how to terminate a sleeping thread in pthread?

Question

I have thread which sleeps for a long time, then wakes up to do something, then sleep again, like this:

while(some_condition)
{
    // do something
    sleep(1000);
}

How could I make this thread exit gracefully and QUICKLY?

I tried to use pthread_cancel(), but sleeping threads could not be canceled. I also tried changing the condition of the while loop, but it will still take long to exit. And I don't want to use pthread_kill(), since it may kill the thread when it's working.

So, are there any good ideas?

Answer 1

As an alternative to sleep, you could use pthread_cond_timedwait with a 1000 ms timeout. Then when you want to exit, signal the condition variable.

This is similar to how you might do this in C#/Java using wait and notify.

Answer 2

The classic UNIX condition variable is the self-pipe.

int fds[2];
pipe2(fds, O_NONBLOCK);  // requires newish kernel and glibc; use pipe + 2*fcntl otherwise

child:
    while (some_condition) {
        // do something
        struct pollfd pd = { .fd = fds[0], .events = POLLIN };
        int rc;
        char c;
        while ((rc = poll(&pd, 1, 1000000)) == -1 && errno == EINTR)
            // not entirely correct; 1000000 should be decreased according to elapsed time when repeating after a signal interruption
            ;
        if (rc > 0 && (pd.revents & POLLIN) && read(fds[0], &c, 1) >= 0)
            break;
    }

parent:
    cancel() {
        char c = 0;
        write(fds[1], &c, 1);
    }

Yeah, it's a lot of fiddly (and untested) code. You should probably just use pthread_cond_wait, it requires a pthread_mutex_t and a pthread_cond_t but is much easier.

Answer 3

Did you use pthread_cleanup_push and pop? Canceling with pthread_cancel doesn't work without them. You must use them in pairs just like I did in the example below. if you forget one it wont compile (fancy macros, one has the '{' and the other has the '}'). You can even nest different levels of cleanup/pops. Anyway, they set a long jump point that cancel jumps to when cancel occurs (pretty cool). Also, if your test program does not wait for the thread to start or to stop, you may not notice the canceling happening.

Example:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

static void *ThreadProc(void * arg);
static void unwind(__attribute__ ((unused)) void *arg);

int _fActive = 0;

int main(int argc, char** argv)
{
pthread_t    Thread;
int      nRet;

    nRet = pthread_create(&Thread, NULL, ThreadProc, NULL);
    printf("MAIN: waiting for thread to startup...\n");
    while (_fActive == 0)
        nanosleep(&(struct timespec){ 0, 0}, NULL);
    printf("MAIN: sending cancel...\n");
    nRet = pthread_cancel(Thread);

    printf("MAIN: waiting for thread to exit...\n");
    while (_fActive)
        nanosleep(&(struct timespec){ 0, 0}, NULL);

    printf("MAIN: done\n");
    return 0;
}

static void unwind(__attribute__ ((unused)) void *arg)
{
    // do some cleanup if u want
    printf("THREAD: unwind (all threads, canceled or normal exit get here)\n");
    _fActive = 0;
}

static void *ThreadProc(void * arg)
{
    pthread_cleanup_push(unwind, arg);
    // optional : pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
    printf("THREAD: Enter Sleep\n");
    _fActive = 1;
    sleep(1000000);
    printf("THREAD: Exit Sleep (canceled thread never gets here)\n");
    pthread_cleanup_pop(1);

    printf("THREAD: Exit (canceled thread never gets here)\n");
    return NULL;
}

Program output:

MAIN: waiting for thread to startup...
THREAD: Enter Sleep
MAIN: sending cancel...
MAIN: waiting for thread to exit...
THREAD: unwind (all threads, canceled or normal exit get here)
MAIN: done

Notice how the cancel blows out of ThreadProc at the cancel point sleep() and executes only the unwind() function.