array of threads creation in c

Question

I have created two array of threads using POSIX thread.There are two thread functions student and teacher. My sample program is given below. I have a small problem which I have commented out in the code . I have written a thread join function at the last of main function. Is it okey or is there anything wrong in writing it in this way?

Sample code fragment:

int main(void)
{
    pthread_t thread[25];
    int i;
    int id1[25];   //for students
    int id2[6];   //for teachers

    for(i=0;i<25;i++)
    {
          id1[i]=i;
          id2[i]=i;
          pthread_create(&thread[i],NULL,student,(void*)&id1[i] );

          if(i<6)
          {
             pthread_create(&thread[i],NULL,teacher,(void*)&id2[i]);
          }
   }



  for (i=0;i<25;i++)
  {
    pthread_join(thread[i],NULL);  //problem in this line
  }

 return 0;

}

Solution 1

      pthread_create(&thread[i],NULL,student,(void*)&id1[i] );

      if(i<6)
      {
         pthread_create(&thread[i],NULL,teacher,(void*)&id2[i]);
      }

For i<6, the value in thread[i] for student is overwritten by later pthread_create(), which makes 6 threads leaked. 6 joinable threads remains running when main() is about to return.

Solution 2

I don't think that your problem is in the end when joining threads, but in the beginning when threads are started.

First of all, id2[i]=i might give a segfault unless that statement is moved to within the check that i<6.

Next, overwriting the same &thread[i] within the if-statement which was already created before the if-statement means that you have lost control of 6 student threads, those will not be joined at the end.

Solution 3

One of the approach to avoid the problem of undefined behavior due to id2[i]=i and also the issue of number of threads left without joining at end can be by having separate array for teacher/student coupled with appropriate changes in creation/joining procedure.

You can have separate array of pthread_t for student and teacher as below :

pthread_t thread_teacher[6];
pthread_t thread_student[25];

Accordingly, the update in creation and joining procedure for covering both student and teacher can be as below :

for(i=0;i<25;i++)
{
    id1[i]=i;
    pthread_create(&thread_student[i],NULL,student,(void*)&id1[i] );
    if(i<6)
    {
        id2[i]=i;
        pthread_create(&thread_teacher[i],NULL,teacher,(void*)&id2[i]);
    }
}

for (i=0;i<25;i++)
{
    pthread_join(thread_student[i],NULL);  
    if(i<6)
        pthread_join(thread_teacher[i],NULL);  
}