ffmpeg結構體以及函數介紹（一）

extern "C"

{

#include "libavcodec/avcodec.h"

#include "libavformat/avformat.h"

#include "libavutil/avutil.h"

#include "libavutil/mem.h"

#include "libavutil/fifo.h"

#include "libswscale/swscale.h"

};

 

/**

 * Initialize libavcodec.

 * If called more than once, does nothing.

 *

 * @warning This function must be called before any other libavcodec

 * function.

 *

 * @warning This function is not thread-safe.

 */

void avcodec_init(void);

// 初始化libavcodec,一般最先調用該函數

// 引入頭文件： #include "libavcodec/avcodec.h"

// 實現在: \ffmpeg\libavcodec\utils.c

// 該函數必須在調用libavcodec裏的其它函數前調用,一般在程序啟動或模塊初始化時調用,如果你調用了多次也無所謂,因為後麵的調用不會做任何事情.從函數的實現裏你可以發現,代碼中對多次調用進行了控製.

// 該函數是非線程安全的

 

2 av_register_all()

/**

 * Initialize libavformat and register all the muxers, demuxers and

 * protocols. If you do not call this function, then you can select

 * exactly which formats you want to support.

 *

 * @see av_register_input_format()

 * @see av_register_output_format()

 * @see av_register_protocol()

 */

void av_register_all(void);

// 初始化 libavformat和注冊所有的muxers、demuxers和protocols，

// 一般在調用avcodec_init後調用該方法

// 引入頭文件：#include "libavformat/avformat.h"

// 實現在:\ffmpeg\libavformat\allformats.c

// 其中會調用avcodec_register_all()注冊多種音視頻格式的編解碼器,並注冊各種文件的編解複用器

// 當然，你也可以不調用該函數，而通過選擇調用特定的方法來提供支持

 

3 avformat_alloc_context()

/**

 * Allocate an AVFormatContext.

 * avformat_free_context() can be used to free the context and everything

 * allocated by the framework within it.

 */

AVFormatContext *avformat_alloc_context(void);

// 分配一個AVFormatContext結構

// 引入頭文件：#include "libavformat/avformat.h"

// 實現在:\ffmpeg\libavformat\options.c

// 其中負責申請一個AVFormatContext結構的內存,並進行簡單初始化

// avformat_free_context()可以用來釋放該結構裏的所有東西以及該結構本身

// 也是就說使用 avformat_alloc_context()分配的結構,需要使用avformat_free_context()來釋放

// 有些版本中函數名可能為: av_alloc_format_context();

 

4 avformat_free_context()

/**

 * Free an AVFormatContext and all its streams.

 * @param s context to free

 */

void avformat_free_context(AVFormatContext *s);

// 釋放一個AVFormatContext結構

// 引入頭文件：#include "libavformat/avformat.h"

// 實現在:\ffmpeg\libavformat\utils.c

// 使用 avformat_alloc_context()分配的結構,采用該函數進行釋放,除釋放AVFormatContext結構本身內存之外,AVFormatContext中指針所指向的內存也會一並釋放

// 有些版本中函數名猜測可能為: av_free_format_context();

 

5 AVFormatContext 結構

/**

 * Format I/O 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(AVFormatContext) must not be used outside libav*.

 */

typedef struct AVFormatContext {

    struct AVInputFormat *iformat;

    struct AVOutputFormat *oformat;

    AVIOContext *pb;

    unsigned int nb_streams;

    AVStream **streams;

    char filename[1024]; /**< input or output filename */

.

.

.

} AVFormatContext;

// AVFormatContext在FFMpeg裏是一個非常重要的的結構，是其它輸入、輸出相關信息的一個容器

// 引入頭文件：#include "libavformat/avformat.h"

// 以上隻列出了其中的部分成員

// 作為輸入容器時 struct AVInputFormat *iformat; 不能為空, 其中包含了輸入文件的音視頻流信息,程序從輸入容器從讀出音視頻包進行解碼處理

// 作為輸出容器時 struct AVOutputFormat *oformat; 不能為空, 程序把編碼好的音視頻包寫入到輸出容器中

// AVIOContext *pb: I/O上下文,通過對該變量賦值可以改變輸入源或輸出目的

// unsigned int nb_streams; 音視頻流數量

// AVStream **streams; 音視頻流

 

6 AVIOContext 結構

/**

 * 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 {

    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 */

#if FF_API_OLD_AVIO

    attribute_deprecated int is_streamed;

#endif

    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;

} AVIOContext;

// 字節流 I/O 上下文

// 在結構的尾部增加變量可以減少版本衝突

// 移除、排序和修改已經存在的變量將會導致較大的版本衝突

// sizeof(AVIOContext)在libav*.外部不可使用

// AVIOContext裏的函數指針不能直接調用,通常使用avio_alloc_context()函數來設置其中的函數指針

// unsigned char *buffer: 緩存的起始指針

// int buffer_size: 緩存的最大值

// void *opaque: 在回調函數中使用的指針

// int (*read_packet)(void *opaque, uint8_t *buf, int buf_size): 讀文件回調方法

// int (*write_packet)(void *opaque, uint8_t *buf, int buf_size): 寫文件回調方法

// int64_t (*seek)(void *opaque, int64_t offset, int whence): seek文件回調方法

 

7 avio_alloc_context()

/**

 * Allocate and initialize an AVIOContext for buffered I/O. It must be later

 * freed with av_free().

 *

 * @param buffer Memory block for input/output operations via AVIOContext.

 *        The buffer must be allocated with av_malloc() and friends.

 * @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.

 * @param write_packet A function for writing the buffer contents, may be NULL.

 * @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));

// 為I/0緩存申請並初始化一個AVIOContext結構,結束使用時必須使用av_free()進行釋放

// unsigned char *buffer: 輸入/輸出緩存內存塊,必須是使用av_malloc()分配的

// int buffer_size: 緩存大小是非常重要的

// int write_flag: 如果緩存為可寫則設置為1,否則設置為0

// void *opaque: 指針,用於回調時使用

// int (*read_packet): 讀包函數指針

// int (*write_packet): 寫包函數指針

// int64_t (*seek): seek文件函數指針

8 av_open_input_file()

/**

 * Open a media file as input. The codecs are not opened. Only the file

 * header (if present) is read.

 *

 * @param ic_ptr The opened media file handle is put here.

 * @param filename filename to open

 * @param fmt If non-NULL, force the file format to use.

 * @param buf_size optional buffer size (zero if default is OK)

 * @param ap Additional parameters needed when opening the file

 *           (NULL if default).

 * @return 0 if OK, AVERROR_xxx otherwise

 *

 * @deprecated use avformat_open_input instead.

 */

attribute_deprecated int av_open_input_file(AVFormatContext **ic_ptr, const char *filename,

                       AVInputFormat *fmt,

                       int buf_size,

                       AVFormatParameters *ap);

// 以輸入方式打開一個媒體文件,也即源文件,codecs並沒有打開,隻讀取了文件的頭信息.

// 引入頭文件：#include "libavformat/avformat.h"

// AVFormatContext **ic_ptr 輸入文件容器

// const char *filename 輸入文件名,全路徑,並且保證文件存在

// AVInputFormat *fmt 輸入文件格式,填NULL即可

// int buf_size,緩衝區大小,直接填0即可

// AVFormatParameters *ap, 格式參數,添NULL即可

// 成功返回0,其它失敗

// 不讚成使用 avformat_open_input 代替

 

9 av_close_input_file()

/**

 * @deprecated use avformat_close_input()

 * Close a media file (but not its codecs).

 *

 * @param s media file handle

 */

void av_close_input_file(AVFormatContext *s);

// 關閉使用avformat_close_input()打開的輸入文件容器,但並不關係它的codecs

// 引入頭文件：#include "libavformat/avformat.h"

// 使用av_open_input_file 打開的文件容器,可以使用該函數關閉

// 使用 av_close_input_file 關閉後,就不再需要使用avformat_free_context 進行釋放了

 

10 av_find_stream_info()

/**

 * Read packets of a media file to get stream information. This

 * is useful for file formats with no headers such as MPEG. This

 * function also computes the real framerate in case of MPEG-2 repeat

 * frame mode.

 * The logical file position is not changed by this function;

 * examined packets may be buffered for later processing.

 *

 * @param ic media file handle

 * @return >=0 if OK, AVERROR_xxx on error

 * @todo Let the user decide somehow what information is needed so that

 *       we do not waste time getting stuff the user does not need.

 */

int av_find_stream_info(AVFormatContext *ic);

// 通過讀取媒體文件的中的包來獲取媒體文件中的流信息,對於沒有頭信息的文件如(mpeg)是非常有用的,

// 該函數通常重算類似mpeg-2幀模式的真實幀率,該函數並未改變邏輯文件的position.

// 引入頭文件：#include "libavformat/avformat.h"

// 也就是把媒體文件中的音視頻流等信息讀出來,保存在容器中,以便解碼時使用

// 返回>=0時成功,否則失敗