Cancelling a pthread and printf behaviour - c

Question

Here is a code sample of pthread cancellation:
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
void *my_routine(void *arg) {
int i;
for (i = 0; i < 10; i++) {
printf("%d\n", i);
}
return NULL;
}
int main(void) {
pthread_t thread;
if (pthread_create(&thread, NULL, my_routine, NULL)) {
fprintf(stderr, "Cannot create pthread\n");
return 1;
}
usleep(20);
pthread_cancel(thread);
pthread_join(thread, NULL);
//fflush(stdout);
sleep(1);
return 0;
}
I compile with:
gcc -pthread -Wall threadtest.c -o threadtest
When executing, sometimes, it prints an additional number after sleep(1).
Sometimes this number is a duplicate:
0
1
2
3
4
4 // printed after sleep(1)
Sometimes not:
0
1
2
3
4
5 // printed after sleep(1)
If I fflush(stdout) before sleep(1), then the additional number is printed immediately.
How can I avoid this weird printf behaviour when cancelling a thread?
Thank you.

Answer 1

You could disable cancellation while doing the printf() and add an explicit cancellation point:
int cancel_state = 0;
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cancel_state);
printf("%d\n", i);
pthread_setcancelstate(cancel_state);
pthread_testcancel();
(error checking left out for the sake of readability)

Answer 2

When using pthread_cancel you should take more care on thread handler.
man page of pthread_cancel explain lot about this.
man pthread_cancel
Check the example program available in the man page
When thread handler start you have to make PTHREAD_CANCEL_DISABLE with the help of pthread_setcancelstate system call, when handler meet the cancellation point set PTHREAD_CANCEL_ENABLE.
In the main check the pthread_cancel success or not with the help of PTHREAD_CANCELED macro.
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
void *my_routine(void *arg) {
int i;
//pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
for (i = 0; i < 10; i++) {
sleep(1);
printf("%d\n", i);
}
//pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
sleep(60);
return NULL;
}
int main(void)
{
pthread_t thread;
if (pthread_create(&thread, NULL, my_routine, NULL)) {
fprintf(stderr, "Cannot create pthread\n");
return 1;
}
usleep(20);
pthread_cancel(thread);
pthread_join(thread, NULL);
//fflush(stdout);
sleep(1);
return 0;
}
Initially compile and run the above program, note the output. It may not display anything and terminate the program because after thread created thread handler will wait 1sec and sleep, in that time parent thread will cancel the thread and finish process.
Then uncomment the two pthread_setcancelstate() from above program, now compile and run the program. It will now display from 1 to 9. Because thread is set as PTHREAD_CANCEL_DISABLE so parent pthread_cancel will not cancel and wait for PTHREAD_CANCEL_ENABLE.When thread handler meet PTHREAD_CANCEL_ENABLE parent thread will cancel the thread.
when you note the output. handler will not stay on sleep(60), because before that parent will cancel the thread.