pthread_equal() in C

The pthread_equal() function is used to check whether two threads are equal or not. This function returns 0 if the threads are different, or a non-zero value if the threads are equal.

Syntax

int pthread_equal(pthread_t th1, pthread_t th2);

Parameters

• th1 − First thread identifier
• th2 − Second thread identifier

Return Value

Returns a non-zero value if threads are equal, otherwise returns 0.

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

Example 1: Comparing Thread with Itself

In this example, we will check if a thread is equal to itself using pthread_self() −

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <pthread.h>

pthread_t sample_thread;

void* my_thread_function(void* p) {
    if (pthread_equal(sample_thread, pthread_self())) { /* pthread_self returns current thread id */
        printf("Threads are equal<br>");
    } else {
        printf("Threads are not equal<br>");
    }
    return NULL;
}

int main() {
    pthread_t th1;
    sample_thread = th1; /* assign the thread th1 to another thread object */
    pthread_create(&th1, NULL, my_thread_function, NULL); /* create a thread using my thread function */
    pthread_join(th1, NULL); /* wait for joining the thread with the main thread */
    return 0;
}

Threads are equal

Example 2: Comparing Two Different Threads

Now we will see the result when we compare between two different threads −

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <pthread.h>

pthread_t sample_thread;

void* my_thread_function1(void* ptr) {
    sample_thread = pthread_self(); /* assign the id of the current thread */
    return NULL;
}

void* my_thread_function2(void* p) {
    if (pthread_equal(sample_thread, pthread_self())) { /* pthread_self returns current thread id */
        printf("Threads are equal<br>");
    } else {
        printf("Threads are not equal<br>");
    }
    return NULL;
}

int main() {
    pthread_t th1, th2;
    pthread_create(&th1, NULL, my_thread_function1, NULL); /* create a thread using my_thread_function1 */
    pthread_join(th1, NULL); /* wait for first thread to complete */
    pthread_create(&th2, NULL, my_thread_function2, NULL); /* create a thread using my_thread_function2 */
    pthread_join(th2, NULL); /* wait for second thread to complete */
    return 0;
}

Threads are not equal

Key Points

• Thread equality is determined by comparing thread identifiers, not thread functions.
• Use pthread_self() to get the current thread's identifier.
• Always include return statements in thread functions to avoid undefined behavior.

Conclusion

The pthread_equal() function provides a portable way to compare thread identifiers. It is essential for thread management and synchronization in multi-threaded applications.