make main program wait for threads to finish

Question

In the following code I create some number of threads, and each threads sleeps for some seconds.

However my main program doesn't wait for the threads to finish, I was under the assumption that threads would continue to run until they finished by themselves.

Is there someway of making threads continue to run even though the calling thread finishes.

#include <pthread.h>
#include <iostream>
#include <cstdio>
#include <cstdlib>


int sample(int min,int max){
  int r=rand();
  return (r %max+min );
}



void *worker(void *p){
  long i = (long) p;
  int s = sample(1,10);
  fprintf(stdout,"\tid:%ld  will sleep: %d \n",i,s);
  sleep(s);
  fprintf(stdout,"\tid:%ld  done sleeping \n",i,s);
}

pthread_t thread1;

int main(){
  int nThreads = sample(1,10);

  for(int i=0;i<nThreads;i++){
    fprintf(stderr,"\t-> Creating: %d of %d\n",i,nThreads);
    int iret1 = pthread_create( &thread1, NULL, worker, (void*) i);
    pthread_detach(thread1);
  }
  //  sleep(10);//work if this is not commented out.
  return 0;
}

Thanks

Edit:

Sorry for not clarifying, is it possible without explicitly keeping track of my current running threads and by using join.

Answer 1

Each program has a main thread. It is the thread in which your main() function executes. When the execution of that thread finishes, the program finishes along with all its threads. If you want your main thread to wait for other threads, use must use pthread_join function

Answer 2

You need to keep track of the threads. You are not doing that because you are using the same thread1 variable to every thread you are creating.

You track threads by creating a list (or array) of pthread_t types that you pass to the pthread_create() function. Then you pthread_join() those threads in the list.

edit:

Well, it's really lazy of you to not keep track of running threads. But, you can accomplish what you want by having a global var (protected by a mutex) that gets incremented just before a thread finishes. Then in you main thread you can check if that var gets to the value you want. Say nThreads in your sample code.

Answer 3

You need to join each thread you create:

int main()
{
    int                    nThreads = sample(1,10);
    std::vector<pthread_t> threads(nThreads);

    for(i=0; i<nThreads; i++)
    {
            pthread_create( &threads[i], NULL, worker, (void*) i)
    }

    /* Wait on the other threads */
    for(i=0; i<nThreads; i++)
    {
            status*   status;
            pthread_join(threads[i], &status);
    }
}