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/* $OpenBSD: pthread_rwlock.c,v 1.3 2014/05/20 01:25:24 guenther Exp $ */
/* PUBLIC DOMAIN Feb 2012 <guenther@openbsd.org> */
#include <sys/types.h>
#include <assert.h>
#include <err.h>
#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
/*
* Set up an rwlock with a few reader threads, start a writer blocking,
* then let go the reader threads one by one. Verify that the writer
* thread gets, gets out, and then the rwlock can be locked for reading
* again.
*/
pthread_rwlock_t rw;
pthread_mutex_t m;
pthread_cond_t c;
enum
{
UNLOCKED,
NUM_READERS = 6,
WRITE_STARTED,
WRITE,
WRITE_DONE,
} state;
time_t write_started;
static void *
reader(void *arg)
{
int me = *(int *)arg;
int diff;
pthread_mutex_lock(&m);
assert(state < NUM_READERS);
pthread_rwlock_rdlock(&rw);
state++;
printf("reader %d locked, state = %d\n", me, state);
pthread_cond_broadcast(&c);
pthread_mutex_unlock(&m);
pthread_mutex_lock(&m);
while (state < WRITE_STARTED) {
pthread_cond_wait(&c, &m);
printf("reader %d woken, state = %d\n", me, state);
}
diff = difftime(time(NULL), write_started);
if (diff < 2)
sleep(3 - diff);
pthread_rwlock_unlock(&rw);
printf("reader %d unlocked\n", me);
sleep(1);
pthread_mutex_unlock(&m);
pthread_mutex_lock(&m);
while (state >= WRITE_STARTED) {
pthread_cond_wait(&c, &m);
printf("reader %d woken, state = %d\n", me, state);
}
state++;
printf("reader %d trying again (%d)\n", me, state);
pthread_rwlock_rdlock(&rw);
printf("reader %d locked again (%d)\n", me, state);
pthread_cond_broadcast(&c);
while (state != NUM_READERS) {
pthread_cond_wait(&c, &m);
printf("reader %d woken, state = %d\n", me, state);
}
pthread_mutex_unlock(&m);
pthread_rwlock_unlock(&rw);
printf("reader %d exiting\n", me);
return NULL;
}
static void *
writer(void *arg)
{
pthread_mutex_lock(&m);
printf("writer started, state = %d\n", state);
while (state != NUM_READERS) {
pthread_cond_wait(&c, &m);
printf("writer woken, state = %d\n", state);
}
state = WRITE_STARTED;
printf("writer starting\n");
write_started = time(NULL);
pthread_cond_broadcast(&c);
pthread_mutex_unlock(&m);
pthread_rwlock_wrlock(&rw);
printf("writer locked\n");
pthread_mutex_lock(&m);
state = WRITE;
pthread_cond_broadcast(&c);
while (state == WRITE)
pthread_cond_wait(&c, &m);
printf("writer unlocking\n");
pthread_rwlock_unlock(&rw);
state = UNLOCKED;
pthread_cond_broadcast(&c);
pthread_mutex_unlock(&m);
printf("writer exiting\n");
return NULL;
}
int
main(void)
{
pthread_t tr[NUM_READERS], tw;
int ids[NUM_READERS], i, r;
pthread_rwlock_init(&rw, NULL);
pthread_mutex_init(&m, NULL);
pthread_cond_init(&c, NULL);
state = UNLOCKED;
for (i = 0; i < NUM_READERS; i++) {
ids[i] = i;
if ((r = pthread_create(&tr[i], NULL, reader, &ids[i])))
errc(1, r, "create %d", i);
}
if ((r = pthread_create(&tw, NULL, writer, NULL)))
errc(1, r, "create writer");
pthread_mutex_lock(&m);
while (state != WRITE)
pthread_cond_wait(&c, &m);
state = WRITE_DONE;
pthread_mutex_unlock(&m);
pthread_cond_broadcast(&c);
pthread_join(tw, NULL);
for (i = 0; i < NUM_READERS; i++)
pthread_join(tr[i], NULL);
return 0;
}
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