Question

I am trying to simulate send and receive functionality using pthreads. I did implement the following code. But some times it hangs or produce wrong results.

int g_sender,g_receiver; //global variables to keep track of the sender and receiver
void send(double ** data,int sender,int receiver, int size[],int block_size){
    int i;
    pthread_mutex_lock(&lock);  
    for (i=0;i<size[0];i++)
            memcpy(sharedA[i],data[i],size[1]*block_size);
    g_sender = sender;
    g_receiver = receiver;
    sem_wait(&sem);
    pthread_mutex_unlock(&lock);

}

void receive(double ** data, int sender,int receiver, int size[],int block_size){
        int i;
    while(!(sender==g_sender && receiver==g_receiver));
    for(i=0;i<size[0];i++)
        memcpy(data[i],sharedA[i],size[1]*block_size);
    g_sender =-1;
    g_receiver = -1;
    sem_post(&sem);
}

what do you think I did miss here?

Thanks

Answer 1

a lot 🙂

• what is the purpose of holding the lock?
• never wait for a semaphore when holding a lock
• always check the return value of library functions. In particular the
  sem_ functions are interruptible, you must check if an
  interrupt happened for sem_wait
• if you are not so experienced with POSIX threads just don’t use
  sem_t. Use mutexes and condition variables instead. They are higher
  level interfaces better suited for application code.