semaphores.c

// Adapted from: https://dextutor.com/process-synchronization-using-semaphores/

#include<pthread.h>
#include<stdio.h>
#include<semaphore.h>
#include<unistd.h> 

void *fun1();       // inrement shared variable by 1
void *fun2();       // decrement shared variable by 1

int shared = 1;     //shared variable
sem_t s;            //semaphore variable

int main() {
    /*
    initialize semaphore variable 
    - 1st argument is address of variable, 
    - 2nd is number of processes sharing semaphore, 
    - 3rd argument is the initial value of semaphore variable
    */
    sem_init(&s, 0, 1);

    // create two threads
    pthread_t thread1, thread2;
    pthread_create(&thread1, NULL, fun1, NULL);
    pthread_create(&thread2, NULL, fun2, NULL);

    // wait for the threads to finish
    pthread_join(thread1, NULL);
    pthread_join(thread2,NULL);

    // prints the last updated value of shared variable
    printf("Final value of shared variable is %d\n",shared);
}

void *fun1() {
    int x;
    sem_wait(&s);   //executes wait operation on s
    x = shared;     //thread1 reads value of shared variable
    printf("Thread1 reads the value as %d\n",x);
    x++;            //thread1 increments its value
    printf("Local updation by Thread1: %d\n",x);
    sleep(1);       //thread1 is preempted by thread 2
    shared = x;     //thread one updates the value of shared variable
    printf("Value of shared variable updated by Thread1 is: %d\n",shared);    
    sleep(5);
    sem_post(&s);
}

void *fun2() {
    int y;
    sem_wait(&s);   //executes wait operation on s
    y = shared;     //thread2 reads value of shared variable
    printf("Thread2 reads the value as %d\n",y);
    y--;            //thread2 increments its value
    printf("Local updation by Thread2: %d\n",y);
    sleep(1);       //thread2 is preempted by thread 1
    shared = y;    //thread2 updates the value of shared variable
    printf("Value of shared variable updated by Thread2 is: %d\n",shared);
    sem_post(&s);
}