C - circular character buffer w/ pthreads
- by Matt
I have a homework assignment where I have to implement a circular buffer and add and remove chars with separate threads:
#include <pthread.h>
#include <stdio.h>
#define QSIZE 10
pthread_cond_t full,/* count == QSIZE */
empty,/* count == 0 */
ready;
pthread_mutex_t m, n; /* implements critical section */
unsigned int iBuf, /* tail of circular queue */
oBuf; /* head of circular queue */
int count; /* count characters */
char buf [QSIZE]; /* the circular queue */
void Put(char s[]) {/* add "ch"; wait if full */
pthread_mutex_lock(&m);
int size = sizeof(s)/sizeof(char);
printf("size: %d", size);
int i;
for(i = 0; i < size; i++) {
while (count >= QSIZE)
pthread_cond_wait(&full, &m);/* is there empty slot? */
buf[iBuf] = s[i]; /* store the character */
iBuf = (iBuf+1) % QSIZE; /* increment mod QSIZE */
count++;
if (count == 1)
pthread_cond_signal(&empty);/* new character available */
}
pthread_mutex_unlock(&m);
}
char Get() {/* remove "ch" from queue; wait if empty */
char ch;
pthread_mutex_lock(&m);
while (count <= 0)
pthread_cond_wait(&empty, &m);/* is a character present? */
ch = buf[oBuf]; /* retrieve from the head of the queue */
oBuf = (oBuf+1) % QSIZE;
count--;
if (count == QSIZE-1)
pthread_cond_signal(&full);/* signal existence of a slot */
pthread_mutex_unlock(&m);
return ch;
}
void * p1(void *arg) {
int i;
for (i = 0; i < 5; i++) {
Put("hella");
}
}
void * p2(void *arg) {
int i;
for (i = 0; i < 5; i++) {
Put("goodby");
}
}
int main() {
pthread_t t1, t2;
void *r1, *r2;
oBuf = 0; iBuf = 0; count=0; /* all slots are empty */
pthread_cond_init(&full, NULL);
pthread_cond_init(&empty, NULL);
pthread_mutex_init(&m, NULL);
pthread_create(&t1, NULL, p1, &r1);
pthread_create(&t2, NULL, p2, &r2);
printf("Main");
char c;
int i = 0;
while (i < 55) {
c = Get();
printf("%c",c);
i++;
}
pthread_join(t1, &r1);
pthread_join(t2, &r2);
return 0;
}
I shouldn't have to change the logic much at all, the requirements are pretty specific. I think my problem lies in the Put() method. I think the first thread is going in and blocking the critical section and causing a deadlock. I was thinking I should make a scheduling attribute? Of course I could be wrong. I am pretty new to pthreads and concurrent programming, so I could really use some help spotting my error.
