FFmpeg内存IO模式(内存区作输入或输出)

本文为作者原创，转载请注明出处：https://www.cnblogs.com/leisure_chn/p/10318145.html

所谓内存IO，在FFmpeg中叫作“buffered IO”或“custom IO”，指的是将一块内存缓冲区用作FFmpeg的输入或输出。与内存IO操作对应的是指定URL作为FFmpeg的输入或输出，比如URL可能是普通文件或网络流地址等。这两种输入输出模式我们暂且称作“内存IO模式”和“URL-IO模式”。

本文源码基于FFmpeg 4.1版本，为帮助理解，可参考FFmpeg工程examples中如下两份代码：

https://github.com/FFmpeg/FFmpeg/blob/n4.1/doc/examples/avio_reading.c

https://github.com/FFmpeg/FFmpeg/blob/n4.1/doc/examples/remuxing.c

1. 内存区作输入

1.1 用法

用法如示例中注释的步骤，如下：

// @opaque  : 是由用户提供的参数，指向用户数据
// @buf     : 作为FFmpeg的输入，此处由用户准备好buf中的数据
// @buf_size: buf的大小
// @return  : 本次IO数据量
static int read_packet(void *opaque, uint8_t *buf, int buf_size)
{
    int fd = *((int *)opaque);
    int ret = read(fd, buf, buf_size);
    return ret;
}

int main()
{
    AVFormatContext *ifmt_ctx = NULL;
    AVIOContext *avio_in = NULL;
    uint8_t *ibuf = NULL;
    size_t ibuf_size = 4096;
    int fd = -1;

    // 打开一个FIFO文件的读端
    fd = open_fifo_for_read("/tmp/test_fifo");

    // 1. 分配缓冲区
    ibuf = av_malloc(ibuf_size);

    // 2. 分配AVIOContext，第三个参数write_flag为0
    avio_in = avio_alloc_context(ibuf, ibuf_size, 0, &fd, &read_packet, NULL, NULL);

    // 3. 分配AVFormatContext，并指定AVFormatContext.pb字段。必须在调用avformat_open_input()之前完成
    ifmt_ctx = avformat_alloc_context();
    ifmt_ctx->pb = avio_in;

    // 4. 打开输入(读取封装格式文件头)
    avformat_open_input(&ifmt_ctx, NULL, NULL, NULL);

    ......
}

当启用内存IO模式后(即ifmt_ctx->pb有效时)，将会忽略avformat_open_input()第二个参数url的值。在上述示例中，打开了FIFO的读端，并在回调函数中将FIFO中的数据填入内存缓冲区ibuf，内存缓冲区ibuf将作为FFmpeg的输入。在上述示例中，因为打开的是一个命名管道FIFO，FIFO的数据虽然在内存中，但FIFO有名字("/tmp/test_fifo")，所以此例也可以使用URL-IO模式，如下：

AVFormatContext *ifmt_ctx = NULL;
avformat_open_input(&ifmt_ctx, "/tmp/test_fifo", NULL, NULL);

而对于其他一些场合，当有效音视频数据位于内存，而这片内存并无一个URL属性可用时，则只能使用内存IO模式来取得输入数据。

1.2 回调时机

回调函数何时被回调呢？所有需要从输入源中读取数据的时刻，都将调用回调函数。和输入源是普通文件相比，只不过输入源变成了内存区，其他各种外在表现并无不同。

如下各函数在不同的阶段从输入源读数据，都会调用回调函数：

avformat_open_input() 从输入源读取封装格式文件头

avformat_find_stream_info() 从输入源读取一段数据，尝试解码，以获取流信息

av_read_frame() 从输入源读取数据包

2. 内存区作输出

2.1 用法

用法如示例中注释的步骤，如下：

// @opaque  : 是由用户提供的参数，指向用户数据
// @buf     : 作为FFmpeg的输出，此处FFmpeg已准备好buf中的数据
// @buf_size: buf的大小
// @return  : 本次IO数据量
static int write_packet(void *opaque, uint8_t *buf, int buf_size)
{
    int fd = *((int *)opaque);
    int ret = write(fd, buf, buf_size);
    return ret;
}

int main()
{
    AVFormatContext *ofmt_ctx = NULL;
    AVIOContext *avio_out = NULL;
    uint8_t *obuf = NULL;
    size_t obuf_size = 4096;
    int fd = -1;

    // 打开一个FIFO文件的写端
    fd = open_fifo_for_write("/tmp/test_fifo");

    // 1. 分配缓冲区
    obuf = av_malloc(obuf_size);

    // 2. 分配AVIOContext，第三个参数write_flag为1
    AVIOContext *avio_out = avio_alloc_context(obuf, obuf_size, 1, &fd, NULL, write_packet, NULL);  

    // 3. 分配AVFormatContext，并指定AVFormatContext.pb字段。必须在调用avformat_write_header()之前完成
    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, NULL);
    ofmt_ctx->pb=avio_out;

    // 4. 将文件头写入输出文件
    avformat_write_header(ofmt_ctx, NULL);

    ......
}

当启用内存IO模式后(即ofmt_ctx->pb有效时)，FFmpeg会将输出写入内存缓冲区obuf，用户可在回调函数中将obuf中的数据取走。在上述示例中，因为打开的是一个命名管道FIFO，FIFO的数据虽然在内存中，但FIFO有名字("/tmp/test_fifo")，所以此例也可以使用URL-IO模式，如下：

AVFormatContext *ofmt_ctx = NULL;
avformat_alloc_output_context2(&ofmt_ctx, "/tmp/test_fifo", NULL, NULL);

而对于其他一些场合，需将数据输出到内存，而这片内存并无一个URL属性可用时，则只能使用内存IO模式。

2.2 回调时机

回调函数何时被回调呢？所有输出数据的时刻，都将调用回调函数。和输出是普通文件相比，只不过输出变成了内存区，其他各种外在表现并无不同。

如下各函数在不同的阶段会输出数据，都会调用回调函数：

avformat_write_header() 将流头部信息写入输出区

av_interleaved_write_frame() 将数据包写入输出区

av_write_trailer() 将流尾部信息写入输出区

3. 实现机制

如下是与内存IO操作相关的一些关键数据结构及函数，我们从API接口层面来看一下内存IO的实现机制，而不深入分析内部源码。FFmpeg的API注释非常详细，从注释中能得到很多有用信息。

3.1 struct AVIOContext

/**
 * Bytestream IO Context.
 * New fields can be added to the end with minor version bumps.
 * Removal, reordering and changes to existing fields require a major
 * version bump.
 * sizeof(AVIOContext) must not be used outside libav*.
 *
 * @note None of the function pointers in AVIOContext should be called
 *       directly, they should only be set by the client application
 *       when implementing custom I/O. Normally these are set to the
 *       function pointers specified in avio_alloc_context()
 */
typedef struct AVIOContext {
    ......
    /*
     * The following shows the relationship between buffer, buf_ptr,
     * buf_ptr_max, buf_end, buf_size, and pos, when reading and when writing
     * (since AVIOContext is used for both):
     *
     **********************************************************************************
     *                                   READING
     **********************************************************************************
     *
     *                            |              buffer_size              |
     *                            |---------------------------------------|
     *                            |                                       |
     *
     *                         buffer          buf_ptr       buf_end
     *                            +---------------+-----------------------+
     *                            |/ / / / / / / /|/ / / / / / /|         |
     *  read buffer:              |/ / consumed / | to be read /|         |
     *                            |/ / / / / / / /|/ / / / / / /|         |
     *                            +---------------+-----------------------+
     *
     *                                                         pos
     *              +-------------------------------------------+-----------------+
     *  input file: |                                           |                 |
     *              +-------------------------------------------+-----------------+
     *
     *
     **********************************************************************************
     *                                   WRITING
     **********************************************************************************
     *
     *                             |          buffer_size                 |
     *                             |--------------------------------------|
     *                             |                                      |
     *
     *                                                buf_ptr_max
     *                          buffer                 (buf_ptr)       buf_end
     *                             +-----------------------+--------------+
     *                             |/ / / / / / / / / / / /|              |
     *  write buffer:              | / / to be flushed / / |              |
     *                             |/ / / / / / / / / / / /|              |
     *                             +-----------------------+--------------+
     *                               buf_ptr can be in this
     *                               due to a backward seek
     *
     *                            pos
     *               +-------------+----------------------------------------------+
     *  output file: |             |                                              |
     *               +-------------+----------------------------------------------+
     *
     */
    unsigned char *buffer;  /**< Start of the buffer. */
    int buffer_size;        /**< Maximum buffer size */
    unsigned char *buf_ptr; /**< Current position in the buffer */
    unsigned char *buf_end; /**< End of the data, may be less than
                                 buffer+buffer_size if the read function returned
                                 less data than requested, e.g. for streams where
                                 no more data has been received yet. */
    void *opaque;           /**< A private pointer, passed to the read/write/seek/...
                                 functions. */
    int (*read_packet)(void *opaque, uint8_t *buf, int buf_size);
    int (*write_packet)(void *opaque, uint8_t *buf, int buf_size);
    ......
} AVIOContext;

注意：此数据结构中的成员不应由用户程序直接访问。当使用内存IO模式时，用户应调用avio_alloc_context()对此结构的read_packet和write_packet函数指针进行赋值。

3.2 AVIOContext* AVFormatContext.pb

/**
 * Format I/O context.
 * ......
 */
typedef struct AVFormatContext {
    ......
    /**
     * I/O context.
     *
     * - demuxing: either set by the user before avformat_open_input() (then
     *             the user must close it manually) or set by avformat_open_input().
     * - muxing: set by the user before avformat_write_header(). The caller must
     *           take care of closing / freeing the IO context.
     *
     * Do NOT set this field if AVFMT_NOFILE flag is set in
     * iformat/oformat.flags. In such a case, the (de)muxer will handle
     * I/O in some other way and this field will be NULL.
     */
    AVIOContext *pb;
    ......
}

struct AVFormatContext结构中与内存IO操作相关的重要成员是AVIOContext *pb，有如下规则：

• 解复用过程：在调用avformat_open_input()前由用户手工设置，因为从avformat_open_input()开始有读输入的操作。
• 复用过程：在调用avformat_write_header()前由用户手工设置，因为从avformat_write_header()开始有写输出的操作。

3.3 输入时：avformat_open_input()

/**
 * Open an input stream and read the header. The codecs are not opened.
 * The stream must be closed with avformat_close_input().
 *
 * @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
 *           May be a pointer to NULL, in which case an AVFormatContext is allocated by this
 *           function and written into ps.
 *           Note that a user-supplied AVFormatContext will be freed on failure.
 * @param url URL of the stream to open.
 * @param fmt If non-NULL, this parameter forces a specific input format.
 *            Otherwise the format is autodetected.
 * @param options  A dictionary filled with AVFormatContext and demuxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return 0 on success, a negative AVERROR on failure.
 *
 * @note If you want to use custom IO, preallocate the format context and set its pb field.
 */
int avformat_open_input(AVFormatContext **ps, const char *url, AVInputFormat *fmt, AVDictionary **options);

打开输入流读取头部信息。如果使用内存IO模式，应在此之前分配AVFormatContext并设置其pb成员。

3.4 输出时：avformat_write_header()

/**
 * Allocate the stream private data and write the stream header to
 * an output media file.
 *
 * @param s Media file handle, must be allocated with avformat_alloc_context().
 *          Its oformat field must be set to the desired output format;
 *          Its pb field must be set to an already opened AVIOContext.
 * @param options  An AVDictionary filled with AVFormatContext and muxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return AVSTREAM_INIT_IN_WRITE_HEADER on success if the codec had not already been fully initialized in avformat_init,
 *         AVSTREAM_INIT_IN_INIT_OUTPUT  on success if the codec had already been fully initialized in avformat_init,
 *         negative AVERROR on failure.
 *
 * @see av_opt_find, av_dict_set, avio_open, av_oformat_next, avformat_init_output.
 */
av_warn_unused_result
int avformat_write_header(AVFormatContext *s, AVDictionary **options);

将流头部信息写入输出文件。在调用此函数前，AVFormatContext.pb成员必须设置为一个已经打开的AVIOContext。AVFormatContext.pb赋值方式分为两种情况：

[1]. URL-IO模式：调用avio_open()或avio_open2()，形如

avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE);

[2]. 内存IO模式：调用avio_alloc_context()分配AVIOContext，然后为pb赋值，形如：

avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, NULL);
ofmt_ctx->pb=avio_out;

3.5 内存IO模式：avio_alloc_context()

/**
 * Allocate and initialize an AVIOContext for buffered I/O. It must be later
 * freed with avio_context_free().
 *
 * @param buffer Memory block for input/output operations via AVIOContext.
 *        The buffer must be allocated with av_malloc() and friends.
 *        It may be freed and replaced with a new buffer by libavformat.
 *        AVIOContext.buffer holds the buffer currently in use,
 *        which must be later freed with av_free().
 * @param buffer_size The buffer size is very important for performance.
 *        For protocols with fixed blocksize it should be set to this blocksize.
 *        For others a typical size is a cache page, e.g. 4kb.
 * @param write_flag Set to 1 if the buffer should be writable, 0 otherwise.
 * @param opaque An opaque pointer to user-specific data.
 * @param read_packet  A function for refilling the buffer, may be NULL.
 *                     For stream protocols, must never return 0 but rather
 *                     a proper AVERROR code.
 * @param write_packet A function for writing the buffer contents, may be NULL.
 *        The function may not change the input buffers content.
 * @param seek A function for seeking to specified byte position, may be NULL.
 *
 * @return Allocated AVIOContext or NULL on failure.
 */
AVIOContext *avio_alloc_context(
                  unsigned char *buffer,
                  int buffer_size,
                  int write_flag,
                  void *opaque,
                  int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),
                  int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
                  int64_t (*seek)(void *opaque, int64_t offset, int whence));

• opaque是read_packet/write_packet的第一个参数，指向用户数据。
• buffer和buffer_size是read_packet/write_packet的第二个和第三个参数，是供FFmpeg使用的数据区。
  
  buffer用作FFmpeg输入时，由用户负责向buffer中填充数据，FFmpeg取走数据。
  
  buffer用作FFmpeg输出时，由FFmpeg负责向buffer中填充数据，用户取走数据。
• write_flag是缓冲区读写标志，读写的主语是指FFmpeg。
  
  write_flag为1时，buffer用于写，即作为FFmpeg输出。
  
  write_flag为0时，buffer用于读，即作为FFmpeg输入。
• read_packet和write_packet是函数指针，指向用户编写的回调函数。

3.6 URL-IO模式：avio_open()

/**
 * Create and initialize a AVIOContext for accessing the
 * resource indicated by url.
 * @note When the resource indicated by url has been opened in
 * read+write mode, the AVIOContext can be used only for writing.
 *
 * @param s Used to return the pointer to the created AVIOContext.
 * In case of failure the pointed to value is set to NULL.
 * @param url resource to access
 * @param flags flags which control how the resource indicated by url
 * is to be opened
 * @return >= 0 in case of success, a negative value corresponding to an
 * AVERROR code in case of failure
 */
int avio_open(AVIOContext **s, const char *url, int flags);

4. 修改记录

2019-01-24 V1.0 初稿