Implementing Producer/Consumer Communication - c

Question

I am attempting to implement producer/consumer communication through a bounded buffer using semaphores and locks I've already implemented in C. I need to have the producer place "hello world" onto a 5-byte buffer, one character at a time. The producer needs to block if the buffer is full. I am struggling to figure out how to place the string on the buffer. Here is what I have so far:
#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;
}

Answer 1

a stack is a Last in, First out. Not what you want.
I would strongly implementing a circular buffer, with a head and tail pointer.
Then use the mutex (no need for a semaphore) to lock the critical code.
(the critical code for the producer is where it is adding/trying to add a char)
(the critical code for the consumer is where it is removing/trying to remove a char)
the mutex is so the two critical variables 'head' and 'tail' are stable
while any one process is accessing the circular queue.