/*! Simple example with read/write locks */

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>

/*--- macro that simplifies handling errors with function calls ------------- */
#define ACT_WARN	0
#define ACT_STOP	1
#define CALL(ACT,FUNC,...)                                \
do {                                                      \
    if ( FUNC ( __VA_ARGS__ ) ) {                         \
        perror ( #FUNC );                                 \
        if ( ACT == ACT_STOP )                            \
            exit (1);                                     \
    }                                                     \
} while(0)

/* for example, instead of:
 *    if ( pthread_create ( &t1, NULL, worker, (void *) p ) ) {
 *        perror ( "pthread_create" );
 *        exit ( -1 );
 *    }
 * use:
 *    CALL ( ACT_STOP, pthread_create, &t1, NULL, worker, (void *) 1 );
 *
 *--------------------------------------------------------------------------- */

#define THREADS  6
static pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;

static void *worker ( void *p )
{
	long id = (long) p;
	struct timespec t = { 2, 0 };

	if ( id & 1 ) {
		printf ( "[%ld] Reader started\n", id );
		CALL ( ACT_STOP, pthread_rwlock_rdlock, &rwlock );
		printf ( "[%ld] Read lock acquired\n", id );
	}
	else {
		printf ( "[%ld] Writter started\n", id );
		CALL ( ACT_STOP, pthread_rwlock_wrlock, &rwlock );
		printf ( "[%ld] Write lock acquired\n", id );
	}

	CALL ( ACT_WARN, nanosleep, &t, NULL );

	if ( id & 1 )
		printf ( "[%ld] Releasing read lock\n", id );
	else
		printf ( "[%ld] Releasing write lock\n", id );
	CALL ( ACT_STOP, pthread_rwlock_unlock, &rwlock );

	return NULL;
}

int main ()
{
	long i;
	pthread_t thr[THREADS];
	struct timespec t = { 1, 0 };

	for ( i = 0; i < THREADS; i++ ) {
		CALL ( ACT_STOP, pthread_create, &thr[i], NULL, worker, (void *) i+1 );
		t.tv_sec = i;
		CALL ( ACT_WARN, nanosleep, &t, NULL );
	}

	for ( i = 0; i < THREADS; i++ )
		CALL ( ACT_WARN, pthread_join, thr[i], NULL );

	return 0;
}

/* Example run:
$ gcc pthread_rwlock.c -pthread -Wall
$ ./a.out
[1] Reader started
[1] Read lock acquired
[2] Writter started
[3] Reader started
[3] Read lock acquired
[1] Releasing read lock
[3] Releasing read lock
[2] Write lock acquired
[4] Writter started
[2] Releasing write lock
[4] Write lock acquired
[5] Reader started
[4] Releasing write lock
[5] Read lock acquired
[5] Releasing read lock
[6] Writter started
[6] Write lock acquired
[6] Releasing write lock
$
*/