pthread reader/write locks question

[beheadedmyway]

I'm just messing around with mutex and rwlock in pthreads. I wrote a quick little sample. but what's interesting is that the rwlock behaves much differently.

#include <stdlib.h>
#include <stdbool.h>
#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

int kill = 0;
char * s = NULL;
bool userw = false;
pthread_t tim;
pthread_t th1;
pthread_t th2;
pthread_mutex_t klock = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t slock = PTHREAD_MUTEX_INITIALIZER;
pthread_rwlock_t slock2 = PTHREAD_RWLOCK_INITIALIZER;

void * thfn1(void * args) {
	printf("thfn1\n");
	bool ext = false;
	while(1) {
		pthread_mutex_lock(&klock);
		if(kill == 1) ext = true;
		pthread_mutex_unlock(&klock);
		if(ext == true) break;
		if(userw) pthread_rwlock_wrlock(&slock2);
		else pthread_mutex_lock(&slock);
		if(s) free(s);
		s = strdup("bbb");
		printf("%s\n",s);
		sleep(arc4random() % 5);
		printf("th1 awake\n");
		if(userw) pthread_rwlock_unlock(&slock2);
		else pthread_mutex_unlock(&slock);
	}
	printf("thread1 finished\n");
	return NULL;
}

void * thfn2(void * args) {
	printf("thfn2\n");
	bool ext = false;
	while(1) {
		pthread_mutex_lock(&klock);
		if(kill == 1) ext = true;
		pthread_mutex_unlock(&klock);
		if(ext == true) break;
		if(userw) pthread_rwlock_wrlock(&slock2);
		else pthread_mutex_lock(&slock);
		if(s) free(s);
		s = strdup("ccc");
		printf("%s\n",s);
		sleep(arc4random() % 5);
		printf("th2 awake\n");
		if(userw) pthread_rwlock_unlock(&slock2);
		else pthread_mutex_unlock(&slock);
	}
	printf("thread 2 finished\n");
	return NULL;
}

void * timfn(void * args) {
	int activetime = *(int *)args;
	sleep(activetime);
	pthread_mutex_lock(&klock);
	kill = 1;
	pthread_mutex_unlock(&klock);
	return NULL;
}

int main(int argc, char ** argv) {
	int activetime = 45; //seconds
	pthread_create(&th1,NULL,thfn1,NULL);
	pthread_create(&th2,NULL,thfn2,NULL);
	pthread_create(&tim,NULL,timfn,&activetime);
	pthread_join(th1,NULL);
	pthread_join(th2,NULL);
	pthread_join(tim,NULL);
	return 0;
}

When this is run with read/write locks on, only the first thread ever acquires the write lock. But with read/write lock turned off, the threads will alternate access to the "s" variable. Note that it's not meant to be a 1to1 alternation.

Any ideas why the second thread never acquires the write lock?

Thanks!

[beheadedmyway]

nvmnd, I think everything was working as expected, I updated the print statements to help be more identifiable and everything is working as I thought it would...

#include <stdlib.h>
#include <stdbool.h>
#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

int kill = 0;
bool userw = true;
pthread_t tim;
pthread_t th1;
pthread_t th2;
pthread_mutex_t klock = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t slock = PTHREAD_MUTEX_INITIALIZER;
pthread_rwlock_t slock2 = PTHREAD_RWLOCK_INITIALIZER;

bool shouldkill() {
	bool ret = false;
	pthread_mutex_lock(&klock);
	if(kill == 1) ret = true;
	pthread_mutex_unlock(&klock);
	return ret;
}

void * thfn2(void * args) {
	printf("entered thfn2\n");
	int res = 0;
	while(1) {
		if(shouldkill())break;
		printf("th2: lock\n");
		if(userw) res = pthread_rwlock_wrlock(&slock2);
		else res = pthread_mutex_lock(&slock);
		printf("th2: locked\n");
		sleep(arc4random() % 5);
		if(userw) res = pthread_rwlock_unlock(&slock2);
		else res = pthread_mutex_unlock(&slock);
		printf("th2: unlocked\n");
	}
	printf("thread2 finished\n");
	return NULL;
}

void * thfn3(void * args) {
	printf("entered thfn3\n");
	int res = 0;
	while(1) {
		if(shouldkill())break;
		printf("th3: lock\n");
		if(userw) res = pthread_rwlock_wrlock(&slock2);
		else res = pthread_mutex_lock(&slock);
		printf("th3: locked\n");
		sleep(arc4random() % 5);
		if(userw) pthread_rwlock_unlock(&slock2);
		else pthread_mutex_unlock(&slock);
		printf("th3 unlocked\n");
	}
	printf("thread3 finished\n");
	return NULL;
}

void * timfn(void * args) {
	int activetime = *(int *)args;
	sleep(activetime);
	pthread_mutex_lock(&klock);
	kill = 1;
	pthread_mutex_unlock(&klock);
	return NULL;
}

int main(int argc, char ** argv) {
	int activetime = 15; //seconds
	printf("th1 entered\n");
	pthread_create(&th1,NULL,thfn2,NULL);
	pthread_create(&th2,NULL,thfn3,NULL);
	pthread_create(&tim,NULL,timfn,&activetime);
	pthread_join(th1,NULL);
	pthread_join(th2,NULL);
	pthread_join(tim,NULL);
	printf("th1 finished\n");
	return 0;
}