multithreading application in c

Question

I'm trying to implement a multithreaded application but got some doubts. Can anybody please clarify one this?

My main goal is to

Create five threads to execute the work concurrently.

The program for this is:

for(i = 0; i < 5; i++)
{
    pthread_create(tid[i], NULL, func, NULL)
}

Then generally we call for the pthread_join():

for(j = 0; j < 5; j++) 
{
    pthread_join(tid[j], NULL);
}

So here my question is that in this call of pthread_join(), if thread 2 finishes first then thread 2 will wait for thread 1 to finish as we have put in a sequential loop for the pthread_join() function.

What do you mean by if thread 2 finishes first then thread 2 will wait for thread 1 to finish as we have put in a sequential loop for the pthread_join function? — tangrs
– tangrs, Commented Aug 5, 2014 at 12:24
My question is all the thread will do their work independently if am not wrong on func. If somehow thread 2 finish it works before thread 1 finishes — user3160866
– user3160866, Commented Aug 5, 2014 at 12:27

Mohit Jain · Accepted Answer · 2014-08-05 12:27:31Z

4

for(j=0; j< 5 ; j++) 
{
   pthread_join(tid[j], NULL);
}

Means wait for thread with tid[0] to finish. If it is already finished, return immediately.
Then wait for thread with tid[1] to finish. If it is already finished, return immediately and so on.

In short, this for loop will make sure, current (possibly main) thread would not exit before children thread.

answered Aug 5, 2014 at 12:27

Mohit Jain

30.6k8 gold badges80 silver badges104 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Soren · Accepted Answer · 2014-08-05 12:25:40Z

3

The for-loop is executed sequentially,

for(j=0; j< 5 ; j++) 
{
   pthread_join(tid[j], NULL);
}

so regardless of which thread finishes first, the for-loop will wait for thread 0 first, and then thread 1 etc. If thread 2 actually completes first then the result of that thread will simply just hang around until the join is executed -- this is similar to the way the wait call works if you were to wait for specific process ids.

answered Aug 5, 2014 at 12:25

Soren

14.8k4 gold badges43 silver badges69 bronze badges

4 Comments

user3160866 Over a year ago

If I wanted to make it non blocking then I do have to remove pthread_join as I understood from you. So my basic am is to create 5 threads in a continuous loop and execute some command independently. for that I wrote the below program which is stopping after sometimes.Can you please have a look on this.

user3160866 Over a year ago

#include <stdio.h> #include <pthread.h> #define NTHREADS 10 void thread_function(void *); int main() { pthread_t thread_id[NTHREADS]; int i, j; while(1) { for(i=0; i < NTHREADS; i++) { pthread_create( &thread_id[i], NULL, thread_function, NULL ); } / for(j=0; j< NTHREADS; j++) { pthread_join(thread_id[j], NULL); } */ } return 0; } void *thread_function(void *dummyPtr) {printf("iniside thread \n");}

Klas Lindbäck Over a year ago

If you let the main thread exit then the other threads will be killed, possibly (probably!) before their work is done, and that is not a good idea.

Soren Over a year ago

If you want to syncronize the exit of the threads you will have to implement some sort of syncronization method -- the possibilities are many and too long for this format, but includes mutex semaphores and many other methods. Alternatively if you are not interested in the join-result, why not just mark the thread non-joinable and it can just exit when it wants to without any syncronization?

Basile Starynkevitch · Accepted Answer · 2014-08-05 12:40:12Z

2

Your loop is not doing something magical. So it calls

  pthread_join(tid[0], NULL);
  pthread_join(tid[1], NULL);
  pthread_join(tid[2], NULL);
  pthread_join(tid[3], NULL);
  pthread_join(tid[4], NULL);

in the above order.

If you want to join threads in some other order, you need to use explicitly some synchronizations inside your func. You may want to use condition variables and mutexes. Read a good pthread tutorial

answered Aug 5, 2014 at 12:40

Basile Starynkevitch

231k18 gold badges323 silver badges578 bronze badges

Comments

Nanc · Accepted Answer · 2014-08-05 12:32:08Z

1

When you use pthread_join() function in a loop, your system will wait until first thread is returned, then the second one and so on. If the second thread is finished, it will wait for the first one and then it will take care of the second one.

answered Aug 5, 2014 at 12:32

Nanc

4625 silver badges16 bronze badges

Collectives™ on Stack Overflow

multithreading application in c

4 Answers 4

Comments

4 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related