Incorrect output of code after using pthreads

Question

I am working in some code trying to make it use threads, with little success. The code is as follows (using some online tuts)

(1) Create an array to save arguments to pass to each thread. (2) The struct where thread args are stored (3) Function each thread executes (4) Main, where I create each pthread and pass args. end and begin are dummy i and i+1 just for testing purposes.

#include <pthread.h>
struct thread_data* thread_data_array;

struct thread_data{
   int  thread_id;
   int  begin;
   int  end;
};
void *proccessData(void *threadarg)
{
  struct thread_data *my_data = (struct thread_data *) threadarg;
  int id = my_data->thread_id;
  int start = my_data->begin;
  int end = my_data->end;
  printf("%s%d%s%d%s%d%s","Process: Id ",id," begin: ",start," end: ",end,"\n");

  // Init
  FILE* fileOut;
  // do stuff
  fileOut = fopen(someNameUsingId, "w");
  // more stuff
  fclose(fileOut);
}
int main(int argc, char **argv)
{
  int n_th = atoi(argv[1]);
  thread_data_array = malloc(n_th*sizeof(struct thread_data));
  pthread_t threads[n_th];
  int i;

  for (i=0; i<n_th; i++)
  {
    thread_data_array[i].thread_id = i;
    thread_data_array[i].begin = i;
    thread_data_array[i].end = i+1;
    pthread_create(&threads[i], NULL, proccessData,(void *) &thread_data_array[i]);
  }
}

What I am getting: Sometimes nothing is printed, Sometimes some ids are printed twice. Files are not created (one in every 5 lets say, files are created, and are empty)

But using this in main works as I am expecting, id 1 from 0 to 1 is printed, id 2 from 1 to 2, and both files are created and have the right content

  thread_data_array[0].thread_id = 1;
  thread_data_array[0].begin = 0;
  thread_data_array[0].end = 1;
  proccessData((void *) &thread_data_array[0]);

  thread_data_array[1].thread_id = 2;
  thread_data_array[1].begin = 1;
  thread_data_array[1].end = 2;
  proccessData((void *) &thread_data_array[1]);

Can someone point out what am I doing wrong, and how to solve it?

Thanks in advance

I think main is ending before threads do, but can't figure out how to solve it — divmermarlav
– divmermarlav, Commented Apr 22, 2015 at 10:54
pthread_join will solve the issue, call it for each created thread in main — Dabo
– Dabo, Commented Apr 22, 2015 at 10:55
Is this necessary? Because I don't really need to join them, just that each one does its work — divmermarlav
– divmermarlav, Commented Apr 22, 2015 at 10:58
pthread_join() function is called to wait for the threads to complete. If main is done before threads are done, they will die before they finished their job — Dabo
– Dabo, Commented Apr 22, 2015 at 10:59
it works! Thanks. Post it as an answer and I will accept it. Could you please explain why is this necessary? — divmermarlav
– divmermarlav, Commented Apr 22, 2015 at 11:01

Jens Gustedt · Accepted Answer · 2015-04-22 10:56:35Z

4

Your main is terminating before the other threads. main is special, as returning from it is equivalent to call exit.

Either use pthread_exit to end main or use pthread_join to wait for the termination of the other threads.

answered Apr 22, 2015 at 10:56

Jens Gustedt

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1 Comment

divmermarlav Over a year ago

since begginers as myself may see this, this is the code I added for (i=0; i<n_th; i++) {pthread_join(threads[i], NULL);}

Dabo · Accepted Answer · 2015-04-22 11:02:45Z

1

pthread_join will solve the issue, call it for each created thread in main. pthread_join() function is called to wait for the threads to complete. If main is done before threads are done, they will die before they finished their job.

answered Apr 22, 2015 at 11:02

Dabo

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1 Comment

divmermarlav Over a year ago

since begginers as myself may see this, this is the code I added for (i=0; i<n_th; i++) {pthread_join(threads[i], NULL);}

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