When to use pthread_exit() and when to use pthread_join() in Linux?

In Linux, pthread_exit() and pthread_join() are two functions in the Pthreads (POSIX threads) library used for managing thread termination and synchronization. Below, I will explain their usage scenarios and provide relevant examples.

pthread_exit()

The pthread_exit() function is used to explicitly terminate a thread. After a thread completes its execution task, you can call this function to exit, optionally providing a return value. This return value can be received and processed by other threads via the pthread_join() function.

Usage Scenarios:

1. Active thread termination: If you need to terminate a thread at a specific point during its execution rather than letting it run to completion, use pthread_exit().
2. Returning from the thread function: Using pthread_exit() within the thread's execution function provides a clear exit point.

Example:

c
#include <pthread.h>
#include <stdio.h>

void* thread_func(void* arg) {
    printf("Hello from the thread\n");
    pthread_exit(NULL);  // Explicitly exit the thread
}

int main() {
    pthread_t thread_id;
    pthread_create(&thread_id, NULL, thread_func, NULL);
    pthread_join(thread_id, NULL);  // Wait for the thread to finish
    printf("Thread has finished execution\n");
    return 0;
}

pthread_join()

The pthread_join() function is used to wait for a specified thread to terminate. After creating a thread, you can use pthread_join() to ensure the main thread (or another thread) waits for the thread to complete its task before proceeding.

Usage Scenarios:

1. Thread synchronization: If your program requires ensuring that a thread completes its task before the main thread (or another thread) continues execution, use pthread_join().
2. Retrieving the thread's return value: If the target thread terminates via pthread_exit() and provides a return value, you can retrieve this value using pthread_join().

Example:

c
#include <pthread.h>
#include <stdio.h>

void* thread_func(void* arg) {
    printf("Thread is running\n");
    pthread_exit("Finished");  // Thread terminates with return value
}

int main() {
    pthread_t thread_id;
    void* retval;
    pthread_create(&thread_id, NULL, thread_func, NULL);
    pthread_join(thread_id, &retval);  // Wait for thread to finish and retrieve return value
    printf("Thread returned: %s\n", (char*)retval);
    return 0;
}

In summary, pthread_exit() is primarily used within a thread to mark its own termination, while pthread_join() is used by other threads to synchronize the execution order of multiple threads or retrieve the thread's return value. These functions are invaluable when precisely controlling thread lifecycles and synchronizing multithreaded operations.