The Linux Programming Interface

Threads: Thread Cancellation 675

printf("New thread started\n"); /* May be a cancellation point */

for (j = 1; ; j++) {

printf("Loop %d\n", j); /* May be a cancellation point */

sleep(1); /* A cancellation point */

}

/* NOTREACHED */

return NULL;

}

int

main(int argc, char *argv[])

{

pthread_t thr;

int s;

void *res;

s = pthread_create(&thr, NULL, threadFunc, NULL);

if (s != 0)

errExitEN(s, "pthread_create");

sleep(3); /* Allow new thread to run a while */

s = pthread_cancel(thr);

if (s != 0)

errExitEN(s, "pthread_cancel");

s = pthread_join(thr, &res);

if (s != 0)

errExitEN(s, "pthread_join");

if (res == PTHREAD_CANCELED)

printf("Thread was canceled\n");

else

printf("Thread was not canceled (should not happen!)\n");

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––– threads/thread_cancel.c

32.4 Testing for Thread Cancellation

In Listing 32-1, the thread created by main() accepted the cancellation request

because it executed a function that was a cancellation point (sleep() is a cancellation

point; printf() may be one). However, suppose a thread executes a loop that con-

tains no cancellation points (e.g., a compute-bound loop). In this case, the thread

would never honor the cancellation request.

The purpose of pthread_testcancel() is simply to be a cancellation point. If a can-

cellation is pending when this function is called, then the calling thread is terminated.