1.描述

AVInputFormat 是类似COM 接口的数据结构，表示输入文件容器格式，着重于功能函数，一种文件容器格式对应一个AVInputFormat 结构，在程序运行时有多个实例，位于avoformat.h文件中。

2.结构体定义

  1 typedef struct AVInputFormat {
  2     /**
  3      * A comma separated list of short names for the format. New names
  4      * may be appended with a minor bump.
  5      */
  6     const char *name;
  7 
  8     /**
  9      * Descriptive name for the format, meant to be more human-readable
 10      * than name. You should use the NULL_IF_CONFIG_SMALL() macro
 11      * to define it.
 12      */
 13     const char *long_name;
 14 
 15     /**
 16      * Can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_SHOW_IDS,
 17      * AVFMT_GENERIC_INDEX, AVFMT_TS_DISCONT, AVFMT_NOBINSEARCH,
 18      * AVFMT_NOGENSEARCH, AVFMT_NO_BYTE_SEEK, AVFMT_SEEK_TO_PTS.
 19      */
 20     int flags;
 21 
 22     /**
 23      * If extensions are defined, then no probe is done. You should
 24      * usually not use extension format guessing because it is not
 25      * reliable enough
 26      */
 27     const char *extensions;
 28 
 29     const struct AVCodecTag * const *codec_tag;
 30 
 31     const AVClass *priv_class; ///< AVClass for the private context
 32 
 33     /**
 34      * Comma-separated list of mime types.
 35      * It is used check for matching mime types while probing.
 36      * @see av_probe_input_format2
 37      */
 38     const char *mime_type;
 39 
 40     /*****************************************************************
 41      * No fields below this line are part of the public API. They
 42      * may not be used outside of libavformat and can be changed and
 43      * removed at will.
 44      * New public fields should be added right above.
 45      *****************************************************************
 46      */
 47     struct AVInputFormat *next;
 48 
 49     /**
 50      * Raw demuxers store their codec ID here.
 51      */
 52     int raw_codec_id;
 53 
 54     /**
 55      * Size of private data so that it can be allocated in the wrapper.
 56      */
 57     int priv_data_size;
 58 
 59     /**
 60      * Tell if a given file has a chance of being parsed as this format.
 61      * The buffer provided is guaranteed to be AVPROBE_PADDING_SIZE bytes
 62      * big so you do not have to check for that unless you need more.
 63      */
 64     int (*read_probe)(AVProbeData *);
 65 
 66     /**
 67      * Read the format header and initialize the AVFormatContext
 68      * structure. Return 0 if OK. 'avformat_new_stream' should be
 69      * called to create new streams.
 70      */
 71     int (*read_header)(struct AVFormatContext *);
 72 
 73     /**
 74      * Used by format which open further nested input.
 75      */
 76     int (*read_header2)(struct AVFormatContext *, AVDictionary **options);
 77 
 78     /**
 79      * Read one packet and put it in 'pkt'. pts and flags are also
 80      * set. 'avformat_new_stream' can be called only if the flag
 81      * AVFMTCTX_NOHEADER is used and only in the calling thread (not in a
 82      * background thread).
 83      * @return 0 on success, < 0 on error.
 84      *         When returning an error, pkt must not have been allocated
 85      *         or must be freed before returning
 86      */
 87     int (*read_packet)(struct AVFormatContext *, AVPacket *pkt);
 88 
 89     /**
 90      * Close the stream. The AVFormatContext and AVStreams are not
 91      * freed by this function
 92      */
 93     int (*read_close)(struct AVFormatContext *);
 94 
 95     /**
 96      * Seek to a given timestamp relative to the frames in
 97      * stream component stream_index.
 98      * @param stream_index Must not be -1.
 99      * @param flags Selects which direction should be preferred if no exact
100      *              match is available.
101      * @return >= 0 on success (but not necessarily the new offset)
102      */
103     int (*read_seek)(struct AVFormatContext *,
104                      int stream_index, int64_t timestamp, int flags);
105 
106     /**
107      * Get the next timestamp in stream[stream_index].time_base units.
108      * @return the timestamp or AV_NOPTS_VALUE if an error occurred
109      */
110     int64_t (*read_timestamp)(struct AVFormatContext *s, int stream_index,
111                               int64_t *pos, int64_t pos_limit);
112 
113     /**
114      * Start/resume playing - only meaningful if using a network-based format
115      * (RTSP).
116      */
117     int (*read_play)(struct AVFormatContext *);
118 
119     /**
120      * Pause playing - only meaningful if using a network-based format
121      * (RTSP).
122      */
123     int (*read_pause)(struct AVFormatContext *);
124 
125     /**
126      * Seek to timestamp ts.
127      * Seeking will be done so that the point from which all active streams
128      * can be presented successfully will be closest to ts and within min/max_ts.
129      * Active streams are all streams that have AVStream.discard < AVDISCARD_ALL.
130      */
131     int (*read_seek2)(struct AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int flags);
132 
133     /**
134      * Returns device list with it properties.
135      * @see avdevice_list_devices() for more details.
136      */
137     int (*get_device_list)(struct AVFormatContext *s, struct AVDeviceInfoList *device_list);
138 
139     /**
140      * Initialize device capabilities submodule.
141      * @see avdevice_capabilities_create() for more details.
142      */
143     int (*create_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);
144 
145     /**
146      * Free device capabilities submodule.
147      * @see avdevice_capabilities_free() for more details.
148      */
149     int (*free_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);
150 } AVInputFormat;

3.常见变量及其作用

const char *name;//格式名列表.也可以分配一个新名字。
const char *long_name;//格式的描述性名称，意味着比名称更易于阅读。
int flags;
//可用的flag有: AVFMT_NOFILE, AVFMT_NEEDNUMBER, AVFMT_SHOW_IDS, 
AVFMT_GENERIC_INDEX, AVFMT_TS_DISCONT, AVFMT_NOBINSEARCH, 
  AVFMT_NOGENSEARCH, AVFMT_NO_BYTE_SEEK, AVFMT_SEEK_TO_PTS.
const char *extensions;//文件扩展名
const AVClass *priv_class; //一个模拟类型列表.用来在probe的时候check匹配的类型。
struct AVInputFormat *next;//用于把所有支持的输入文件容器格式连接成链表，便于遍历查找。
int priv_data_size;//标示具体的文件容器格式对应的Context 的大小。
int (*read_probe)(AVProbeData *);//判断一个给定的文件是否有可能被解析为这种格式。 给定的buf足够大，所以你没有必要去检查它，除非你需要更多 。
int (*read_header)(struct AVFormatContext *);//读取format头并初始化AVFormatContext结构体，如果成功，返回0。创建新的流需要调用avformat_new_stream。
int (*read_header2)(struct AVFormatContext *, AVDictionary **options);//新加的函数指针，用于打开进一步嵌套输入的格式。
int (*read_packet)(struct AVFormatContext *, AVPacket *pkt);//读取一个数据包并将其放在“pkt”中。 pts和flag也被设置。
int (*read_close)(struct AVFormatContext *);//关闭流。 AVFormatContext和Streams不会被此函数释放。
int (*read_seek)(struct AVFormatContext *, int stream_index, int64_t timestamp, int flags);
int64_t (*read_timestamp)(struct AVFormatContext *s, int stream_index, int64_t *pos, int64_t pos_limit);//获取stream [stream_index] .time_base单位中的下一个时间戳。
int (*read_play)(struct AVFormatContext *);//开始/继续播放 - 仅当使用基于网络的（RTSP）格式时才有意义。
int (*read_pause)(struct AVFormatContext *);//暂停播放 - 仅当使用基于网络的（RTSP）格式时才有意义。
int (*read_seek2)(struct AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int flags);//寻求时间戳ts。
int (*get_device_list)(struct AVFormatContext *s, struct AVDeviceInfoList *device_list);返回设备列表及其属性。
int (*create_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);//初始化设备能力子模块。
int (*free_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);//释放设备能力子模块。