C Why is the # of threads created in my code inconsistent?

Question

The goal of my assignment is to create a loop to spawn 5 threads with integer arguments 0 through 4. I have 3 files: thread_demo.c that contains the main function, worker.c that contains the function to compute square of the argument, and header.h to keep the 2 files together.

thread_demo.c

#include "header.h"
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <readline/readline.h>

#define NUM_THREADS 5
#define PROMPT_SIZE 5

int main(){
        pthread_t threads[NUM_THREADS];
        pthread_attr_t pthread_attributes;

        int *prompt;
        scanf("%d", &prompt);

        for(int i = 0; i < NUM_THREADS; i++){
                pthread_create(&threads[i], &pthread_attributes, &worker, (void *) prompt);
        } 
} 

worker.c

#include "header.h"
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

void *worker(void * num){
        int *input;
        input = (int*) &num;

        // Calculate square
        int output = *input * *input;

        printf("The square of %d", *input);
        printf(" is %d.\n", output);

        pthread_exit(NULL);
}

I have no compile errors, but the number of threads my code spawns is inconsistent. The pictures shown are my outputs from entering "5" 3 different times in which I have not made changes to the code. I'm not sure what causes this, I know I'm missing something but I don't know what it is. I also have to make "the main thread wait until all threads have completed", which is confusing to me. When I spawn the 5 threads, how do I know which is the main thread? I've never wrote code related to processes and threads so I'm completely lost. Any help would be greatly appreciated!

Answer 1

Your program is probably exiting before all your threads are complete. This would explain why you see different outputs on different runs: sometimes your program exits more or less quickly allowing less or more of the work to complete.

You need to keep your program from exiting (returning from main() ) until all your thread are finished. To do this, you can use pthread_join for each of the threads. Add after you create all the threads:

for(int i = 0; i < NUM_THREADS; i++) {
    pthread_join(threads[i]);
}

ptread_join will block (stop execution at that line) until the thread has terminated.

From the docs:

The pthread_join() function waits for the thread specified by thread to terminate. If that thread has already terminated, then pthread_join() returns immediately. The thread specified by thread must be joinable.

http://man7.org/linux/man-pages/man3/pthread_join.3.html