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