Semaphore Concept (Operating System)

Concept of semaphore was devised by Dijkastra in 1965. Semaphore is an integer variable that is used to record number of weakups and had been saved. If it is equal to zero it indicates that no wakeup's are saved. A positive value shows that one or more wakeup's are pending. The DOWN operation (sleep) checks the semaphore to see if it is greater than zero. If it is, it decrements the value (UP) and continues. If the semaphore is zero the process sleeps. The UP operation (weakup) increments the value of the semaphore. If one or more proces were sleeping on that semaphore then one of the process is chosen and allowed to compute its DOWN. Checking and updating the semaphore must be done as an atomic action to avoid race conditions. Product Customer problem can be solved using semaphore as below.

 
#define N 100 /* number of slots in the buffer */
typedef int semaphore; /* semaphores are a special kind of int */
semaphore mutex = 1; /* controls access to critical region */
semaphore empty = N; /* counts empty buffer slots */
semaphore null = 0; /* counts full buffer slots */
void producer(void){
    int item; 
    message m;      // message buffer. 
    while(TRUE){
        item = produce_item(); // generate something to put in buffer 
        receive(consumer, &m); // Receive an empty message, if any 
        build_message(&m, item); // construct a message to send 
        send(consumer, &m);  // send item to consumer 
    }
} 
void consumer(void){
    int item, i; 
    message m; 
    for(i=0; i<N; i++)
        send(producer, &m);  // send N empty messages 
    while(TRUE){
        receive(produce, &m);  // get message containing item   
        item = extract_item(&m); // extract item from message
        send(producer, &m); //send back empty replay 
        consumer_item(term); //do something with the item
    }
}