reader_writer.cpp

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

sem_t mutex;
sem_t db;
int readcount;

void *reader(void *arg)
{
    int f;
    sem_getvalue(&mutex, &f);
    printf("Reader %d is trying to enter, readcount = %d, mutex = %d\n", *((int*)arg), readcount, f);
    sem_wait(&mutex);
    readcount++;
    if (readcount == 1)
        sem_wait(&db);
    printf("Reader %d is inside, readcount = %d\n", *((int*)arg), readcount);
    sem_post(&mutex);

    // reading is performed here

    sem_wait(&mutex);
    readcount--;
    if (readcount == 0)
        sem_post(&db);
    sem_post(&mutex);
    printf("Reader %d is leaving, readcount = %d\n", *((int*)arg), readcount);
}

void *writer(void *arg)
{
    int f;
    sem_getvalue(&db, &f);
    printf("Writer %d is trying to enter, mutex = %d, db = %d\n", *((int*)arg), f, f);
    sem_wait(&db);
    printf("Writer %d is inside\n", *((int*)arg));
    // writing is performed here
    sem_post(&db);
    printf("Writer %d is leaving\n", *((int*)arg));
}

int main()
{
    pthread_t r1, r2, r3, r4, r5, w1, w2;
    sem_init(&mutex, 0, 1);
    sem_init(&db, 0, 1);
    readcount = 0;

    int a1 = 1, a2 = 2, a3 = 3, a4 = 4, a5 = 5, a6 = 6, a7 = 7;
    pthread_create(&r1, NULL, reader, (void*)&a1);
    pthread_create(&r2, NULL, reader, (void*)&a2);
    pthread_create(&r3, NULL, reader, (void*)&a3);
    pthread_create(&r4, NULL, reader, (void*)&a4);
    pthread_create(&r5, NULL, reader, (void*)&a5);
    pthread_create(&w1, NULL, writer, (void*)&a6);
    pthread_create(&w2, NULL, writer, (void*)&a7);

    pthread_join(r1, NULL);
    pthread_join(r2, NULL);
    pthread_join(r3, NULL);
    pthread_join(r4, NULL);
    pthread_join(r5, NULL);
    pthread_join(w1, NULL);
    pthread_join(w2, NULL);

    sem_destroy(&mutex);
    sem_destroy(&db);

    return 0;
}