ffmpeg4.2.3--libavutil模块内存管理

核心模块

内存管理

buffer.h / buffer_internal.h

主要是ffmpeg缓存数据的主要接口，主要包含以下几个数据结构：AVBuffer，AVBufferRef，BufferPoolEntry，AVBufferPool。

AVBuffer/AVBufferRef 结构

struct AVBuffer {
    uint8_t *data; /**< data described by this buffer */
    //缓冲区地址
    int      size; /**< size of data in bytes */
    //缓冲区大小
    /**
     *  number of existing AVBufferRef instances referring to this buffer
     */
    atomic_uint refcount;
    //引用计数值 ，原子操作
    /**
     * a callback for freeing the data
     */
    void (*free)(void *opaque, uint8_t *data);
    //用于释放缓冲区内存的回调函数
    /**
     * an opaque pointer, to be used by the freeing callback
     */
    void *opaque;
    //提供给free回调函数的参数
    /**
     * A combination of BUFFER_FLAG_*
     */
    int flags;
    // 缓冲区标志
};

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.
     */
  //缓冲区地址，实际等于buffer->data
    int      size;
  //缓冲区大小，实际等于buffer->size
} AVBufferRef;

1. AVBufferRef则对AVBuffer缓冲区提供了一层封装，最主要的是作引用计数处理，实现了一种安全机制
2. 用户不应直接访问AVBuffer，应通过AVBufferRef来访问AVBuffer，以保证安全。
3. FFmpeg中很多基础的数据结构都包含了AVBufferRef成员，来间接使用AVBuffer缓冲区。
4. 内部，对于refcount的访问都是原子函数，比如：atomic_load(&buf->buffer->refcount)。

AVBuffer/AVBufferRef 关键函数

1. av_buffer_alloc

AVBufferRef *av_buffer_alloc(int size)
{
    AVBufferRef *ret = NULL;
    uint8_t    *data = NULL;

    data = av_malloc(size);
    //分配缓冲区
    if (!data)
        return NULL;

    ret = av_buffer_create(data, size, av_buffer_default_free, NULL, 0);
    if (!ret)
        av_freep(&data);

    return ret;
}

非常非常标准的c语言内存申请模式，返回AVBufferRef的指针来操作。

1. av_buffer_create

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;
}

两个函数的关系很清楚，av_buffer_create会给AVBuffer里面字段进行赋值

av_buffer_default_free是默认的内存回收函数

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

1. av_buffer_ref

AVBufferRef *av_buffer_ref(AVBufferRef *buf)
{
    AVBufferRef *ret = av_mallocz(sizeof(*ret));

    if (!ret)
        return NULL;

    *ret = *buf;
    //共用同一份缓冲区
    atomic_fetch_add_explicit(&buf->buffer->refcount, 1, memory_order_relaxed);
    //引用计数加1
    return ret;
}

1. av_buffer_unref

static void buffer_replace(AVBufferRef **dst, AVBufferRef **src)
{
    AVBuffer *b;

    b = (*dst)->buffer;

    if (src) {
        **dst = **src;
        av_freep(src);
    } else
        av_freep(dst);
    //引用计数减1
    if (atomic_fetch_add_explicit(&b->refcount, -1, memory_order_acq_rel) == 1) {
        b->free(b->opaque, b->data);
        av_freep(&b);
      // 缓冲区引用计数变为0，则将缓冲区也回收
    }
}

void av_buffer_unref(AVBufferRef **buf)
{
    if (!buf || !*buf)
        return;

    buffer_replace(buf, NULL);
}

AVBufferPool 结构

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 相关的函数

1. av_buffer_pool_init2

   创建缓冲池，得到AVBufferPool指针pool， pool的refcount为1,pool的pool变量值为NULL

   AVBufferPool主要解决用户需要使用同样长度的缓冲区的情况，比如原始音视频帧。
   在开始用户可以调用av_buffer_pool_init来创建缓冲池。然后在任何时间都可以调用av_buffer_pool_get()来获得buffer，在该buffer的引用计数为0时，将会返回给缓冲池，这样就可以被循环使用了。

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;
}

1. av_buffer_pool_uninit

   释放内存池

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);
}

1. pool_release_buffer

   回收内存池的buffer

   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); //加锁
    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);
}

1. pool_alloc_buffer / av_buffer_pool_get

   内存池中无法获取到buffer就创建

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;
    //加锁
    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;
}

• pool是一个缓冲池，管理者众多的内存缓冲区AVBuffer
• 从池里生成的buffer，在释放的时候，是再回到池里，并且池的引用计数-1。也就是这是一个循环使用的缓冲池，使用引用计数来标记内部的缓冲区。
• av_buffer_pool_init的时候,引用计数为初始值1，调用av_buffer_pool_uninit标记为可销毁，引用计数减1。
• 内部每生成一个buffer，引用计数+1,回收一个buffer,引用计数-1。这两者也是匹配的。
• 音视频编解码中大量使用内存池的方式
• 因为有锁的控制，可以在多线程的环境中使用。
• c语言中结合引用计数来管理内存基本上大多数开源库的解决方案。