POSIX pthread_join hangs after thousands of threads

Question

I am new to POSIX and I cant find a solution to this particular problem.

Are there any known issues with creating pthreads inside of loop with big number of iterations(>100000)?

It seems like every time I execute, it hangs on a random pthread_join.

• I've tested for memory leaks and thread stack usage with valgrind.
• If I interrupt gdb when the program hangs it will trace the problem to pthread_join.

This is example code recreates my problem.

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

    void* task(void* args);
    int main(int argc, char **argv)
    {
        int num_threads = 4;
        int m = 100000;
        int i;
        for(i = 0; i < m; i++)
        {
            fprintf(stdout, "i:%d\n",i);
            //parallel region starts
                pthread_t threads[num_threads];
                int t,rc;

                for(t = 0; t < num_threads; t++)
                {
                    rc = pthread_create(&threads[t],NULL,task,NULL);
                    if(rc){
                        fprintf(stderr,"ERROR; return code from pthread_create() is %d\n", rc);
                        exit(-1);
                    }
                }
                for(t = 0; t < num_threads; t++)
                {
                    rc = pthread_join(threads[t],NULL);
                    if(rc){
                    fprintf(stderr,"ERROR; return code from pthread_join() is %d\n", rc);
                        exit(-1);
                    }
                }
            //parallel region ends
        }
        pthread_exit(NULL);
    }

    //thread task
    void* task(void* args)
    {
        pthread_exit(NULL);
    }

Answer 1

Verified your code on an Ubuntu machine i7-4600U CPU @ 2.10GHz I don't see any issue with your code, just to verify i compiled and run your code on my local machine it seems to work fine.

i got the output as expected:

i:999,j:999

so I am assuming the issue you you are experiancing has something to do with your environment. what CPU OS and GCC are you using?