转自：http://blog.chinaunix.net/uid-20718335-id-3024436.html，尊重原创
头文件引入方法:
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,一般最先调用该函数
// 引入头文件： #include "libavcodec/avcodec.h"
// 实现在: \ffmpeg\libavcodec\utils.c
// 该函数必须在调用libavcodec里的其它函数前调用,一般在程序启动或模块初始化时调用,如果你调用了多次也无所谓,因为后面的调用不会做任何事情.从函数的实现里你可以发现,代码中对多次调用进行了控制.
// 该函数是非线程安全的
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和注册所有的muxers、demuxers和protocols，
// 一般在调用avcodec_init后调用该方法
// 引入头文件：#include "libavformat/avformat.h"
// 实现在:\ffmpeg\libavformat\allformats.c
// 其中会调用avcodec_register_all()注册多种音视频格式的编解码器,并注册各种文件的编解复用器
// 当然，你也可以不调用该函数，而通过选择调用特定的方法来提供支持
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);
// 分配一个AVFormatContext结构
// 引入头文件：#include "libavformat/avformat.h"
// 实现在:\ffmpeg\libavformat\options.c
// 其中负责申请一个AVFormatContext结构的内存,并进行简单初始化
// avformat_free_context()可以用来释放该结构里的所有东西以及该结构本身
// 也是就说使用 avformat_alloc_context()分配的结构,需要使用avformat_free_context()来释放
// 有些版本中函数名可能为: 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);
// 释放一个AVFormatContext结构
// 引入头文件：#include "libavformat/avformat.h"
// 实现在:\ffmpeg\libavformat\utils.c
// 使用 avformat_alloc_context()分配的结构,采用该函数进行释放,除释放AVFormatContext结构本身内存之外,AVFormatContext中指针所指向的内存也会一并释放
// 有些版本中函数名猜测可能为: av_free_format_context();
5 AVFormatContext 结构
/**
* 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里是一个非常重要的的结构，是其它输入、输出相关信息的一个容器
// 引入头文件：#include "libavformat/avformat.h"
// 以上只列出了其中的部分成员
// 作为输入容器时 struct AVInputFormat *iformat; 不能为空, 其中包含了输入文件的音视频流信息,程序从输入容器从读出音视频包进行解码处理
// 作为输出容器时 struct AVOutputFormat *oformat; 不能为空, 程序把编码好的音视频包写入到输出容器中
// AVIOContext *pb: I/O上下文,通过对该变量赋值可以改变输入源或输出目的
// unsigned int nb_streams; 音视频流数量
// AVStream **streams; 音视频流
6 AVIOContext 结构
/**
* 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;
// 字节流 I/O 上下文
// 在结构的尾部增加变量可以减少版本冲突
// 移除、排序和修改已经存在的变量将会导致较大的版本冲突
// sizeof(AVIOContext)在libav*.外部不可使用
// AVIOContext里的函数指针不能直接调用,通常使用avio_alloc_context()函数来设置其中的函数指针
// unsigned char *buffer: 缓存的起始指针
// int buffer_size: 缓存的最大值
// void *opaque: 在回调函数中使用的指针
// int (*read_packet)(void *opaque, uint8_t *buf, int buf_size): 读文件回调方法
// int (*write_packet)(void *opaque, uint8_t *buf, int buf_size): 写文件回调方法
// int64_t (*seek)(void *opaque, int64_t offset, int whence): seek文件回调方法
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));
// 为I/0缓存申请并初始化一个AVIOContext结构,结束使用时必须使用av_free()进行释放
// unsigned char *buffer: 输入/输出缓存内存块,必须是使用av_malloc()分配的
// int buffer_size: 缓存大小是非常重要的
// int write_flag: 如果缓存为可写则设置为1,否则设置为0
// void *opaque: 指针,用于回调时使用
// int (*read_packet): 读包函数指针
// int (*write_packet): 写包函数指针
// int64_t (*seek): seek文件函数指针
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);
// 以输入方式打开一个媒体文件,也即源文件,codecs并没有打开,只读取了文件的头信息.
// 引入头文件：#include "libavformat/avformat.h"
// AVFormatContext **ic_ptr 输入文件容器
// const char *filename 输入文件名,全路径,并且保证文件存在
// AVInputFormat *fmt 输入文件格式,填NULL即可
// int buf_size,缓冲区大小,直接填0即可
// AVFormatParameters *ap, 格式参数,添NULL即可
// 成功返回0,其它失败
// 不赞成使用 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);
// 关闭使用avformat_close_input()打开的输入文件容器,但并不关系它的codecs
// 引入头文件：#include "libavformat/avformat.h"
// 使用av_open_input_file 打开的文件容器,可以使用该函数关闭
// 使用 av_close_input_file 关闭后,就不再需要使用avformat_free_context 进行释放了
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);
// 通过读取媒体文件的中的包来获取媒体文件中的流信息,对于没有头信息的文件如(mpeg)是非常有用的,
// 该函数通常重算类似mpeg-2帧模式的真实帧率,该函数并未改变逻辑文件的position.
// 引入头文件：#include "libavformat/avformat.h"
// 也就是把媒体文件中的音视频流等信息读出来,保存在容器中,以便解码时使用
// 返回>=0时成功,否则失败