one semaphore object for more than one consumer thread.

This is a discussion on one semaphore object for more than one consumer thread. within the Linux Programming forums, part of the Platform Specific Boards category; Iam having a simple c program, with one main thread 4 consumer threds all consumebr threads are waiting on the ...

  1. #1
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    one semaphore object for more than one consumer thread.

    Iam having a simple c program,

    with one main thread

    4 consumer threds

    all consumebr threads are waiting on the same semaphore object.

    my expectation is when main thread do the sem_post all consumer threads will come out of

    blocked state and start printing the data which has been buffered by the main thread.

    Is this right?

    But in reality it does not unblock all the threads only first thread is coming out of the blocked state

    and printing the result.

    Can any body help me is this!

    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <errno.h>
    #include <stdlib.h>
    #include <string.h>
    
    #include <pthread.h>
    #include <semaphore.h>
    
    #define MAX_MSG_LEN 256
    
    sem_t sem1;
    
    char msg1[MAX_MSG_LEN] = "1";
    
    void *thrdFun1(void *arg);
    void *thrdFun2(void *arg);
    void *thrdFun3(void *arg);
    void *thrdFun4(void *arg);
    
    int main()
    {
         pthread_t thrd1,thrd2,thrd3,thrd4;
         char argmsg1[] = "Thread1: ";
         char argmsg2[] = "Thread2: ";
         char argmsg3[] = "Thread3: ";
         char argmsg4[] = "Thread4: ";
         int res;
         int thNum;
    
         res = sem_init(&sem1,0,0);
         res = pthread_create(&thrd1, NULL, thrdFun1, argmsg1);
         res = pthread_create(&thrd2, NULL, thrdFun2, argmsg2);
         res = pthread_create(&thrd3, NULL, thrdFun3, argmsg3);
         res = pthread_create(&thrd4, NULL, thrdFun4, argmsg4);
    
         while(1)
         {
              printf("Enter message to end to thread \n");
              fgets(msg1,MAX_MSG_LEN,stdin);
              sem_post(&sem1);
         }
    
         return 0;          /* writing the comments */
    }
    
    void *thrdFun1(void *arg)
    {
         while(1)
         {
              sem_wait(&sem1);
              printf("Iam %s message is %s\n",arg,msg1);
         }
    }
    
    
    void *thrdFun4(void *arg)
    {
         while(1)
         {
              sem_wait(&sem1);
              printf("Iam %s message is %s\n",arg,msg1);
         }
    }
    
    
    void *thrdFun3(void *arg)
    {
         while(1)
         {
              sem_wait(&sem1);
              printf("Iam %s message is %s\n",arg,msg1);
         }
    }
    
    
    void *thrdFun2(void *arg)
    {
         while(1)
         {
              sem_wait(&sem1);
              printf("Iam %s message is %s\n",arg,msg1);

  2. #2
    Captain Crash brewbuck's Avatar
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    I'm not trying to sound harsh, but... Duh? You've initialized the semaphore with a value of zero, meaning only one thread can hold it at once.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  3. #3
    Registered User
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    it's not dependent on the initialized value from sem_init()

    if i give 0 or 4 its not showing any difference .

    The threads are coming out of blocked state sequentially i.e for thread1 to thread4 after each buffer from the user.

    Does this mean like simultaneously all the threads will not come out of blocked state with the single semaphore .

    can any body give there thoughts..

  4. #4
    Registered User Codeplug's Avatar
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    Calling sem_post increments the semaphore value by 1.
    Calling sem_wait tries to decrement the semaphore value by 1. If the value is 0, then it blocks.

    If more than one thread is blocked on the semaphore, calling sem_post will release exactly one thread. You have no control over which thread is released.

    So even if you called sem_post 4 times in a row, the first thread may run 4 times, or the first and second threads may run twice, etc...

    If you want to signal all 4 threads to do something at once, then a condition variable would be more appropriate.

    gg

  5. #5
    Unregistered User Yarin's Avatar
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    Your desired functionality can be achieved by placing a "semaphore bomb" in each of the consumer threads.
    Basically, each time one of the threads wake from their sem_wait(), have them call sem_post() right away, it'll domino through, waking each thread... and finishing with a value of 1, so make sure you reset the semaphore's value back to zero before trying to lock them again.

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