How to cancel a thread from another thread (Glib threads)? - multithreading

Question

I am developing an application library using GTK and the functions for threads in GLib. I have a thread (from now on will be called thread A) that is created when I hit an "ok" button in a certain graphical window. Thread A starts doing some heavy tasks. Another button named "cancel" is available to stop and finish thread A at any moment.
My aim is to code a function for the thread created when I hit the "cancel" button (thread B) that has the ability to end the thread A.
I create thread A with the function g_thread_create. However I cannot find any function similar to g_thread_cancel to stop thread A using thread B. Is this possible or cannot be done?
Thank you so much for any kind of information provided.

Answer 1

You might want to consider using GTask to run your task in a thread, rather than using a manually-created thread. If you use g_task_run_in_thread(), the operation will run in a separate thread automatically.
GTask is integrated with GCancellable, so to cancel the operation you would call g_cancellable_cancel() in the callback from your ‘Cancel’ button.
As OznOg says, you should treat the GCancellable as a way of gently (and thread-safely) telling your task that it should cancel. Depending on how your long-running task is written, you could either check g_cancellable_is_cancelled() once per loop iteration, or you could add the GSource from g_cancellable_source_new() to a poll loop in your task.
The advice about using threads with GLib is probably also relevant here.

Answer 2

I have developed a code that is able to cancel a thread from another, both of them created from a main one. The code works correctly according to my tests:
#include <pthread.h>
#include <stdio.h>
/* these variables are references to the first and second threads */
pthread_t inc_x_thread, inc_y_thread;
/* this function is run by the first thread */
void *inc_x(void *x_void_ptr)
{
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
/* increment x to 100 */
int *x_ptr = (int *)x_void_ptr;
while(++(*x_ptr) < 100000000);
printf("x increment finished\n");
/* the function must return something - NULL will do */
return NULL;
}
/* this function is run by the second thread */
void *inc_y(void *x_void_ptr)
{
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
/* increment y to 100 */
int *x_ptr = (int *)x_void_ptr;
pthread_cancel(inc_x_thread);
while(++(*x_ptr) < 100);
printf("y increment finished\n");
return NULL;
}
/* this is the main thread */
int main()
{
int x = 0, y = 0;
void *res;
/* show the initial values of x and y */
printf("x: %d, y: %d\n", x, y);
/* create a first thread */
if(pthread_create(&inc_x_thread, NULL, inc_x, &x)) {
fprintf(stderr, "Error creating thread\n");
return 1;
}
/* create a second thread */
if(pthread_create(&inc_y_thread, NULL, inc_y, &y)) {
fprintf(stderr, "Error creating thread\n");
return 1;
}
/* wait for the first thread to finish */
if(pthread_join(inc_x_thread, &res)) {
fprintf(stderr, "Error joining thread\n");
return 2;
}
if (res == PTHREAD_CANCELED)
printf(" thread was canceled\n");
else
printf(" thread wasn't canceled\n");
/* wait for the second thread to finish */
if(pthread_join(inc_y_thread, &res)) {
fprintf(stderr, "Error joining thread\n");
return 2;
}
if (res == PTHREAD_CANCELED)
printf(" thread was canceled\n");
else
printf(" thread wasn't canceled\n");
/* show the results*/
printf("x: %d, y: %d\n", x, y);
return 0;
}
You can compile the code by using: gcc example.c -lpthread
However, as OznOg and Philip Withnall have said, this is not the correct way of cancelling a thread. It is only a quick way of doing it that might not work in some specific situations. A better and safer way is to gently ask the thread to stop itself.