/* 
 * Readers-writers solution with weak reader priority
 * Originally From Courtois et al, CACM, 1971.
 * via code accompanying Computer Systems: A Programmer's Perspective, 2/E (CS:APP2e)
 * Randal E. Bryant and David R. O'Hallaron, Carnegie Mellon University
 *
 * Minor adaptation by Brighten Godfrey <pbg@illinois.edu>
 *
 * NOTE: This program is incomplete.  We haven't initialized
 * the global variables and haven't actually started any
 * readers / writers. So it doesn't do very much. :-)
 */

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

/* Global variables */
int readcnt;    /* Initially = 0 */
sem_t mutex, w; /* Both initially = 1 */

void reader(void) 
{
    while (1) {
        sem_wait(&mutex);
        readcnt++;
        if (readcnt == 1) /* First in */
            sem_wait(&w); /* lock out writers */
        sem_post(&mutex);

        /* Main critical section */
        /* Reading would happen here */

        sem_wait(&mutex);
        readcnt--;
        if (readcnt == 0) /* Last out */
            sem_post(&w); /* let in writers */
        sem_post(&mutex);
    }
}

void writer(void) 
{
    while (1) {
        sem_wait(&w);

        /* Critical section */
        /* Writing would happen here */

        sem_post(&w);
    }
}