FFmpeg-基础组件-AVBufferPool

FFmpeg实现了一个AVBufferPool ，这个pool可以用来提前做些内存分配等，在ffmpeg cuvid插件中hwcontext_cuda.c文件夹中可以看到这个Pool的用法。

下面是关键结构体的定义，可以看到几个比较重要的函数指针，比如：

void (*free)(void *opaque, uint8_t *data);

这个是用来释放AVBuffer 中的data数据的，可以由用户来指定。


/**
 * The buffer was av_realloc()ed, so it is reallocatable.
 */
#define BUFFER_FLAG_REALLOCATABLE (1 << 0)

struct AVBuffer {
    uint8_t *data; /**< data described by this buffer */
    buffer_size_t size; /**< size of data in bytes */

    /**
     *  number of existing AVBufferRef instances referring to this buffer
     */
    atomic_uint refcount;

    /**
     * a callback for freeing the data
     */
    void (*free)(void *opaque, uint8_t *data);

    /**
     * an opaque pointer, to be used by the freeing callback
     */
    void *opaque;

    /**
     * A combination of AV_BUFFER_FLAG_*
     */
    int flags;

    /**
     * A combination of BUFFER_FLAG_*
     */
    int flags_internal;
};

下面是Pool的元素，BufferPoolEntry，可以看到一个next指针，其实就是一个单向链表。
其中free用来释放buffer


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;

下面是一个bufferPoll的定义，其中有一个refcount，作为ref来使用，另外有两个alloc函数和pool_free


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;

    buffer_size_t size;
    void *opaque;
    AVBufferRef* (*alloc)(buffer_size_t size);
    AVBufferRef* (*alloc2)(void *opaque, buffer_size_t size);
    void         (*pool_free)(void *opaque);
};

下面的注释，其实已经说明了该怎么去使用buffer poll。

/**
 * @defgroup lavu_bufferpool AVBufferPool
 * @ingroup lavu_data
 *
 * @{
 * AVBufferPool is an API for a lock-free thread-safe pool of AVBuffers.
 *
 * Frequently allocating and freeing large buffers may be slow. AVBufferPool is
 * meant to solve this in cases when the caller needs a set of buffers of the
 * same size (the most obvious use case being buffers for raw video or audio
 * frames).
 *
 * At the beginning, the user must call av_buffer_pool_init() to create the
 * buffer pool. Then whenever a buffer is needed, call av_buffer_pool_get() to
 * get a reference to a new buffer, similar to av_buffer_alloc(). This new
 * reference works in all aspects the same way as the one created by
 * av_buffer_alloc(). However, when the last reference to this buffer is
 * unreferenced, it is returned to the pool instead of being freed and will be
 * reused for subsequent av_buffer_pool_get() calls.
 *
 * When the caller is done with the pool and no longer needs to allocate any new
 * buffers, av_buffer_pool_uninit() must be called to mark the pool as freeable.
 * Once all the buffers are released, it will automatically be freed.
 *
 * Allocating and releasing buffers with this API is thread-safe as long as
 * either the default alloc callback is used, or the user-supplied one is
 * thread-safe.
 */

/**
 * The buffer pool. This structure is opaque and not meant to be accessed
 * directly. It is allocated with av_buffer_pool_init() and freed with
 * av_buffer_pool_uninit().
 */
typedef struct AVBufferPool AVBufferPool;

/**
 * Allocate and initialize a buffer pool.
 *
 * @param size size of each buffer in this pool
 * @param alloc a function that will be used to allocate new buffers when the
 * pool is empty. May be NULL, then the default allocator will be used
 * (av_buffer_alloc()).
 * @return newly created buffer pool on success, NULL on error.
 */
#if FF_API_BUFFER_SIZE_T
AVBufferPool *av_buffer_pool_init(int size, AVBufferRef* (*alloc)(int size));
#else
AVBufferPool *av_buffer_pool_init(size_t size, AVBufferRef* (*alloc)(size_t size));
#endif

/**
 * Allocate and initialize a buffer pool with a more complex allocator.
 *
 * @param size size of each buffer in this pool
 * @param opaque arbitrary user data used by the allocator
 * @param alloc a function that will be used to allocate new buffers when the
 *              pool is empty. May be NULL, then the default allocator will be
 *              used (av_buffer_alloc()).
 * @param pool_free a function that will be called immediately before the pool
 *                  is freed. I.e. after av_buffer_pool_uninit() is called
 *                  by the caller and all the frames are returned to the pool
 *                  and freed. It is intended to uninitialize the user opaque
 *                  data. May be NULL.
 * @return newly created buffer pool on success, NULL on error.
 */
#if FF_API_BUFFER_SIZE_T
AVBufferPool *av_buffer_pool_init2(int size, void *opaque,
                                   AVBufferRef* (*alloc)(void *opaque, int size),
#else
AVBufferPool *av_buffer_pool_init2(size_t size, void *opaque,
                                   AVBufferRef* (*alloc)(void *opaque, size_t size),
#endif
                                   void (*pool_free)(void *opaque));

/**
 * Mark the pool as being available for freeing. It will actually be freed only
 * once all the allocated buffers associated with the pool are released. Thus it
 * is safe to call this function while some of the allocated buffers are still
 * in use.
 *
 * @param pool pointer to the pool to be freed. It will be set to NULL.
 */
void av_buffer_pool_uninit(AVBufferPool **pool);

/**
 * Allocate a new AVBuffer, reusing an old buffer from the pool when available.
 * This function may be called simultaneously from multiple threads.
 *
 * @return a reference to the new buffer on success, NULL on error.
 */
AVBufferRef *av_buffer_pool_get(AVBufferPool *pool);

/**
 * Query the original opaque parameter of an allocated buffer in the pool.
 *
 * @param ref a buffer reference to a buffer returned by av_buffer_pool_get.
 * @return the opaque parameter set by the buffer allocator function of the
 *         buffer pool.
 *
 * @note the opaque parameter of ref is used by the buffer pool implementation,
 * therefore you have to use this function to access the original opaque
 * parameter of an allocated buffer.
 */
void *av_buffer_pool_buffer_get_opaque(AVBufferRef *ref);