How to make main thread wait for all child threads finish?

user_888051 0
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I intend to fire 2 threads in the main thread, and the main thread should wait till all the 2 child threads finish, this is how I do it.

void *routine(void *arg)
{
    sleep(3);
}

int main()
{
    for (int i = 0; i < 2; i++) {
        pthread_t tid;
        pthread_create(&tid, NULL, routine, NULL);
        pthread_join(&tid, NULL);  //This function will block main thread, right?
    }
}

In the above code, pthread_join indeed makes main thread wait for the child threads, but the problem is, the second thread won't be created untill the first one finishes. This is not what I want.

What I want is, the 2 threads get created immediatly in the main thread, and then main thread waits for them to finish. Seems like pthread_join cannot do the trick, can it?

I thought, maybe via a semaphore I can do the job, but any other way?

multithreading c
4 Answers
#include<stdio.h>
#include<pthread.h>

int icnt = 0;   //in non_bss data segment
pthread_mutex_t lock;     //lock variable created stored into bss data segment


void *Thread_count(void* args)      //syncronization 
{
pthread_mutex_lock(&lock);              //lock aquire 
    
icnt++;

for(int x = 1; x <= icnt; x++)
{
    printf("Hello from Thread_count : %d \n",icnt);
}
printf("\n");

pthread_mutex_unlock(&lock);            //release lock  
pthread_exit(NULL);                     //exit from child thread
}


int main()
{

pthread_t threads[4];  //created array of {unsigned long int}
int status = 0;

//creating threads in loop      
for(int i = 1; i <= sizeof(threads)/sizeof(threads[0]); i++)
{
    pthread_create(&threads[i], NULL, &Thread_count, NULL);
}

//waiting for threads in loop
for(int j = 1; j <= sizeof(threads)/sizeof(threads[0]); j++)
{
    pthread_join(threads[j], &status);
    
    printf("Thread number : %d     <-->  Thread status : %d\n",j, status);
}


pthread_exit(0);  //end of main thread
}
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