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C++ || Snippet – Multi-Process Synchronization Producer Consumer Problem Using Threads

October 03, 2013 admin 2 Comments

The following is sample code which demonstrates the use of POSIX threads (pthreads), aswell as pthread mutex and condition variables for use on Unix based systems.

The use of mutual exclusion (mutex) variables refers to the requirement of ensuring that no two processes or threads are in their critical section at the same time. Here, a critical section refers to the period of time in which a process accesses a shared resource, such as shared memory. This procedure highlights a classic example of a multi-process synchronization problem known as the producer–consumer problem.

The producer–consumer problem describes a scenario in which two processes (the producer and the consumer) share a common resource (i.e: a string buffer). In this scenario, the producer’s job is to generate a piece of data, update that data with the shared resource (the buffer), and repeat. At the same time, the consumer is consuming that shared resource (i.e: removing something from that same buffer) one piece at a time. The problem is to make sure that the producer wont try to manipulate that shared resource (i.e: add data into the buffer) while the consumer is accessing it; and that the consumer wont try to remove data from that shared resource (the buffer) while the producer is updating it.

In this instance, a solution for the producer can be to go to sleep if a synchronization conflict occurs, and the next time the consumer removes an item from the buffer, it can notify the producer to wake up and start to fill the buffer again. In the same way, the consumer can go to sleep when the producer is accessing it. The next time the producer puts data into the buffer, it wakes up the sleeping consumer and the process continues.

The example on this page demonstrates the use of the above solution using the “pthread_mutex_lock()” function call to limit access to the critical section, and the “pthread_cond_wait()” function call to simulate the sleeping process.





		
		
			Producer Consumer Problem Using Pthreads
			
C++
			
			// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Oct 3, 2013
//    Taken From: http://programmingnotes.org/
//    File:  Condvar.cpp
//    Description: Demonstrate the use of the producer consumer problem
//       using mutex and condition variables
// ============================================================================
#include <iostream>
#include <cstdlib>
#include <pthread.h>
using namespace std;

// Compile & Run
// g++ Condvar.cpp -lpthread -o Condvar
// ./Condvar

// function prototypes
void* Produce(void* arg);
void* Consume(void* arg);

// global variables
// the mutex and the condition variable
pthread_mutex_t mutex;
pthread_cond_t cond;

// the condition flag
bool condition = false;

// the item produced
int count = 0;

// how much to produce
const int NUM_TO_PRODUCE = 6;

int main()
{
    // declare variables
    pthread_t producerThread;
    pthread_t consumerThread;

    // initialize the mutex and the condition variable
    pthread_mutex_init(&mutex, NULL);
    pthread_cond_init(&cond, NULL);

    cout <<"\nThe Parent is creating the producer and consumer threads..\n" << endl;

    // create a producer thread
    if(pthread_create(&producerThread, NULL, Produce, NULL) < 0)
    {
        perror("pthread_create");
        exit(1);
    }

    // create a consumer thread
    if(pthread_create(&consumerThread, NULL, Consume, NULL) < 0)
    {
        perror("pthread_create");
        exit(1);
    }

    // wait for the threads to complete
    // similar to the wait() system call for fork() processes
    if(pthread_join(producerThread, NULL) < 0)
    {
        perror("pthread_join");
        exit(1);
    }

    if(pthread_join(consumerThread, NULL) < 0)
    {
        perror("pthread_join");
        exit(1);
    }

    cout <<"\nBoth threads have completed and have terminated!\n"
        <<"\nThe Parent is now exiting...\n";

    return 0;
}// end of main

/**
 * The producer thread function
 * @param arg - pointer to the thread local data - unused
 */
void* Produce(void* arg)
{
    // produce things until the loop condition is met
    while(count < NUM_TO_PRODUCE)
    {
        // lock the mutex to protect the condition variable
        if(pthread_mutex_lock(&mutex) < 0)
        {
            perror("pthread_mutex_lock");
            exit(1);
        }

        // we have produced something that has not been
        // consumed yet, so we sleep (wait) until the consumer
        // wakes us up.
        while(condition)
        {
            // sleep (wait) on a condition variable until the
            // the consumer wakes the producer up.
            if(pthread_cond_wait(&cond, &mutex) < 0)
            {
                perror("pthread_cond_wait");
                exit(1);
            }
        }

        // produce an item
        cout <<"Produced: "<<++count<<endl;

        // we have produced something
        condition = true;

        // wake up the sleeping consumer
        if(pthread_cond_signal(&cond) < 0)
        {
            perror("pthread_cond_signal");
            exit(1);
        }

        // release the mutex lock
        if(pthread_mutex_unlock(&mutex) < 0)
        {
            perror("pthread_mutex_unlock");
            exit(1);
        }
    }
    return 0;
}// end of Produce

/**
 * The consumer function
 * @param arg - pointer to the thread local data - unused
 */
void* Consume(void* arg)
{
    // consume things until the loop condition is met
    while(count < NUM_TO_PRODUCE)
    {
        // lock the mutex to protect the condition variable
        if(pthread_mutex_lock(&mutex) < 0)
        {
            perror("pthread_mutex_lock");
            exit(1);
        }

        // if there is nothing to consume, then sleep
        while(!condition)
        {
            // sleep (wait) on a condition variable until the
            // producer wakes the consumer up.
            if(pthread_cond_wait(&cond, &mutex) < 0)
            {
                perror("pthread_cond_wait");
                exit(1);
            }
        }

        cout <<"Consumed: "<<count<<endl<<endl;

        // consume the item
        condition = false;

        // wake up the sleeping producer
        if(pthread_cond_signal(&cond) < 0)
        {
            perror("pthread_cond_signal");
            exit(1);
        }

        // unlock the mutex
        if(pthread_mutex_unlock(&mutex) < 0)
        {
            perror("pthread_mutex_unlock");
            exit(1);
        }
    }
    return 0;
}// http://programmingnotes.org/
			
				
					
				
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						// ============================================================================
//    Author: Kenneth Perkins
//    Date:   Oct 3, 2013
//    Taken From: http://programmingnotes.org/
//    File:  Condvar.cpp
//    Description: Demonstrate the use of the producer consumer problem
//       using mutex and condition variables
// ============================================================================
#include <iostream>
#include <cstdlib>
#include <pthread.h>
using namespace std;
 
// Compile & Run
// g++ Condvar.cpp -lpthread -o Condvar
// ./Condvar
 
// function prototypes
void* Produce(void* arg);
void* Consume(void* arg);
 
// global variables
// the mutex and the condition variable
pthread_mutex_t mutex;
pthread_cond_t cond;
 
// the condition flag
bool condition = false;
 
// the item produced
int count = 0;
 
// how much to produce
const int NUM_TO_PRODUCE = 6;
 
int main()
{
    // declare variables
    pthread_t producerThread;
    pthread_t consumerThread;
 
    // initialize the mutex and the condition variable
    pthread_mutex_init(&mutex, NULL);
    pthread_cond_init(&cond, NULL);
 
    cout <<"\nThe Parent is creating the producer and consumer threads..\n" << endl;
 
    // create a producer thread
    if(pthread_create(&producerThread, NULL, Produce, NULL) < 0)
    {
        perror("pthread_create");
        exit(1);
    }
 
    // create a consumer thread
    if(pthread_create(&consumerThread, NULL, Consume, NULL) < 0)
    {
        perror("pthread_create");
        exit(1);
    }
 
    // wait for the threads to complete
    // similar to the wait() system call for fork() processes
    if(pthread_join(producerThread, NULL) < 0)
    {
        perror("pthread_join");
        exit(1);
    }
 
    if(pthread_join(consumerThread, NULL) < 0)
    {
        perror("pthread_join");
        exit(1);
    }
 
    cout <<"\nBoth threads have completed and have terminated!\n"
        <<"\nThe Parent is now exiting...\n";
 
    return 0;
}// end of main
 
/**
 * The producer thread function
 * @param arg - pointer to the thread local data - unused
 */
void* Produce(void* arg)
{
    // produce things until the loop condition is met
    while(count < NUM_TO_PRODUCE)
    {
        // lock the mutex to protect the condition variable
        if(pthread_mutex_lock(&mutex) < 0)
        {
            perror("pthread_mutex_lock");
            exit(1);
        }
 
        // we have produced something that has not been
        // consumed yet, so we sleep (wait) until the consumer
        // wakes us up.
        while(condition)
        {
            // sleep (wait) on a condition variable until the
            // the consumer wakes the producer up.
            if(pthread_cond_wait(&cond, &mutex) < 0)
            {
                perror("pthread_cond_wait");
                exit(1);
            }
        }
 
        // produce an item
        cout <<"Produced: "<<++count<<endl;
 
        // we have produced something
        condition = true;
 
        // wake up the sleeping consumer
        if(pthread_cond_signal(&cond) < 0)
        {
            perror("pthread_cond_signal");
            exit(1);
        }
 
        // release the mutex lock
        if(pthread_mutex_unlock(&mutex) < 0)
        {
            perror("pthread_mutex_unlock");
            exit(1);
        }
    }
    return 0;
}// end of Produce
 
/**
 * The consumer function
 * @param arg - pointer to the thread local data - unused
 */
void* Consume(void* arg)
{
    // consume things until the loop condition is met
    while(count < NUM_TO_PRODUCE)
    {
        // lock the mutex to protect the condition variable
        if(pthread_mutex_lock(&mutex) < 0)
        {
            perror("pthread_mutex_lock");
            exit(1);
        }
 
        // if there is nothing to consume, then sleep
        while(!condition)
        {
            // sleep (wait) on a condition variable until the
            // producer wakes the consumer up.
            if(pthread_cond_wait(&cond, &mutex) < 0)
            {
                perror("pthread_cond_wait");
                exit(1);
            }
        }
 
        cout <<"Consumed: "<<count<<endl<<endl;
 
        // consume the item
        condition = false;
 
        // wake up the sleeping producer
        if(pthread_cond_signal(&cond) < 0)
        {
            perror("pthread_cond_signal");
            exit(1);
        }
 
        // unlock the mutex
        if(pthread_mutex_unlock(&mutex) < 0)
        {
            perror("pthread_mutex_unlock");
            exit(1);
        }
    }
    return 0;
}// http://programmingnotes.org/

QUICK NOTES:
The highlighted lines are sections of interest to look out for.

To use pthreads, the compiler flag “-lpthread” must be set.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

The following is sample output:

The Parent is creating the producer and consumer threads..
Produced: 1 Consumed: 1 Produced: 2 Consumed: 2 Produced: 3 Consumed: 3 Produced: 4 Consumed: 4 Produced: 5 Consumed: 5 Produced: 6 Consumed: 6 Both threads have completed and have terminated!
The Parent is now exiting...

Tag Archives: mutex

C++ || Snippet – Multi-Process Synchronization Producer Consumer Problem Using Threads

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