Reader-Writer problem using Monitors (pthreads)

Difficulty Level : Hard
Last Updated : 13 Aug, 2019

Prerequisite – Monitors, Readers-Writers Problem
There is a shared resource which is accessed by multiple processes i.e. readers and writers. Any number of readers can read from the shared resource simultaneously, but only one writer can write to the shared resource at a time. When a writer is writing data to the resource, no other process can access the resource. A writer cannot write to the resource if there are any readers accessing the resource at that time. Similarly, a reader can not read if there is a writer accessing the resource or if there are any waiting writers.

The Reader-Writer problem using monitor can be implemented using pthreads. The POSIX threads (or pthread) libraries are a standards based thread API for C/C++. The library provides following synchronization mechanisms:

Mutexes (pthread_mutex_t) – Mutual exclusion lock:
Block access to variables by other threads. This enforces exclusive access by a thread to a variable or set of variables.
Condition Variables – (pthread_cond_t):
The condition variable mechanism allows threads to suspend execution and relinquish the processor until some condition is true.

Implementation of Reader-Writer solution using pthread library:

Execute the program using following command in your Linux system

$g++ -pthread program_name.cpp
$./a.out
or
$g++ -pthread program_name.cpp -o object_name
$./object_name

Code:

// Reader-Writer problem using monitors
  
#include <iostream>
#include <pthread.h>
#include <unistd.h>
using namespace std;
  
class monitor {
private:
    // no. of readers
    int rcnt;
  
    // no. of writers
    int wcnt;
  
    // no. of readers waiting
    int waitr;
  
    // no. of writers waiting
    int waitw;
  
    // condition variable to check whether reader can read
    pthread_cond_t canread;
  
    // condition variable to check whether writer can write
    pthread_cond_t canwrite;
  
    // mutex for synchronisation
    pthread_mutex_t condlock;
  
public:
    monitor()
    {
        rcnt = 0;
        wcnt = 0;
        waitr = 0;
        waitw = 0;
  
        pthread_cond_init(&canread, NULL);
        pthread_cond_init(&canwrite, NULL);
        pthread_mutex_init(&condlock, NULL);
    }
  
    // mutex provide synchronisation so that no other thread
    // can change the value of data
    void beginread(int i)
    {
        pthread_mutex_lock(&condlock);
  
        // if there are active or waiting writers
        if (wcnt == 1 || waitw > 0) {
            // incrementing waiting readers
            waitr++;
  
            // reader suspended
            pthread_cond_wait(&canread, &condlock);
            waitr--;
        }
  
        // else reader reads the resource
        rcnt++;
        cout << "reader " << i << " is reading\n";
        pthread_mutex_unlock(&condlock);
        pthread_cond_broadcast(&canread);
    }
  
    void endread(int i)
    {
  
        // if there are no readers left then writer enters monitor
        pthread_mutex_lock(&condlock);
  
        if (--rcnt == 0)
            pthread_cond_signal(&canwrite);
  
        pthread_mutex_unlock(&condlock);
    }
  
    void beginwrite(int i)
    {
        pthread_mutex_lock(&condlock);
  
        // a writer can enter when there are no active
        // or waiting readers or other writer
        if (wcnt == 1 || rcnt > 0) {
            ++waitw;
            pthread_cond_wait(&canwrite, &condlock);
            --waitw;
        }
        wcnt = 1;
        cout << "writer " << i << " is writing\n";
        pthread_mutex_unlock(&condlock);
    }
  
    void endwrite(int i)
    {
        pthread_mutex_lock(&condlock);
        wcnt = 0;
  
        // if any readers are waiting, threads are unblocked
        if (waitr > 0)
            pthread_cond_signal(&canread);
        else
            pthread_cond_signal(&canwrite);
        pthread_mutex_unlock(&condlock);
    }
  
}
  
// global object of monitor class
M;
  
void* reader(void* id)
{
    int c = 0;
    int i = *(int*)id;
  
    // each reader attempts to read 5 times
    while (c < 5) {
        usleep(1);
        M.beginread(i);
        M.endread(i);
        c++;
    }
}
  
void* writer(void* id)
{
    int c = 0;
    int i = *(int*)id;
  
    // each writer attempts to write 5 times
    while (c < 5) {
        usleep(1);
        M.beginwrite(i);
        M.endwrite(i);
        c++;
    }
}
  
int main()
{
    pthread_t r[5], w[5];
    int id[5];
    for (int i = 0; i < 5; i++) {
        id[i] = i;
  
        // creating threads which execute reader function
        pthread_create(&r[i], NULL, &reader, &id[i]);
  
        // creating threads which execute writer function
        pthread_create(&w[i], NULL, &writer, &id[i]);
    }
  
    for (int i = 0; i < 5; i++) {
        pthread_join(r[i], NULL);
    }
    for (int i = 0; i < 5; i++) {
        pthread_join(w[i], NULL);
    }
}

Output:

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