUP"); // Should not happen
}
if (sigaction(SIGUSR1, &sa, NULL) == -1) {
perror("Error: cannot handle SIGUSR1"); // Should not happen
}
// Will always fail, SIGKILL is intended to force kill your process
if (sigaction(SIGKILL, &sa, NULL) == -1) {
perror("Cannot handle SIGKILL"); // Will always happen
printf("You can never handle SIGKILL anyway...\n");
}
if (sigaction(SIGINT, &sa, NULL) == -1) {
perror("Error: cannot handle SIGINT"); // Should not happen
}
for (;;) {
printf("\nSleeping for ~3 seconds\n");
do_sleep(3); // Later to be replaced with a SIGALRM
}
}
void handle_signal(int signal) {
const char *signal_name;
sigset_t pending;
// Find out which signal we're handling
switch (signal) {
case SIGHUP:
signal_name = "SIGHUP";
break;
case SIGUSR1:
signal_name = "SIGUSR1";
break;
case SIGINT:
printf("Caught SIGINT, exiting now\n");
exit(0);
default:
fprintf(stderr, "Caught wrong signal: %d\n", signal);
return;
}
/*
* Please note that printf et al. are NOT safe to use in signal handlers.
* Look for async safe functions.
*/
printf("Caught %s, sleeping for ~3 seconds\n"
"Try sending another SIGHUP / SIGINT / SIGALRM "
"(or more) meanwhile\n", signal_name);
/*
* Indeed, all signals are blocked during this handler
* But, at least on OSX, if you send 2 other SIGHUP,
* only one will be delivered: signals are not queued
* However, if you send HUP, INT, HUP,
* you'll see that both INT and HUP are queued
* Even more, on my system, HUP has priority over INT
*/
do_sleep(3);
printf("Done sleeping for %s\n", signal_name);
// So what did you send me while I was asleep?
sigpending(&pending);
if (sigismember(&pending, SIGHUP)) {
printf("A SIGHUP is waiting\n");
}
if (sigismember(&pending, SIGUSR1)) {
printf("A SIGUSR1 is waiting\n");
}
printf("Done handling %s\n\n", signal_name);
}
void handle_sigalrm(int signal) {
if (signal != SIGALRM) {
fprintf(stderr, "Caught wrong signal: %d\n", signal);
}
printf("Got sigalrm, do_sleep() will end\n");
}
void do_sleep(int seconds) {
struct sigaction sa;
sigset_t mask;
sa.sa_handler = &handle_sigalrm; // Intercept and ignore SIGALRM
sa.sa_flags = SA_RESETHAND; // Remove the handler after first signal
sigfillset(&sa.sa_mask);
sigaction(SIGALRM, &sa, NULL);
// Get the current signal mask
sigprocmask(0, NULL, &mask);
// Unblock SIGALRM
sigdelset(&mask, SIGALRM);
// Wait with this mask
alarm(seconds);
sigsuspend(&mask);
printf("sigsuspend() returned\n");
}

Good example man, many thanks.

idning commented Jun 17, 2014

you should not use printf in signal handler. see: http://man7.org/linux/man-pages/man7/signal.7.html

I'm reading man pages and getting buried in technical details. Not fun.

It really helps to read some neatly written code like this. I learned a lot. Thanks for this!

Owner

aspyct commented Jun 8, 2015

@idning Thanks for the remark! I added a comment to reflect that :)

Sure did help!

JoschK commented Dec 5, 2016

thanks a lot bro very helpful! replace printf with this line:
const char buf[ ] = "your message\n";
write(STDOUT_FILENO, buf, strlen(buf));

sachind commented Mar 10, 2017

Thanks.

golh30 commented Apr 11, 2017

Thanks a lot.

Instead of calling printf() from within your signal handler (bad idea) you could implement a FIFO of messages to be written and check that (calling printf() on non-empty) from your main event loop. (Basically any case where you're going to handle signals in a non-terminating fashion entails implementing one or more event loops to handle the interruption by signals and resume whatever was being done by main()).

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