ffmpeg4.2.3--libavutil模塊內存管理

核心模塊

內存管理

buffer.h / buffer_internal.h

主要是ffmpeg緩存數據的主要接口，主要包含如下幾個數據結構：AVBuffer，AVBufferRef，BufferPoolEntry，AVBufferPool。

AVBuffer/AVBufferRef 結構

struct AVBuffer {
uint8_t *data; /**< data described by this buffer */
//緩衝區地址
int      size; /**< size of data in bytes */
//緩衝區大小
/**
* number of existing AVBufferRef instances referring to this buffer
*/
atomic_uint refcount;
//引用計數值 ，原子操做
/**
* a callback for freeing the data
*/
void (*free)(void *opaque, uint8_t *data);
//用於釋放緩衝區內存的回調函數
/**
* an opaque pointer, to be used by the freeing callback
*/
void *opaque;
//提供給free回調函數的參數
/**
* A combination of BUFFER_FLAG_*
*/
int flags;
// 緩衝區標誌
};

typedef struct AVBufferRef {
AVBuffer *buffer;
​
/**
* The data buffer. It is considered writable if and only if
* this is the only reference to the buffer, in which case
* av_buffer_is_writable() returns 1.
*/
uint8_t *data;
/**
* Size of data in bytes.
*/
//緩衝區地址，實際等於buffer->data
int      size;
//緩衝區大小，實際等於buffer->size
} AVBufferRef;

AVBufferRef *av_buffer_alloc(int size)
{
AVBufferRef *ret = NULL;
uint8_t    *data = NULL;
​
data = av_malloc(size);
//分配緩衝區
if (!data)
return NULL;
​
ret = av_buffer_create(data, size, av_buffer_default_free, NULL, 0);
if (!ret)
av_freep(&data);
​
return ret;
}

AVBufferRef *av_buffer_create(uint8_t *data, int size,
void (*free)(void *opaque, uint8_t *data),
void *opaque, int flags)
{
AVBufferRef *ref = NULL;
AVBuffer    *buf = NULL;
​
buf = av_mallocz(sizeof(*buf));
if (!buf)
return NULL;
​
buf->data     = data;
buf->size     = size;
buf->free     = free ? free : av_buffer_default_free;
buf->opaque   = opaque;
​
atomic_init(&buf->refcount, 1);
​
if (flags & AV_BUFFER_FLAG_READONLY)
buf->flags |= BUFFER_FLAG_READONLY;
​
ref = av_mallocz(sizeof(*ref));
if (!ref) {
av_freep(&buf);
return NULL;
}
​
ref->buffer = buf;
ref->data   = data;
ref->size   = size;
​
return ref;
}

void av_buffer_default_free(void *opaque, uint8_t *data)
{
av_free(data);
}
​

AVBufferRef *av_buffer_ref(AVBufferRef *buf)
{
AVBufferRef *ret = av_mallocz(sizeof(*ret));
​
if (!ret)
return NULL;
​
*ret = *buf;
//共用同一份緩衝區
atomic_fetch_add_explicit(&buf->buffer->refcount, 1, memory_order_relaxed);
//引用計數加1
return ret;
}

static void buffer_replace(AVBufferRef **dst, AVBufferRef **src)
{
AVBuffer *b;
​
b = (*dst)->buffer;
​
if (src) {
**dst = **src;
av_freep(src);
} else
av_freep(dst);
//引用計數減1
if (atomic_fetch_add_explicit(&b->refcount, -1, memory_order_acq_rel) == 1) {
b->free(b->opaque, b->data);
av_freep(&b);
// 緩衝區引用計數變爲0，則將緩衝區也回收
}
}
​
void av_buffer_unref(AVBufferRef **buf)
{
if (!buf || !*buf)
return;
​
buffer_replace(buf, NULL);
}

typedef struct BufferPoolEntry {
uint8_t *data;
​
/*
* Backups of the original opaque/free of the AVBuffer corresponding to
* data. They will be used to free the buffer when the pool is freed.
*/
void *opaque;
void (*free)(void *opaque, uint8_t *data);
​
AVBufferPool *pool;
struct BufferPoolEntry *next;
} BufferPoolEntry;
​
struct AVBufferPool {
AVMutex mutex;
BufferPoolEntry *pool;
​
/*
* This is used to track when the pool is to be freed.
* The pointer to the pool itself held by the caller is considered to
* be one reference. Each buffer requested by the caller increases refcount
* by one, returning the buffer to the pool decreases it by one.
* refcount reaches zero when the buffer has been uninited AND all the
* buffers have been released, then it's safe to free the pool and all
* the buffers in it.
*/
atomic_uint refcount;
​
int size;
void *opaque;
AVBufferRef* (*alloc)(int size);
AVBufferRef* (*alloc2)(void *opaque, int size);
void         (*pool_free)(void *opaque);
};

AVBufferPool *av_buffer_pool_init2(int size, void *opaque,
AVBufferRef* (*alloc)(void *opaque, int size),
void (*pool_free)(void *opaque))
{
AVBufferPool *pool = av_mallocz(sizeof(*pool));
if (!pool)
return NULL;
​
ff_mutex_init(&pool->mutex, NULL);
​
pool->size      = size;
pool->opaque    = opaque;
pool->alloc2    = alloc;
pool->pool_free = pool_free;
​
atomic_init(&pool->refcount, 1);
​
return pool;
}

void av_buffer_pool_uninit(AVBufferPool **ppool)
{
AVBufferPool *pool;
​
if (!ppool || !*ppool)
return;
pool   = *ppool;
*ppool = NULL;
​
if (atomic_fetch_add_explicit(&pool->refcount, -1, memory_order_acq_rel) == 1)
buffer_pool_free(pool);
}

static void pool_release_buffer(void *opaque, uint8_t *data)
{
BufferPoolEntry *buf = opaque;
AVBufferPool *pool = buf->pool;
​
if(CONFIG_MEMORY_POISONING)
memset(buf->data, FF_MEMORY_POISON, pool->size);
​
ff_mutex_lock(&pool->mutex); //加鎖
buf->next = pool->pool;
pool->pool = buf;
ff_mutex_unlock(&pool->mutex);
​
if (atomic_fetch_add_explicit(&pool->refcount, -1, memory_order_acq_rel) == 1)
buffer_pool_free(pool);
}

static AVBufferRef *pool_alloc_buffer(AVBufferPool *pool)
{
BufferPoolEntry *buf;
AVBufferRef     *ret;
​
ret = pool->alloc2 ? pool->alloc2(pool->opaque, pool->size) :
pool->alloc(pool->size);
if (!ret)
return NULL;
​
buf = av_mallocz(sizeof(*buf));
if (!buf) {
av_buffer_unref(&ret);
return NULL;
}
​
buf->data   = ret->buffer->data;
buf->opaque = ret->buffer->opaque;
buf->free   = ret->buffer->free;
buf->pool   = pool;
​
ret->buffer->opaque = buf;
ret->buffer->free   = pool_release_buffer;
​
return ret;
}
​
AVBufferRef *av_buffer_pool_get(AVBufferPool *pool)
{
AVBufferRef *ret;
BufferPoolEntry *buf;
//加鎖
ff_mutex_lock(&pool->mutex);
buf = pool->pool;
if (buf) {
ret = av_buffer_create(buf->data, pool->size, pool_release_buffer,
buf, 0);
if (ret) {
pool->pool = buf->next;
buf->next = NULL;
}
} else {
ret = pool_alloc_buffer(pool);
}
ff_mutex_unlock(&pool->mutex);
​
if (ret)
atomic_fetch_add_explicit(&pool->refcount, 1, memory_order_relaxed);
​
return ret;
}

• 內部每生成一個buffer，引用計數+1,回收一個buffer,引用計數-1。這二者也是匹配的。
• 音視頻編解碼中大量使用內存池的方式
• 由於有鎖的控制，能夠在多線程的環境中使用。
• c語言中結合引用計數來管理內存基本上大多數開源庫的解決方案。
• av_buffer_pool_init的時候,引用計數爲初始值1，調用av_buffer_pool_uninit標記爲可銷燬，引用計數減1。
• 從池裏生成的buffer，在釋放的時候，是再回到池裏，而且池的引用計數-1。也就是這是一個循環使用的緩衝池，使用引用計數來標記內部的緩衝區。
• pool是一個緩衝池，管理者衆多的內存緩衝區AVBuffer

1. pool_alloc_buffer / av_buffer_pool_get

   內存池中沒法獲取到buffer就建立

2. pool_release_buffer

   回收內存池的buffer

3. av_buffer_pool_uninit

   釋放內存池

4. av_buffer_pool_init2

   建立緩衝池，獲得AVBufferPool指針pool， pool的refcount爲1,pool的pool變量值爲NULL

   AVBufferPool主要解決用戶須要使用一樣長度的緩衝區的狀況，好比原始音視頻幀。 在開始用戶能夠調用av_buffer_pool_init來建立緩衝池。而後在任什麼時候間均可以調用av_buffer_pool_get()來得到buffer，在該buffer的引用計數爲0時，將會返回給緩衝池，這樣就能夠被循環使用了。

AVBufferPool 相關的函數

AVBufferPool 結構

1. av_buffer_unref
2. av_buffer_ref

av_buffer_default_free是默認的內存回收函數

兩個函數的關係很清楚，av_buffer_create會給AVBuffer裏面字段進行賦值

1. av_buffer_create

很是很是標準的c語言內存申請模式，返回AVBufferRef的指針來操做。

1. av_buffer_alloc

AVBuffer/AVBufferRef 關鍵函數

1. AVBufferRef則對AVBuffer緩衝區提供了一層封裝，最主要的是做引用計數處理，實現了一種安全機制
2. 用戶不該直接訪問AVBuffer，應經過AVBufferRef來訪問AVBuffer，以保證安全。
3. FFmpeg中不少基礎的數據結構都包含了AVBufferRef成員，來間接使用AVBuffer緩衝區。
4. 內部，對於refcount的訪問都是原子函數，好比：atomic_load(&buf->buffer->refcount)。