pthreads in C - pthread_exit

Question

For some reason I thought that calling pthread_exit(NULL) at the end of a main function would guarantee that all running threads (at least created in the main function) would finish running before main could exit. However when I run this code below without calling the two pthread_join functions (at the end of main) explicitly I get a segmentation fault, which seems to happen because the main function has been exited before the two threads finish their job, and therefore the char buffer is not available anymore. However when I include these two pthread_join function calls at the end of main it runs as it should. To guarantee that main will not exit before all running threads have finished, is it necessary to call pthread_join explicitly for all threads initialized directly in main?

#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <assert.h>
#include <semaphore.h>
#define NUM_CHAR 1024
#define BUFFER_SIZE 8

typedef struct {
    pthread_mutex_t mutex; 
    sem_t full;
    sem_t empty;
    char* buffer;
} Context;

void *Reader(void* arg) {
    Context* context = (Context*) arg;
    for (int i = 0; i < NUM_CHAR; ++i) {
        sem_wait(&context->full);
        pthread_mutex_lock(&(context->mutex));
        char c = context->buffer[i % BUFFER_SIZE];
        pthread_mutex_unlock(&(context->mutex));
        sem_post(&context->empty);

        printf("%c", c);
    }
    printf("\n");
    return NULL;
}

void *Writer(void* arg) {
    Context* context = (Context*) arg;
    for (int i = 0; i < NUM_CHAR; ++i) {
        sem_wait(&context->empty);
        pthread_mutex_lock(&(context->mutex));
        context->buffer[i % BUFFER_SIZE] = 'a' + (rand() % 26);
        float ranFloat = (float) rand() / RAND_MAX;
        if (ranFloat < 0.5) sleep(0.2);
        pthread_mutex_unlock(&(context->mutex));
        sem_post(&context->full);
    }
    return NULL;
}

int main() {
    char buffer[BUFFER_SIZE];
    pthread_t reader, writer;
    Context context;
    srand(time(NULL));
    int status = 0;
    status = pthread_mutex_init(&context.mutex, NULL);
    status = sem_init(&context.full,0,0);
    status = sem_init(&context.empty,0, BUFFER_SIZE);
    context.buffer = buffer;

    status = pthread_create(&reader, NULL, Reader, &context);
    status = pthread_create(&writer, NULL, Writer, &context);

    pthread_join(reader,NULL);   // This line seems to be necessary
    pthread_join(writer,NULL);   // This line seems to be necessary

    pthread_exit(NULL);
    return 0;
}

If that is the case, how could I handle the case where plenty of identical threads (like in the code below) would be created using the same thread identifier? In that case, how can I make sure that all the threads will have finished before main exits? Do I really have to keep an array of NUM_STUDENTS pthread_t identifiers to be able to do this? I guess I could do this by letting the Student threads signal a semaphore and then let the main function wait on that semaphore, but is there really no easier way to do this?

int main()
{
    pthread_t thread;
    for (int i = 0; i < NUM_STUDENTS; i++)
        pthread_create(&thread,NULL,Student,NULL);  // Threads 
    // Make sure that all student threads have finished
    exit(0);
}

Answer 1

pthread_exit() is a function called by a thread to terminate its own execution. For the situation you've given it is not to be called from your main program thread.

As you have figured out, pthread_join() is the correct means to wait for the completion of a joinable thread from main().

Also as you've figured out, you need to maintain the value returned from pthread_create() to pass to pthread_join().

What this means is that you cannot use the same pthread_t variable for all the threads you create if you intend to use pthread_join().

Rather, build an array of pthread_t so that you have a copy of each thread's ID.