buffer.c

/*
 * Copyright (c) 2002-2007 Niels Provos <provos@citi.umich.edu>
 * Copyright (c) 2007-2012 Niels Provos and Nick Mathewson
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */


#include "event2/event-config.h"

#ifdef WIN32
#include <winsock2.h>
#include <windows.h>
#include <io.h>
#endif

#ifdef _EVENT_HAVE_VASPRINTF
/* If we have vasprintf, we need to define this before we include stdio.h. */
#define _GNU_SOURCE
#endif

#include <sys/types.h>

#ifdef _EVENT_HAVE_SYS_TIME_H
#include <sys/time.h>
#endif

#ifdef _EVENT_HAVE_SYS_SOCKET_H
#include <sys/socket.h>
#endif

#ifdef _EVENT_HAVE_SYS_UIO_H
#include <sys/uio.h>
#endif

#ifdef _EVENT_HAVE_SYS_IOCTL_H
#include <sys/ioctl.h>
#endif

#ifdef _EVENT_HAVE_SYS_MMAN_H
#include <sys/mman.h>
#endif

#ifdef _EVENT_HAVE_SYS_SENDFILE_H
#include <sys/sendfile.h>
#endif

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#ifdef _EVENT_HAVE_STDARG_H
#include <stdarg.h>
#endif
#ifdef _EVENT_HAVE_UNISTD_H
#include <unistd.h>
#endif
#include <limits.h>

#include "event2/event.h"
#include "event2/buffer.h"
#include "event2/buffer_compat.h"
#include "event2/bufferevent.h"
#include "event2/bufferevent_compat.h"
#include "event2/bufferevent_struct.h"
#include "event2/thread.h"
#include "event2/event-config.h"
#include "log-internal.h"
#include "mm-internal.h"
#include "util-internal.h"
#include "evthread-internal.h"
#include "evbuffer-internal.h"
#include "bufferevent-internal.h"

/* some systems do not have MAP_FAILED */
#ifndef MAP_FAILED
#define MAP_FAILED	((void *)-1)
#endif

/* send file support */
#if defined(_EVENT_HAVE_SYS_SENDFILE_H) && defined(_EVENT_HAVE_SENDFILE) && defined(__linux__)
#define USE_SENDFILE		1
#define SENDFILE_IS_LINUX	1
#elif defined(_EVENT_HAVE_SENDFILE) && defined(__FreeBSD__)
#define USE_SENDFILE		1
#define SENDFILE_IS_FREEBSD	1
#elif defined(_EVENT_HAVE_SENDFILE) && defined(__APPLE__)
#define USE_SENDFILE		1
#define SENDFILE_IS_MACOSX	1
#elif defined(_EVENT_HAVE_SENDFILE) && defined(__sun__) && defined(__svr4__)
#define USE_SENDFILE		1
#define SENDFILE_IS_SOLARIS	1
#endif

#ifdef USE_SENDFILE
static int use_sendfile = 1;
#endif
#ifdef _EVENT_HAVE_MMAP
static int use_mmap = 1;
#endif


/* Mask of user-selectable callback flags. */
#define EVBUFFER_CB_USER_FLAGS	    0xffff
/* Mask of all internal-use-only flags. */
#define EVBUFFER_CB_INTERNAL_FLAGS  0xffff0000

/* Flag set if the callback is using the cb_obsolete function pointer  */
#define EVBUFFER_CB_OBSOLETE	       0x00040000

/* evbuffer_chain support */
#define CHAIN_SPACE_PTR(ch) ((ch)->buffer + (ch)->misalign + (ch)->off)
#define CHAIN_SPACE_LEN(ch) ((ch)->flags & EVBUFFER_IMMUTABLE ? \
	    0 : (ch)->buffer_len - ((ch)->misalign + (ch)->off))

#define CHAIN_PINNED(ch)  (((ch)->flags & EVBUFFER_MEM_PINNED_ANY) != 0)
#define CHAIN_PINNED_R(ch)  (((ch)->flags & EVBUFFER_MEM_PINNED_R) != 0)

static void evbuffer_chain_align(struct evbuffer_chain *chain);
static int evbuffer_chain_should_realign(struct evbuffer_chain *chain,
    size_t datalen);
static void evbuffer_deferred_callback(struct deferred_cb *cb, void *arg);
static int evbuffer_ptr_memcmp(const struct evbuffer *buf,
    const struct evbuffer_ptr *pos, const char *mem, size_t len);
static struct evbuffer_chain *evbuffer_expand_singlechain(struct evbuffer *buf,
    size_t datlen);

#ifdef WIN32
static int evbuffer_readfile(struct evbuffer *buf, evutil_socket_t fd,
    ev_ssize_t howmuch);
#else
#define evbuffer_readfile evbuffer_read
#endif

static struct evbuffer_chain *
evbuffer_chain_new(size_t size)
{
	struct evbuffer_chain *chain;
	size_t to_alloc;

    //申请缓冲区大小超过系统限制
	if (size > EVBUFFER_CHAIN_MAX - EVBUFFER_CHAIN_SIZE)
		return (NULL);

	size += EVBUFFER_CHAIN_SIZE;

	/* 开辟内存大小以MIN_BUFFER_SIZE成倍增大 */
	if (size < EVBUFFER_CHAIN_MAX / 2) {
		to_alloc = MIN_BUFFER_SIZE;
		while (to_alloc < size) {
			to_alloc <<= 1;
		}
	} else {
	    /*size大于EVBUFFER_CHAIN_MAX/2， 直接申请size大小，此时size已非常大。*/
		to_alloc = size;
	}

	/* 空间前面部分用作chain结构体，后面部分用作buffer，这样设计的原因是
	 * 可以将buffer指向内存映射等其他结构
	 */
	if ((chain = mm_malloc(to_alloc)) == NULL)
		return (NULL);

	memset(chain, 0, EVBUFFER_CHAIN_SIZE);

	chain->buffer_len = to_alloc - EVBUFFER_CHAIN_SIZE;

	chain->buffer = EVBUFFER_CHAIN_EXTRA(u_char, chain);

	return (chain);
}

static inline void
evbuffer_chain_free(struct evbuffer_chain *chain)
{
	if (CHAIN_PINNED(chain)) {
		chain->flags |= EVBUFFER_DANGLING;
		return;
	}
	if (chain->flags & (EVBUFFER_MMAP|EVBUFFER_SENDFILE|
		EVBUFFER_REFERENCE)) {
		if (chain->flags & EVBUFFER_REFERENCE) {
			struct evbuffer_chain_reference *info =
			    EVBUFFER_CHAIN_EXTRA(
				    struct evbuffer_chain_reference,
				    chain);
			if (info->cleanupfn)
				(*info->cleanupfn)(chain->buffer,
				    chain->buffer_len,
				    info->extra);
		}
#ifdef _EVENT_HAVE_MMAP
		if (chain->flags & EVBUFFER_MMAP) {
			struct evbuffer_chain_fd *info =
			    EVBUFFER_CHAIN_EXTRA(struct evbuffer_chain_fd,
				chain);
			if (munmap(chain->buffer, chain->buffer_len) == -1)
				event_warn("%s: munmap failed", __func__);
			if (close(info->fd) == -1)
				event_warn("%s: close(%d) failed",
				    __func__, info->fd);
		}
#endif
#ifdef USE_SENDFILE
		if (chain->flags & EVBUFFER_SENDFILE) {
			struct evbuffer_chain_fd *info =
			    EVBUFFER_CHAIN_EXTRA(struct evbuffer_chain_fd,
				chain);
			if (close(info->fd) == -1)
				event_warn("%s: close(%d) failed",
				    __func__, info->fd);
		}
#endif
	}

	mm_free(chain);
}

static void
evbuffer_free_all_chains(struct evbuffer_chain *chain)
{
	struct evbuffer_chain *next;
	for (; chain; chain = next) {
		next = chain->next;
		evbuffer_chain_free(chain);
	}
}

#ifndef NDEBUG
static int
evbuffer_chains_all_empty(struct evbuffer_chain *chain)
{
	for (; chain; chain = chain->next) {
		if (chain->off)
			return 0;
	}
	return 1;
}
#else
/* The definition is needed for EVUTIL_ASSERT, which uses sizeof to avoid
"unused variable" warnings. */
static inline int evbuffer_chains_all_empty(struct evbuffer_chain *chain) {
	return 1;
}
#endif

/* Free all trailing chains in 'buf' that are neither pinned nor empty, prior
 * to replacing them all with a new chain.  Return a pointer to the place
 * where the new chain will go.
 *
 * Internal; requires lock.  The caller must fix up buf->last and buf->first
 * as needed; they might have been freed.
 */
static struct evbuffer_chain **
evbuffer_free_trailing_empty_chains(struct evbuffer *buf)
{
	struct evbuffer_chain **ch = buf->last_with_datap;
	/* Find the first victim chain.  It might be *last_with_datap */
	while ((*ch) && ((*ch)->off != 0 || CHAIN_PINNED(*ch)))
		ch = &(*ch)->next;
	if (*ch) {
		EVUTIL_ASSERT(evbuffer_chains_all_empty(*ch));
		evbuffer_free_all_chains(*ch);
		*ch = NULL;
	}
	return ch;
}

/* 将chain插入到buf最后。如果*last_with_datap后存在未使用空数据节点，释放空节点。*/
static void
evbuffer_chain_insert(struct evbuffer *buf,
    struct evbuffer_chain *chain)
{
	ASSERT_EVBUFFER_LOCKED(buf);
	if (*buf->last_with_datap == NULL) {
		/* 第一次插入链表节点 */
		EVUTIL_ASSERT(buf->last_with_datap == &buf->first);
		EVUTIL_ASSERT(buf->first == NULL);
		buf->first = buf->last = chain;
	} else {
		struct evbuffer_chain **ch = buf->last_with_datap;
		/* Find the first victim chain.  It might be *last_with_datap */
		while ((*ch) && ((*ch)->off != 0 || CHAIN_PINNED(*ch)))
			ch = &(*ch)->next;
		if (*ch == NULL) {
			/* *last_with_datap已经是最后一个节点，直接插入 */
			buf->last->next = chain;
			if (chain->off)
				buf->last_with_datap = &buf->last->next;
		} else {
			/* *last_with_datap后面存在未使用的空节点，释放后续所有未使用空节点。 */
			EVUTIL_ASSERT(evbuffer_chains_all_empty(*ch));
			evbuffer_free_all_chains(*ch);
			*ch = chain;
		}
		buf->last = chain;
	}
	buf->total_len += chain->off;
}

static inline struct evbuffer_chain *
evbuffer_chain_insert_new(struct evbuffer *buf, size_t datlen)
{
	struct evbuffer_chain *chain;
	if ((chain = evbuffer_chain_new(datlen)) == NULL)
		return NULL;
	evbuffer_chain_insert(buf, chain);
	return chain;
}

void
_evbuffer_chain_pin(struct evbuffer_chain *chain, unsigned flag)
{
	EVUTIL_ASSERT((chain->flags & flag) == 0);
	chain->flags |= flag;
}

void
_evbuffer_chain_unpin(struct evbuffer_chain *chain, unsigned flag)
{
	EVUTIL_ASSERT((chain->flags & flag) != 0);
	chain->flags &= ~flag;
	if (chain->flags & EVBUFFER_DANGLING)
		evbuffer_chain_free(chain);
}

struct evbuffer *
evbuffer_new(void)
{
	struct evbuffer *buffer;

	buffer = mm_calloc(1, sizeof(struct evbuffer));
	if (buffer == NULL)
		return (NULL);

	TAILQ_INIT(&buffer->callbacks);
	buffer->refcnt = 1;
	buffer->last_with_datap = &buffer->first;

	return (buffer);
}

int
evbuffer_set_flags(struct evbuffer *buf, ev_uint64_t flags)
{
	EVBUFFER_LOCK(buf);
	buf->flags |= (ev_uint32_t)flags;
	EVBUFFER_UNLOCK(buf);
	return 0;
}

int
evbuffer_clear_flags(struct evbuffer *buf, ev_uint64_t flags)
{
	EVBUFFER_LOCK(buf);
	buf->flags &= ~(ev_uint32_t)flags;
	EVBUFFER_UNLOCK(buf);
	return 0;
}

void
_evbuffer_incref(struct evbuffer *buf)
{
	EVBUFFER_LOCK(buf);
	++buf->refcnt;
	EVBUFFER_UNLOCK(buf);
}

void
_evbuffer_incref_and_lock(struct evbuffer *buf)
{
	EVBUFFER_LOCK(buf);
	++buf->refcnt;
}

int
evbuffer_defer_callbacks(struct evbuffer *buffer, struct event_base *base)
{
	EVBUFFER_LOCK(buffer);
	buffer->cb_queue = event_base_get_deferred_cb_queue(base);
	buffer->deferred_cbs = 1;
	event_deferred_cb_init(&buffer->deferred,
	    evbuffer_deferred_callback, buffer);
	EVBUFFER_UNLOCK(buffer);
	return 0;
}

int
evbuffer_enable_locking(struct evbuffer *buf, void *lock)
{
#ifdef _EVENT_DISABLE_THREAD_SUPPORT
	return -1;
#else
	if (buf->lock)
		return -1;

	if (!lock) {
		EVTHREAD_ALLOC_LOCK(lock, EVTHREAD_LOCKTYPE_RECURSIVE);
		if (!lock)
			return -1;
		buf->lock = lock;
		buf->own_lock = 1;
	} else {
		buf->lock = lock;
		buf->own_lock = 0;
	}

	return 0;
#endif
}

void
evbuffer_set_parent(struct evbuffer *buf, struct bufferevent *bev)
{
	EVBUFFER_LOCK(buf);
	buf->parent = bev;
	EVBUFFER_UNLOCK(buf);
}

static void
evbuffer_run_callbacks(struct evbuffer *buffer, int running_deferred)
{
	struct evbuffer_cb_entry *cbent, *next;
	struct evbuffer_cb_info info;
	size_t new_size;
	ev_uint32_t mask, masked_val;
	int clear = 1;

	if (running_deferred) {
		mask = EVBUFFER_CB_NODEFER|EVBUFFER_CB_ENABLED;
		masked_val = EVBUFFER_CB_ENABLED;
	} else if (buffer->deferred_cbs) {
		mask = EVBUFFER_CB_NODEFER|EVBUFFER_CB_ENABLED;
		masked_val = EVBUFFER_CB_NODEFER|EVBUFFER_CB_ENABLED;
		/* Don't zero-out n_add/n_del, since the deferred callbacks
		   will want to see them. */
		clear = 0;
	} else {
		mask = EVBUFFER_CB_ENABLED;
		masked_val = EVBUFFER_CB_ENABLED;
	}

	ASSERT_EVBUFFER_LOCKED(buffer);

	if (TAILQ_EMPTY(&buffer->callbacks)) {
		buffer->n_add_for_cb = buffer->n_del_for_cb = 0;
		return;
	}
	if (buffer->n_add_for_cb == 0 && buffer->n_del_for_cb == 0)
		return;

	new_size = buffer->total_len;
	info.orig_size = new_size + buffer->n_del_for_cb - buffer->n_add_for_cb;
	info.n_added = buffer->n_add_for_cb;
	info.n_deleted = buffer->n_del_for_cb;
	if (clear) {
		buffer->n_add_for_cb = 0;
		buffer->n_del_for_cb = 0;
	}
	for (cbent = TAILQ_FIRST(&buffer->callbacks);
	     cbent != TAILQ_END(&buffer->callbacks);
	     cbent = next) {
		/* Get the 'next' pointer now in case this callback decides
		 * to remove itself or something. */
		next = TAILQ_NEXT(cbent, next);

		if ((cbent->flags & mask) != masked_val)
			continue;

		if ((cbent->flags & EVBUFFER_CB_OBSOLETE))
			cbent->cb.cb_obsolete(buffer,
			    info.orig_size, new_size, cbent->cbarg);
		else
			cbent->cb.cb_func(buffer, &info, cbent->cbarg);
	}
}

void
evbuffer_invoke_callbacks(struct evbuffer *buffer)
{
	if (TAILQ_EMPTY(&buffer->callbacks)) {
		buffer->n_add_for_cb = buffer->n_del_for_cb = 0;
		return;
	}

	if (buffer->deferred_cbs) {
		if (buffer->deferred.queued)
			return;
		_evbuffer_incref_and_lock(buffer);
		if (buffer->parent)
			bufferevent_incref(buffer->parent);
		EVBUFFER_UNLOCK(buffer);
		event_deferred_cb_schedule(buffer->cb_queue, &buffer->deferred);
	}

	evbuffer_run_callbacks(buffer, 0);
}

static void
evbuffer_deferred_callback(struct deferred_cb *cb, void *arg)
{
	struct bufferevent *parent = NULL;
	struct evbuffer *buffer = arg;

	/* XXXX It would be better to run these callbacks without holding the
	 * lock */
	EVBUFFER_LOCK(buffer);
	parent = buffer->parent;
	evbuffer_run_callbacks(buffer, 1);
	_evbuffer_decref_and_unlock(buffer);
	if (parent)
		bufferevent_decref(parent);
}

static void
evbuffer_remove_all_callbacks(struct evbuffer *buffer)
{
	struct evbuffer_cb_entry *cbent;

	while ((cbent = TAILQ_FIRST(&buffer->callbacks))) {
	    TAILQ_REMOVE(&buffer->callbacks, cbent, next);
	    mm_free(cbent);
	}
}

void
_evbuffer_decref_and_unlock(struct evbuffer *buffer)
{
	struct evbuffer_chain *chain, *next;
	ASSERT_EVBUFFER_LOCKED(buffer);

	EVUTIL_ASSERT(buffer->refcnt > 0);

	if (--buffer->refcnt > 0) {
		EVBUFFER_UNLOCK(buffer);
		return;
	}

	for (chain = buffer->first; chain != NULL; chain = next) {
		next = chain->next;
		evbuffer_chain_free(chain);
	}
	evbuffer_remove_all_callbacks(buffer);
	if (buffer->deferred_cbs)
		event_deferred_cb_cancel(buffer->cb_queue, &buffer->deferred);

	EVBUFFER_UNLOCK(buffer);
	if (buffer->own_lock)
		EVTHREAD_FREE_LOCK(buffer->lock, EVTHREAD_LOCKTYPE_RECURSIVE);
	mm_free(buffer);
}

void
evbuffer_free(struct evbuffer *buffer)
{
	EVBUFFER_LOCK(buffer);
	_evbuffer_decref_and_unlock(buffer);
}

void
evbuffer_lock(struct evbuffer *buf)
{
	EVBUFFER_LOCK(buf);
}

void
evbuffer_unlock(struct evbuffer *buf)
{
	EVBUFFER_UNLOCK(buf);
}

size_t
evbuffer_get_length(const struct evbuffer *buffer)
{
	size_t result;

	EVBUFFER_LOCK(buffer);

	result = (buffer->total_len);

	EVBUFFER_UNLOCK(buffer);

	return result;
}

size_t
evbuffer_get_contiguous_space(const struct evbuffer *buf)
{
	struct evbuffer_chain *chain;
	size_t result;

	EVBUFFER_LOCK(buf);
	chain = buf->first;
	result = (chain != NULL ? chain->off : 0);
	EVBUFFER_UNLOCK(buf);

	return result;
}

int
evbuffer_reserve_space(struct evbuffer *buf, ev_ssize_t size,
    struct evbuffer_iovec *vec, int n_vecs)
{
	struct evbuffer_chain *chain, **chainp;
	int n = -1;

	EVBUFFER_LOCK(buf);
	if (buf->freeze_end)
		goto done;
	if (n_vecs < 1)
		goto done;
	if (n_vecs == 1) {
		if ((chain = evbuffer_expand_singlechain(buf, size)) == NULL)
			goto done;

		vec[0].iov_base = CHAIN_SPACE_PTR(chain);
		vec[0].iov_len = (size_t) CHAIN_SPACE_LEN(chain);
		EVUTIL_ASSERT(size<0 || (size_t)vec[0].iov_len >= (size_t)size);
		n = 1;
	} else {
		if (_evbuffer_expand_fast(buf, size, n_vecs)<0)
			goto done;
		n = _evbuffer_read_setup_vecs(buf, size, vec, n_vecs,
				&chainp, 0);
	}

done:
	EVBUFFER_UNLOCK(buf);
	return n;

}

static int
advance_last_with_data(struct evbuffer *buf)
{
	int n = 0;
	ASSERT_EVBUFFER_LOCKED(buf);

	if (!*buf->last_with_datap)
		return 0;

	while ((*buf->last_with_datap)->next && (*buf->last_with_datap)->next->off) {
		buf->last_with_datap = &(*buf->last_with_datap)->next;
		++n;
	}
	return n;
}

int
evbuffer_commit_space(struct evbuffer *buf,
    struct evbuffer_iovec *vec, int n_vecs)
{
	struct evbuffer_chain *chain, **firstchainp, **chainp;
	int result = -1;
	size_t added = 0;
	int i;

	EVBUFFER_LOCK(buf);

	if (buf->freeze_end)
		goto done;
	if (n_vecs == 0) {
		result = 0;
		goto done;
	} else if (n_vecs == 1 &&
	    (buf->last && vec[0].iov_base == (void*)CHAIN_SPACE_PTR(buf->last))) {
		/* The user only got or used one chain; it might not
		 * be the first one with space in it. */
		if ((size_t)vec[0].iov_len > (size_t)CHAIN_SPACE_LEN(buf->last))
			goto done;
		buf->last->off += vec[0].iov_len;
		added = vec[0].iov_len;
		if (added)
			advance_last_with_data(buf);
		goto okay;
	}

	/* Advance 'firstchain' to the first chain with space in it. */
	firstchainp = buf->last_with_datap;
	if (!*firstchainp)
		goto done;
	if (CHAIN_SPACE_LEN(*firstchainp) == 0) {
		firstchainp = &(*firstchainp)->next;
	}

	chain = *firstchainp;
	/* pass 1: make sure that the pointers and lengths of vecs[] are in
	 * bounds before we try to commit anything. */
	for (i=0; i<n_vecs; ++i) {
		if (!chain)
			goto done;
		if (vec[i].iov_base != (void*)CHAIN_SPACE_PTR(chain) ||
		    (size_t)vec[i].iov_len > CHAIN_SPACE_LEN(chain))
			goto done;
		chain = chain->next;
	}
	/* pass 2: actually adjust all the chains. */
	chainp = firstchainp;
	for (i=0; i<n_vecs; ++i) {
		(*chainp)->off += vec[i].iov_len;
		added += vec[i].iov_len;
		if (vec[i].iov_len) {
			buf->last_with_datap = chainp;
		}
		chainp = &(*chainp)->next;
	}

okay:
	buf->total_len += added;
	buf->n_add_for_cb += added;
	result = 0;
	evbuffer_invoke_callbacks(buf);

done:
	EVBUFFER_UNLOCK(buf);
	return result;
}

static inline int
HAS_PINNED_R(struct evbuffer *buf)
{
	return (buf->last && CHAIN_PINNED_R(buf->last));
}

static inline void
ZERO_CHAIN(struct evbuffer *dst)
{
	ASSERT_EVBUFFER_LOCKED(dst);
	dst->first = NULL;
	dst->last = NULL;
	dst->last_with_datap = &(dst)->first;
	dst->total_len = 0;
}

/* Prepares the contents of src to be moved to another buffer by removing
 * read-pinned chains. The first pinned chain is saved in first, and the
 * last in last. If src has no read-pinned chains, first and last are set
 * to NULL. */
static int
PRESERVE_PINNED(struct evbuffer *src, struct evbuffer_chain **first,
		struct evbuffer_chain **last)
{
	struct evbuffer_chain *chain, **pinned;

	ASSERT_EVBUFFER_LOCKED(src);

	if (!HAS_PINNED_R(src)) {
		*first = *last = NULL;
		return 0;
	}

	pinned = src->last_with_datap;
	if (!CHAIN_PINNED_R(*pinned))
		pinned = &(*pinned)->next;
	EVUTIL_ASSERT(CHAIN_PINNED_R(*pinned));
	chain = *first = *pinned;
	*last = src->last;

	/* If there's data in the first pinned chain, we need to allocate
	 * a new chain and copy the data over. */
	if (chain->off) {
		struct evbuffer_chain *tmp;

		EVUTIL_ASSERT(pinned == src->last_with_datap);
		tmp = evbuffer_chain_new(chain->off);
		if (!tmp)
			return -1;
		memcpy(tmp->buffer, chain->buffer + chain->misalign,
			chain->off);
		tmp->off = chain->off;
		*src->last_with_datap = tmp;
		src->last = tmp;
		chain->misalign += chain->off;
		chain->off = 0;
	} else {
		src->last = *src->last_with_datap;
		*pinned = NULL;
	}

	return 0;
}

static inline void
RESTORE_PINNED(struct evbuffer *src, struct evbuffer_chain *pinned,
		struct evbuffer_chain *last)
{
	ASSERT_EVBUFFER_LOCKED(src);

	if (!pinned) {
		ZERO_CHAIN(src);
		return;
	}

	src->first = pinned;
	src->last = last;
	src->last_with_datap = &src->first;
	src->total_len = 0;
}

static inline void
COPY_CHAIN(struct evbuffer *dst, struct evbuffer *src)
{
	ASSERT_EVBUFFER_LOCKED(dst);
	ASSERT_EVBUFFER_LOCKED(src);
	dst->first = src->first;
	if (src->last_with_datap == &src->first)
		dst->last_with_datap = &dst->first;
	else
		dst->last_with_datap = src->last_with_datap;
	dst->last = src->last;
	dst->total_len = src->total_len;
}

static void
APPEND_CHAIN(struct evbuffer *dst, struct evbuffer *src)
{
	ASSERT_EVBUFFER_LOCKED(dst);
	ASSERT_EVBUFFER_LOCKED(src);
	dst->last->next = src->first;
	if (src->last_with_datap == &src->first)
		dst->last_with_datap = &dst->last->next;
	else
		dst->last_with_datap = src->last_with_datap;
	dst->last = src->last;
	dst->total_len += src->total_len;
}

static void
PREPEND_CHAIN(struct evbuffer *dst, struct evbuffer *src)
{
	ASSERT_EVBUFFER_LOCKED(dst);
	ASSERT_EVBUFFER_LOCKED(src);
	src->last->next = dst->first;
	dst->first = src->first;
	dst->total_len += src->total_len;
	if (*dst->last_with_datap == NULL) {
		if (src->last_with_datap == &(src)->first)
			dst->last_with_datap = &dst->first;
		else
			dst->last_with_datap = src->last_with_datap;
	} else if (dst->last_with_datap == &dst->first) {
		dst->last_with_datap = &src->last->next;
	}
}

int
evbuffer_add_buffer(struct evbuffer *outbuf, struct evbuffer *inbuf)
{
	struct evbuffer_chain *pinned, *last;
	size_t in_total_len, out_total_len;
	int result = 0;

	EVBUFFER_LOCK2(inbuf, outbuf);
	in_total_len = inbuf->total_len;
	out_total_len = outbuf->total_len;

	if (in_total_len == 0 || outbuf == inbuf)
		goto done;

	if (outbuf->freeze_end || inbuf->freeze_start) {
		result = -1;
		goto done;
	}

	if (PRESERVE_PINNED(inbuf, &pinned, &last) < 0) {
		result = -1;
		goto done;
	}

	if (out_total_len == 0) {
		/* There might be an empty chain at the start of outbuf; free
		 * it. */
		evbuffer_free_all_chains(outbuf->first);
		COPY_CHAIN(outbuf, inbuf);
	} else {
		APPEND_CHAIN(outbuf, inbuf);
	}

	RESTORE_PINNED(inbuf, pinned, last);

	inbuf->n_del_for_cb += in_total_len;
	outbuf->n_add_for_cb += in_total_len;

	evbuffer_invoke_callbacks(inbuf);
	evbuffer_invoke_callbacks(outbuf);

done:
	EVBUFFER_UNLOCK2(inbuf, outbuf);
	return result;
}

int
evbuffer_prepend_buffer(struct evbuffer *outbuf, struct evbuffer *inbuf)
{
	struct evbuffer_chain *pinned, *last;
	size_t in_total_len, out_total_len;
	int result = 0;

	EVBUFFER_LOCK2(inbuf, outbuf);

	in_total_len = inbuf->total_len;
	out_total_len = outbuf->total_len;

	if (!in_total_len || inbuf == outbuf)
		goto done;

	if (outbuf->freeze_start || inbuf->freeze_start) {
		result = -1;
		goto done;
	}

	if (PRESERVE_PINNED(inbuf, &pinned, &last) < 0) {
		result = -1;
		goto done;
	}

	if (out_total_len == 0) {
		/* There might be an empty chain at the start of outbuf; free
		 * it. */
		evbuffer_free_all_chains(outbuf->first);
		COPY_CHAIN(outbuf, inbuf);
	} else {
		PREPEND_CHAIN(outbuf, inbuf);
	}

	RESTORE_PINNED(inbuf, pinned, last);

	inbuf->n_del_for_cb += in_total_len;
	outbuf->n_add_for_cb += in_total_len;

	evbuffer_invoke_callbacks(inbuf);
	evbuffer_invoke_callbacks(outbuf);
done:
	EVBUFFER_UNLOCK2(inbuf, outbuf);
	return result;
}

//删除数据
int
evbuffer_drain(struct evbuffer *buf, size_t len)
{
	struct evbuffer_chain *chain, *next;
	size_t remaining, old_len;
	int result = 0;

	EVBUFFER_LOCK(buf);
	old_len = buf->total_len;

	if (old_len == 0)
		goto done;

	if (buf->freeze_start) {
		result = -1;
		goto done;
	}

	if (len >= old_len && !HAS_PINNED_R(buf)) {
		len = old_len;
		for (chain = buf->first; chain != NULL; chain = next) {
			next = chain->next;
			evbuffer_chain_free(chain);
		}

		ZERO_CHAIN(buf);
	} else {
		if (len >= old_len)
			len = old_len;

		buf->total_len -= len;
		remaining = len;
		for (chain = buf->first;
		     remaining >= chain->off;
		     chain = next) {
			next = chain->next;
			remaining -= chain->off;

			if (chain == *buf->last_with_datap) {
				buf->last_with_datap = &buf->first;
			}
			if (&chain->next == buf->last_with_datap)
				buf->last_with_datap = &buf->first;

			if (CHAIN_PINNED_R(chain)) {
				EVUTIL_ASSERT(remaining == 0);
				chain->misalign += chain->off;
				chain->off = 0;
				break;
			} else
				evbuffer_chain_free(chain);
		}

		buf->first = chain;
		if (chain) {
			EVUTIL_ASSERT(remaining <= chain->off);
			chain->misalign += remaining;
			chain->off -= remaining;
		}
	}

	buf->n_del_for_cb += len;
	/* buffer数据长度发生改变，执行设置的回调。 */
	evbuffer_invoke_callbacks(buf);

done:
	EVBUFFER_UNLOCK(buf);
	return result;
}

/* Reads data from an event buffer and drains the bytes read */
int
evbuffer_remove(struct evbuffer *buf, void *data_out, size_t datlen)
{
	ev_ssize_t n;
	EVBUFFER_LOCK(buf);
	n = evbuffer_copyout(buf, data_out, datlen);
	if (n > 0) {
		if (evbuffer_drain(buf, n)<0)
			n = -1;
	}
	EVBUFFER_UNLOCK(buf);
	return (int)n;
}

//读取数据，不改变evbuffer。
ev_ssize_t
evbuffer_copyout(struct evbuffer *buf, void *data_out, size_t datlen)
{
	/*XXX fails badly on sendfile case. */
	struct evbuffer_chain *chain;
	char *data = data_out;
	size_t nread;
	ev_ssize_t result = 0;

	EVBUFFER_LOCK(buf);

	chain = buf->first;

	if (datlen >= buf->total_len)
		datlen = buf->total_len;  /*最大能提供数据*/

	if (datlen == 0)
		goto done;

	if (buf->freeze_start) {
		result = -1;
		goto done;
	}

	nread = datlen;

	while (datlen && datlen >= chain->off) {
		memcpy(data, chain->buffer + chain->misalign, chain->off);
		data += chain->off;
		datlen -= chain->off;

		chain = chain->next;
		EVUTIL_ASSERT(chain || datlen==0);
	}

	if (datlen) {
		EVUTIL_ASSERT(chain);
		EVUTIL_ASSERT(datlen <= chain->off);
		memcpy(data, chain->buffer + chain->misalign, datlen);
	}

	result = nread;
done:
	EVBUFFER_UNLOCK(buf);
	return result;
}

/* reads data from the src buffer to the dst buffer, avoids memcpy as
 * possible. */
/*  XXXX should return ev_ssize_t */
int
evbuffer_remove_buffer(struct evbuffer *src, struct evbuffer *dst,
    size_t datlen)
{
	/*XXX We should have an option to force this to be zero-copy.*/

	/*XXX can fail badly on sendfile case. */
	struct evbuffer_chain *chain, *previous;
	size_t nread = 0;
	int result;

	EVBUFFER_LOCK2(src, dst);

	chain = previous = src->first;

	if (datlen == 0 || dst == src) {
		result = 0;
		goto done;
	}

	if (dst->freeze_end || src->freeze_start) {
		result = -1;
		goto done;
	}

	/* short-cut if there is no more data buffered */
	if (datlen >= src->total_len) {
		datlen = src->total_len;
		evbuffer_add_buffer(dst, src);
		result = (int)datlen; /*XXXX should return ev_ssize_t*/
		goto done;
	}

	/* removes chains if possible */
	while (chain->off <= datlen) {
		/* We can't remove the last with data from src unless we
		 * remove all chains, in which case we would have done the if
		 * block above */
		EVUTIL_ASSERT(chain != *src->last_with_datap);
		nread += chain->off;
		datlen -= chain->off;
		previous = chain;
		if (src->last_with_datap == &chain->next)
			src->last_with_datap = &src->first;
		chain = chain->next;
	}

	if (nread) {
		/* we can remove the chain */
		struct evbuffer_chain **chp;
		chp = evbuffer_free_trailing_empty_chains(dst);

		if (dst->first == NULL) {
			dst->first = src->first;
		} else {
			*chp = src->first;
		}
		dst->last = previous;
		previous->next = NULL;
		src->first = chain;
		advance_last_with_data(dst);

		dst->total_len += nread;
		dst->n_add_for_cb += nread;
	}

	/* we know that there is more data in the src buffer than
	 * we want to read, so we manually drain the chain */
	evbuffer_add(dst, chain->buffer + chain->misalign, datlen);
	chain->misalign += datlen;
	chain->off -= datlen;
	nread += datlen;

	/* You might think we would want to increment dst->n_add_for_cb
	 * here too.  But evbuffer_add above already took care of that.
	 */
	src->total_len -= nread;
	src->n_del_for_cb += nread;

	if (nread) {
		evbuffer_invoke_callbacks(dst);
		evbuffer_invoke_callbacks(src);
	}
	result = (int)nread;/*XXXX should change return type */

done:
	EVBUFFER_UNLOCK2(src, dst);
	return result;
}

unsigned char *
evbuffer_pullup(struct evbuffer *buf, ev_ssize_t size)
{
	struct evbuffer_chain *chain, *next, *tmp, *last_with_data;
	unsigned char *buffer, *result = NULL;
	ev_ssize_t remaining;
	int removed_last_with_data = 0;
	int removed_last_with_datap = 0;

	EVBUFFER_LOCK(buf);

	chain = buf->first;

	if (size < 0)
		size = buf->total_len;
	/* if size > buf->total_len, we cannot guarantee to the user that she
	 * is going to have a long enough buffer afterwards; so we return
	 * NULL */
	if (size == 0 || (size_t)size > buf->total_len)
		goto done;

	/* No need to pull up anything; the first size bytes are
	 * already here. */
	if (chain->off >= (size_t)size) {
		result = chain->buffer + chain->misalign;
		goto done;
	}

	/* Make sure that none of the chains we need to copy from is pinned. */
	remaining = size - chain->off;
	EVUTIL_ASSERT(remaining >= 0);
	for (tmp=chain->next; tmp; tmp=tmp->next) {
		if (CHAIN_PINNED(tmp))
			goto done;
		if (tmp->off >= (size_t)remaining)
			break;
		remaining -= tmp->off;
	}

	if (CHAIN_PINNED(chain)) {
		size_t old_off = chain->off;
		if (CHAIN_SPACE_LEN(chain) < size - chain->off) {
			/* not enough room at end of chunk. */
			goto done;
		}
		buffer = CHAIN_SPACE_PTR(chain);
		tmp = chain;
		tmp->off = size;
		size -= old_off;
		chain = chain->next;
	} else if (chain->buffer_len - chain->misalign >= (size_t)size) {
		/* already have enough space in the first chain */
		size_t old_off = chain->off;
		buffer = chain->buffer + chain->misalign + chain->off;
		tmp = chain;
		tmp->off = size;
		size -= old_off;
		chain = chain->next;
	} else {
		if ((tmp = evbuffer_chain_new(size)) == NULL) {
			event_warn("%s: out of memory", __func__);
			goto done;
		}
		buffer = tmp->buffer;
		tmp->off = size;
		buf->first = tmp;
	}

	/* TODO(niels): deal with buffers that point to NULL like sendfile */

	/* Copy and free every chunk that will be entirely pulled into tmp */
	last_with_data = *buf->last_with_datap;
	for (; chain != NULL && (size_t)size >= chain->off; chain = next) {
		next = chain->next;

		memcpy(buffer, chain->buffer + chain->misalign, chain->off);
		size -= chain->off;
		buffer += chain->off;
		if (chain == last_with_data)
			removed_last_with_data = 1;
		if (&chain->next == buf->last_with_datap)
			removed_last_with_datap = 1;

		evbuffer_chain_free(chain);
	}

	if (chain != NULL) {
		memcpy(buffer, chain->buffer + chain->misalign, size);
		chain->misalign += size;
		chain->off -= size;
	} else {
		buf->last = tmp;
	}

	tmp->next = chain;

	if (removed_last_with_data) {
		buf->last_with_datap = &buf->first;
	} else if (removed_last_with_datap) {
		if (buf->first->next && buf->first->next->off)
			buf->last_with_datap = &buf->first->next;
		else
			buf->last_with_datap = &buf->first;
	}

	result = (tmp->buffer + tmp->misalign);

done:
	EVBUFFER_UNLOCK(buf);
	return result;
}

/*
 * Reads a line terminated by either '\r\n', '\n\r' or '\r' or '\n'.
 * The returned buffer needs to be freed by the called.
 */
char *
evbuffer_readline(struct evbuffer *buffer)
{
	return evbuffer_readln(buffer, NULL, EVBUFFER_EOL_ANY);
}

static inline ev_ssize_t
evbuffer_strchr(struct evbuffer_ptr *it, const char chr)
{
	struct evbuffer_chain *chain = it->_internal.chain;
	size_t i = it->_internal.pos_in_chain;
	while (chain != NULL) {
		char *buffer = (char *)chain->buffer + chain->misalign;
		char *cp = memchr(buffer+i, chr, chain->off-i);
		if (cp) {
			it->_internal.chain = chain;
			it->_internal.pos_in_chain = cp - buffer;
			it->pos += (cp - buffer - i);
			return it->pos;
		}
		it->pos += chain->off - i;
		i = 0;
		chain = chain->next;
	}

	return (-1);
}

static inline char *
find_eol_char(char *s, size_t len)
{
#define CHUNK_SZ 128
	/* Lots of benchmarking found this approach to be faster in practice
	 * than doing two memchrs over the whole buffer, doin a memchr on each
	 * char of the buffer, or trying to emulate memchr by hand. */
	char *s_end, *cr, *lf;
	s_end = s+len;
	while (s < s_end) {
		size_t chunk = (s + CHUNK_SZ < s_end) ? CHUNK_SZ : (s_end - s);
		cr = memchr(s, '\r', chunk);
		lf = memchr(s, '\n', chunk);
		if (cr) {
			if (lf && lf < cr)
				return lf;
			return cr;
		} else if (lf) {
			return lf;
		}
		s += CHUNK_SZ;
	}

	return NULL;
#undef CHUNK_SZ
}

static ev_ssize_t
evbuffer_find_eol_char(struct evbuffer_ptr *it)
{
	struct evbuffer_chain *chain = it->_internal.chain;
	size_t i = it->_internal.pos_in_chain;
	while (chain != NULL) {
		char *buffer = (char *)chain->buffer + chain->misalign;
		char *cp = find_eol_char(buffer+i, chain->off-i);
		if (cp) {
			it->_internal.chain = chain;
			it->_internal.pos_in_chain = cp - buffer;
			it->pos += (cp - buffer) - i;
			return it->pos;
		}
		it->pos += chain->off - i;
		i = 0;
		chain = chain->next;
	}

	return (-1);
}

static inline int
evbuffer_strspn(
	struct evbuffer_ptr *ptr, const char *chrset)
{
	int count = 0;
	struct evbuffer_chain *chain = ptr->_internal.chain;
	size_t i = ptr->_internal.pos_in_chain;

	if (!chain)
		return -1;

	while (1) {
		char *buffer = (char *)chain->buffer + chain->misalign;
		for (; i < chain->off; ++i) {
			const char *p = chrset;
			while (*p) {
				if (buffer[i] == *p++)
					goto next;
			}
			ptr->_internal.chain = chain;
			ptr->_internal.pos_in_chain = i;
			ptr->pos += count;
			return count;
		next:
			++count;
		}
		i = 0;

		if (! chain->next) {
			ptr->_internal.chain = chain;
			ptr->_internal.pos_in_chain = i;
			ptr->pos += count;
			return count;
		}

		chain = chain->next;
	}
}


static inline char
evbuffer_getchr(struct evbuffer_ptr *it)
{
	struct evbuffer_chain *chain = it->_internal.chain;
	size_t off = it->_internal.pos_in_chain;

	return chain->buffer[chain->misalign + off];
}

struct evbuffer_ptr
evbuffer_search_eol(struct evbuffer *buffer,
    struct evbuffer_ptr *start, size_t *eol_len_out,
    enum evbuffer_eol_style eol_style)
{
	struct evbuffer_ptr it, it2;
	size_t extra_drain = 0;
	int ok = 0;

	EVBUFFER_LOCK(buffer);

	if (start) {
		memcpy(&it, start, sizeof(it));
	} else {
		it.pos = 0;
		it._internal.chain = buffer->first;
		it._internal.pos_in_chain = 0;
	}

	/* the eol_style determines our first stop character and how many
	 * characters we are going to drain afterwards. */
	switch (eol_style) {
	case EVBUFFER_EOL_ANY:
		if (evbuffer_find_eol_char(&it) < 0)
			goto done;
		memcpy(&it2, &it, sizeof(it));
		extra_drain = evbuffer_strspn(&it2, "\r\n");
		break;
	case EVBUFFER_EOL_CRLF_STRICT: {
		it = evbuffer_search(buffer, "\r\n", 2, &it);
		if (it.pos < 0)
			goto done;
		extra_drain = 2;
		break;
	}
	case EVBUFFER_EOL_CRLF:
		while (1) {
			if (evbuffer_find_eol_char(&it) < 0)
				goto done;
			if (evbuffer_getchr(&it) == '\n') {
				extra_drain = 1;
				break;
			} else if (!evbuffer_ptr_memcmp(
				    buffer, &it, "\r\n", 2)) {
				extra_drain = 2;
				break;
			} else {
				if (evbuffer_ptr_set(buffer, &it, 1,
					EVBUFFER_PTR_ADD)<0)
					goto done;
			}
		}
		break;
	case EVBUFFER_EOL_LF:
		if (evbuffer_strchr(&it, '\n') < 0)
			goto done;
		extra_drain = 1;
		break;
	default:
		goto done;
	}

	ok = 1;
done:
	EVBUFFER_UNLOCK(buffer);

	if (!ok) {
		it.pos = -1;
	}
	if (eol_len_out)
		*eol_len_out = extra_drain;

	return it;
}

char *
evbuffer_readln(struct evbuffer *buffer, size_t *n_read_out,
		enum evbuffer_eol_style eol_style)
{
	struct evbuffer_ptr it;
	char *line;
	size_t n_to_copy=0, extra_drain=0;
	char *result = NULL;

	EVBUFFER_LOCK(buffer);

	if (buffer->freeze_start) {
		goto done;
	}

	it = evbuffer_search_eol(buffer, NULL, &extra_drain, eol_style);
	if (it.pos < 0)
		goto done;
	n_to_copy = it.pos;

	if ((line = mm_malloc(n_to_copy+1)) == NULL) {
		event_warn("%s: out of memory", __func__);
		goto done;
	}

	evbuffer_remove(buffer, line, n_to_copy);
	line[n_to_copy] = '\0';

	evbuffer_drain(buffer, extra_drain);
	result = line;
done:
	EVBUFFER_UNLOCK(buffer);

	if (n_read_out)
		*n_read_out = result ? n_to_copy : 0;

	return result;
}

#define EVBUFFER_CHAIN_MAX_AUTO_SIZE 4096

/* 添加数据到evbuffer，evbuffer_add是copy一份数据，用户调用该函数后可以释放data_in。 */

int
evbuffer_add(struct evbuffer *buf, const void *data_in, size_t datlen)
{
	struct evbuffer_chain *chain, *tmp;
	const unsigned char *data = data_in;
	size_t remain, to_alloc;
	int result = -1;

	EVBUFFER_LOCK(buf);

    //buffer尾部被冻结，进制追加数据。
	if (buf->freeze_end) {
		goto done;
	}
	/* 追加数据和现有数据长度大于EV_SIZE_MAX，禁止追加。*/
	if (datlen > EV_SIZE_MAX - buf->total_len) {
		goto done;
	}

    //从最后buffer追加数据
	chain = buf->last;

	/* 第一次追加数据。 */
	if (chain == NULL) {
		chain = evbuffer_chain_new(datlen);
		if (!chain)
			goto done;
		evbuffer_chain_insert(buf, chain);
	}

	if ((chain->flags & EVBUFFER_IMMUTABLE) == 0) {
		/* buffer可写 */
		EVUTIL_ASSERT(chain->misalign >= 0 &&
		    (ev_uint64_t)chain->misalign <= EVBUFFER_CHAIN_MAX);
		//可写入的字节长度
		remain = chain->buffer_len - (size_t)chain->misalign - chain->off;
		if (remain >= datlen) {
			/* 最后buffer可以放下本次写入数据 */
			memcpy(chain->buffer + chain->misalign + chain->off,
			    data, datlen);
			chain->off += datlen;
			buf->total_len += datlen;
			buf->n_add_for_cb += datlen;
			goto out;
		} else if (!CHAIN_PINNED(chain) &&  /* 该buffer可以修改 */
		    evbuffer_chain_should_realign(chain, datlen)) {
			/* 通过把chain->misalign值为0后可以放下本次数据。 */
			evbuffer_chain_align(chain);

			memcpy(chain->buffer + chain->off, data, datlen);
			chain->off += datlen;
			buf->total_len += datlen;
			buf->n_add_for_cb += datlen;
			goto out;
		}
	} else {
		/* we cannot write any data to the last chain */
		remain = 0;
	}

	/* 最后一个evbuffer_chain只读或者放不下本次数据，新建一个evbuffer_chain。 */
	to_alloc = chain->buffer_len;
	/* 当最后evbuffer_chain的缓冲区小于等于2048时，那么新建的evbuffer_chain的 
	 * 大小将是最后一个节点缓冲区的2倍。
	 */
	if (to_alloc <= EVBUFFER_CHAIN_MAX_AUTO_SIZE/2)
		to_alloc <<= 1;
	/* 分配大小最终以插入数据大小为准 */
	if (datlen > to_alloc)
		to_alloc = datlen;
	tmp = evbuffer_chain_new(to_alloc);
	if (tmp == NULL)
		goto done;

    //链表最后节点还可以放下部分数据，先填满最后节点。
	if (remain) {
		memcpy(chain->buffer + chain->misalign + chain->off,
		    data, remain);
		chain->off += remain;
		buf->total_len += remain;
		buf->n_add_for_cb += remain;
	}

	data += remain;
	datlen -= remain;

    //复制剩余数据到新chain中
	memcpy(tmp->buffer, data, datlen);
	tmp->off = datlen;
	evbuffer_chain_insert(buf, tmp);
	buf->n_add_for_cb += datlen;

out:
	//成功添加数据，调用回调。
	evbuffer_invoke_callbacks(buf);
	result = 0;
done:
	EVBUFFER_UNLOCK(buf);
	return result;
}

//在evbuffer头部插入数据。
int
evbuffer_prepend(struct evbuffer *buf, const void *data, size_t datlen)
{
	struct evbuffer_chain *chain, *tmp;
	int result = -1;

	EVBUFFER_LOCK(buf);

    //缓冲区头部被冻结，禁止添加数据。
	if (buf->freeze_start) {
		goto done;
	}
	//添加数据后总数据长度大于EV_SIZE_MAX，禁止添加。
	if (datlen > EV_SIZE_MAX - buf->total_len) {
		goto done;
	}

	chain = buf->first;

    //链表目前还没有节点
	if (chain == NULL) {
		chain = evbuffer_chain_new(datlen);
		if (!chain)
			goto done;
		evbuffer_chain_insert(buf, chain);
	}

	/* 该chain可以修改 */
	if ((chain->flags & EVBUFFER_IMMUTABLE) == 0) {
		/* Always true for mutable buffers */
		EVUTIL_ASSERT(chain->misalign >= 0 &&
		    (ev_uint64_t)chain->misalign <= EVBUFFER_CHAIN_MAX);

		/* 设置misalgn为buffer_len，后续插入数据时多余空间将出现在前面。 */
		if (chain->off == 0)
			chain->misalign = chain->buffer_len;

		if ((size_t)chain->misalign >= datlen) {
			/* misalgn预留空间足够大 */
			memcpy(chain->buffer + chain->misalign - datlen,
			    data, datlen);
			chain->off += datlen;
			chain->misalign -= datlen;
			buf->total_len += datlen;
			buf->n_add_for_cb += datlen;
			goto out;
		} else if (chain->misalign) {
			/* 不够大，填充部分数据. */
			memcpy(chain->buffer,
			    (char*)data + datlen - chain->misalign,
			    (size_t)chain->misalign);
			chain->off += (size_t)chain->misalign;
			buf->total_len += (size_t)chain->misalign;
			buf->n_add_for_cb += (size_t)chain->misalign;
			datlen -= (size_t)chain->misalign;
			chain->misalign = 0;
		}
	}

	/* chain不能修改或者不够大(只填充部分数据)时，新增一个chain。 */
	if ((tmp = evbuffer_chain_new(datlen)) == NULL)
		goto done;
	buf->first = tmp;
	if (buf->last_with_datap == &buf->first)
		buf->last_with_datap = &tmp->next;

	tmp->next = chain;

	tmp->off = datlen;
	EVUTIL_ASSERT(datlen <= tmp->buffer_len);
	tmp->misalign = tmp->buffer_len - datlen;

	memcpy(tmp->buffer + tmp->misalign, data, datlen);
	buf->total_len += datlen;
	buf->n_add_for_cb += (size_t)chain->misalign;

out:
	evbuffer_invoke_callbacks(buf);
	result = 0;
done:
	EVBUFFER_UNLOCK(buf);
	return result;
}

/** Helper: realigns the memory in chain->buffer so that misalign is 0. */
static void
evbuffer_chain_align(struct evbuffer_chain *chain)
{
	EVUTIL_ASSERT(!(chain->flags & EVBUFFER_IMMUTABLE));
	EVUTIL_ASSERT(!(chain->flags & EVBUFFER_MEM_PINNED_ANY));
	memmove(chain->buffer, chain->buffer + chain->misalign, chain->off);
	chain->misalign = 0;
}

#define MAX_TO_COPY_IN_EXPAND 4096
#define MAX_TO_REALIGN_IN_EXPAND 2048

/** 返回true，如果通过调整chain可以放下datlen长度数据,且目前存储数据少于整个buffer长度一半
 *  且小于MAX_TO_REALIGN_IN_EXPAND
 *  调用该函数后一般通过memmove移动数据使misalign为0，通过控制off小于一定长度
 *  避免off过长带来的copy开销。
 */
static int
evbuffer_chain_should_realign(struct evbuffer_chain *chain,
    size_t datlen)
{
	return chain->buffer_len - chain->off >= datlen &&
	    (chain->off < chain->buffer_len / 2) &&
	    (chain->off <= MAX_TO_REALIGN_IN_EXPAND);
}

/* 扩展evbuffer空间，使能容纳datlen长度数据。扩展空间在一个chain里面，并返回这个chain. */
static struct evbuffer_chain *
evbuffer_expand_singlechain(struct evbuffer *buf, size_t datlen)
{
	struct evbuffer_chain *chain, **chainp;
	struct evbuffer_chain *result = NULL;
	ASSERT_EVBUFFER_LOCKED(buf);

	chainp = buf->last_with_datap;

	/* 如果last_with_datap不可写，使用last_with_datap的下一个节点。
	 * 如果last_with_datap不可写但有足够空间这可能是个糟糕的注意。
	 */
	if (*chainp && CHAIN_SPACE_LEN(*chainp) == 0)
		chainp = &(*chainp)->next;

	/* chain 此时指向一个可写的空间。*/
	chain = *chainp;

	if (chain == NULL ||
	    (chain->flags & (EVBUFFER_IMMUTABLE|EVBUFFER_MEM_PINNED_ANY))) {
		/* 如果chain指向NULL或者chain不可修改，新插入一个节点。 */
		goto insert_new;
	}

	/* chain有足够可用空间 */
	if (CHAIN_SPACE_LEN(chain) >= datlen) {
		result = chain;
		goto ok;
	}

	/* 如果最后一个节点没有数据，插入一个空节点替换他。*/
	if (chain->off == 0) {
		goto insert_new;
	}

	/* 通过使用misalign错位空间，也能使得可用空间大于等于预留空间。
	 * 由于使用misalign空间需要移动现有数据，只有当现有数据较小时才使用misalign空间，
	 * 否则将带来较大的移动开销。
	 */
	if (evbuffer_chain_should_realign(chain, datlen)) {
		evbuffer_chain_align(chain);
		result = chain;
		goto ok;
	}

	/* 可以通过扩展当前chain或在链表尾部新增一个chain扩展空间。
	 * 如果新增一个chain，将浪费当前chain CHAIN_SPACE_LEN(chain)字节空间。
	 * 如果扩展当前chain，需要拷贝chain->off字节
	 */

	/* 如果剩余空间小于chain->buffer_len/8或已有数据大于MAX_TO_COPY_IN_EXPAND，
	 * copy开销较大，使用下一个节点，不在此节点上扩展空间。(将浪费掉当前节点剩余空间)
	 * */
	if (CHAIN_SPACE_LEN(chain) < chain->buffer_len / 8 ||
	    chain->off > MAX_TO_COPY_IN_EXPAND ||
	    (datlen < EVBUFFER_CHAIN_MAX &&
		EVBUFFER_CHAIN_MAX - datlen >= chain->off)) {
		if (chain->next && CHAIN_SPACE_LEN(chain->next) >= datlen) {
			/* 下一个节点可用 (which should
			 * be empty) */
			result = chain->next;
			goto ok;
		} else {
			/* 下一个节点不可用，新增一个节点。 (which will free all
			 * terminal empty chains.) */
			goto insert_new;
		}
	} else {
		/* 现有数据较少，在此节点上扩充空间。*/
		/* 计算需要多少空间 */
		size_t length = chain->off + datlen;
		struct evbuffer_chain *tmp = evbuffer_chain_new(length);
		if (tmp == NULL)
			goto err;

		/* 数据迁移 */
		tmp->off = chain->off;
		memcpy(tmp->buffer, chain->buffer + chain->misalign,
		    chain->off);
		/* fix up the list */
		EVUTIL_ASSERT(*chainp == chain);
		result = *chainp = tmp;

		if (buf->last == chain)
			buf->last = tmp;

		tmp->next = chain->next;
		/*释放原节点*/
		evbuffer_chain_free(chain);
		goto ok;
	}

insert_new:
	result = evbuffer_chain_insert_new(buf, datlen);
	if (!result)
		goto err;
ok:
	EVUTIL_ASSERT(result);
	EVUTIL_ASSERT(CHAIN_SPACE_LEN(result) >= datlen);
err:
	return result;
}

/* 确保最后n个chain能提供datlen长度空间，没有数据拷贝或移动。
 * 仅供libevent内部使用。
 */
int
_evbuffer_expand_fast(struct evbuffer *buf, size_t datlen, int n)
{
	struct evbuffer_chain *chain = buf->last, *tmp, *next;
	size_t avail;
	int used;

	ASSERT_EVBUFFER_LOCKED(buf);
	EVUTIL_ASSERT(n >= 2);

	if (chain == NULL || (chain->flags & EVBUFFER_IMMUTABLE)) {
		/* 最后一个节点不可用，直接新建一个足够大的chain */
		chain = evbuffer_chain_new(datlen);
		if (chain == NULL)
			return (-1);

		evbuffer_chain_insert(buf, chain);
		return (0);
	}

	used = 0; /* 从*last_with_datap开始，有剩余空间chain数目。 */
	avail = 0; /* 剩余空间大小 */
	/* 遍历*last_with_datap以后的节点，计算剩余空间。 */
	for (chain = *buf->last_with_datap; chain; chain = chain->next) {
		if (chain->off) {
			size_t space = (size_t) CHAIN_SPACE_LEN(chain);
			EVUTIL_ASSERT(chain == *buf->last_with_datap);
			if (space) {
				avail += space;
				++used;
			}
		} else {
			/* 如果off为0，使用所有空间。 */
			chain->misalign = 0;
			avail += chain->buffer_len;
			++used;
		}
		if (avail >= datlen) {
			/* 所需空间已满足 */
			return (0);
		}
		/* 达到可接受最大长度 */
		if (used == n)
			break;
	}

	/* 后面n个chain提供不了足够的空间。
	 * 根据n大小选择新增一个足够空间的chain或用一个足够空间的chain替换掉所有空chain。
	 */
	if (used < n) {
		/* 链表*last_with_datap以后未使用节点数目小于n，新增一个chain */
		EVUTIL_ASSERT(chain == NULL);

		tmp = evbuffer_chain_new(datlen - avail);
		if (tmp == NULL)
			return (-1);

		buf->last->next = tmp;
		buf->last = tmp;
		/* (we would only set last_with_data if we added the first
		 * chain. But if the buffer had no chains, we would have
		 * just allocated a new chain earlier) */
		return (0);
	} else {
		/* 后面n个节点用了还是不够空间，释放这些节点用一个足够大的chain替换。 */
		int rmv_all = 0; /* True iff we removed last_with_data. */
		chain = *buf->last_with_datap;
		if (!chain->off) {
			//第一个没有数据的节点是头结点，可以释放整个链表。
			EVUTIL_ASSERT(chain == buf->first);
			rmv_all = 1;
			avail = 0;
		} else {
			/* 最后一个有数据的chain剩余avail空间，这个空间可以用。 */
			avail = (size_t) CHAIN_SPACE_LEN(chain);
			chain = chain->next;
		}


        //释放掉所有off为0的chain。
		for (; chain; chain = next) {
			next = chain->next;
			EVUTIL_ASSERT(chain->off == 0);
			evbuffer_chain_free(chain);
		}
		EVUTIL_ASSERT(datlen >= avail);
		//new一个足够大chain。
		tmp = evbuffer_chain_new(datlen - avail);
		if (tmp == NULL) {
			if (rmv_all) {
				ZERO_CHAIN(buf);
			} else {
				buf->last = *buf->last_with_datap;
				(*buf->last_with_datap)->next = NULL;
			}
			return (-1);
		}

		if (rmv_all) {
			buf->first = buf->last = tmp;
			buf->last_with_datap = &buf->first;
		} else {
			(*buf->last_with_datap)->next = tmp;
			buf->last = tmp;
		}
		return (0);
	}
}

//扩展evbuffer空间，使能容纳datlen长度数据。
int
evbuffer_expand(struct evbuffer *buf, size_t datlen)
{
	struct evbuffer_chain *chain;

	EVBUFFER_LOCK(buf);
	chain = evbuffer_expand_singlechain(buf, datlen);
	EVBUFFER_UNLOCK(buf);
	return chain ? 0 : -1;
}

/*
 * Reads data from a file descriptor into a buffer.
 */

#if defined(_EVENT_HAVE_SYS_UIO_H) || defined(WIN32)
#define USE_IOVEC_IMPL
#endif

#ifdef USE_IOVEC_IMPL

#ifdef _EVENT_HAVE_SYS_UIO_H
/* number of iovec we use for writev, fragmentation is going to determine
 * how much we end up writing */

#define DEFAULT_WRITE_IOVEC 128

#if defined(UIO_MAXIOV) && UIO_MAXIOV < DEFAULT_WRITE_IOVEC
#define NUM_WRITE_IOVEC UIO_MAXIOV
#elif defined(IOV_MAX) && IOV_MAX < DEFAULT_WRITE_IOVEC
#define NUM_WRITE_IOVEC IOV_MAX
#else
#define NUM_WRITE_IOVEC DEFAULT_WRITE_IOVEC
#endif

#define IOV_TYPE struct iovec
#define IOV_PTR_FIELD iov_base
#define IOV_LEN_FIELD iov_len
#define IOV_LEN_TYPE size_t
#else
#define NUM_WRITE_IOVEC 16
#define IOV_TYPE WSABUF
#define IOV_PTR_FIELD buf
#define IOV_LEN_FIELD len
#define IOV_LEN_TYPE unsigned long
#endif
#endif
#define NUM_READ_IOVEC 4

#define EVBUFFER_MAX_READ	4096

/** Helper function to figure out which space to use for reading data into
    an evbuffer.  Internal use only.

    @param buf The buffer to read into
    @param howmuch How much we want to read.
    @param vecs An array of two or more iovecs or WSABUFs.
    @param n_vecs_avail The length of vecs
    @param chainp A pointer to a variable to hold the first chain we're
      reading into.
    @param exact Boolean: if true, we do not provide more than 'howmuch'
      space in the vectors, even if more space is available.
    @return The number of buffers we're using.
 */
int
_evbuffer_read_setup_vecs(struct evbuffer *buf, ev_ssize_t howmuch,
    struct evbuffer_iovec *vecs, int n_vecs_avail,
    struct evbuffer_chain ***chainp, int exact)
{
	struct evbuffer_chain *chain;
	struct evbuffer_chain **firstchainp;
	size_t so_far;
	int i;
	ASSERT_EVBUFFER_LOCKED(buf);

	if (howmuch < 0)
		return -1;

	so_far = 0;
	/* Let firstchain be the first chain with any space on it */
	firstchainp = buf->last_with_datap;
	if (CHAIN_SPACE_LEN(*firstchainp) == 0) {
		firstchainp = &(*firstchainp)->next;
	}

	chain = *firstchainp;
	for (i = 0; i < n_vecs_avail && so_far < (size_t)howmuch; ++i) {
		size_t avail = (size_t) CHAIN_SPACE_LEN(chain);
		if (avail > (howmuch - so_far) && exact)
			avail = howmuch - so_far;
		vecs[i].iov_base = CHAIN_SPACE_PTR(chain);
		vecs[i].iov_len = avail;
		so_far += avail;
		chain = chain->next;
	}

	*chainp = firstchainp;
	return i;
}

static int
get_n_bytes_readable_on_socket(evutil_socket_t fd)
{
#if defined(FIONREAD) && defined(WIN32)
	unsigned long lng = EVBUFFER_MAX_READ;
	if (ioctlsocket(fd, FIONREAD, &lng) < 0)
		return -1;
	/* Can overflow, but mostly harmlessly. XXXX */
	return (int)lng;
#elif defined(FIONREAD)
	int n = EVBUFFER_MAX_READ;
	if (ioctl(fd, FIONREAD, &n) < 0)
		return -1;
	return n;
#else
	return EVBUFFER_MAX_READ;
#endif
}

/* TODO(niels): should this function return ev_ssize_t and take ev_ssize_t
 * as howmuch? */
int
evbuffer_read(struct evbuffer *buf, evutil_socket_t fd, int howmuch)
{
	struct evbuffer_chain **chainp;
	int n;
	int result;

#ifdef USE_IOVEC_IMPL
	int nvecs, i, remaining;
#else
	struct evbuffer_chain *chain;
	unsigned char *p;
#endif

	EVBUFFER_LOCK(buf);

	if (buf->freeze_end) {
		result = -1;
		goto done;
	}

	n = get_n_bytes_readable_on_socket(fd);
	if (n <= 0 || n > EVBUFFER_MAX_READ)
		n = EVBUFFER_MAX_READ;
	if (howmuch < 0 || howmuch > n)
		howmuch = n;

#ifdef USE_IOVEC_IMPL
	/* Since we can use iovecs, we're willing to use the last
	 * NUM_READ_IOVEC chains. */
	if (_evbuffer_expand_fast(buf, howmuch, NUM_READ_IOVEC) == -1) {
		result = -1;
		goto done;
	} else {
		IOV_TYPE vecs[NUM_READ_IOVEC];
#ifdef _EVBUFFER_IOVEC_IS_NATIVE
		nvecs = _evbuffer_read_setup_vecs(buf, howmuch, vecs,
		    NUM_READ_IOVEC, &chainp, 1);
#else
		/* We aren't using the native struct iovec.  Therefore,
		   we are on win32. */
		struct evbuffer_iovec ev_vecs[NUM_READ_IOVEC];
		nvecs = _evbuffer_read_setup_vecs(buf, howmuch, ev_vecs, 2,
		    &chainp, 1);

		for (i=0; i < nvecs; ++i)
			WSABUF_FROM_EVBUFFER_IOV(&vecs[i], &ev_vecs[i]);
#endif

#ifdef WIN32
		{
			DWORD bytesRead;
			DWORD flags=0;
			if (WSARecv(fd, vecs, nvecs, &bytesRead, &flags, NULL, NULL)) {
				/* The read failed. It might be a close,
				 * or it might be an error. */
				if (WSAGetLastError() == WSAECONNABORTED)
					n = 0;
				else
					n = -1;
			} else
				n = bytesRead;
		}
#else
		n = readv(fd, vecs, nvecs);
#endif
	}

#else /*!USE_IOVEC_IMPL*/
	/* If we don't have FIONREAD, we might waste some space here */
	/* XXX we _will_ waste some space here if there is any space left
	 * over on buf->last. */
	if ((chain = evbuffer_expand_singlechain(buf, howmuch)) == NULL) {
		result = -1;
		goto done;
	}

	/* We can append new data at this point */
	p = chain->buffer + chain->misalign + chain->off;

#ifndef WIN32
	n = read(fd, p, howmuch);
#else
	n = recv(fd, p, howmuch, 0);
#endif
#endif /* USE_IOVEC_IMPL */

	if (n == -1) {
		result = -1;
		goto done;
	}
	if (n == 0) {
		result = 0;
		goto done;
	}

#ifdef USE_IOVEC_IMPL
	remaining = n;
	for (i=0; i < nvecs; ++i) {
		/* can't overflow, since only mutable chains have
		 * huge misaligns. */
		size_t space = (size_t) CHAIN_SPACE_LEN(*chainp);
		/* XXXX This is a kludge that can waste space in perverse
		 * situations. */
		if (space > EVBUFFER_CHAIN_MAX)
			space = EVBUFFER_CHAIN_MAX;
		if ((ev_ssize_t)space < remaining) {
			(*chainp)->off += space;
			remaining -= (int)space;
		} else {
			(*chainp)->off += remaining;
			buf->last_with_datap = chainp;
			break;
		}
		chainp = &(*chainp)->next;
	}
#else
	chain->off += n;
	advance_last_with_data(buf);
#endif
	buf->total_len += n;
	buf->n_add_for_cb += n;

	/* Tell someone about changes in this buffer */
	evbuffer_invoke_callbacks(buf);
	result = n;
done:
	EVBUFFER_UNLOCK(buf);
	return result;
}

#ifdef WIN32
static int
evbuffer_readfile(struct evbuffer *buf, evutil_socket_t fd, ev_ssize_t howmuch)
{
	int result;
	int nchains, n;
	struct evbuffer_iovec v[2];

	EVBUFFER_LOCK(buf);

	if (buf->freeze_end) {
		result = -1;
		goto done;
	}

	if (howmuch < 0)
		howmuch = 16384;


	/* XXX we _will_ waste some space here if there is any space left
	 * over on buf->last. */
	nchains = evbuffer_reserve_space(buf, howmuch, v, 2);
	if (nchains < 1 || nchains > 2) {
		result = -1;
		goto done;
	}
	n = read((int)fd, v[0].iov_base, (unsigned int)v[0].iov_len);
	if (n <= 0) {
		result = n;
		goto done;
	}
	v[0].iov_len = (IOV_LEN_TYPE) n; /* XXXX another problem with big n.*/
	if (nchains > 1) {
		n = read((int)fd, v[1].iov_base, (unsigned int)v[1].iov_len);
		if (n <= 0) {
			result = (unsigned long) v[0].iov_len;
			evbuffer_commit_space(buf, v, 1);
			goto done;
		}
		v[1].iov_len = n;
	}
	evbuffer_commit_space(buf, v, nchains);

	result = n;
done:
	EVBUFFER_UNLOCK(buf);
	return result;
}
#endif

#ifdef USE_IOVEC_IMPL
static inline int
evbuffer_write_iovec(struct evbuffer *buffer, evutil_socket_t fd,
    ev_ssize_t howmuch)
{
	IOV_TYPE iov[NUM_WRITE_IOVEC];
	struct evbuffer_chain *chain = buffer->first;
	int n, i = 0;

	if (howmuch < 0)
		return -1;

	ASSERT_EVBUFFER_LOCKED(buffer);
	/* XXX make this top out at some maximal data length?  if the
	 * buffer has (say) 1MB in it, split over 128 chains, there's
	 * no way it all gets written in one go. */
	while (chain != NULL && i < NUM_WRITE_IOVEC && howmuch) {
#ifdef USE_SENDFILE
		/* we cannot write the file info via writev */
		if (chain->flags & EVBUFFER_SENDFILE)
			break;
#endif
		iov[i].IOV_PTR_FIELD = (void *) (chain->buffer + chain->misalign);
		if ((size_t)howmuch >= chain->off) {
			/* XXXcould be problematic when windows supports mmap*/
			iov[i++].IOV_LEN_FIELD = (IOV_LEN_TYPE)chain->off;
			howmuch -= chain->off;
		} else {
			/* XXXcould be problematic when windows supports mmap*/
			iov[i++].IOV_LEN_FIELD = (IOV_LEN_TYPE)howmuch;
			break;
		}
		chain = chain->next;
	}
	if (! i)
		return 0;
#ifdef WIN32
	{
		DWORD bytesSent;
		if (WSASend(fd, iov, i, &bytesSent, 0, NULL, NULL))
			n = -1;
		else
			n = bytesSent;
	}
#else
	n = writev(fd, iov, i);
#endif
	return (n);
}
#endif

#ifdef USE_SENDFILE
static inline int
evbuffer_write_sendfile(struct evbuffer *buffer, evutil_socket_t fd,
    ev_ssize_t howmuch)
{
	struct evbuffer_chain *chain = buffer->first;
	struct evbuffer_chain_fd *info =
	    EVBUFFER_CHAIN_EXTRA(struct evbuffer_chain_fd, chain);
#if defined(SENDFILE_IS_MACOSX) || defined(SENDFILE_IS_FREEBSD)
	int res;
	off_t len = chain->off;
#elif defined(SENDFILE_IS_LINUX) || defined(SENDFILE_IS_SOLARIS)
	ev_ssize_t res;
	off_t offset = chain->misalign;
#endif

	ASSERT_EVBUFFER_LOCKED(buffer);

#if defined(SENDFILE_IS_MACOSX)
	res = sendfile(info->fd, fd, chain->misalign, &len, NULL, 0);
	if (res == -1 && !EVUTIL_ERR_RW_RETRIABLE(errno))
		return (-1);

	return (len);
#elif defined(SENDFILE_IS_FREEBSD)
	res = sendfile(info->fd, fd, chain->misalign, chain->off, NULL, &len, 0);
	if (res == -1 && !EVUTIL_ERR_RW_RETRIABLE(errno))
		return (-1);

	return (len);
#elif defined(SENDFILE_IS_LINUX)
	/* TODO(niels): implement splice */
	res = sendfile(fd, info->fd, &offset, chain->off);
	if (res == -1 && EVUTIL_ERR_RW_RETRIABLE(errno)) {
		/* if this is EAGAIN or EINTR return 0; otherwise, -1 */
		return (0);
	}
	return (res);
#elif defined(SENDFILE_IS_SOLARIS)
	{
		const off_t offset_orig = offset;
		res = sendfile(fd, info->fd, &offset, chain->off);
		if (res == -1 && EVUTIL_ERR_RW_RETRIABLE(errno)) {
			if (offset - offset_orig)
				return offset - offset_orig;
			/* if this is EAGAIN or EINTR and no bytes were
			 * written, return 0 */
			return (0);
		}
		return (res);
	}
#endif
}
#endif

int
evbuffer_write_atmost(struct evbuffer *buffer, evutil_socket_t fd,
    ev_ssize_t howmuch)
{
	int n = -1;

	EVBUFFER_LOCK(buffer);

	if (buffer->freeze_start) {
		goto done;
	}

	if (howmuch < 0 || (size_t)howmuch > buffer->total_len)
		howmuch = buffer->total_len;

	if (howmuch > 0) {
#ifdef USE_SENDFILE
		struct evbuffer_chain *chain = buffer->first;
		if (chain != NULL && (chain->flags & EVBUFFER_SENDFILE))
			n = evbuffer_write_sendfile(buffer, fd, howmuch);
		else {
#endif
#ifdef USE_IOVEC_IMPL
		n = evbuffer_write_iovec(buffer, fd, howmuch);
#elif defined(WIN32)
		/* XXX(nickm) Don't disable this code until we know if
		 * the WSARecv code above works. */
		void *p = evbuffer_pullup(buffer, howmuch);
		EVUTIL_ASSERT(p || !howmuch);
		n = send(fd, p, howmuch, 0);
#else
		void *p = evbuffer_pullup(buffer, howmuch);
		EVUTIL_ASSERT(p || !howmuch);
		n = write(fd, p, howmuch);
#endif
#ifdef USE_SENDFILE
		}
#endif
	}

	if (n > 0)
		evbuffer_drain(buffer, n);

done:
	EVBUFFER_UNLOCK(buffer);
	return (n);
}

int
evbuffer_write(struct evbuffer *buffer, evutil_socket_t fd)
{
	return evbuffer_write_atmost(buffer, fd, -1);
}

unsigned char *
evbuffer_find(struct evbuffer *buffer, const unsigned char *what, size_t len)
{
	unsigned char *search;
	struct evbuffer_ptr ptr;

	EVBUFFER_LOCK(buffer);

	ptr = evbuffer_search(buffer, (const char *)what, len, NULL);
	if (ptr.pos < 0) {
		search = NULL;
	} else {
		search = evbuffer_pullup(buffer, ptr.pos + len);
		if (search)
			search += ptr.pos;
	}
	EVBUFFER_UNLOCK(buffer);
	return search;
}

int
evbuffer_ptr_set(struct evbuffer *buf, struct evbuffer_ptr *pos,
    size_t position, enum evbuffer_ptr_how how)
{
	size_t left = position;
	struct evbuffer_chain *chain = NULL;

	EVBUFFER_LOCK(buf);

	switch (how) {
	case EVBUFFER_PTR_SET:
		chain = buf->first;
		pos->pos = position;
		position = 0;
		break;
	case EVBUFFER_PTR_ADD:
		/* this avoids iterating over all previous chains if
		   we just want to advance the position */
		if (pos->pos < 0 || EV_SIZE_MAX - position < (size_t)pos->pos) {
			EVBUFFER_UNLOCK(buf);
			return -1;
		}
		chain = pos->_internal.chain;
		pos->pos += position;
		position = pos->_internal.pos_in_chain;
		break;
	}

	EVUTIL_ASSERT(EV_SIZE_MAX - left >= position);
	while (chain && position + left >= chain->off) {
		left -= chain->off - position;
		chain = chain->next;
		position = 0;
	}
	if (chain) {
		pos->_internal.chain = chain;
		pos->_internal.pos_in_chain = position + left;
	} else {
		pos->_internal.chain = NULL;
		pos->pos = -1;
	}

	EVBUFFER_UNLOCK(buf);

	return chain != NULL ? 0 : -1;
}

/**
   Compare the bytes in buf at position pos to the len bytes in mem.  Return
   less than 0, 0, or greater than 0 as memcmp.
 */
static int
evbuffer_ptr_memcmp(const struct evbuffer *buf, const struct evbuffer_ptr *pos,
    const char *mem, size_t len)
{
	struct evbuffer_chain *chain;
	size_t position;
	int r;

	ASSERT_EVBUFFER_LOCKED(buf);

	if (pos->pos < 0 ||
	    EV_SIZE_MAX - len < (size_t)pos->pos ||
	    pos->pos + len > buf->total_len)
		return -1;

	chain = pos->_internal.chain;
	position = pos->_internal.pos_in_chain;
	while (len && chain) {
		size_t n_comparable;
		if (len + position > chain->off)
			n_comparable = chain->off - position;
		else
			n_comparable = len;
		r = memcmp(chain->buffer + chain->misalign + position, mem,
		    n_comparable);
		if (r)
			return r;
		mem += n_comparable;
		len -= n_comparable;
		position = 0;
		chain = chain->next;
	}

	return 0;
}

struct evbuffer_ptr
evbuffer_search(struct evbuffer *buffer, const char *what, size_t len, const struct evbuffer_ptr *start)
{
	return evbuffer_search_range(buffer, what, len, start, NULL);
}

struct evbuffer_ptr
evbuffer_search_range(struct evbuffer *buffer, const char *what, size_t len, const struct evbuffer_ptr *start, const struct evbuffer_ptr *end)
{
	struct evbuffer_ptr pos;
	struct evbuffer_chain *chain, *last_chain = NULL;
	const unsigned char *p;
	char first;

	EVBUFFER_LOCK(buffer);

	if (start) {
		memcpy(&pos, start, sizeof(pos));
		chain = pos._internal.chain;
	} else {
		pos.pos = 0;
		chain = pos._internal.chain = buffer->first;
		pos._internal.pos_in_chain = 0;
	}

	if (end)
		last_chain = end->_internal.chain;

	if (!len || len > EV_SSIZE_MAX)
		goto done;

	first = what[0];

	while (chain) {
		const unsigned char *start_at =
		    chain->buffer + chain->misalign +
		    pos._internal.pos_in_chain;
		p = memchr(start_at, first,
		    chain->off - pos._internal.pos_in_chain);
		if (p) {
			pos.pos += p - start_at;
			pos._internal.pos_in_chain += p - start_at;
			if (!evbuffer_ptr_memcmp(buffer, &pos, what, len)) {
				if (end && pos.pos + (ev_ssize_t)len > end->pos)
					goto not_found;
				else
					goto done;
			}
			++pos.pos;
			++pos._internal.pos_in_chain;
			if (pos._internal.pos_in_chain == chain->off) {
				chain = pos._internal.chain = chain->next;
				pos._internal.pos_in_chain = 0;
			}
		} else {
			if (chain == last_chain)
				goto not_found;
			pos.pos += chain->off - pos._internal.pos_in_chain;
			chain = pos._internal.chain = chain->next;
			pos._internal.pos_in_chain = 0;
		}
	}

not_found:
	pos.pos = -1;
	pos._internal.chain = NULL;
done:
	EVBUFFER_UNLOCK(buffer);
	return pos;
}

int
evbuffer_peek(struct evbuffer *buffer, ev_ssize_t len,
    struct evbuffer_ptr *start_at,
    struct evbuffer_iovec *vec, int n_vec)
{
	struct evbuffer_chain *chain;
	int idx = 0;
	ev_ssize_t len_so_far = 0;

	EVBUFFER_LOCK(buffer);

	if (start_at) {
		chain = start_at->_internal.chain;
		len_so_far = chain->off
		    - start_at->_internal.pos_in_chain;
		idx = 1;
		if (n_vec > 0) {
			vec[0].iov_base = chain->buffer + chain->misalign
			    + start_at->_internal.pos_in_chain;
			vec[0].iov_len = len_so_far;
		}
		chain = chain->next;
	} else {
		chain = buffer->first;
	}

	if (n_vec == 0 && len < 0) {
		/* If no vectors are provided and they asked for "everything",
		 * pretend they asked for the actual available amount. */
		len = buffer->total_len;
		if (start_at) {
			len -= start_at->pos;
		}
	}

	while (chain) {
		if (len >= 0 && len_so_far >= len)
			break;
		if (idx<n_vec) {
			vec[idx].iov_base = chain->buffer + chain->misalign;
			vec[idx].iov_len = chain->off;
		} else if (len<0) {
			break;
		}
		++idx;
		len_so_far += chain->off;
		chain = chain->next;
	}

	EVBUFFER_UNLOCK(buffer);

	return idx;
}


int
evbuffer_add_vprintf(struct evbuffer *buf, const char *fmt, va_list ap)
{
	char *buffer;
	size_t space;
	int sz, result = -1;
	va_list aq;
	struct evbuffer_chain *chain;


	EVBUFFER_LOCK(buf);

	if (buf->freeze_end) {
		goto done;
	}

	/* make sure that at least some space is available */
	if ((chain = evbuffer_expand_singlechain(buf, 64)) == NULL)
		goto done;

	for (;;) {
#if 0
		size_t used = chain->misalign + chain->off;
		buffer = (char *)chain->buffer + chain->misalign + chain->off;
		EVUTIL_ASSERT(chain->buffer_len >= used);
		space = chain->buffer_len - used;
#endif
		buffer = (char*) CHAIN_SPACE_PTR(chain);
		space = (size_t) CHAIN_SPACE_LEN(chain);

#ifndef va_copy
#define	va_copy(dst, src)	memcpy(&(dst), &(src), sizeof(va_list))
#endif
		va_copy(aq, ap);

		sz = evutil_vsnprintf(buffer, space, fmt, aq);

		va_end(aq);

		if (sz < 0)
			goto done;
		if (INT_MAX >= EVBUFFER_CHAIN_MAX &&
		    (size_t)sz >= EVBUFFER_CHAIN_MAX)
			goto done;
		if ((size_t)sz < space) {
			chain->off += sz;
			buf->total_len += sz;
			buf->n_add_for_cb += sz;

			advance_last_with_data(buf);
			evbuffer_invoke_callbacks(buf);
			result = sz;
			goto done;
		}
		if ((chain = evbuffer_expand_singlechain(buf, sz + 1)) == NULL)
			goto done;
	}
	/* NOTREACHED */

done:
	EVBUFFER_UNLOCK(buf);
	return result;
}

int
evbuffer_add_printf(struct evbuffer *buf, const char *fmt, ...)
{
	int res = -1;
	va_list ap;

	va_start(ap, fmt);
	res = evbuffer_add_vprintf(buf, fmt, ap);
	va_end(ap);

	return (res);
}

int
evbuffer_add_reference(struct evbuffer *outbuf,
    const void *data, size_t datlen,
    evbuffer_ref_cleanup_cb cleanupfn, void *extra)
{
	struct evbuffer_chain *chain;
	struct evbuffer_chain_reference *info;
	int result = -1;

	chain = evbuffer_chain_new(sizeof(struct evbuffer_chain_reference));
	if (!chain)
		return (-1);
	chain->flags |= EVBUFFER_REFERENCE | EVBUFFER_IMMUTABLE;
	chain->buffer = (u_char *)data;
	chain->buffer_len = datlen;
	chain->off = datlen;

	info = EVBUFFER_CHAIN_EXTRA(struct evbuffer_chain_reference, chain);
	info->cleanupfn = cleanupfn;
	info->extra = extra;

	EVBUFFER_LOCK(outbuf);
	if (outbuf->freeze_end) {
		/* don't call chain_free; we do not want to actually invoke
		 * the cleanup function */
		mm_free(chain);
		goto done;
	}
	evbuffer_chain_insert(outbuf, chain);
	outbuf->n_add_for_cb += datlen;

	evbuffer_invoke_callbacks(outbuf);

	result = 0;
done:
	EVBUFFER_UNLOCK(outbuf);

	return result;
}

/* TODO(niels): maybe we don't want to own the fd, however, in that
 * case, we should dup it - dup is cheap.  Perhaps, we should use a
 * callback instead?
 */
/* TODO(niels): we may want to add to automagically convert to mmap, in
 * case evbuffer_remove() or evbuffer_pullup() are being used.
 */
int
evbuffer_add_file(struct evbuffer *outbuf, int fd,
    ev_off_t offset, ev_off_t length)
{
#if defined(USE_SENDFILE) || defined(_EVENT_HAVE_MMAP)
	struct evbuffer_chain *chain;
	struct evbuffer_chain_fd *info;
#endif
#if defined(USE_SENDFILE)
	int sendfile_okay = 1;
#endif
	int ok = 1;

	if (offset < 0 || length < 0 ||
	    ((ev_uint64_t)length > EVBUFFER_CHAIN_MAX) ||
	    (ev_uint64_t)offset > (ev_uint64_t)(EVBUFFER_CHAIN_MAX - length))
		return (-1);

#if defined(USE_SENDFILE)
	if (use_sendfile) {
		EVBUFFER_LOCK(outbuf);
		sendfile_okay = outbuf->flags & EVBUFFER_FLAG_DRAINS_TO_FD;
		EVBUFFER_UNLOCK(outbuf);
	}

	if (use_sendfile && sendfile_okay) {
		chain = evbuffer_chain_new(sizeof(struct evbuffer_chain_fd));
		if (chain == NULL) {
			event_warn("%s: out of memory", __func__);
			return (-1);
		}

		chain->flags |= EVBUFFER_SENDFILE | EVBUFFER_IMMUTABLE;
		chain->buffer = NULL;	/* no reading possible */
		chain->buffer_len = length + offset;
		chain->off = length;
		chain->misalign = offset;

		info = EVBUFFER_CHAIN_EXTRA(struct evbuffer_chain_fd, chain);
		info->fd = fd;

		EVBUFFER_LOCK(outbuf);
		if (outbuf->freeze_end) {
			mm_free(chain);
			ok = 0;
		} else {
			outbuf->n_add_for_cb += length;
			evbuffer_chain_insert(outbuf, chain);
		}
	} else
#endif
#if defined(_EVENT_HAVE_MMAP)
	if (use_mmap) {
		void *mapped = mmap(NULL, length + offset, PROT_READ,
#ifdef MAP_NOCACHE
		    MAP_NOCACHE |
#endif
#ifdef MAP_FILE
		    MAP_FILE |
#endif
		    MAP_PRIVATE,
		    fd, 0);
		/* some mmap implementations require offset to be a multiple of
		 * the page size.  most users of this api, are likely to use 0
		 * so mapping everything is not likely to be a problem.
		 * TODO(niels): determine page size and round offset to that
		 * page size to avoid mapping too much memory.
		 */
		if (mapped == MAP_FAILED) {
			event_warn("%s: mmap(%d, %d, %zu) failed",
			    __func__, fd, 0, (size_t)(offset + length));
			return (-1);
		}
		chain = evbuffer_chain_new(sizeof(struct evbuffer_chain_fd));
		if (chain == NULL) {
			event_warn("%s: out of memory", __func__);
			munmap(mapped, length);
			return (-1);
		}

		chain->flags |= EVBUFFER_MMAP | EVBUFFER_IMMUTABLE;
		chain->buffer = mapped;
		chain->buffer_len = length + offset;
		chain->off = length + offset;

		info = EVBUFFER_CHAIN_EXTRA(struct evbuffer_chain_fd, chain);
		info->fd = fd;

		EVBUFFER_LOCK(outbuf);
		if (outbuf->freeze_end) {
			info->fd = -1;
			evbuffer_chain_free(chain);
			ok = 0;
		} else {
			outbuf->n_add_for_cb += length;

			evbuffer_chain_insert(outbuf, chain);

			/* we need to subtract whatever we don't need */
			evbuffer_drain(outbuf, offset);
		}
	} else
#endif
	{
		/* the default implementation */
		struct evbuffer *tmp = evbuffer_new();
		ev_ssize_t read;

		if (tmp == NULL)
			return (-1);

#ifdef WIN32
#define lseek _lseeki64
#endif
		if (lseek(fd, offset, SEEK_SET) == -1) {
			evbuffer_free(tmp);
			return (-1);
		}

		/* we add everything to a temporary buffer, so that we
		 * can abort without side effects if the read fails.
		 */
		while (length) {
			ev_ssize_t to_read = length > EV_SSIZE_MAX ? EV_SSIZE_MAX : (ev_ssize_t)length;
			read = evbuffer_readfile(tmp, fd, to_read);
			if (read == -1) {
				evbuffer_free(tmp);
				return (-1);
			}

			length -= read;
		}

		EVBUFFER_LOCK(outbuf);
		if (outbuf->freeze_end) {
			evbuffer_free(tmp);
			ok = 0;
		} else {
			evbuffer_add_buffer(outbuf, tmp);
			evbuffer_free(tmp);

#ifdef WIN32
#define close _close
#endif
			close(fd);
		}
	}

	if (ok)
		evbuffer_invoke_callbacks(outbuf);
	EVBUFFER_UNLOCK(outbuf);

	return ok ? 0 : -1;
}


void
evbuffer_setcb(struct evbuffer *buffer, evbuffer_cb cb, void *cbarg)
{
	EVBUFFER_LOCK(buffer);

	if (!TAILQ_EMPTY(&buffer->callbacks))
		evbuffer_remove_all_callbacks(buffer);

	if (cb) {
		struct evbuffer_cb_entry *ent =
		    evbuffer_add_cb(buffer, NULL, cbarg);
		ent->cb.cb_obsolete = cb;
		ent->flags |= EVBUFFER_CB_OBSOLETE;
	}
	EVBUFFER_UNLOCK(buffer);
}

struct evbuffer_cb_entry *
evbuffer_add_cb(struct evbuffer *buffer, evbuffer_cb_func cb, void *cbarg)
{
	struct evbuffer_cb_entry *e;
	if (! (e = mm_calloc(1, sizeof(struct evbuffer_cb_entry))))
		return NULL;
	EVBUFFER_LOCK(buffer);
	e->cb.cb_func = cb;
	e->cbarg = cbarg;
	e->flags = EVBUFFER_CB_ENABLED;
	TAILQ_INSERT_HEAD(&buffer->callbacks, e, next);
	EVBUFFER_UNLOCK(buffer);
	return e;
}

int
evbuffer_remove_cb_entry(struct evbuffer *buffer,
			 struct evbuffer_cb_entry *ent)
{
	EVBUFFER_LOCK(buffer);
	TAILQ_REMOVE(&buffer->callbacks, ent, next);
	EVBUFFER_UNLOCK(buffer);
	mm_free(ent);
	return 0;
}

int
evbuffer_remove_cb(struct evbuffer *buffer, evbuffer_cb_func cb, void *cbarg)
{
	struct evbuffer_cb_entry *cbent;
	int result = -1;
	EVBUFFER_LOCK(buffer);
	TAILQ_FOREACH(cbent, &buffer->callbacks, next) {
		if (cb == cbent->cb.cb_func && cbarg == cbent->cbarg) {
			result = evbuffer_remove_cb_entry(buffer, cbent);
			goto done;
		}
	}
done:
	EVBUFFER_UNLOCK(buffer);
	return result;
}

int
evbuffer_cb_set_flags(struct evbuffer *buffer,
		      struct evbuffer_cb_entry *cb, ev_uint32_t flags)
{
	/* the user isn't allowed to mess with these. */
	flags &= ~EVBUFFER_CB_INTERNAL_FLAGS;
	EVBUFFER_LOCK(buffer);
	cb->flags |= flags;
	EVBUFFER_UNLOCK(buffer);
	return 0;
}

int
evbuffer_cb_clear_flags(struct evbuffer *buffer,
		      struct evbuffer_cb_entry *cb, ev_uint32_t flags)
{
	/* the user isn't allowed to mess with these. */
	flags &= ~EVBUFFER_CB_INTERNAL_FLAGS;
	EVBUFFER_LOCK(buffer);
	cb->flags &= ~flags;
	EVBUFFER_UNLOCK(buffer);
	return 0;
}

int
evbuffer_freeze(struct evbuffer *buffer, int start)
{
	EVBUFFER_LOCK(buffer);
	if (start)
		buffer->freeze_start = 1;
	else
		buffer->freeze_end = 1;
	EVBUFFER_UNLOCK(buffer);
	return 0;
}

int
evbuffer_unfreeze(struct evbuffer *buffer, int start)
{
	EVBUFFER_LOCK(buffer);
	if (start)
		buffer->freeze_start = 0;
	else
		buffer->freeze_end = 0;
	EVBUFFER_UNLOCK(buffer);
	return 0;
}

#if 0
void
evbuffer_cb_suspend(struct evbuffer *buffer, struct evbuffer_cb_entry *cb)
{
	if (!(cb->flags & EVBUFFER_CB_SUSPENDED)) {
		cb->size_before_suspend = evbuffer_get_length(buffer);
		cb->flags |= EVBUFFER_CB_SUSPENDED;
	}
}

void
evbuffer_cb_unsuspend(struct evbuffer *buffer, struct evbuffer_cb_entry *cb)
{
	if ((cb->flags & EVBUFFER_CB_SUSPENDED)) {
		unsigned call = (cb->flags & EVBUFFER_CB_CALL_ON_UNSUSPEND);
		size_t sz = cb->size_before_suspend;
		cb->flags &= ~(EVBUFFER_CB_SUSPENDED|
			       EVBUFFER_CB_CALL_ON_UNSUSPEND);
		cb->size_before_suspend = 0;
		if (call && (cb->flags & EVBUFFER_CB_ENABLED)) {
			cb->cb(buffer, sz, evbuffer_get_length(buffer), cb->cbarg);
		}
	}
}
#endif

/* These hooks are exposed so that the unit tests can temporarily disable
 * sendfile support in order to test mmap, or both to test linear
 * access. Don't use it; if we need to add a way to disable sendfile support
 * in the future, it will probably be via an alternate version of
 * evbuffer_add_file() with a 'flags' argument.
 */
int _evbuffer_testing_use_sendfile(void);
int _evbuffer_testing_use_mmap(void);
int _evbuffer_testing_use_linear_file_access(void);

int
_evbuffer_testing_use_sendfile(void)
{
	int ok = 0;
#ifdef USE_SENDFILE
	use_sendfile = 1;
	ok = 1;
#endif
#ifdef _EVENT_HAVE_MMAP
	use_mmap = 0;
#endif
	return ok;
}
int
_evbuffer_testing_use_mmap(void)
{
	int ok = 0;
#ifdef USE_SENDFILE
	use_sendfile = 0;
#endif
#ifdef _EVENT_HAVE_MMAP
	use_mmap = 1;
	ok = 1;
#endif
	return ok;
}
int
_evbuffer_testing_use_linear_file_access(void)
{
#ifdef USE_SENDFILE
	use_sendfile = 0;
#endif
#ifdef _EVENT_HAVE_MMAP
	use_mmap = 0;
#endif
	return 1;
}