本篇介绍
最近遇到一个问题，需要让ffmpeg支持android的URI路径媒体文件。最后用自定义IO解决了，而且对外部没有任何影响。本篇总结下解决过程。
自定义IO
自定义IO:
在使用ffmpeg打开文件的时候，可以是绝对路径，也可以是http路径，还可以其他路径，ffmpeg是如何支持该能力的呢？ 在ffmpeg内部有很多歌URLProtocol，表示某个来源的文件，ffmpeg会使用这些protocol挨个匹配输入的文件，看哪个protocol可以打开。那什么是URLProtocol呢?
代码语言：javascript复制typedef struct URLProtocol {
const char *name;
int (*url_open)( URLContext *h, const char *url, int flags);
/**
* This callback is to be used by protocols which open further nested
* protocols. options are then to be passed to ffurl_open_whitelist()
* or ffurl_connect() for those nested protocols.
*/
int (*url_open2)(URLContext *h, const char *url, int flags, AVDictionary **options);
int (*url_accept)(URLContext *s, URLContext **c);
int (*url_handshake)(URLContext *c);
/**
* Read data from the protocol.
* If data is immediately available (even less than size), EOF is
* reached or an error occurs (including EINTR), return immediately.
* Otherwise:
* In non-blocking mode, return AVERROR(EAGAIN) immediately.
* In blocking mode, wait for data/EOF/error with a short timeout (0.1s),
* and return AVERROR(EAGAIN) on timeout.
* Checking interrupt_callback, looping on EINTR and EAGAIN and until
* enough data has been read is left to the calling function; see
* retry_transfer_wrapper in avio.c.
*/
int (*url_read)( URLContext *h, unsigned char *buf, int size);
int (*url_write)(URLContext *h, const unsigned char *buf, int size);
int64_t (*url_seek)( URLContext *h, int64_t pos, int whence);
int (*url_close)(URLContext *h);
int (*url_read_pause)(URLContext *h, int pause);
int64_t (*url_read_seek)(URLContext *h, int stream_index,
int64_t timestamp, int flags);
int (*url_get_file_handle)(URLContext *h);
int (*url_get_multi_file_handle)(URLContext *h, int **handles,
int *numhandles);
int (*url_get_short_seek)(URLContext *h);
int (*url_shutdown)(URLContext *h, int flags);
const AVClass *priv_data_class;
int priv_data_size;
int flags;
int (*url_check)(URLContext *h, int mask);
int (*url_open_dir)(URLContext *h);
int (*url_read_dir)(URLContext *h, AVIODirEntry **next);
int (*url_close_dir)(URLContext *h);
int (*url_delete)(URLContext *h);
int (*url_move)(URLContext *h_src, URLContext *h_dst);
const char *default_whitelist;
} URLProtocol;看起来就是文件的一些基本操作函数指针，不同的Protocol只要实现对应的文件操作方法即可，目前ffmpeg有以下这几种protocol:
代码语言：javascript复制extern const URLProtocol ff_async_protocol;
extern const URLProtocol ff_bluray_protocol;
extern const URLProtocol ff_cache_protocol;
extern const URLProtocol ff_concat_protocol;
extern const URLProtocol ff_concatf_protocol;
extern const URLProtocol ff_crypto_protocol;
extern const URLProtocol ff_data_protocol;
extern const URLProtocol ff_ffrtmpcrypt_protocol;
extern const URLProtocol ff_ffrtmphttp_protocol;
extern const URLProtocol ff_file_protocol;
extern const URLProtocol ff_ftp_protocol;
extern const URLProtocol ff_gopher_protocol;
extern const URLProtocol ff_gophers_protocol;
extern const URLProtocol ff_hls_protocol;
extern const URLProtocol ff_http_protocol;
extern const URLProtocol ff_httpproxy_protocol;
extern const URLProtocol ff_https_protocol;
extern const URLProtocol ff_icecast_protocol;
extern const URLProtocol ff_mmsh_protocol;
extern const URLProtocol ff_mmst_protocol;
extern const URLProtocol ff_md5_protocol;
extern const URLProtocol ff_pipe_protocol;
extern const URLProtocol ff_prompeg_protocol;
extern const URLProtocol ff_rtmp_protocol;
extern const URLProtocol ff_rtmpe_protocol;
extern const URLProtocol ff_rtmps_protocol;
extern const URLProtocol ff_rtmpt_protocol;
extern const URLProtocol ff_rtmpte_protocol;
extern const URLProtocol ff_rtmpts_protocol;
extern const URLProtocol ff_rtp_protocol;
extern const URLProtocol ff_sctp_protocol;
extern const URLProtocol ff_srtp_protocol;
extern const URLProtocol ff_subfile_protocol;
extern const URLProtocol ff_tee_protocol;
extern const URLProtocol ff_tcp_protocol;
extern const URLProtocol ff_tls_protocol;
extern const URLProtocol ff_udp_protocol;
extern const URLProtocol ff_udplite_protocol;
extern const URLProtocol ff_unix_protocol;
extern const URLProtocol ff_libamqp_protocol;
extern const URLProtocol ff_librist_protocol;
extern const URLProtocol ff_librtmp_protocol;
extern const URLProtocol ff_librtmpe_protocol;
extern const URLProtocol ff_librtmps_protocol;
extern const URLProtocol ff_librtmpt_protocol;
extern const URLProtocol ff_librtmpte_protocol;
extern const URLProtocol ff_libsrt_protocol;
extern const URLProtocol ff_libssh_protocol;
extern const URLProtocol ff_libsmbclient_protocol;
extern const URLProtocol ff_libzmq_protocol;如果需要让ffmpeg支持其他来源的媒体文件，只需要再定义一个新的URLProtocol就行。可是这样就需要修改ffmpeg源码，至少需要让自己添加的protocol添加到ffmpeg的protocol_list里面。这样在很多场景下是行不通的，毕竟修改开源代码带来的维护成本是很高的。ffmpeg已经帮我们想好了解决方法，就是用自定义io，让调用方来告诉ffmpeg如何read，write文件。具体使用的结构就是aviocontext.
代码语言：javascript复制typedef struct AVIOContext {
/**
* A class for private options.
*
* If this AVIOContext is created by avio_open2(), av_class is set and
* passes the options down to protocols.
*
* If this AVIOContext is manually allocated, then av_class may be set by
* the caller.
*
* warning -- this field can be NULL, be sure to not pass this AVIOContext
* to any av_opt_* functions in that case.
*/
const AVClass *av_class;
/*
* 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);
int64_t (*seek)(void *opaque, int64_t offset, int whence);
int64_t pos; /**< position in the file of the current buffer */
int eof_reached; /**< true if was unable to read due to error or eof */
int error; /**< contains the error code or 0 if no error happened */
int write_flag; /**< true if open for writing */
int max_packet_size;
int min_packet_size; /**< Try to buffer at least this amount of data
before flushing it. */
unsigned long checksum;
unsigned char *checksum_ptr;
unsigned long (*update_checksum)(unsigned long checksum, const uint8_t *buf, unsigned int size);
/**
* Pause or resume playback for network streaming protocols - e.g. MMS.
*/
int (*read_pause)(void *opaque, int pause);
/**
* Seek to a given timestamp in stream with the specified stream_index.
* Needed for some network streaming protocols which don't support seeking
* to byte position.
*/
int64_t (*read_seek)(void *opaque, int stream_index,
int64_t timestamp, int flags);
/**
* A combination of AVIO_SEEKABLE_ flags or 0 when the stream is not seekable.
*/
int seekable;
/**
* avio_read and avio_write should if possible be satisfied directly
* instead of going through a buffer, and avio_seek will always
* call the underlying seek function directly.
*/
int direct;
/**
* ',' separated list of allowed protocols.
*/
const char *protocol_whitelist;
/**
* ',' separated list of disallowed protocols.
*/
const char *protocol_blacklist;
/**
* A callback that is used instead of write_packet.
*/
int (*write_data_type)(void *opaque, uint8_t *buf, int buf_size,
enum AVIODataMarkerType type, int64_t time);
/**
* If set, don't call write_data_type separately for AVIO_DATA_MARKER_BOUNDARY_POINT,
* but ignore them and treat them as AVIO_DATA_MARKER_UNKNOWN (to avoid needlessly
* small chunks of data returned from the callback).
*/
int ignore_boundary_point;
#if FF_API_AVIOCONTEXT_WRITTEN
/**
* @deprecated field utilized privately by libavformat. For a public
* statistic of how many bytes were written out, see
* AVIOContext::bytes_written.
*/
attribute_deprecated
int64_t written;
#endif
/**
* Maximum reached position before a backward seek in the write buffer,
* used keeping track of already written data for a later flush.
*/
unsigned char *buf_ptr_max;
/**
* Read-only statistic of bytes read for this AVIOContext.
*/
int64_t bytes_read;
/**
* Read-only statistic of bytes written for this AVIOContext.
*/
int64_t bytes_written;
} AVIOContext;可以看到有很多成员，实际上只需要关心以下三个就够了。
代码语言：javascript复制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);读、写、seek，这三个操作基本就覆盖了ffmpeg需要操作一个媒体文件常用的能力。这儿的opaque就是一个私有数据，在实现自定义io的时候，可以将opaque作为一个实现了读写，seek的对象指针。 接下来可以看下是如何让ffmpeg支持uri的。
image.png
关键代码演示：
代码语言：javascript复制 avio_ctx = avio_alloc_context(avio_ctx_buffer, avio_ctx_buffer_size,
0, &bd, &read_packet, &write_packet, &seek_packet);
if (!avio_ctx) {
ret = AVERROR(ENOMEM);
goto end;
}
fmt_ctx->pb = avio_ctx;
ret = avformat_open_input(&fmt_ctx, NULL, NULL, NULL);
if (ret < 0) {
fprintf(stderr, "Could not open inputn");
goto end;
}通过avio_alloc_context来分配自定义io对象，并且传入read_packet，write_packet，seek_packet。 形式如上，而bd就是自定义的数据，对于我们，就是用FFmpegProtocolFactory拿到的protocol对象。 这时候剩下的操作就简单了，比如：
代码语言：javascript复制int (*read_packet)(void *opaque, uint8_t *buf, int buf_size) {
ProtocolInterface *p = static_cast<ProtocolInterface*>(opaque);
return p->Read(buf, buf_size);
}
int (*write_packet)(void *opaque, uint8_t *buf, int buf_size) {
ProtocolInterface *p = static_cast<ProtocolInterface*>(opaque);
return p->Write(buf, buf_size);
}
int64_t (*seek)(void *opaque, int64_t offset, int whence) {
ProtocolInterface *p = static_cast<ProtocolInterface*>(opaque);
return p->Seek(offset, whence);
}如果要支持URI，那么在URIProtocol里面，就直接反射到Java，用ContentResolver获取InputStream，就可以支持读写了。
