1.描述
AVIOContext是FFmpeg管理輸入輸出數(shù)據(jù)的結(jié)構(gòu)體趋急，位于avio.h文件中。
2.結(jié)構(gòu)體定義
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_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 |
* |-------------------------------|
* | |
*
* buffer buf_ptr buf_end
* +-------------------+-----------+
* |/ / / / / / / / / /| |
* write buffer: | / to be flushed / | |
* |/ / / / / / / / / /| |
* +-------------------+-----------+
*
* 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 must_flush; /**< true if the next seek should flush */
int eof_reached; /**< true if eof reached */
int write_flag; /**< true if open for writing */
int max_packet_size;
unsigned long checksum;
unsigned char *checksum_ptr;
unsigned long (*update_checksum)(unsigned long checksum, const uint8_t *buf, unsigned int size);
int error; /**< contains the error code or 0 if no error happened */
/**
* 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;
/**
* max filesize, used to limit allocations
* This field is internal to libavformat and access from outside is not allowed.
*/
int64_t maxsize;
/**
* 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;
/**
* Bytes read statistic
* This field is internal to libavformat and access from outside is not allowed.
*/
int64_t bytes_read;
/**
* seek statistic
* This field is internal to libavformat and access from outside is not allowed.
*/
int seek_count;
/**
* writeout statistic
* This field is internal to libavformat and access from outside is not allowed.
*/
int writeout_count;
/**
* Original buffer size
* used internally after probing and ensure seekback to reset the buffer size
* This field is internal to libavformat and access from outside is not allowed.
*/
int orig_buffer_size;
/**
* Threshold to favor readahead over seek.
* This is current internal only, do not use from outside.
*/
int short_seek_threshold;
/**
* ',' 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;
/**
* Internal, not meant to be used from outside of AVIOContext.
*/
enum AVIODataMarkerType current_type;
int64_t last_time;
} AVIOContext;
3.常見變量及其作用
unsigned char *buffer; //緩存開始位置势誊。
int buffer_size; //緩沖區(qū)的大小呜达。
unsigned char *buf_ptr;//當(dāng)前指針在緩沖區(qū)中的位置，即當(dāng)前指針讀取到的位置粟耻。
unsigned char *buf_end; //緩存結(jié)束的位置
void *opaque;//一個私有指針查近，傳遞給read / write / seek / ...函數(shù)。
opaque指向了URLContext挤忙。URLContext結(jié)構(gòu)體中還有一個結(jié)構(gòu)體URLProtocol霜威。
ps:每種協(xié)議（rtp，rtmp饭玲，file等）對應(yīng)一個URLProtocol侥祭。
int64_t pos; //當(dāng)前緩沖區(qū)在文件中的位置。
int must_flush; //如果下一次seek需要刷新則為真茄厘。
int eof_reached; //是否讀到eof矮冬，文件結(jié)尾。
int (*read_pause)(void *opaque, int pause);//暫痛喂或恢復(fù)播放網(wǎng)絡(luò)流媒體協(xié)議 - 例如 MMS胎署。
int64_t (*read_seek)(void *opaque, int stream_index, int64_t timestamp, int flags);
//使用指定的stream_index查找流中給定的時間戳（對于不支持尋找字節(jié)位置的網(wǎng)絡(luò)流協(xié)議）。
