Simple pthreads and signal program on linux wont run

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_destroy).

My code compiles fine, just when I run it, absolutely nothing happens, not even the first printf which I put there as a test. Ive been staring at it for an hour and there are no errors, so why wont this run?

EDIT: This runs now, thanks charlie, however when it creates the thread, it outputs "[thread] sleeping for 1 m[thread] sleeping for 1 minute" and then ends, it never waits for the 2nd signal

#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);
}

Add a newline to the initial printf to make it flush. Or call fflush. In fact, adding a newline to all your printfs will probably convince you it actually works. Also, checking fork() for -1 is a good idea. — Charlie Burns
@charlie what would adding a newline do? It never even prints the first "hello" — Cyberhawk94
stdio is buffered, which means that characters sent to stdout don't get flushed until a newline is seen. Try it, just change printf("hello"); to printf("hello\n"); and you will see. I ran your code, it works. — Charlie Burns
holy crap, thank you so much. got to love the little weird things — Cyberhawk94
new weird problem tho, it exits out of the process after the thread is created, never waits for the USR2 signal — Cyberhawk94

Charlie Burns Charlie Burns · Accepted Answer · 2013-10-17T02:02:00

Add a newline "\n" to all your printf's. Without it, stdout will not flush and it will appear your program is not working even though it is.

Also, checking fork() for failure is a good idea. fork() returns -1 on failure and sets errno.

Simple pthreads and signal program on linux wont run

2 Answers