pthreads and signal handling C ending early

Question

This program is supposed to

The parent simply waits indefinitely for any child to return (hint, waitpid). b. The child sets up two signal handlers (hint, signal) and goes to sleep for 5 minutes. i. The first signal handler listens for the USR1 signal, and upon receiving it: 1. Creates a thread (hint, pthread_create). a. Basically, all that the thread needs to do is “say hello” and sleep for 60 seconds. ii. The second signal handler listens for the USR2 signal, and upon receiving it: 1. Destroys the thread (hint, pthread_cancel).

When this program receives the first signal to create the thread, it outputs "[thread] sleeping for 1 m[thread] sleeping for 1 minute" and then ends, it never waits for the 2nd signal, what am i doing wrong?

#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <pthread.h>
#include <signal.h>

pthread_t thread;

void* temp()
{
    printf("[thread] hello professor\n");
    printf("[thread] sleeping for 1 minute\n");
    sleep(60);
}
void handle_USR1(int x)
{
    int s;
    printf("[signal] creating the thread\n");
    s = pthread_create(&thread, NULL, &temp, NULL);
}

void handle_USR2(int x)
{
    int s;
    printf("[signal] destroying the thread\n");
    s = pthread_cancel(thread);
}

int main(void)
{
    int status = 0;

    if(fork() != 0)
    {
     printf("[parent] waiting.....\n");
     waitpid(-1, &status, 0);
    }
    else
    {
     printf("[child] to create the thread: kill -USR1 %d\n", getpid());
     printf("[child] to end the thread: kill -USR2 %d\n", getpid());
     printf("[child] setting up signal handlers\n");

     signal(SIGUSR1, handle_USR1);
     signal(SIGUSR2, handle_USR2);

     printf("[child] waiting for signals\n");
     sleep(300);
    }
    return (0);
}

The OP's issue is that sending a kill SIGUSR1 to the child process calls the handler and then kills the child so the waitpid() in the parent returns. The question is how the OP can prevent the waitpid() from returning after a SIGUSR1 is sent to the child. — Charlie Burns
@charlie Thank you, I wasnt sure exactly what was going on, still no idea how to fix it lol — Cyberhawk94
pthread_create isn't a safe function to call from a signal handler so this looks like it is inviting trouble from the start. — Duck
@duck Thats how our professor said he did it, and his worked fine :/ — Cyberhawk94
possible duplicate of Simple pthreads and signal program on linux wont run — Basile Starynkevitch

simark simark · Accepted Answer · 2013-10-17T03:39:31

As Charlie Burns pointed out, both processes eventually exit as a consequence of the signal, but for different reasons.

Child

During its sleep, the child is blocked in a system call (the actual system call is nanosleep, used to implement the sleep() function). When a process receives a signal while in a system call, the corresponding signal handler is executed and the system call returns an error, EINTR, which means it has been interrupted and couldn't fulfill its duty. You can then decide if you want to restart the system call or not. Upon receiving SIGUSR1, the nanosleep system call executed by the child is interrupted, the handler is executed and sleep() returns immediately. Notice what man 3 sleep says about the return value of sleep():

Zero if the requested time has elapsed, or the number of seconds left to sleep, if the call was interrupted by a signal handler.

The correct way would be for the child to check for the return value of sleep (number of seconds left to sleep), and sleep again for that duration.

Parent

Unlike what Charlie Burns pointed out, waitpid() in the parent does not return because of the child receiving a signal. It returns because of the child exiting. It would return because of the child IF the child did not handle the signal, and thus was killed by it (an unhandled signal causes the process to die). You can (and should) check that using the WIFEXITED macro and its companions as described in man 2 waitpid. The example at the bottom of this man page is very good:

do {
   w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
   if (w == -1) {
       perror("waitpid");
       exit(EXIT_FAILURE);
   }

   if (WIFEXITED(status)) {
       printf("exited, status=%d\n", WEXITSTATUS(status));
   } else if (WIFSIGNALED(status)) {
       printf("killed by signal %d\n", WTERMSIG(status));
   } else if (WIFSTOPPED(status)) {
       printf("stopped by signal %d\n", WSTOPSIG(status));
   } else if (WIFCONTINUED(status)) {
       printf("continued\n");
   }
} while (!WIFEXITED(status) && !WIFSIGNALED(status));

Basically, what this code does is wait on the child until it has exited normally or has exited because of an unhandled signal. In your case, it would be a good idea for the parent to check the status variable to make sure that waitpid returned because of the event it expects (a child exiting) and not something else.

pthreads and signal handling C ending early

3 Answers

Child

Parent