使用pthread_join的pthread的同步问题

Question

I have the following loops ,

for (i = 1; i < nDiag; ++i)
{
    elem = findelemnum();
    taskarr[3]= elem;
    if(threadnum > elem) limit = elem;
    else limit=threadnum;

    if (i <= lenA) 
    {
        si = i;
        sj = 1;
    } 
    else 
    {
        si = lenA;
        sj = i - lenA + 1;
    }
    taskarr[0] = si, taskarr[1] = sj;  

    for (j = 0; j < limit; ++j) 
    {
        taskarr[2] = j;
        wakeup = 0;
        pthread_create(&threads[j], NULL, mytask, taskarr);
        while(!wakeup){
        }
    }
    for (int j=0; j < limit ;j++){
        pthread_join(threads[j],NULL);  
    } 
}

I want to synchronize my threads so that all threads have finished the inner loop first and then a new loop (i++) will start. For that reason I used the pthread_join function. Though a new for i loop starts before its previous ends. What I'm doing wrong?

Answer 1

If you want to wait for a specific condition to happen use the class of functions pthread_cond_* .

If you just want to wait for the thread to do their stuff remove while(!wakeup){} . This works because

int pthread_join(pthread_t thread, void **value_ptr);

The pthread_join() function suspends execution of the calling thread until the target thread terminates, unless the target thread has already terminated.

So in the following snippet you first create limit threads and they start running right away. Then you wait for every thread to end, that is you suspend the calling thread until the termination of limit threads.

for (j = 0; j < limit; ++j) 
{
    taskarr[2] = j;
    wakeup = 0;
    pthread_create(&threads[j], NULL, mytask, taskarr);
}

for (int j=0; j < limit ;j++){
    pthread_join(threads[j],NULL);  
} 

This is a little demo

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

#define NTHREAD 4
#define ITERATIONS 5

typedef struct _pcontext
{
    int id;
    int iteration;
}pcontext_t;

void* work(void* arg)
{
    pcontext_t* ctx = (pcontext_t*)arg;

    sleep(2); 

    printf("Thread ID: %d\n", ctx->id);
    printf("Iteration nr: %d\n", ctx->iteration);
    printf("Done.\n\n");

    return NULL;
}

int main()
{
    pthread_t thread[NTHREAD];
    pcontext_t ctx[NTHREAD];
    int err;
    int i,j;

    for(j = 0; j < ITERATIONS; j++)
    {
        for(i = 0; i < NTHREAD; i++)
        {
            ctx[i].id = i;
            ctx[i].iteration = j;
            err = pthread_create(&thread[i], NULL, work, (void*)(&(ctx[i])));
            if (err)
            {
                printf("An error occured: %d", err);
                return err;
            }
        }

        printf("Waiting for the threads to end...\n");

        for(i = 0; i < NTHREAD; i++)
        {
          pthread_join(thread[i], (void**)NULL);    
        }

        printf("Threads ended.\n");
        printf("Iteration %d ended.\n\n", j);
    } 

    return 0;
}

Output:

gcc -Wall -Wextra -pedantic -pthread -o main main.c 
Waiting for the threads to end...
Thread ID: 1
Iteration nr: 0
Done.

Thread ID: 0
Iteration nr: 0
Done.

Thread ID: 2
Iteration nr: 0
Done.

Thread ID: 3
Iteration nr: 0
Done.

Threads ended.
Iteration 0 ended.

Waiting for the threads to end...
Thread ID: 0
Iteration nr: 1
Done.

Thread ID: 3
Iteration nr: 1
Done.

Thread ID: 2
Iteration nr: 1
Done.

Thread ID: 1
Iteration nr: 1
Done.

Threads ended.
Iteration 1 ended.