pthread_cancel() in C

The pthread_cancel() function is used to send a cancellation request to a specific thread identified by its thread ID. This function allows one thread to request the termination of another thread in a controlled manner.

Note: To compile and run pthread programs, use: gcc -pthread filename.c -o output

Syntax

int pthread_cancel(pthread_t thread);

Parameters

• thread − The thread ID of the thread to be cancelled

Return Value

• Returns 0 on success
• Returns an error number on failure

Example: Thread Cancellation

This example demonstrates how one thread can cancel another thread after a certain condition is met −

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

int count = 0;
pthread_t sample_thread;

void* thread_one_func(void* p) {
    while (1) {
        printf("This is thread 1<br>");
        sleep(1); /* wait for 1 second */
        count++;
        if (count == 5) {
            /* if counter reaches 5, cancel thread 2 and exit */
            printf("Thread 1: Cancelling thread 2<br>");
            pthread_cancel(sample_thread);
            pthread_exit(NULL);
        }
    }
    return NULL;
}

void* thread_two_func(void* p) {
    sample_thread = pthread_self(); /* store thread 2's ID */
    while (1) {
        printf("This is thread 2<br>");
        sleep(2); /* wait for 2 seconds */
    }
    return NULL;
}

int main() {
    pthread_t t1, t2;
    
    /* create two threads */
    pthread_create(&t1, NULL, thread_one_func, NULL);
    pthread_create(&t2, NULL, thread_two_func, NULL);
    
    /* wait for threads to complete */
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    
    printf("Main thread exiting<br>");
    return 0;
}

This is thread 2
This is thread 1
This is thread 1
This is thread 2
This is thread 1
This is thread 2
This is thread 1
This is thread 1
Thread 1: Cancelling thread 2
Main thread exiting

Key Points

• pthread_cancel() only sends a cancellation request − the target thread must be in a cancellable state
• Thread cancellation points include functions like sleep(), read(), write(), etc.
• Use pthread_setcancelstate() and pthread_setcanceltype() to control cancellation behavior
• Always handle thread cancellation gracefully to avoid resource leaks

Conclusion

The pthread_cancel() function provides a mechanism for cooperative thread termination. It's important to understand that cancellation is a request, not a forced termination, and proper cleanup should always be considered.