#include <stdlib.h>#include <pthread.h>#define NHASH 29#define HASH(fp) (((unsigned long)fp)%NHASH)struct foo *fh[NHASH];pthread_mutex_t hashlock = PTHREAD_MUTEX_INITIALIZER;struct foo {	int             f_count;	pthread_mutex_t f_lock;	struct foo     *f_next; /* protected by hashlock */	int             f_id;	/* ... more stuff here ... */};struct foo *foo_alloc(void) /* allocate the object */{	struct foo	*fp;	int			idx;	if ((fp = malloc(sizeof(struct foo))) != NULL) {		fp->f_count = 1;		if (pthread_mutex_init(&fp->f_lock, NULL) != 0) {			free(fp);			return(NULL);		}		idx = HASH(fp);		pthread_mutex_lock(&hashlock);		fp->f_next = fh[idx];		fh[idx] = fp->f_next;		pthread_mutex_lock(&fp->f_lock);		pthread_mutex_unlock(&hashlock);		/* ... continue initialization ... */		pthread_mutex_unlock(&fp->f_lock);	}	return(fp);}voidfoo_hold(struct foo *fp) /* add a reference to the object */{	pthread_mutex_lock(&fp->f_lock);	fp->f_count++;	pthread_mutex_unlock(&fp->f_lock);}struct foo *foo_find(int id) /* find an existing object */{	struct foo	*fp;	int			idx;	idx = HASH(fp);	pthread_mutex_lock(&hashlock);	for (fp = fh[idx]; fp != NULL; fp = fp->f_next) {		if (fp->f_id == id) {			foo_hold(fp);			break;		}	}	pthread_mutex_unlock(&hashlock);	return(fp);}voidfoo_rele(struct foo *fp) /* release a reference to the object */{	struct foo	*tfp;	int			idx;	pthread_mutex_lock(&fp->f_lock);	if (fp->f_count == 1) { /* last reference */		pthread_mutex_unlock(&fp->f_lock);		pthread_mutex_lock(&hashlock);		pthread_mutex_lock(&fp->f_lock);		/* need to recheck the condition */		if (fp->f_count != 1) {			fp->f_count--;			pthread_mutex_unlock(&fp->f_lock);			pthread_mutex_unlock(&hashlock);			return;		}		/* remove from list */		idx = HASH(fp);		tfp = fh[idx];		if (tfp == fp) {			fh[idx] = fp->f_next;		} else {			while (tfp->f_next != fp)				tfp = tfp->f_next;			tfp->f_next = fp->f_next;		}		pthread_mutex_unlock(&hashlock);		pthread_mutex_unlock(&fp->f_lock);		pthread_mutex_destroy(&fp->f_lock);		free(fp);	} else {		fp->f_count--;		pthread_mutex_unlock(&fp->f_lock);	}}