Multi threading using POSIX Semaphore in c

Question

I am trying to use semaphores to capture frames from the video camera and do object recognition in parallel , I have a doubt here:

main.c(with edits)

sem_t sem_1;
sem_init(&sem_1, 0, 1);  //Initial value of 1
sem_init(&sem_2, 0, 1);  //Initial value of 1

int val_sem1,val_sem2;
sem_getvalue(&mutex_ping1, &val_sem1);
printf("%d %d \r\n", val_sem1,val_sem2);   //Output = 1 1(Correct)

//Create thread
trc = pthread_create(&tid_1, NULL, objrecognition_2, &obj_num[0]);
trc = pthread_create(&tid_2, NULL, objrecognition_3, &obj_num[1]);

Sleep(5000);


 sem_getvalue(&sem_1, &val_sem1);
 sem_getvalue(&sem_2, &val_sem2);
 printf("%d %d \r\n", val_sem1,val_sem2);  //Ideal output? 

 //Few line of code

while(1)
{
   //Get camera frame from video camera
   ....
   ....

   frame[index%3] = currentframe; //Using 3 backup buffers to avoid race around


   //For the very first time always sem_post(logic to keep index > index_use)
   if ((check))   //Initial value of check =1
  {
      check = 0;//Check is made 0 here after permanently
      sem_post(&sem1);
      sem_post(&sem2);
  }


   sem_getvalue(&sem_1, &val_sem1);//Get the present semaphore1 value
   sem_getvalue(&sem_2, &val_sem2);//Get the present semaphore2 value


   //This part of the code is activated from the second run because of check variable

   //Check if thread has completed one loop run and is waiting on sem_wait()
   if ((val_sem_1 == 0) &&  (val_sem_2 == 0) && (check==0)) //Checking if thread has completed one loop run
   {
     index_use++;    //The thread uses frame[index_use % 3] to process 
                    //so that it does not collide with frame[index % 3]
     sem_post(&sem_1);
     sem_post(&sem_2);
   }

   index++;
}

Output should be 0 since sem_wait in thread(in functions.c below) must have decremented the value to 0 and should have got blocked

But I am getting random outputs such as 1, -1 and sometimes 0.

Could anyone please help me out here, Is my understanding of semaphores wrong??

functions.c

void*  objrecognition_2(void* arg2)
{
   while (1)
  {

    sem_wait(&mutex_ping2);

   ...
   ...
  }
}

edits

I have put a break point before calling sem_post() and also keeping a 5 second delay after creating the thread.

Hence the thread gets created and has to decrement semaphore by 1 and become zero and should wait untill sem_post() gets activated.

Now it prints only -1 at the second printf.

StoryTeller - Unslander Monica · Accepted Answer · 2017-03-01 12:04:00Z

4

From the documentation:

If one or more processes or threads are blocked waiting to lock the semaphore with sem_wait(3), POSIX.1 permits two possibilities for the value returned in sval: either 0 is returned; or a negative number whose absolute value is the count of the number of processes and threads currently blocked in sem_wait(3). Linux adopts the former behavior.

So the value isn't random, it has meaning. In your particular sample code:

-1 means objrecognition_2 calls sem_wait again without calling sem_post before. So it's in a deadlock.
0 means objrecognition_2 hasn't deadlocked itself yet.
1 means objrecognition_2 didn't call sem_wait on the semaphore at all yet.

Following your edits and discussion in the comments to my answer, it's clear to me you are trying to implement a thread barrier by yourself. I suggest you just use the pthread barrier implementation instead.

edited Mar 1, 2017 at 12:04

answered Mar 1, 2017 at 7:03

StoryTeller - Unslander Monica

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13 Comments

Nithin G A Over a year ago

Why would sem_wait be called again , since initial semaphore value is 1, so sem_wait must decrement the semaphore value to 0 and go to wait mode till sem_post increments the semaphore value.

StoryTeller - Unslander Monica Over a year ago

@NithinGA - In the code you posted you sem_wait at the start of a loop. And you don't show yourself calling sem_post. So in the next iteration, sem_wait will see that the semaphore is 0 and wait. The thread is deadlocked with itself.

Nithin G A Over a year ago

Actually I have put a break point before calling sem post(Please check the edits)...Now in the thread the first statement is sem_wait...So if I allow 5 seconds after creating the thread and then print the value of semaphore value, It has to get blocked at sem_wait right??

StoryTeller - Unslander Monica Over a year ago

@NithinGA - Your edit doesn't have any call to sem_post in sight. Your code sample still has the same symptoms I described. If you want to discuss something about sem_post, then please post code that calls it.

Nithin G A Over a year ago

@StorTeller, I have edited the question , does it give some clarity as to what I am doing

|

Mahmoud Fayez · Accepted Answer · 2017-03-01 08:21:27Z

0

You are getting random value as their is a timing issue here. To make sure the thread already started and decremented the semaphore I suggest you to wait for 1 second see the code below:

//After say 20 lines of code, such that the thread actually gets created

// put the main thread to sleep for 1 second as 
// 20 lines of code may not be too much to give 
// the opportunity to another thread to work and capture the semaphore.

sem_getvalue(&mutex_ping1, &val_sem1);

answered Mar 1, 2017 at 8:21

Mahmoud Fayez

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1 Comment

Nithin G A Over a year ago

Good Idea, I tried it, gave a 5 second delay to be safe, Now it is always -1, so as @StoryTeller pointed out it may be calling sem_wait() again and decrementing twice and hence -1 always...But why is this happening I do not understand

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