环形缓冲区

环形缓冲区的基本概念

环形缓冲区的基本概念来自Wikipedia：环形缓冲器

圆形缓冲区（circular buffer），也称作圆形队列（circular queue），循环缓冲区（cyclic buffer），环形缓冲区（ring buffer），是一种用于表示一个固定尺寸、头尾相连的缓冲区的数据结构，适合缓存数据流。

圆形缓冲区的一个有用特性是：当一个数据元素被用掉后，其余数据元素不需要移动其存储位置。相反，一个非圆形缓冲区（例如一个普通的队列）在用掉一个数据元素后，其余数据元素需要向前搬移。换句话说，圆形缓冲区适合实现先进先出缓冲区，而非圆形缓冲区适合后进先出缓冲区。

圆形缓冲区适合于事先明确了缓冲区的最大容量的情形。扩展一个圆形缓冲区的容量，需要搬移其中的数据。因此一个缓冲区如果需要经常调整其容量，用链表实现更为合适。

写操作覆盖圆形缓冲区中未被处理的数据在某些情况下是允许的。特别是在多媒体处理时。例如，音频的生产者可以覆盖掉声卡尚未来得及处理的音频数据。

圆形缓冲区的实现

环形缓冲区的特点：

1. 环形缓冲区的大小是固定的，在整个过程中只申请一次内存。

2. 是一种先进先出的缓冲区。

3. 写操作时，当要写入的size大于available write size时有两种处理方式：覆盖、不覆盖（哈哈，废话），依情况选择。

我只是代码的搬运工，代码来自webrtc的ring_buffer.h，具体位置是

webrtc/common_audio/ring_buffer.h

。读写操作是非线程安全的，所以需要调用者自己去保证线程安全。写操作遇到要写入的大小大于可用大小时选择的方式是：部分写入的策略。所以在使用的时候不顺心可以改改改！

对外提供的API代码：

是一个完全独立的功能，不依赖任何webrtc的头文件，可以脱离webrtc在任意地方使用。考虑到并非每一个人都能访问googlesource，so直接列出源码。官方-ring_buffer.h

#ifndef WEBRTC_COMMON_AUDIO_RING_BUFFER_H_
#define WEBRTC_COMMON_AUDIO_RING_BUFFER_H_

#ifdef __cplusplus
extern "C" {
#endif

#include <stddef.h>  // size_t

typedef struct RingBuffer RingBuffer;

// Creates and initializes the buffer. Returns NULL on failure.
RingBuffer* WebRtc_CreateBuffer(size_t element_count, size_t element_size);
void WebRtc_InitBuffer(RingBuffer* handle);
void WebRtc_FreeBuffer(void* handle);

// Reads data from the buffer. The |data_ptr| will point to the address where
// it is located. If all |element_count| data are feasible to read without
// buffer wrap around |data_ptr| will point to the location in the buffer.
// Otherwise, the data will be copied to |data| (memory allocation done by the
// user) and |data_ptr| points to the address of |data|. |data_ptr| is only
// guaranteed to be valid until the next call to WebRtc_WriteBuffer().
//
// To force a copying to |data|, pass a NULL |data_ptr|.
//
// Returns number of elements read.
size_t WebRtc_ReadBuffer(RingBuffer* handle,
void** data_ptr,
void* data,
size_t element_count);

// Writes |data| to buffer and returns the number of elements written.
size_t WebRtc_WriteBuffer(RingBuffer* handle, const void* data,
size_t element_count);

// Moves the buffer read position and returns the number of elements moved.
// Positive |element_count| moves the read position towards the write position,
// that is, flushing the buffer. Negative |element_count| moves the read
// position away from the the write position, that is, stuffing the buffer.
// Returns number of elements moved.
int WebRtc_MoveReadPtr(RingBuffer* handle, int element_count);

// Returns number of available elements to read.
size_t WebRtc_available_read(const RingBuffer* handle);

// Returns number of available elements for write.
size_t WebRtc_available_write(const RingBuffer* handle);

#ifdef __cplusplus
}
#endif

#endif  // WEBRTC_COMMON_AUDIO_RING_BUFFER_H_

实现代码：

只依赖标准库，足够的说明，Good。官方-ring_buffer.c

#include "ring_buffer.h"

#include <stddef.h>  // size_t
#include <stdlib.h>
#include <string.h>

enum Wrap {
SAME_WRAP,
DIFF_WRAP
};

struct RingBuffer {
size_t read_pos;
size_t write_pos;
size_t element_count;
size_t element_size;
enum Wrap rw_wrap;
char* data;
};

// Get address of region(s) from which we can read data.
// If the region is contiguous, |data_ptr_bytes_2| will be zero.
// If non-contiguous, |data_ptr_bytes_2| will be the size in bytes of the second
// region. Returns room available to be read or |element_count|, whichever is
// smaller.
static size_t GetBufferReadRegions(RingBuffer* buf,
size_t element_count,
void** data_ptr_1,
size_t* data_ptr_bytes_1,
void** data_ptr_2,
size_t* data_ptr_bytes_2) {

const size_t readable_elements = WebRtc_available_read(buf);
const size_t read_elements = (readable_elements < element_count ?
readable_elements : element_count);
const size_t margin = buf->element_count - buf->read_pos;

// Check to see if read is not contiguous.
if (read_elements > margin) {
// Write data in two blocks that wrap the buffer.
*data_ptr_1 = buf->data + buf->read_pos * buf->element_size;
*data_ptr_bytes_1 = margin * buf->element_size;
*data_ptr_2 = buf->data;
*data_ptr_bytes_2 = (read_elements - margin) * buf->element_size;
} else {
*data_ptr_1 = buf->data + buf->read_pos * buf->element_size;
*data_ptr_bytes_1 = read_elements * buf->element_size;
*data_ptr_2 = NULL;
*data_ptr_bytes_2 = 0;
}

return read_elements;
}

RingBuffer* WebRtc_CreateBuffer(size_t element_count, size_t element_size) {
RingBuffer* self = NULL;
if (element_count == 0 || element_size == 0) {
return NULL;
}

self = malloc(sizeof(RingBuffer));
if (!self) {
return NULL;
}

self->data = malloc(element_count * element_size);
if (!self->data) {
free(self);
self = NULL;
return NULL;
}

self->element_count = element_count;
self->element_size = element_size;
WebRtc_InitBuffer(self);

return self;
}

void WebRtc_InitBuffer(RingBuffer* self) {
self->read_pos = 0;
self->write_pos = 0;
self->rw_wrap = SAME_WRAP;

// Initialize buffer to zeros
memset(self->data, 0, self->element_count * self->element_size);
}

void WebRtc_FreeBuffer(void* handle) {
RingBuffer* self = (RingBuffer*)handle;
if (!self) {
return;
}

free(self->data);
free(self);
}

size_t WebRtc_ReadBuffer(RingBuffer* self,
void** data_ptr,
void* data,
size_t element_count) {

if (self == NULL) {
return 0;
}
if (data == NULL) {
return 0;
}

{
void* buf_ptr_1 = NULL;
void* buf_ptr_2 = NULL;
size_t buf_ptr_bytes_1 = 0;
size_t buf_ptr_bytes_2 = 0;
const size_t read_count = GetBufferReadRegions(self,
element_count,
&buf_ptr_1,
&buf_ptr_bytes_1,
&buf_ptr_2,
&buf_ptr_bytes_2);

if (buf_ptr_bytes_2 > 0) {
// We have a wrap around when reading the buffer. Copy the buffer data to
// |data| and point to it.
memcpy(data, buf_ptr_1, buf_ptr_bytes_1);
memcpy(((char*) data) + buf_ptr_bytes_1, buf_ptr_2, buf_ptr_bytes_2);
buf_ptr_1 = data;
} else if (!data_ptr) {
// No wrap, but a memcpy was requested.
memcpy(data, buf_ptr_1, buf_ptr_bytes_1);
}
if (data_ptr) {
// |buf_ptr_1| == |data| in the case of a wrap.
*data_ptr = buf_ptr_1;
}

// Update read position
WebRtc_MoveReadPtr(self, (int) read_count);

return read_count;
}
}

size_t WebRtc_WriteBuffer(RingBuffer* self,
const void* data,
size_t element_count) {
if (!self) {
return 0;
}
if (!data) {
return 0;
}

{
const size_t free_elements = WebRtc_available_write(self);
const size_t write_elements = (free_elements < element_count ? free_elements
: element_count);
size_t n = write_elements;
const size_t margin = self->element_count - self->write_pos;

if (write_elements > margin) {
// Buffer wrap around when writing.
memcpy(self->data + self->write_pos * self->element_size,
data, margin * self->element_size);
self->write_pos = 0;
n -= margin;
self->rw_wrap = DIFF_WRAP;
}
memcpy(self->data + self->write_pos * self->element_size,
((const char*) data) + ((write_elements - n) * self->element_size),
n * self->element_size);
self->write_pos += n;

return write_elements;
}
}

int WebRtc_MoveReadPtr(RingBuffer* self, int element_count) {
if (!self) {
return 0;
}

{
// We need to be able to take care of negative changes, hence use "int"
// instead of "size_t".
const int free_elements = (int) WebRtc_available_write(self);
const int readable_elements = (int) WebRtc_available_read(self);
int read_pos = (int) self->read_pos;

if (element_count > readable_elements) {
element_count = readable_elements;
}
if (element_count < -free_elements) {
element_count = -free_elements;
}

read_pos += element_count;
if (read_pos > (int) self->element_count) {
// Buffer wrap around. Restart read position and wrap indicator.
read_pos -= (int) self->element_count;
self->rw_wrap = SAME_WRAP;
}
if (read_pos < 0) {
// Buffer wrap around. Restart read position and wrap indicator.
read_pos += (int) self->element_count;
self->rw_wrap = DIFF_WRAP;
}

self->read_pos = (size_t) read_pos;

return element_count;
}
}

size_t WebRtc_available_read(const RingBuffer* self) {
if (!self) {
return 0;
}

if (self->rw_wrap == SAME_WRAP) {
return self->write_pos - self->read_pos;
} else {
return self->element_count - self->read_pos + self->write_pos;
}
}

size_t WebRtc_available_write(const RingBuffer* self) {
if (!self) {
return 0;
}

return self->element_count - WebRtc_available_read(self);
}

功能测试

Google的代码文档少，但是必定会有测试代码，一般是Gtest的单元测试，很专业。

ring_buffer_unittest.cc