Sending signal in C from child to Parents. "User defined signal 1: 30"

Question

I'm beginner to signal and practicing by writing a function kill_parent_process that will be used as a signal handler.

This func will asks parents to exit() (either sending SIGKILL or other methods). The way I did is let child sending a signal to parents and the handler exit(). But I do feel this is where the problem because when I exit, I might just exit in child process. However, it looks like the handler never been called. Here is my code

void kill_parent_process(int code) {
    fprintf(stderr, "KIll the Parent\n");
    exit(1);
}

int main() {
    struct sigaction sa;
    sa.sa_handler = kill_parent_process;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);

    int r = fork();

    if (r == 0) {


        sigaction(SIGUSR1, &sa, NULL);

        kill(getppid(), SIGUSR1);
        exit(1);
    } else if (r > 0) {

        while (1) {
            sleep(1);
            printf("%d\n",getpid());
        }
        int status;
        wait(&status);
        if (WIFEXITED(status)) {
            int result = WEXITSTATUS(status);
            fprintf(stderr, "%d\n", result);
        }

      } 
    }

I receive a message User defined signal 1: 30 when I ran the program and kill_parent_process never been called. What is the problem with this code?

@WilliamPursell sorry, missing some crucial part of code. I have updated the post — ElleryL, Dec 18 '17 at 03:51
You're supposed to set the signal handler in the **parent**, you're now setting it only in the child process. — Antti Haapala -- Слава Україні, Dec 18 '17 at 04:28
@AnttiHaapala Thanks for the point out. I see where I misunderstood the concept of signal handler — ElleryL, Dec 18 '17 at 04:41
A process's signal handler is called to handle the specified signal when it is received. — ikegami, Dec 18 '17 at 05:29
Note [Avoid using `printf()` in a signal handler?](https://stackoverflow.com/questions/16891019/). — Jonathan Leffler, Dec 18 '17 at 05:30
You don't set any signal handler in the parent process, so when the child sends the signal, the parent undergoes the default signal handling action. For SIGUSR1, that terminates the process. The child doesn't receive a signal, so it doesn't need signal handling. — Jonathan Leffler, Dec 18 '17 at 05:32
Note the potential race condition. When you change the code to set the signal handler in the parent, you need to ensure that you are setting the signal handler before the signal is sent. — William Pursell, Dec 18 '17 at 13:47

score 0 · Answer 1 · answered Dec 18 '17 at 15:40

the following proposed code:

performs the desired functionality
cleanly compiles
documents why most of the header files are being included
does not invoke any signal handlers, etc, rather uses the default action
properly checks for and handles errors

and now the proposed code:

#include <stdio.h>   // perror(), puts()
#include <stdlib.h>  // exit(), EXIT_FAILURE, EXIT_SUCCESS
#include <sys/types.h>  // for 'kill()' and 'getppid()'
#include <signal.h>  // kill()
#include <unistd.h>  // getppid(), fork()

int main( void )
{
    pid_t pid = fork();

    if( pid < 0 )
    {
        perror( "fork failed" );
        exit( EXIT_FAILURE );
    }

    else if( pid == 0 )
    {  // then child
        puts( "in child process" );
        kill( getppid(), SIGUSR1 );
        exit( EXIT_SUCCESS );
    }

    else // if ( pid > 0 )
    { // then parent
        while(1)
        {
            puts( "in parent process" );
            sleep(1);
        }
    }
}

the (typical) output of the proposed code is:

in parent process
in child process
User defined signal 1

Sending signal in C from child to Parents. "User defined signal 1: 30"

1 Answers1