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/*
 * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, you can obtain one at https://mozilla.org/MPL/2.0/.
 *
 * See the COPYRIGHT file distributed with this work for additional
 * information regarding copyright ownership.
 */

#ifndef ISC_REFCOUNT_H
#define ISC_REFCOUNT_H 1

#include <inttypes.h>

#include <isc/assertions.h>
#include <isc/atomic.h>
#include <isc/error.h>
#include <isc/lang.h>
#include <isc/mutex.h>
#include <isc/platform.h>
#include <isc/types.h>

#if defined(ISC_PLATFORM_HAVESTDATOMIC)
#if defined (__cplusplus)
#include <isc/stdatomic.h>
#else
#include <stdatomic.h>
#endif
#endif

/*! \file isc/refcount.h
 * \brief Implements a locked reference counter.
 *
 * These functions may actually be
 * implemented using macros, and implementations of these macros are below.
 * The isc_refcount_t type should not be accessed directly, as its contents
 * depend on the implementation.
 */

ISC_LANG_BEGINDECLS

/*
 * Function prototypes
 */

/*
 * isc_result_t
 * isc_refcount_init(isc_refcount_t *ref, unsigned int n);
 *
 * Initialize the reference counter.  There will be 'n' initial references.
 *
 * Requires:
 *	ref != NULL
 */

/*
 * void
 * isc_refcount_destroy(isc_refcount_t *ref);
 *
 * Destroys a reference counter.
 *
 * Requires:
 *	ref != NULL
 *	The number of references is 0.
 */

/*
 * void
 * isc_refcount_increment(isc_refcount_t *ref, unsigned int *targetp);
 * isc_refcount_increment0(isc_refcount_t *ref, unsigned int *targetp);
 *
 * Increments the reference count, returning the new value in targetp if it's
 * not NULL.  The reference counter typically begins with the initial counter
 * of 1, and will be destroyed once the counter reaches 0.  Thus,
 * isc_refcount_increment() additionally requires the previous counter be
 * larger than 0 so that an error which violates the usage can be easily
 * caught.  isc_refcount_increment0() does not have this restriction.
 *
 * Requires:
 *	ref != NULL.
 */

/*
 * void
 * isc_refcount_decrement(isc_refcount_t *ref, unsigned int *targetp);
 *
 * Decrements the reference count,  returning the new value in targetp if it's
 * not NULL.
 *
 * Requires:
 *	ref != NULL.
 */

/*
 * Sample implementations
 */
#ifdef ISC_PLATFORM_USETHREADS

#if defined(ISC_PLATFORM_HAVESTDATOMIC)
# define ISC_REFCOUNT_HAVEATOMIC 1
# define ISC_REFCOUNT_HAVESTDATOMIC 1
#else /* defined(ISC_PLATFORM_HAVESTDATOMIC) */
# if defined(ISC_PLATFORM_HAVEXADD)
#  define ISC_REFCOUNT_HAVEATOMIC 1
# endif /* defined(ISC_PLATFORM_HAVEXADD */
#endif /* !defined(ISC_REFCOUNT_HAVEATOMIC) */

#if defined(ISC_REFCOUNT_HAVEATOMIC)

typedef struct isc_refcount {
#if defined(ISC_REFCOUNT_HAVESTDATOMIC)
	atomic_int_fast32_t refs;
#else
	int32_t refs;
#endif
} isc_refcount_t;

#if defined(ISC_REFCOUNT_HAVESTDATOMIC)

#define isc_refcount_current(rp)					\
	((unsigned int)(atomic_load_explicit(&(rp)->refs,		\
					     memory_order_acquire)))
#define isc_refcount_destroy(rp) ISC_REQUIRE(isc_refcount_current(rp) == 0)

#define isc_refcount_increment0(rp, tp)				\
	do {							\
		unsigned int *_tmp = (unsigned int *)(tp);	\
		int32_t prev;				\
		prev = atomic_fetch_add_explicit		\
			(&(rp)->refs, 1, memory_order_relaxed); \
		if (_tmp != NULL)				\
			*_tmp = prev + 1;			\
	} while (0)

#define isc_refcount_increment(rp, tp)				\
	do {							\
		unsigned int *_tmp = (unsigned int *)(tp);	\
		int32_t prev;				\
		prev = atomic_fetch_add_explicit		\
			(&(rp)->refs, 1, memory_order_relaxed); \
		ISC_REQUIRE(prev > 0);				\
		if (_tmp != NULL)				\
			*_tmp = prev + 1;			\
	} while (0)

#define isc_refcount_decrement(rp, tp)				\
	do {							\
		unsigned int *_tmp = (unsigned int *)(tp);	\
		int32_t prev;				\
		prev = atomic_fetch_sub_explicit		\
			(&(rp)->refs, 1, memory_order_acq_rel); \
		ISC_REQUIRE(prev > 0);				\
		if (_tmp != NULL)				\
			*_tmp = prev - 1;			\
	} while (0)

#else /* defined(ISC_REFCOUNT_HAVESTDATOMIC) */

#define isc_refcount_current(rp)				\
	((unsigned int)(isc_atomic_xadd(&(rp)->refs, 0)))
#define isc_refcount_destroy(rp) ISC_REQUIRE(isc_refcount_current(rp) == 0)

#define isc_refcount_increment0(rp, tp)				\
	do {							\
		unsigned int *_tmp = (unsigned int *)(tp);	\
		int32_t prev;				\
		prev = isc_atomic_xadd(&(rp)->refs, 1);		\
		if (_tmp != NULL)				\
			*_tmp = prev + 1;			\
	} while (0)

#define isc_refcount_increment(rp, tp)				\
	do {							\
		unsigned int *_tmp = (unsigned int *)(tp);	\
		int32_t prev;				\
		prev = isc_atomic_xadd(&(rp)->refs, 1);		\
		ISC_REQUIRE(prev > 0);				\
		if (_tmp != NULL)				\
			*_tmp = prev + 1;			\
	} while (0)

#define isc_refcount_decrement(rp, tp)				\
	do {							\
		unsigned int *_tmp = (unsigned int *)(tp);	\
		int32_t prev;				\
		prev = isc_atomic_xadd(&(rp)->refs, -1);	\
		ISC_REQUIRE(prev > 0);				\
		if (_tmp != NULL)				\
			*_tmp = prev - 1;			\
	} while (0)

#endif /* defined(ISC_REFCOUNT_HAVESTDATOMIC) */

#else /* defined(ISC_REFCOUNT_HAVEATOMIC) */

typedef struct isc_refcount {
	int refs;
	isc_mutex_t lock;
} isc_refcount_t;

/*% Destroys a reference counter. */
#define isc_refcount_destroy(rp)					\
	do {								\
		isc_result_t _result;					\
		ISC_REQUIRE((rp)->refs == 0);				\
		_result = isc_mutex_destroy(&(rp)->lock);		\
		ISC_ERROR_RUNTIMECHECK(_result == ISC_R_SUCCESS);	\
	} while (0)

unsigned int isc_refcount_current(isc_refcount_t *rp);

/*%
 * Increments the reference count, returning the new value in
 * 'tp' if it's not NULL.
 */
#define isc_refcount_increment0(rp, tp)					\
	do {								\
		isc_result_t _result;					\
		unsigned int *_tmp = (unsigned int *)(tp);		\
		_result = isc_mutex_lock(&(rp)->lock);			\
		ISC_ERROR_RUNTIMECHECK(_result == ISC_R_SUCCESS);	\
		++((rp)->refs);						\
		if (_tmp != NULL)					\
			*_tmp = ((rp)->refs);				\
		_result = isc_mutex_unlock(&(rp)->lock);		\
		ISC_ERROR_RUNTIMECHECK(_result == ISC_R_SUCCESS);	\
	} while (0)

#define isc_refcount_increment(rp, tp)					\
	do {								\
		isc_result_t _result;					\
		unsigned int *_tmp = (unsigned int *)(tp);		\
		_result = isc_mutex_lock(&(rp)->lock);			\
		ISC_ERROR_RUNTIMECHECK(_result == ISC_R_SUCCESS);	\
		ISC_REQUIRE((rp)->refs > 0);				\
		++((rp)->refs);						\
		if (_tmp != NULL)					\
			*_tmp = ((rp)->refs);				\
		_result = isc_mutex_unlock(&(rp)->lock);		\
		ISC_ERROR_RUNTIMECHECK(_result == ISC_R_SUCCESS);	\
	} while (0)

/*%
 * Decrements the reference count, returning the new value in 'tp'
 * if it's not NULL.
 */
#define isc_refcount_decrement(rp, tp)					\
	do {								\
		isc_result_t _result;					\
		unsigned int *_tmp = (unsigned int *)(tp);		\
		_result = isc_mutex_lock(&(rp)->lock);			\
		ISC_ERROR_RUNTIMECHECK(_result == ISC_R_SUCCESS);	\
		ISC_REQUIRE((rp)->refs > 0);				\
		--((rp)->refs);						\
		if (_tmp != NULL)					\
			*_tmp = ((rp)->refs);				\
		_result = isc_mutex_unlock(&(rp)->lock);		\
		ISC_ERROR_RUNTIMECHECK(_result == ISC_R_SUCCESS);	\
	} while (0)

#endif /* defined(ISC_REFCOUNT_ATOMIC) */

#else  /* ISC_PLATFORM_USETHREADS */

typedef struct isc_refcount {
	int refs;
} isc_refcount_t;

#define isc_refcount_destroy(rp) ISC_REQUIRE((rp)->refs == 0)
#define isc_refcount_current(rp) ((unsigned int)((rp)->refs))

#define isc_refcount_increment0(rp, tp)					\
	do {								\
		unsigned int *_tmp = (unsigned int *)(tp);		\
		int _n = ++(rp)->refs;					\
		if (_tmp != NULL)					\
			*_tmp = _n;					\
	} while (0)

#define isc_refcount_increment(rp, tp)					\
	do {								\
		unsigned int *_tmp = (unsigned int *)(tp);		\
		int _n;							\
		ISC_REQUIRE((rp)->refs > 0);				\
		_n = ++(rp)->refs;					\
		if (_tmp != NULL)					\
			*_tmp = _n;					\
	} while (0)

#define isc_refcount_decrement(rp, tp)					\
	do {								\
		unsigned int *_tmp = (unsigned int *)(tp);		\
		int _n;							\
		ISC_REQUIRE((rp)->refs > 0);				\
		_n = --(rp)->refs;					\
		if (_tmp != NULL)					\
			*_tmp = _n;					\
	} while (0)

#endif /* ISC_PLATFORM_USETHREADS */

isc_result_t
isc_refcount_init(isc_refcount_t *ref, unsigned int n);

ISC_LANG_ENDDECLS

#endif /* ISC_REFCOUNT_H */

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✏️ refcount.h