FFmpeg结构体：AVIOContext
1.描述
AVIOContext是FFmpeg管理输入输出数据的结构体，位于avio.h文件中。
2.结构体定义
1 typedef struct AVIOContext { 2 /** 3 * A class for private options. 4 * 5 * If this AVIOContext is created by avio_open2(), av_class is set and 6 * passes the options down to protocols. 7 * 8 * If this AVIOContext is manually allocated, then av_class may be set by 9 * the caller. 10 * 11 * warning -- this field can be NULL, be sure to not pass this AVIOContext 12 * to any av_opt_* functions in that case. 13 */ 14 const AVClass *av_class; 15 16 /* 17 * The following shows the relationship between buffer, buf_ptr, buf_end, buf_size, 18 * and pos, when reading and when writing (since AVIOContext is used for both): 19 * 20 ********************************************************************************** 21 * READING 22 ********************************************************************************** 23 * 24 * | buffer_size | 25 * |---------------------------------------| 26 * | | 27 * 28 * buffer buf_ptr buf_end 29 * +---------------+-----------------------+ 30 * |/ / / / / / / /|/ / / / / / /| | 31 * read buffer: |/ / consumed / | to be read /| | 32 * |/ / / / / / / /|/ / / / / / /| | 33 * +---------------+-----------------------+ 34 * 35 * pos 36 * +-------------------------------------------+-----------------+ 37 * input file: | | | 38 * +-------------------------------------------+-----------------+ 39 * 40 * 41 ********************************************************************************** 42 * WRITING 43 ********************************************************************************** 44 * 45 * | buffer_size | 46 * |-------------------------------| 47 * | | 48 * 49 * buffer buf_ptr buf_end 50 * +-------------------+-----------+ 51 * |/ / / / / / / / / /| | 52 * write buffer: | / to be flushed / | | 53 * |/ / / / / / / / / /| | 54 * +-------------------+-----------+ 55 * 56 * pos 57 * +--------------------------+-----------------------------------+ 58 * output file: | | | 59 * +--------------------------+-----------------------------------+ 60 * 61 */ 62 unsigned char *buffer; /**< Start of the buffer. */ 63 int buffer_size; /**< Maximum buffer size */ 64 unsigned char *buf_ptr; /**< Current position in the buffer */ 65 unsigned char *buf_end; /**< End of the data, may be less than 66 buffer+buffer_size if the read function returned 67 less data than requested, e.g. for streams where 68 no more data has been received yet. */ 69 void *opaque; /**< A private pointer, passed to the read/write/seek/... 70 functions. */ 71 int (*read_packet)(void *opaque, uint8_t *buf, int buf_size); 72 int (*write_packet)(void *opaque, uint8_t *buf, int buf_size); 73 int64_t (*seek)(void *opaque, int64_t offset, int whence); 74 int64_t pos; /**< position in the file of the current buffer */ 75 int must_flush; /**< true if the next seek should flush */ 76 int eof_reached; /**< true if eof reached */ 77 int write_flag; /**< true if open for writing */ 78 int max_packet_size; 79 unsigned long checksum; 80 unsigned char *checksum_ptr; 81 unsigned long (*update_checksum)(unsigned long checksum, const uint8_t *buf, unsigned int size); 82 int error; /**< contains the error code or 0 if no error happened */ 83 /** 84 * Pause or resume playback for network streaming protocols - e.g. MMS. 85 */ 86 int (*read_pause)(void *opaque, int pause); 87 /** 88 * Seek to a given timestamp in stream with the specified stream_index. 89 * Needed for some network streaming protocols which don't support seeking 90 * to byte position. 91 */ 92 int64_t (*read_seek)(void *opaque, int stream_index, 93 int64_t timestamp, int flags); 94 /** 95 * A combination of AVIO_SEEKABLE_ flags or 0 when the stream is not seekable. 96 */ 97 int seekable; 98 99 /** 100 * max filesize, used to limit allocations 101 * This field is internal to libavformat and access from outside is not allowed. 102 */ 103 int64_t maxsize; 104 105 /** 106 * avio_read and avio_write should if possible be satisfied directly 107 * instead of going through a buffer, and avio_seek will always 108 * call the underlying seek function directly. 109 */ 110 int direct; 111 112 /** 113 * Bytes read statistic 114 * This field is internal to libavformat and access from outside is not allowed. 115 */ 116 int64_t bytes_read; 117 118 /** 119 * seek statistic 120 * This field is internal to libavformat and access from outside is not allowed. 121 */ 122 int seek_count; 123 124 /** 125 * writeout statistic 126 * This field is internal to libavformat and access from outside is not allowed. 127 */ 128 int writeout_count; 129 130 /** 131 * Original buffer size 132 * used internally after probing and ensure seekback to reset the buffer size 133 * This field is internal to libavformat and access from outside is not allowed. 134 */ 135 int orig_buffer_size; 136 137 /** 138 * Threshold to favor readahead over seek. 139 * This is current internal only, do not use from outside. 140 */ 141 int short_seek_threshold; 142 143 /** 144 * ',' separated list of allowed protocols. 145 */ 146 const char *protocol_whitelist; 147 148 /** 149 * ',' separated list of disallowed protocols. 150 */ 151 const char *protocol_blacklist; 152 153 /** 154 * A callback that is used instead of write_packet. 155 */ 156 int (*write_data_type)(void *opaque, uint8_t *buf, int buf_size, 157 enum AVIODataMarkerType type, int64_t time); 158 /** 159 * If set, don't call write_data_type separately for AVIO_DATA_MARKER_BOUNDARY_POINT, 160 * but ignore them and treat them as AVIO_DATA_MARKER_UNKNOWN (to avoid needlessly 161 * small chunks of data returned from the callback). 162 */ 163 int ignore_boundary_point; 164 165 /** 166 * Internal, not meant to be used from outside of AVIOContext. 167 */ 168 enum AVIODataMarkerType current_type; 169 int64_t last_time; 170 } AVIOContext;
3.常见变量及其作用
1 unsigned char *buffer; //缓存开始位置。 2 int buffer_size; //缓冲区的大小。 3 unsigned char *buf_ptr;//当前指针在缓冲区中的位置，即当前指针读取到的位置。 4 unsigned char *buf_end; //缓存结束的位置 5 6 void *opaque;//一个私有指针，传递给read / write / seek / ...函数。 7 opaque指向了URLContext。URLContext结构体中还有一个结构体URLProtocol。 8 ps:每种协议（rtp，rtmp，file等）对应一个URLProtocol。 9 10 int64_t pos; //当前缓冲区在文件中的位置。 11 int must_flush; //如果下一次seek需要刷新则为真。 12 int eof_reached; //是否读到eof，文件结尾。 13 int (*read_pause)(void *opaque, int pause);//暂停或恢复播放网络流媒体协议 - 例如 MMS。 14 int64_t (*read_seek)(void *opaque, int stream_index, int64_t timestamp, int flags); 15 //使用指定的stream_index查找流中给定的时间戳（对于不支持寻找字节位置的网络流协议）。
