Added Reader/Writer lock code from publicily available examples for

Nichols, Butler, Farrell's book "Pthreads Programming" (O'Reilly).

libraries/liblthread/rdwr.c

+/********************************************************
+ * An example source module to accompany...
+ *
+ * "Using POSIX Threads: Programming with Pthreads"
+ *		 by Brad nichols, Dick Buttlar, Jackie Farrell
+ *		 O'Reilly & Associates, Inc.
+ *
+ ********************************************************
+ * rdwr.c --
+ * 
+ * Library of functions implementing reader/writer locks
+ */
+#include <pthread.h>
+#include "rdwr.h"
+
+int pthread_rdwr_init_np(pthread_rdwr_t *rdwrp, pthread_rdwrattr_t *attrp)
+{
+	rdwrp->readers_reading = 0;
+	rdwrp->writer_writing = 0;
+	pthread_mutex_init(&(rdwrp->mutex), NULL);
+	pthread_cond_init(&(rdwrp->lock_free), NULL);
+	return 0;
+}
+
+int pthread_rdwr_rlock_np(pthread_rdwr_t *rdwrp){
+	pthread_mutex_lock(&(rdwrp->mutex));
+	while(rdwrp->writer_writing) {
+		pthread_cond_wait(&(rdwrp->lock_free), &(rdwrp->mutex));
+	}
+	rdwrp->readers_reading++;
+	pthread_mutex_unlock(&(rdwrp->mutex));
+	return 0;
+}
+
+int pthread_rdwr_runlock_np(pthread_rdwr_t *rdwrp)
+{
+	pthread_mutex_lock(&(rdwrp->mutex));
+	if (rdwrp->readers_reading == 0) {
+		pthread_mutex_unlock(&(rdwrp->mutex));
+		return -1;
+	}
+	else {
+		rdwrp->readers_reading--;
+		if (rdwrp->readers_reading == 0) {
+			pthread_cond_signal(&(rdwrp->lock_free));
+		}
+		pthread_mutex_unlock(&(rdwrp->mutex));
+		return 0;
+	}
+}
+
+int pthread_rdwr_wlock_np(pthread_rdwr_t *rdwrp)
+{
+	pthread_mutex_lock(&(rdwrp->mutex));
+	while(rdwrp->writer_writing || rdwrp->readers_reading) {
+		pthread_cond_wait(&(rdwrp->lock_free), &(rdwrp->mutex));
+	}
+	rdwrp->writer_writing++;
+	pthread_mutex_unlock(&(rdwrp->mutex));
+	return 0;
+}
+
+int pthread_rdwr_wunlock_np(pthread_rdwr_t *rdwrp)
+{
+	pthread_mutex_lock(&(rdwrp->mutex));
+	if (rdwrp->writer_writing == 0) {
+		pthread_mutex_unlock(&(rdwrp->mutex));
+		return -1;
+	}
+	else {
+		rdwrp->writer_writing = 0;
+		pthread_cond_broadcast(&(rdwrp->lock_free));
+		pthread_mutex_unlock(&(rdwrp->mutex));
+		return 0;
+	}
+}

libraries/liblthread/rdwr.h

+/********************************************************
+ * An example source module to accompany...
+ *
+ * "Using POSIX Threads: Programming with Pthreads"
+ *     by Brad nichols, Dick Buttlar, Jackie Farrell
+ *     O'Reilly & Associates, Inc.
+ *
+ ********************************************************
+ * rdwr.h --
+ * 
+ * Include file for reader/writer locks
+ */
+typedef struct rdwr_var {
+	int readers_reading;
+	int writer_writing;
+	pthread_mutex_t mutex;
+	pthread_cond_t lock_free;
+} pthread_rdwr_t;
+
+typedef void * pthread_rdwrattr_t;
+
+#define pthread_rdwrattr_default NULL;
+
+int pthread_rdwr_init_np(pthread_rdwr_t *rdwrp, pthread_rdwrattr_t *attrp);
+int pthread_rdwr_rlock_np(pthread_rdwr_t *rdwrp);
+int pthread_rdwr_runlock_np(pthread_rdwr_t *rdwrp);
+int pthread_rdwr_wlock_np(pthread_rdwr_t *rdwrp);
+int pthread_rdwr_wunlock_np(pthread_rdwr_t *rdwrp);