I am a complete newbie and I have been coding producer-consumer problem with fixed buffer size what I observed for buffer-size 1 is very different from what I expected. I print "x-->" when I produce x and "-->x" when x is consumed. The output I get is like this: \
0 --->
1 --->
2 --->
---> 0
---> 1
---> 2
3 --->
4 --->
5 --->
---> 3
---> 4
---> 5
I am confused about how 1,2,3 reproduced at once and then 1,2,3 consumed at once can someone explain, please.
Here is my code
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#define BUFFER_SIZE 1
#define OVER (-1)
int filled = 0;
struct prodcons {
int buffer[BUFFER_SIZE]; /* the actual data */
pthread_mutex_t lock; /* mutex ensuring exclusive access to buffer */
int readpos, writepos; /* positions for reading and writing */
pthread_cond_t notempty; /* signaled when buffer is not empty */
pthread_cond_t notfull; /* signaled when buffer is not full */
};
void init(struct prodcons * b)
{
pthread_mutex_init(&b->lock, NULL);
pthread_cond_init(&b->notempty, NULL);
pthread_cond_init(&b->notfull, NULL);
b->readpos = 0;
b->writepos = 0;
}
void put(struct prodcons * b, int data)
{
pthread_mutex_lock(&b->lock);
if(BUFFER_SIZE == 1) {
if(filled) {
pthread_cond_wait(&b->notfull, &b->lock);
}
b->buffer[0] = data;
filled = 1;
}
else {
/* Wait until buffer is not full */
while ((b->writepos + 1) % BUFFER_SIZE == b->readpos) {
pthread_cond_wait(&b->notfull, &b->lock);
/* pthread_cond_wait reacquired b->lock before returning */
}
/* Write the data and advance write pointer */
b->buffer[b->writepos] = data;
b->writepos++;
if (b->writepos >= BUFFER_SIZE) b->writepos = 0;
/* Signal that the buffer is now not empty */
}
pthread_cond_signal(&b->notempty);
pthread_mutex_unlock(&b->lock);
}
int get(struct prodcons * b)
{
int data = 0;
pthread_mutex_lock(&b->lock);
if(BUFFER_SIZE == 1) {
if(!filled) {
pthread_cond_wait(&b->notempty, &b->lock);
}
data = b->buffer[0];
filled = 0;
}
else {
/* Wait until buffer is not empty */
while (b->writepos == b->readpos) {
pthread_cond_wait(&b->notempty, &b->lock);
}
/* Read the data and advance read pointer */
data = b->buffer[b->readpos];
b->readpos++;
if (b->readpos >= BUFFER_SIZE) b->readpos = 0;
/* Signal that the buffer is now not full */
}
pthread_cond_signal(&b->notfull);
pthread_mutex_unlock(&b->lock);
return data;
}
struct prodcons buffer;
void * producer(void * data)
{
int n;
for (n = 0; n < 10000; n++) {
printf("%d --->\n", n);
put(&buffer, n);
}
put(&buffer, OVER);
return NULL;
}
void * consumer(void * data)
{
int d;
while (1) {
d = get(&buffer);
if (d == OVER) break;
printf("---> %d\n", d);
}
return NULL;
}
int main(void)
{
pthread_t th_a, th_b;
void * retval;
init(&buffer);
/* Create the threads */
pthread_create(&th_a, NULL, producer, 0);
pthread_create(&th_b, NULL, consumer, 0);
/* Wait until producer and consumer finish. */
pthread_join(th_a, &retval);
pthread_join(th_b, &retval);
return 0;
}
