event2/buffer.h(3)

event2/buffer.h(3) | Library Functions Manual | event2/buffer.h(3)

# NAME

event2/buffer.h - Functions for buffering data for network sending or receiving.

# SYNOPSIS

#include <event2/visibility.h>
#include <event2/event-config.h>
#include <stdarg.h>
#include <event2/util.h>

## Data Structures

struct evbuffer
An evbuffer is an opaque data type for efficiently buffering data to be sent or received on the network. struct evbuffer_cb_info
Structure passed to an evbuffer_cb_func evbuffer callback. struct evbuffer_iovec
Describes a single extent of memory inside an evbuffer. struct evbuffer_ptr
Pointer to a position within an evbuffer.

## Macros

#define EVBUF_FS_CLOSE_ON_FREE 0x01
Flag for creating evbuffer_file_segment: If this flag is set, then when the evbuffer_file_segment is freed and no longer in use by any evbuffer, the underlying fd is closed. #define EVBUF_FS_DISABLE_LOCKING 0x08
Flag for creating evbuffer_file_segment: Do not allocate a lock for this segment. #define EVBUF_FS_DISABLE_MMAP 0x02
Flag for creating evbuffer_file_segment: Disable memory-map based implementations. #define EVBUF_FS_DISABLE_SENDFILE 0x04
Flag for creating evbuffer_file_segment: Disable direct fd-to-fd implementations (including sendfile and splice). #define EVBUFFER_CB_ENABLED 1
If this flag is not set, then a callback is temporarily disabled, and should not be invoked. #define EVBUFFER_FLAG_DRAINS_TO_FD 1
If this flag is set, then we will not use evbuffer_peek(), evbuffer_remove(), evbuffer_remove_buffer(), and so on to read bytes from this buffer: we'll only take bytes out of this buffer by writing them to the network (as with evbuffer_write_atmost), by removing them without observing them (as with evbuffer_drain), or by copying them all out at once (as with evbuffer_add_buffer).

## Typedefs

typedef void(* evbuffer_cb_func) (struct evbuffer *buffer, const struct evbuffer_cb_info *info, void *arg)
Type definition for a callback that is invoked whenever data is added or removed from an evbuffer. typedef void(* evbuffer_file_segment_cleanup_cb) (struct evbuffer_file_segment const *seg, int flags, void *arg)
A cleanup function for a evbuffer_file_segment added to an evbuffer for reference. typedef void(* evbuffer_ref_cleanup_cb) (const void *data, size_t datalen, void *extra)
A cleanup function for a piece of memory added to an evbuffer by reference.

## Enumerations

enum evbuffer_eol_style { EVBUFFER_EOL_ANY, EVBUFFER_EOL_CRLF, EVBUFFER_EOL_CRLF_STRICT, EVBUFFER_EOL_LF, EVBUFFER_EOL_NUL } Used to tell evbuffer_readln what kind of line-ending to look for. "
enum evbuffer_ptr_how { EVBUFFER_PTR_SET, EVBUFFER_PTR_ADD } Defines how to adjust an evbuffer_ptr by evbuffer_ptr_set() "

## Functions

EVENT2_EXPORT_SYMBOL int evbuffer_add (struct evbuffer *buf, const void *data, size_t datlen)
Append data to the end of an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_add_buffer (struct evbuffer *outbuf, struct evbuffer *inbuf)
Move all data from one evbuffer into another evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_add_buffer_reference (struct evbuffer *outbuf, struct evbuffer *inbuf)
Copy data from one evbuffer into another evbuffer. EVENT2_EXPORT_SYMBOL struct evbuffer_cb_entry * evbuffer_add_cb (struct evbuffer *buffer, evbuffer_cb_func cb, void *cbarg)
Add a new callback to an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_add_file (struct evbuffer *outbuf, int fd, ev_off_t offset, ev_off_t length)
Copy data from a file into the evbuffer for writing to a socket. EVENT2_EXPORT_SYMBOL int evbuffer_add_file_segment (struct evbuffer *buf, struct evbuffer_file_segment *seg, ev_off_t offset, ev_off_t length)
Insert some or all of an evbuffer_file_segment at the end of an evbuffer. EVENT2_EXPORT_SYMBOL size_t evbuffer_add_iovec (struct evbuffer *buffer, struct evbuffer_iovec *vec, int n_vec)
Append data from 1 or more iovec's to an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_add_printf (struct evbuffer *buf, const char *fmt,...)
Append a formatted string to the end of an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_add_reference (struct evbuffer *outbuf, const void *data, size_t datlen, evbuffer_ref_cleanup_cb cleanupfn, void *cleanupfn_arg)
Reference memory into an evbuffer without copying. EVENT2_EXPORT_SYMBOL int evbuffer_add_vprintf (struct evbuffer *buf, const char *fmt, va_list ap)
Append a va_list formatted string to the end of an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_cb_clear_flags (struct evbuffer *buffer, struct evbuffer_cb_entry *cb, ev_uint32_t flags)
Change the flags that are set for a callback on a buffer by removing some. EVENT2_EXPORT_SYMBOL int evbuffer_cb_set_flags (struct evbuffer *buffer, struct evbuffer_cb_entry *cb, ev_uint32_t flags)
Change the flags that are set for a callback on a buffer by adding more. EVENT2_EXPORT_SYMBOL int evbuffer_clear_flags (struct evbuffer *buf, ev_uint64_t flags)
Change the flags that are set for an evbuffer by removing some. EVENT2_EXPORT_SYMBOL int evbuffer_commit_space (struct evbuffer *buf, struct evbuffer_iovec *vec, int n_vecs)
Commits previously reserved space. EVENT2_EXPORT_SYMBOL ev_ssize_t evbuffer_copyout (struct evbuffer *buf, void *data_out, size_t datlen)
Read data from an evbuffer, and leave the buffer unchanged. EVENT2_EXPORT_SYMBOL ev_ssize_t evbuffer_copyout_from (struct evbuffer *buf, const struct evbuffer_ptr *pos, void *data_out, size_t datlen)
Read data from the middle of an evbuffer, and leave the buffer unchanged. EVENT2_EXPORT_SYMBOL int evbuffer_defer_callbacks (struct evbuffer *buffer, struct event_base *base)
Force all the callbacks on an evbuffer to be run, not immediately after the evbuffer is altered, but instead from inside the event loop. EVENT2_EXPORT_SYMBOL int evbuffer_drain (struct evbuffer *buf, size_t len)
Remove a specified number of bytes data from the beginning of an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_enable_locking (struct evbuffer *buf, void *lock)
Enable locking on an evbuffer so that it can safely be used by multiple threads at the same time. EVENT2_EXPORT_SYMBOL int evbuffer_expand (struct evbuffer *buf, size_t datlen)
Expands the available space in an evbuffer. EVENT2_EXPORT_SYMBOL void evbuffer_file_segment_add_cleanup_cb (struct evbuffer_file_segment *seg, evbuffer_file_segment_cleanup_cb cb, void *arg)
Add cleanup callback and argument for the callback to an evbuffer_file_segment. EVENT2_EXPORT_SYMBOL void evbuffer_file_segment_free (struct evbuffer_file_segment *seg)
Free an evbuffer_file_segment. EVENT2_EXPORT_SYMBOL struct evbuffer_file_segment * evbuffer_file_segment_new (int fd, ev_off_t offset, ev_off_t length, unsigned flags)
Create and return a new evbuffer_file_segment for reading data from a file and sending it out via an evbuffer. EVENT2_EXPORT_SYMBOL void evbuffer_free (struct evbuffer *buf)
Deallocate storage for an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_freeze (struct evbuffer *buf, int at_front)
Prevent calls that modify an evbuffer from succeeding. EVENT2_EXPORT_SYMBOL size_t evbuffer_get_contiguous_space (const struct evbuffer *buf)
Returns the number of contiguous available bytes in the first buffer chain. EVENT2_EXPORT_SYMBOL size_t evbuffer_get_length (const struct evbuffer *buf)
Returns the total number of bytes stored in the evbuffer. EVENT2_EXPORT_SYMBOL void evbuffer_lock (struct evbuffer *buf)
Acquire the lock on an evbuffer. EVENT2_EXPORT_SYMBOL struct evbuffer * evbuffer_new (void)
Allocate storage for a new evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_peek (struct evbuffer *buffer, ev_ssize_t len, struct evbuffer_ptr *start_at, struct evbuffer_iovec *vec_out, int n_vec)
Function to peek at data inside an evbuffer without removing it or copying it out. EVENT2_EXPORT_SYMBOL int evbuffer_prepend (struct evbuffer *buf, const void *data, size_t size)
Prepends data to the beginning of the evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_prepend_buffer (struct evbuffer *dst, struct evbuffer *src)
Prepends all data from the src evbuffer to the beginning of the dst evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_ptr_set (struct evbuffer *buffer, struct evbuffer_ptr *ptr, size_t position, enum evbuffer_ptr_how how)
Sets the search pointer in the buffer to position. EVENT2_EXPORT_SYMBOL unsigned char * evbuffer_pullup (struct evbuffer *buf, ev_ssize_t size)
Makes the data at the beginning of an evbuffer contiguous. EVENT2_EXPORT_SYMBOL int evbuffer_read (struct evbuffer *buffer, evutil_socket_t fd, int howmuch)
Read from a file descriptor and store the result in an evbuffer. EVENT2_EXPORT_SYMBOL char * evbuffer_readln (struct evbuffer *buffer, size_t *n_read_out, enum evbuffer_eol_style eol_style)
Read a single line from an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_remove (struct evbuffer *buf, void *data, size_t datlen)
Read data from an evbuffer and drain the bytes read. EVENT2_EXPORT_SYMBOL int evbuffer_remove_buffer (struct evbuffer *src, struct evbuffer *dst, size_t datlen)
Read data from an evbuffer into another evbuffer, draining the bytes from the source buffer. EVENT2_EXPORT_SYMBOL int evbuffer_remove_cb (struct evbuffer *buffer, evbuffer_cb_func cb, void *cbarg)
Remove a callback from an evbuffer, given the function and argument used to add it. EVENT2_EXPORT_SYMBOL int evbuffer_remove_cb_entry (struct evbuffer *buffer, struct evbuffer_cb_entry *ent)
Remove a callback from an evbuffer, given a handle returned from evbuffer_add_cb. EVENT2_EXPORT_SYMBOL int evbuffer_reserve_space (struct evbuffer *buf, ev_ssize_t size, struct evbuffer_iovec *vec, int n_vec)
Reserves space in the last chain or chains of an evbuffer. EVENT2_EXPORT_SYMBOL struct evbuffer_ptr evbuffer_search (struct evbuffer *buffer, const char *what, size_t len, const struct evbuffer_ptr *start)
Search for a string within an evbuffer. EVENT2_EXPORT_SYMBOL struct evbuffer_ptr evbuffer_search_eol (struct evbuffer *buffer, struct evbuffer_ptr *start, size_t *eol_len_out, enum evbuffer_eol_style eol_style)
Search for an end-of-line string within an evbuffer. EVENT2_EXPORT_SYMBOL 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)
Search for a string within part of an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_set_flags (struct evbuffer *buf, ev_uint64_t flags)
Change the flags that are set for an evbuffer by adding more. EVENT2_EXPORT_SYMBOL int evbuffer_unfreeze (struct evbuffer *buf, int at_front)
Re-enable calls that modify an evbuffer. EVENT2_EXPORT_SYMBOL void evbuffer_unlock (struct evbuffer *buf)
Release the lock on an evbuffer. EVENT2_EXPORT_SYMBOL int evbuffer_write (struct evbuffer *buffer, evutil_socket_t fd)
Write the contents of an evbuffer to a file descriptor. EVENT2_EXPORT_SYMBOL int evbuffer_write_atmost (struct evbuffer *buffer, evutil_socket_t fd, ev_ssize_t howmuch)
Write some of the contents of an evbuffer to a file descriptor.

# Detailed Description

Functions for buffering data for network sending or receiving.
An evbuffer can be used for preparing data before sending it to the network or conversely for reading data from the network. Evbuffers try to avoid memory copies as much as possible. As a result, evbuffers can be used to pass data around without actually incurring the overhead of copying the data.
A new evbuffer can be allocated with evbuffer_new(), and can be freed with evbuffer_free(). Most users will be using evbuffers via the bufferevent interface. To access a bufferevent's evbuffers, use bufferevent_get_input() and bufferevent_get_output().
There are several guidelines for using evbuffers.

•
  if you already know how much data you are going to add as a result of calling evbuffer_add() multiple times, it makes sense to use evbuffer_expand() first to make sure that enough memory is allocated before hand.

•
  evbuffer_add_buffer() adds the contents of one buffer to the other without incurring any unnecessary memory copies.

•
  evbuffer_add() and evbuffer_add_buffer() do not mix very well: if you use them, you will wind up with fragmented memory in your buffer.

•
  For high-performance code, you may want to avoid copying data into and out of buffers. You can skip the copy step by using evbuffer_reserve_space()/evbuffer_commit_space() when writing into a buffer, and evbuffer_peek() when reading.

In Libevent 2.0 and later, evbuffers are represented using a linked list of memory chunks, with pointers to the first and last chunk in the chain.
As the contents of an evbuffer can be stored in multiple different memory blocks, it cannot be accessed directly. Instead, evbuffer_pullup() can be used to force a specified number of bytes to be contiguous. This will cause memory reallocation and memory copies if the data is split across multiple blocks. It is more efficient, however, to use evbuffer_peek() if you don't require that the memory to be contiguous.

# Macro Definition Documentation

## #define EVBUF_FS_DISABLE_LOCKING 0x08

Flag for creating evbuffer_file_segment: Do not allocate a lock for this segment. If this option is set, then neither the segment nor any evbuffer it is added to may ever be accessed from more than one thread at a time.

## #define EVBUF_FS_DISABLE_SENDFILE 0x04

Flag for creating evbuffer_file_segment: Disable direct fd-to-fd implementations (including sendfile and splice). You might want to use this option if data needs to be taken from the evbuffer by any means other than writing it to the network: the sendfile backend is fast, but it only works for sending files directly to the network.

## #define EVBUFFER_CB_ENABLED 1

If this flag is not set, then a callback is temporarily disabled, and should not be invoked.
See also:
evbuffer_cb_set_flags(), evbuffer_cb_clear_flags()

## #define EVBUFFER_FLAG_DRAINS_TO_FD 1

If this flag is set, then we will not use evbuffer_peek(), evbuffer_remove(), evbuffer_remove_buffer(), and so on to read bytes from this buffer: we'll only take bytes out of this buffer by writing them to the network (as with evbuffer_write_atmost), by removing them without observing them (as with evbuffer_drain), or by copying them all out at once (as with evbuffer_add_buffer). Using this option allows the implementation to use sendfile-based operations for evbuffer_add_file(); see that function for more information.
This flag is on by default for bufferevents that can take advantage of it; you should never actually need to set it on a bufferevent's output buffer.

# Typedef Documentation

## typedef void(* evbuffer_cb_func) (struct evbuffer *buffer, const struct evbuffer_cb_info *info, void *arg)

Type definition for a callback that is invoked whenever data is added or removed from an evbuffer. An evbuffer may have one or more callbacks set at a time. The order in which they are executed is undefined.
A callback function may add more callbacks, or remove itself from the list of callbacks, or add or remove data from the buffer. It may not remove another callback from the list.
If a callback adds or removes data from the buffer or from another buffer, this can cause a recursive invocation of your callback or other callbacks. If you ask for an infinite loop, you might just get one: watch out!
Parameters:
buffer the buffer whose size has changed
info a structure describing how the buffer changed.
arg a pointer to user data

## typedef void(* evbuffer_ref_cleanup_cb) (const void *data, size_t datalen, void *extra)

A cleanup function for a piece of memory added to an evbuffer by reference.
See also:
evbuffer_add_reference()

# Enumeration Type Documentation

## enum evbuffer_eol_style

Used to tell evbuffer_readln what kind of line-ending to look for.
Enumerator

EVBUFFER_EOL_ANY
  Any sequence of CR and LF characters is acceptable as an EOL. Note that this style can produce ambiguous results: the sequence 'CRLF' will be treated as a single EOL if it is all in the buffer at once, but if you first read a CR from the network and later read an LF from the network, it will be treated as two EOLs.

EVBUFFER_EOL_CRLF
  An EOL is an LF, optionally preceded by a CR. This style is most useful for implementing text-based internet protocols.

EVBUFFER_EOL_CRLF_STRICT
  An EOL is a CR followed by an LF.

EVBUFFER_EOL_LF
  An EOL is a LF.

EVBUFFER_EOL_NUL
  An EOL is a NUL character (that is, a single byte with value 0)

## enum evbuffer_ptr_how

Defines how to adjust an evbuffer_ptr by evbuffer_ptr_set()
See also:
evbuffer_ptr_set()
Enumerator

EVBUFFER_PTR_SET
  Sets the pointer to the position; can be called on with an uninitialized evbuffer_ptr.

EVBUFFER_PTR_ADD
  Advances the pointer by adding to the current position.

# Function Documentation

## EVENT2_EXPORT_SYMBOL int evbuffer_add (struct evbuffer * buf, const void * data, size_t datlen)

Append data to the end of an evbuffer.
Parameters:
buf the evbuffer to be appended to
data pointer to the beginning of the data buffer
datlen the number of bytes to be copied from the data buffer
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL int evbuffer_add_buffer (struct evbuffer * outbuf, struct evbuffer * inbuf)

Move all data from one evbuffer into another evbuffer. This is a destructive add. The data from one buffer moves into the other buffer. However, no unnecessary memory copies occur.
Parameters:
outbuf the output buffer
inbuf the input buffer
Returns:
0 if successful, or -1 if an error occurred
See also:
evbuffer_remove_buffer()

## EVENT2_EXPORT_SYMBOL int evbuffer_add_buffer_reference (struct evbuffer * outbuf, struct evbuffer * inbuf)

Copy data from one evbuffer into another evbuffer. This is a non-destructive add. The data from one buffer is copied into the other buffer. However, no unnecessary memory copies occur.
Note that buffers already containing buffer references can't be added to other buffers.
Parameters:
outbuf the output buffer
inbuf the input buffer
Returns:
0 if successful, or -1 if an error occurred

## EVENT2_EXPORT_SYMBOL struct evbuffer_cb_entry* evbuffer_add_cb (struct evbuffer * buffer, evbuffer_cb_func cb, void * cbarg)

Add a new callback to an evbuffer. Subsequent calls to evbuffer_add_cb() add new callbacks. To remove this callback, call evbuffer_remove_cb or evbuffer_remove_cb_entry.
Parameters:
buffer the evbuffer to be monitored
cb the callback function to invoke when the evbuffer is modified, or NULL to remove all callbacks.
cbarg an argument to be provided to the callback function
Returns:
a handle to the callback on success, or NULL on failure.

## EVENT2_EXPORT_SYMBOL int evbuffer_add_file (struct evbuffer * outbuf, int fd, ev_off_t offset, ev_off_t length)

Copy data from a file into the evbuffer for writing to a socket. This function avoids unnecessary data copies between userland and kernel. If sendfile is available and the EVBUFFER_FLAG_DRAINS_TO_FD flag is set, it uses those functions. Otherwise, it tries to use mmap (or CreateFileMapping on Windows).
The function owns the resulting file descriptor and will close it when finished transferring data.
The results of using evbuffer_remove() or evbuffer_pullup() on evbuffers whose data was added using this function are undefined.
For more fine-grained control, use evbuffer_add_file_segment.
Parameters:
outbuf the output buffer
fd the file descriptor
offset the offset from which to read data
length how much data to read, or -1 to read as much as possible. (-1 requires that 'fd' support fstat.)
Returns:
0 if successful, or -1 if an error occurred

## EVENT2_EXPORT_SYMBOL int evbuffer_add_file_segment (struct evbuffer * buf, struct evbuffer_file_segment * seg, ev_off_t offset, ev_off_t length)

Insert some or all of an evbuffer_file_segment at the end of an evbuffer. Note that the offset and length parameters of this function have a different meaning from those provided to evbuffer_file_segment_new: When you create the segment, the offset is the offset within the file, and the length is the length of the segment, whereas when you add a segment to an evbuffer, the offset is within the segment and the length is the length of the _part of the segment you want to use.
In other words, if you have a 10 KiB file, and you create an evbuffer_file_segment for it with offset 20 and length 1000, it will refer to bytes 20..1019 inclusive. If you then pass this segment to evbuffer_add_file_segment and specify an offset of 20 and a length of 50, you will be adding bytes 40..99 inclusive.
Parameters:
buf the evbuffer to append to
seg the segment to add
offset the offset within the segment to start from
length the amount of data to add, or -1 to add it all.
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL size_t evbuffer_add_iovec (struct evbuffer * buffer, struct evbuffer_iovec * vec, int n_vec)

Append data from 1 or more iovec's to an evbuffer. Calculates the number of bytes needed for an iovec structure and guarantees all data will fit into a single chain. Can be used in lieu of functionality which calls evbuffer_add() constantly before being used to increase performance.
Parameters:
buffer the destination buffer
vec the source iovec
n_vec the number of iovec structures.
Returns:
the number of bytes successfully written to the output buffer.

## EVENT2_EXPORT_SYMBOL int evbuffer_add_printf (struct evbuffer * buf, const char * fmt, ...)

Append a formatted string to the end of an evbuffer. The string is formated as printf.
Parameters:
buf the evbuffer that will be appended to
fmt a format string
... arguments that will be passed to printf(3)
Returns:
The number of bytes added if successful, or -1 if an error occurred.
See also:
evutil_printf(), evbuffer_add_vprintf()

## EVENT2_EXPORT_SYMBOL int evbuffer_add_reference (struct evbuffer * outbuf, const void * data, size_t datlen, evbuffer_ref_cleanup_cb cleanupfn, void * cleanupfn_arg)

Reference memory into an evbuffer without copying. The memory needs to remain valid until all the added data has been read. This function keeps just a reference to the memory without actually incurring the overhead of a copy.
Parameters:
outbuf the output buffer
data the memory to reference
datlen how memory to reference
cleanupfn callback to be invoked when the memory is no longer referenced by this evbuffer.
cleanupfn_arg optional argument to the cleanup callback
Returns:
0 if successful, or -1 if an error occurred

## EVENT2_EXPORT_SYMBOL int evbuffer_add_vprintf (struct evbuffer * buf, const char * fmt, va_list ap)

Append a va_list formatted string to the end of an evbuffer.
Parameters:
buf the evbuffer that will be appended to
fmt a format string
ap a varargs va_list argument array that will be passed to vprintf(3)
Returns:
The number of bytes added if successful, or -1 if an error occurred.

## EVENT2_EXPORT_SYMBOL int evbuffer_cb_clear_flags (struct evbuffer * buffer, struct evbuffer_cb_entry * cb, ev_uint32_t flags)

Change the flags that are set for a callback on a buffer by removing some.
Parameters:
buffer the evbuffer that the callback is watching.
cb the callback whose status we want to change.
flags EVBUFFER_CB_ENABLED to disable the callback.
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL int evbuffer_cb_set_flags (struct evbuffer * buffer, struct evbuffer_cb_entry * cb, ev_uint32_t flags)

Change the flags that are set for a callback on a buffer by adding more.
Parameters:
buffer the evbuffer that the callback is watching.
cb the callback whose status we want to change.
flags EVBUFFER_CB_ENABLED to re-enable the callback.
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL int evbuffer_clear_flags (struct evbuffer * buf, ev_uint64_t flags)

Change the flags that are set for an evbuffer by removing some.
Parameters:
buffer the evbuffer that the callback is watching.
cb the callback whose status we want to change.
flags One or more EVBUFFER_FLAG_* options
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL int evbuffer_commit_space (struct evbuffer * buf, struct evbuffer_iovec * vec, int n_vecs)

Commits previously reserved space. Commits some of the space previously reserved with evbuffer_reserve_space(). It then becomes available for reading.
This function may return an error if the pointer in the extents do not match those returned from evbuffer_reserve_space, or if data has been added to the buffer since the space was reserved.
If you want to commit less data than you got reserved space for, modify the iov_len pointer of the appropriate extent to a smaller value. Note that you may have received more space than you requested if it was available!
Parameters:
buf the evbuffer in which to reserve space.
vec one or two extents returned by evbuffer_reserve_space.
n_vecs the number of extents.
Returns:
0 on success, -1 on error
See also:
evbuffer_reserve_space()

## EVENT2_EXPORT_SYMBOL ev_ssize_t evbuffer_copyout (struct evbuffer * buf, void * data_out, size_t datlen)

Read data from an evbuffer, and leave the buffer unchanged. If more bytes are requested than are available in the evbuffer, we only extract as many bytes as were available.
Parameters:
buf the evbuffer to be read from
data_out the destination buffer to store the result
datlen the maximum size of the destination buffer
Returns:
the number of bytes read, or -1 if we can't drain the buffer.

## EVENT2_EXPORT_SYMBOL ev_ssize_t evbuffer_copyout_from (struct evbuffer * buf, const struct evbuffer_ptr * pos, void * data_out, size_t datlen)

Read data from the middle of an evbuffer, and leave the buffer unchanged. If more bytes are requested than are available in the evbuffer, we only extract as many bytes as were available.
Parameters:
buf the evbuffer to be read from
pos the position to start reading from
data_out the destination buffer to store the result
datlen the maximum size of the destination buffer
Returns:
the number of bytes read, or -1 if we can't drain the buffer.

## EVENT2_EXPORT_SYMBOL int evbuffer_defer_callbacks (struct evbuffer * buffer, struct event_base * base)

Force all the callbacks on an evbuffer to be run, not immediately after the evbuffer is altered, but instead from inside the event loop. This can be used to serialize all the callbacks to a single thread of execution.

## EVENT2_EXPORT_SYMBOL int evbuffer_drain (struct evbuffer * buf, size_t len)

Remove a specified number of bytes data from the beginning of an evbuffer.
Parameters:
buf the evbuffer to be drained
len the number of bytes to drain from the beginning of the buffer
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL int evbuffer_enable_locking (struct evbuffer * buf, void * lock)

Enable locking on an evbuffer so that it can safely be used by multiple threads at the same time. NOTE: when locking is enabled, the lock will be held when callbacks are invoked. This could result in deadlock if you aren't careful. Plan accordingly!
Parameters:
buf An evbuffer to make lockable.
lock A lock object, or NULL if we should allocate our own.
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL int evbuffer_expand (struct evbuffer * buf, size_t datlen)

Expands the available space in an evbuffer. Expands the available space in the evbuffer to at least datlen, so that appending datlen additional bytes will not require any new allocations.
Parameters:
buf the evbuffer to be expanded
datlen the new minimum length requirement
Returns:
0 if successful, or -1 if an error occurred

## EVENT2_EXPORT_SYMBOL void evbuffer_file_segment_add_cleanup_cb (struct evbuffer_file_segment * seg, evbuffer_file_segment_cleanup_cb cb, void * arg)

Add cleanup callback and argument for the callback to an evbuffer_file_segment. The cleanup callback will be invoked when no more references to the evbuffer_file_segment exist.

## EVENT2_EXPORT_SYMBOL void evbuffer_file_segment_free (struct evbuffer_file_segment * seg)

Free an evbuffer_file_segment. It is safe to call this function even if the segment has been added to one or more evbuffers. The evbuffer_file_segment will not be freed until no more references to it exist.

## EVENT2_EXPORT_SYMBOL struct evbuffer_file_segment* evbuffer_file_segment_new (int fd, ev_off_t offset, ev_off_t length, unsigned flags)

Create and return a new evbuffer_file_segment for reading data from a file and sending it out via an evbuffer. This function avoids unnecessary data copies between userland and kernel. Where available, it uses sendfile or splice.
The file descriptor must not be closed so long as any evbuffer is using this segment.
The results of using evbuffer_remove() or evbuffer_pullup() or any other function that reads bytes from an evbuffer on any evbuffer containing the newly returned segment are undefined, unless you pass the EVBUF_FS_DISABLE_SENDFILE flag to this function.
Parameters:
fd an open file to read from.
offset an index within the file at which to start reading
length how much data to read, or -1 to read as much as possible. (-1 requires that 'fd' support fstat.)
flags any number of the EVBUF_FS_* flags
Returns:
a new evbuffer_file_segment, or NULL on failure.

## EVENT2_EXPORT_SYMBOL void evbuffer_free (struct evbuffer * buf)

Deallocate storage for an evbuffer.
Parameters:
buf pointer to the evbuffer to be freed

## EVENT2_EXPORT_SYMBOL int evbuffer_freeze (struct evbuffer * buf, int at_front)

Prevent calls that modify an evbuffer from succeeding. A buffer may frozen at the front, at the back, or at both the front and the back.
If the front of a buffer is frozen, operations that drain data from the front of the buffer, or that prepend data to the buffer, will fail until it is unfrozen. If the back a buffer is frozen, operations that append data from the buffer will fail until it is unfrozen.
Parameters:
buf The buffer to freeze
at_front If true, we freeze the front of the buffer. If false, we freeze the back.
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL size_t evbuffer_get_contiguous_space (const struct evbuffer * buf)

Returns the number of contiguous available bytes in the first buffer chain. This is useful when processing data that might be split into multiple chains, or that might all be in the first chain. Calls to evbuffer_pullup() that cause reallocation and copying of data can thus be avoided.
Parameters:
buf pointer to the evbuffer
Returns:
0 if no data is available, otherwise the number of available bytes in the first buffer chain.

## EVENT2_EXPORT_SYMBOL size_t evbuffer_get_length (const struct evbuffer * buf)

Returns the total number of bytes stored in the evbuffer.
Parameters:
buf pointer to the evbuffer
Returns:
the number of bytes stored in the evbuffer

## EVENT2_EXPORT_SYMBOL void evbuffer_lock (struct evbuffer * buf)

Acquire the lock on an evbuffer. Has no effect if locking was not enabled with evbuffer_enable_locking.

## EVENT2_EXPORT_SYMBOL struct evbuffer* evbuffer_new (void)

Allocate storage for a new evbuffer.
Returns:
a pointer to a newly allocated evbuffer struct, or NULL if an error occurred

## EVENT2_EXPORT_SYMBOL int evbuffer_peek (struct evbuffer * buffer, ev_ssize_t len, struct evbuffer_ptr * start_at, struct evbuffer_iovec * vec_out, int n_vec)

Function to peek at data inside an evbuffer without removing it or copying it out. Pointers to the data are returned by filling the 'vec_out' array with pointers to one or more extents of data inside the buffer.
The total data in the extents that you get back may be more than you requested (if there is more data last extent than you asked for), or less (if you do not provide enough evbuffer_iovecs, or if the buffer does not have as much data as you asked to see).
Parameters:
buffer the evbuffer to peek into,
len the number of bytes to try to peek. If len is negative, we will try to fill as much of vec_out as we can. If len is negative and vec_out is not provided, we return the number of evbuffer_iovecs that would be needed to get all the data in the buffer.
start_at an evbuffer_ptr indicating the point at which we should start looking for data. NULL means, 'At the start of the
buffer.'
vec_out an array of evbuffer_iovec
n_vec the length of vec_out. If 0, we only count how many extents would be necessary to point to the requested amount of data.
Returns:
The number of extents needed. This may be less than n_vec if we didn't need all the evbuffer_iovecs we were given, or more than n_vec if we would need more to return all the data that was requested.

## EVENT2_EXPORT_SYMBOL int evbuffer_prepend (struct evbuffer * buf, const void * data, size_t size)

Prepends data to the beginning of the evbuffer.
Parameters:
buf the evbuffer to which to prepend data
data a pointer to the memory to prepend
size the number of bytes to prepend
Returns:
0 if successful, or -1 otherwise

## EVENT2_EXPORT_SYMBOL int evbuffer_prepend_buffer (struct evbuffer * dst, struct evbuffer * src)

Prepends all data from the src evbuffer to the beginning of the dst evbuffer.
Parameters:
dst the evbuffer to which to prepend data
src the evbuffer to prepend; it will be emptied as a result
Returns:
0 if successful, or -1 otherwise

## EVENT2_EXPORT_SYMBOL int evbuffer_ptr_set (struct evbuffer * buffer, struct evbuffer_ptr * ptr, size_t position, enum evbuffer_ptr_how how)

Sets the search pointer in the buffer to position. There are two ways to use this function: you can call evbuffer_ptr_set(buf, &pos, N, EVBUFFER_PTR_SET) to move 'pos' to a position 'N' bytes after the start of the buffer, or evbuffer_ptr_set(buf, &pos, N, EVBUFFER_PTR_ADD) to move 'pos' forward by 'N' bytes.
If evbuffer_ptr is not initialized, this function can only be called with EVBUFFER_PTR_SET.
An evbuffer_ptr can represent any position from the start of the buffer to a position immediately after the end of the buffer.
Parameters:
buffer the evbuffer to be search
ptr a pointer to a struct evbuffer_ptr
position the position at which to start the next search
how determines how the pointer should be manipulated.
Returns:
0 on success or -1 otherwise

## EVENT2_EXPORT_SYMBOL unsigned char* evbuffer_pullup (struct evbuffer * buf, ev_ssize_t size)

Makes the data at the beginning of an evbuffer contiguous.
Parameters:
buf the evbuffer to make contiguous
size the number of bytes to make contiguous, or -1 to make the entire buffer contiguous.
Returns:
a pointer to the contiguous memory array, or NULL if param size requested more data than is present in the buffer.

## EVENT2_EXPORT_SYMBOL int evbuffer_read (struct evbuffer * buffer, evutil_socket_t fd, int howmuch)

Read from a file descriptor and store the result in an evbuffer.
Parameters:
buffer the evbuffer to store the result
fd the file descriptor to read from
howmuch the number of bytes to be read
Returns:
the number of bytes read, or -1 if an error occurred
See also:
evbuffer_write()

## EVENT2_EXPORT_SYMBOL char* evbuffer_readln (struct evbuffer * buffer, size_t * n_read_out, enum evbuffer_eol_style eol_style)

Read a single line from an evbuffer. Reads a line terminated by an EOL as determined by the evbuffer_eol_style argument. Returns a newly allocated nul-terminated string; the caller must free the returned value. The EOL is not included in the returned string.
Parameters:
buffer the evbuffer to read from
n_read_out if non-NULL, points to a size_t that is set to the number of characters in the returned string. This is useful for strings that can contain NUL characters.
eol_style the style of line-ending to use.
Returns:
pointer to a single line, or NULL if an error occurred

## EVENT2_EXPORT_SYMBOL int evbuffer_remove (struct evbuffer * buf, void * data, size_t datlen)

Read data from an evbuffer and drain the bytes read. If more bytes are requested than are available in the evbuffer, we only extract as many bytes as were available.
Parameters:
buf the evbuffer to be read from
data the destination buffer to store the result
datlen the maximum size of the destination buffer
Returns:
the number of bytes read, or -1 if we can't drain the buffer.

## EVENT2_EXPORT_SYMBOL int evbuffer_remove_buffer (struct evbuffer * src, struct evbuffer * dst, size_t datlen)

Read data from an evbuffer into another evbuffer, draining the bytes from the source buffer. This function avoids copy operations to the extent possible.
If more bytes are requested than are available in src, the src buffer is drained completely.
Parameters:
src the evbuffer to be read from
dst the destination evbuffer to store the result into
datlen the maximum numbers of bytes to transfer
Returns:
the number of bytes read

## EVENT2_EXPORT_SYMBOL int evbuffer_remove_cb (struct evbuffer * buffer, evbuffer_cb_func cb, void * cbarg)

Remove a callback from an evbuffer, given the function and argument used to add it.
Returns:
0 if a callback was removed, or -1 if no matching callback was found.

## EVENT2_EXPORT_SYMBOL int evbuffer_remove_cb_entry (struct evbuffer * buffer, struct evbuffer_cb_entry * ent)

Remove a callback from an evbuffer, given a handle returned from evbuffer_add_cb. Calling this function invalidates the handle.
Returns:
0 if a callback was removed, or -1 if no matching callback was found.

## EVENT2_EXPORT_SYMBOL int evbuffer_reserve_space (struct evbuffer * buf, ev_ssize_t size, struct evbuffer_iovec * vec, int n_vec)

Reserves space in the last chain or chains of an evbuffer. Makes space available in the last chain or chains of an evbuffer that can be arbitrarily written to by a user. The space does not become available for reading until it has been committed with evbuffer_commit_space().
The space is made available as one or more extents, represented by an initial pointer and a length. You can force the memory to be available as only one extent. Allowing more extents, however, makes the function more efficient.
Multiple subsequent calls to this function will make the same space available until evbuffer_commit_space() has been called.
It is an error to do anything that moves around the buffer's internal memory structures before committing the space.
NOTE: The code currently does not ever use more than two extents. This may change in future versions.
Parameters:
buf the evbuffer in which to reserve space.
size how much space to make available, at minimum. The total length of the extents may be greater than the requested length.
vec an array of one or more evbuffer_iovec structures to hold pointers to the reserved extents of memory.
n_vec The length of the vec array. Must be at least 1; 2 is more efficient.
Returns:
the number of provided extents, or -1 on error.
See also:
evbuffer_commit_space()

## EVENT2_EXPORT_SYMBOL struct evbuffer_ptr evbuffer_search (struct evbuffer * buffer, const char * what, size_t len, const struct evbuffer_ptr * start)

Search for a string within an evbuffer.
Parameters:
buffer the evbuffer to be searched
what the string to be searched for
len the length of the search string
start NULL or a pointer to a valid struct evbuffer_ptr.
Returns:
a struct evbuffer_ptr whose 'pos' field has the offset of the first occurrence of the string in the buffer after 'start'. The 'pos' field of the result is -1 if the string was not found.

## EVENT2_EXPORT_SYMBOL struct evbuffer_ptr evbuffer_search_eol (struct evbuffer * buffer, struct evbuffer_ptr * start, size_t * eol_len_out, enum evbuffer_eol_style eol_style)

Search for an end-of-line string within an evbuffer.
Parameters:
buffer the evbuffer to be searched
start NULL or a pointer to a valid struct evbuffer_ptr to start searching at.
eol_len_out If non-NULL, the pointed-to value will be set to the length of the end-of-line string.
eol_style The kind of EOL to look for; see evbuffer_readln() for more information
Returns:
a struct evbuffer_ptr whose 'pos' field has the offset of the first occurrence EOL in the buffer after 'start'. The 'pos' field of the result is -1 if the string was not found.

## EVENT2_EXPORT_SYMBOL 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)

Search for a string within part of an evbuffer.
Parameters:
buffer the evbuffer to be searched
what the string to be searched for
len the length of the search string
start NULL or a pointer to a valid struct evbuffer_ptr that indicates where we should start searching.
end NULL or a pointer to a valid struct evbuffer_ptr that indicates where we should stop searching.
Returns:
a struct evbuffer_ptr whose 'pos' field has the offset of the first occurrence of the string in the buffer after 'start'. The 'pos' field of the result is -1 if the string was not found.

## EVENT2_EXPORT_SYMBOL int evbuffer_set_flags (struct evbuffer * buf, ev_uint64_t flags)

Change the flags that are set for an evbuffer by adding more.
Parameters:
buffer the evbuffer that the callback is watching.
cb the callback whose status we want to change.
flags One or more EVBUFFER_FLAG_* options
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL int evbuffer_unfreeze (struct evbuffer * buf, int at_front)

Re-enable calls that modify an evbuffer.
Parameters:
buf The buffer to un-freeze
at_front If true, we unfreeze the front of the buffer. If false, we unfreeze the back.
Returns:
0 on success, -1 on failure.

## EVENT2_EXPORT_SYMBOL void evbuffer_unlock (struct evbuffer * buf)

Release the lock on an evbuffer. Has no effect if locking was not enabled with evbuffer_enable_locking.

## EVENT2_EXPORT_SYMBOL int evbuffer_write (struct evbuffer * buffer, evutil_socket_t fd)

Write the contents of an evbuffer to a file descriptor. The evbuffer will be drained after the bytes have been successfully written.
Parameters:
buffer the evbuffer to be written and drained
fd the file descriptor to be written to
Returns:
the number of bytes written, or -1 if an error occurred
See also:
evbuffer_read()

## EVENT2_EXPORT_SYMBOL int evbuffer_write_atmost (struct evbuffer * buffer, evutil_socket_t fd, ev_ssize_t howmuch)

Write some of the contents of an evbuffer to a file descriptor. The evbuffer will be drained after the bytes have been successfully written.
Parameters:
buffer the evbuffer to be written and drained
fd the file descriptor to be written to
howmuch the largest allowable number of bytes to write, or -1 to write as many bytes as we can.
Returns:
the number of bytes written, or -1 if an error occurred
See also:
evbuffer_read()

# Author

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Tue Jan 31 2017 | libevent