Producer / Consumer / Conditions variables / pthreads

consumer.c

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

#define BUF_SIZE 5

// the buffer works like a stack for
// the sake of simplicity, if needed
// we may implement a queue
typedef struct {
    int buf[BUF_SIZE]; // the buffer
    size_t len; // number of items in the buffer
    pthread_mutex_t mutex; // needed to add/remove data from the buffer
    pthread_cond_t can_produce; // signaled when items are removed
    pthread_cond_t can_consume; // signaled when items are added
} buffer_t;

// produce random numbers
void* producer(void *arg) {
    buffer_t *buffer = (buffer_t*)arg;

    while(1) {
#ifdef UNDERFLOW
        // used to show that if the producer is somewhat "slow"
        // the consumer will not fail (i.e. it'll just wait
        // for new items to consume)
        sleep(rand() % 3);
#endif

        pthread_mutex_lock(&buffer->mutex);

        if(buffer->len == BUF_SIZE) { // full
            // wait until some elements are consumed
            pthread_cond_wait(&buffer->can_produce, &buffer->mutex);
        }

        // in real life it may be some data fetched from
        // sensors, the web, or just some I/O
        int t = rand();
        printf("Produced: %d\n", t);

        // append data to the buffer
        buffer->buf[buffer->len] = t;
        ++buffer->len;

        // signal the fact that new items may be consumed
        pthread_cond_signal(&buffer->can_consume);
        pthread_mutex_unlock(&buffer->mutex);
    }

    // never reached
    return NULL;
}

// consume random numbers
void* consumer(void *arg) {
    buffer_t *buffer = (buffer_t*)arg;

    while(1) {
#ifdef OVERFLOW
        // show that the buffer won't overflow if the consumer
        // is slow (i.e. the producer will wait)
        sleep(rand() % 3);
#endif
        pthread_mutex_lock(&buffer->mutex);

        if(buffer->len == 0) { // empty
            // wait for new items to be appended to the buffer
            pthread_cond_wait(&buffer->can_consume, &buffer->mutex);
        }

        // grab data
        --buffer->len;
        printf("Consumed: %d\n", buffer->buf[buffer->len]);

        // signal the fact that new items may be produced
        pthread_cond_signal(&buffer->can_produce);
        pthread_mutex_unlock(&buffer->mutex);
    }

    // never reached
    return NULL;
}

int main(int argc, char *argv[]) {
    buffer_t buffer = {
        .len = 0,
        .mutex = PTHREAD_MUTEX_INITIALIZER,
        .can_produce = PTHREAD_COND_INITIALIZER,
        .can_consume = PTHREAD_COND_INITIALIZER
    };

    pthread_t prod, cons;
    pthread_create(&prod, NULL, producer, (void*)&buffer);
    pthread_create(&cons, NULL, consumer, (void*)&buffer);

    pthread_join(prod, NULL); // will wait forever
    pthread_join(cons, NULL);

    return 0;
}

Why can't use only one condition?
@ktsaou @Alexis-D

Hi, to allow multiple consumers, this if statement:

        if(buffer->len == 0) { // empty
            // wait for new items to be appended to the buffer
            pthread_cond_wait(&buffer->can_consume, &buffer->mutex);
        }

needs to be a while statement:

        while(buffer->len == 0) { // empty
            // wait for new items to be appended to the buffer
            pthread_cond_wait(&buffer->can_consume, &buffer->mutex);
        }

+1

Hi, to allow multiple consumers, this if statement:

        if(buffer->len == 0) { // empty
            // wait for new items to be appended to the buffer
            pthread_cond_wait(&buffer->can_consume, &buffer->mutex);
        }

needs to be a while statement:

        while(buffer->len == 0) { // empty
            // wait for new items to be appended to the buffer
            pthread_cond_wait(&buffer->can_consume, &buffer->mutex);
        }

👍

Hi, to allow multiple consumers, this if statement:

        if(buffer->len == 0) { // empty
            // wait for new items to be appended to the buffer
            pthread_cond_wait(&buffer->can_consume, &buffer->mutex);
        }

needs to be a while statement:

        while(buffer->len == 0) { // empty
            // wait for new items to be appended to the buffer
            pthread_cond_wait(&buffer->can_consume, &buffer->mutex);
        }

but why?

I get this error when compiling:

/tmp/cc1yUZ5e.o: In function main': pthread.c:(.text+0x1aa): undefined reference to pthread_create'
pthread.c:(.text+0x1cd): undefined reference to pthread_create' pthread.c:(.text+0x1e1): undefined reference to pthread_join'
pthread.c:(.text+0x1f5): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status

I get this error when compiling:

/tmp/cc1yUZ5e.o: In function main': pthread.c:(.text+0x1aa): undefined reference to pthread_create'
pthread.c:(.text+0x1cd): undefined reference to pthread_create' pthread.c:(.text+0x1e1): undefined reference to pthread_join'
pthread.c:(.text+0x1f5): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status

compile with -lpthread

if we want to allow 1 consumer and 1 producer to be able to get/append data at the same time would we need two mutexes?