CODEWITHSUNDEEP
cterminalsignalsctrl
FlipFlopSquid
FlipFlopSquid

Reputation: 99

Better understanding of signal catching (ie. SIGINT/SIGQUIT) in C

I wrote this program to catch the Ctrl-C and -\ or sigint and sigquit functions, I commented in my understanding of what this program does. Could you correct me if I am wrong and/or maybe explain what is going on so I can have a better understanding?

//
//  main.c
//  Project 4
//
//  Found help with understanding and coding at
// http://www.thegeekstuff.com/2012/03/catch-signals-sample-c-code/
//

#include<stdio.h>
#include<signal.h>
#include<unistd.h>
//signal handling function that will except ctrl-\ and ctrl-c
void sig_handler(int signo)
{
    //looks for ctrl-c which has a value of 2
    if (signo == SIGINT)
        printf("\nreceived SIGINT\n");
    //looks for ctrl-\ which has a value of 9
    else if (signo == SIGQUIT)
        printf("\nreceived SIGQUIT\n");
}

int main(void)
{
    //these if statement catch errors
    if (signal(SIGINT, sig_handler) == SIG_ERR)
        printf("\ncan't catch SIGINT\n");
    if (signal(SIGQUIT, sig_handler) == SIG_ERR)
        printf("\ncan't catch SIGQUIT\n");
    //Runs the program infinitely so we can continue to input signals
    while(1)
        sleep(1);
    return 0;
}

Upvotes: 2

Views: 735

Answers (1)

Niklas Rosencrantz
Niklas Rosencrantz

Reputation: 26647

There is an online tutorial.

SIGINT is straightfoward to use, but you should use sigaction rather than signal.

To catch SIGCHLD you use the code from the tutorial and you can reap the child status with one of the wait() functions.

void handle_sigchld(int sig) {
    int saved_errno = errno;
    while (waitpid((pid_t)(-1), 0, WNOHANG) > 0) {}
    errno = saved_errno;
}

struct sigaction sa;
sa.sa_handler = &handle_sigchld;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART | SA_NOCLDSTOP;
if (sigaction(SIGCHLD, &sa, 0) == -1) {
    perror(0);
    exit(1);
}

Upvotes: 1

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