ffmpeg函数介绍
本文对在使用ffmpeg进行音视频编解码时使用到的一些函数做一个简单介绍,我当前使用的ffmpeg版本为:0.8.5,因为本人发现在不同的版本中,有些函数名称会有点小改动,所以在此有必要说明下ffmpeg的版本号。
ffmpeg本人也是刚接触，本文将采用累加的方法逐个介绍我使用到的函数，如有不妥之处，还望谅解！
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"
};
1 avcodec_init()
/**
* 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,constchar *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
//constchar *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
/***********************************************************/
1 avcodec_find_decoder()
/**
* Find a registered decoder with a matching codec ID.
*
* @param id CodecID of the requested decoder
* @return A decoder if one was found, NULL otherwise.
*/
AVCodec *avcodec_find_decoder(enum CodecID id);
// 通过code ID查找一个已经注册的音视频解码器
// 引入 #include "libavcodec/avcodec.h"
// 实现在: \ffmpeg\libavcodec\utils.c
// 查找解码器之前,必须先调用av_register_all注册所有支持的解码器
// 查找成功返回解码器指针,否则返回NULL
// 音视频解码器保存在一个链表中,查找过程中,函数从头到尾遍历链表,通过比较解码器的ID来查找
2 avcodec_find_decoder_by_name()
/**
* Find a registered decoder with the specified name.
*
* @param name name of the requested decoder
* @return A decoder if one was found, NULL otherwise.
*/
AVCodec *avcodec_find_decoder_by_name(constchar *name);
// 通过一个指定的名称查找一个已经注册的音视频解码器
// 引入 #include "libavcodec/avcodec.h"
// 实现在: \ffmpeg\libavcodec\utils.c
// 查找解码器之前,必须先调用av_register_all注册所有支持的解码器
// 查找成功返回解码器指针,否则返回NULL
// 音视频解码器保存在一个链表中,查找过程中,函数从头到尾遍历链表,通过比较解码器的name来查找
3 avcodec_find_encoder()
/**
* Find a registered encoder with a matching codec ID.
*
* @param id CodecID of the requested encoder
* @return An encoder if one was found, NULL otherwise.
*/
AVCodec *avcodec_find_encoder(enum CodecID id);
// 通过code ID查找一个已经注册的音视频编码器
// 引入 #include "libavcodec/avcodec.h"
// 实现在: \ffmpeg\libavcodec\utils.c
// 查找编码器之前,必须先调用av_register_all注册所有支持的编码器
// 查找成功返回编码器指针,否则返回NULL
// 音视频编码器保存在一个链表中,查找过程中,函数从头到尾遍历链表,通过比较编码器的ID来查找
4 avcodec_find_encoder_by_name()
/**
* Find a registered encoder with the specified name.
*
* @param name name of the requested encoder
* @return An encoder if one was found, NULL otherwise.
*/
AVCodec *avcodec_find_encoder_by_name(constchar *name);
// 通过一个指定的名称查找一个已经注册的音视频编码器
// 引入 #include "libavcodec/avcodec.h"
// 实现在: \ffmpeg\libavcodec\utils.c
// 查找编码器之前,必须先调用av_register_all注册所有支持的编码器
// 查找成功返回编码器指针,否则返回NULL
// 音视频编码器保存在一个链表中,查找过程中,函数从头到尾遍历链表,通过比较编码器的名称来查找
5 avcodec_open()
/**
* Initialize the AVCodecContext to use the given AVCodec. Prior to using this
* function the context has to be allocated.
*
* The functions avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(),
* avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for
* retrieving a codec.
*
* @warning This function is not thread safe!
*
* @code
* avcodec_register_all();
* codec = avcodec_find_decoder(CODEC_ID_H264);
* if (!codec)
* exit(1);
*
* context = avcodec_alloc_context();
*
* if (avcodec_open(context, codec) < 0)
* exit(1);
* @endcode
*
* @param avctx The context which will be set up to use the given codec.
* @param codec The codec to use within the context.
* @return zero on success, a negative value on error
* @see avcodec_alloc_context, avcodec_find_decoder, avcodec_find_encoder, avcodec_close
*/
int avcodec_open(AVCodecContext *avctx, AVCodec *codec);
// 使用给定的AVCodec初始化AVCodecContext
// 引入#include "libavcodec/avcodec.h"
// 方法: avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(), avcodec_find_decoder() and avcodec_find_encoder() 提供了快速获取一个codec的途径
// 该方法在编码和解码时都会用到
// 返回0时成功,打开作为输出时,参数设置不对的话,调用会失败
6 av_guess_format()
/**
* Return the output format in the list of registered output formats
* which best matches the provided parameters, or return NULL if
* there is no match.
*
* @param short_name if non-NULL checks if short_name matches with the
* names of the registered formats
* @param filename if non-NULL checks if filename terminates with the
* extensions of the registered formats
* @param mime_type if non-NULL checks if mime_type matches with the
* MIME type of the registered formats
*/
AVOutputFormat *av_guess_format(constchar *short_name,
constchar *filename,
constchar *mime_type);
// 返回一个已经注册的最合适的输出格式
// 引入#include "libavformat/avformat.h"
// 可以通过 const char *short_name 获取,如"mpeg"
// 也可以通过 const char *filename 获取,如"E:\a.mp4"
7 av_new_stream()
/**
* Add a new stream to a media file.
*
* Can only be called in the read_header() function. If the flag
* AVFMTCTX_NOHEADER is in the format context, then new streams
* can be added in read_packet too.
*
* @param s media file handle
* @param id file-format-dependent stream ID
*/
AVStream *av_new_stream(AVFormatContext *s, int id);
// 为媒体文件添加一个流,一般为作为输出的媒体文件容器添加音视频流
// 引入 #include "libavformat/avformat.h"
// 再打开源文件时用户一般不需要直接调用该方法
8 dump_format()
#if FF_API_DUMP_FORMAT
/**
* @deprecated Deprecated in favor of av_dump_format().
*/
attribute_deprecated void dump_format(AVFormatContext *ic,
int index,
constchar *url,
int is_output);
#endif
// 该函数的作用就是检查下初始化过程中设置的参数是否符合规范
// 有些版本中为 av_ dump_format
9 av_set_parameters()
#if FF_API_FORMAT_PARAMETERS
/**
* @deprecated pass the options to avformat_write_header directly.
*/
attribute_deprecated int av_set_parameters(AVFormatContext *s, AVFormatParameters *ap);
#endif
// 设置初始化参数
// 不赞成跳过该方法,直接调用 avformat_write_header/av_write_header
10 av_write_header()
#if FF_API_FORMAT_PARAMETERS
/**
* Allocate the stream private data and write the stream header to an
* output media file.
* @note: this sets stream time-bases, if possible to stream->codec->time_base
* but for some formats it might also be some other time base
*
* @param s media file handle
* @return 0 if OK, AVERROR_xxx on error
*
* @deprecated use avformat_write_header.
*/
attribute_deprecated int av_write_header(AVFormatContext *s);
#endif
// 把流头信息写入到媒体文件中
// 返回0成功
/**********************************************************/
1 AVPacket
typedef struct AVPacket {
/**
* Presentation timestamp in AVStream->time_base units; the time at which
* the decompressed packet will be presented to the user.
* Can be AV_NOPTS_VALUE if it is not stored in the file.
* pts MUST be larger or equal to dts as presentation cannot happen before
* decompression, unless one wants to view hex dumps. Some formats misuse
* the terms dts and pts/cts to mean something different. Such timestamps
* must be converted to true pts/dts before they are stored in AVPacket.
*/
int64_t pts;
/**
* Decompression timestamp in AVStream->time_base units; the time at which
* the packet is decompressed.
* Can be AV_NOPTS_VALUE if it is not stored in the file.
*/
int64_t dts;
uint8_t *data;
int size;
int stream_index;
int flags;
int duration;
.
.
.
} AVPacket
// AVPacket是个很重要的结构,该结构在读媒体源文件和写输出文件时都需要用到
// int64_t pts; 显示时间戳
// int64_t dts; 解码时间戳
// uint8_t *data; 包数据
// int size; 包数据长度
// int stream_index; 包所属流序号
// int duration; 时长
// 以上信息,如果是在读媒体源文件那么avcodec会初始化,如果是输出文件,用户需要对以上信息赋值
2 av_init_packet()
/**
* Initialize optional fields of a packet with default values.
*
* @param pkt packet
*/
void av_init_packet(AVPacket *pkt);
// 使用默认值初始化AVPacket
// 定义AVPacket对象后,请使用av_init_packet进行初始化
3 av_free_packet()
/**
* Free a packet.
*
* @param pkt packet to free
*/
void av_free_packet(AVPacket *pkt);
// 释放AVPacket对象
4 av_read_frame()
/**
* Return the next frame of a stream.
* This function returns what is stored in the file, and does not validate
* that what is there are valid frames for the decoder. It will split what is
* stored in the file into frames and return one for each call. It will not
* omit invalid data between valid frames so as to give the decoder the maximum
* information possible for decoding.
*
* The returned packet is valid
* until the next av_read_frame() or until av_close_input_file() and
* must be freed with av_free_packet. For video, the packet contains
* exactly one frame. For audio, it contains an integer number of
* frames if each frame has a known fixed size (e.g. PCM or ADPCM
* data). If the audio frames have a variable size (e.g. MPEG audio),
* then it contains one frame.
*
* pkt->pts, pkt->dts and pkt->duration are always set to correct
* values in AVStream.time_base units (and guessed if the format cannot
* provide them). pkt->pts can be AV_NOPTS_VALUE if the video format
* has B-frames, so it is better to rely on pkt->dts if you do not
* decompress the payload.
*
* @return 0 if OK, < 0 on error or end of file
*/
int av_read_frame(AVFormatContext *s, AVPacket *pkt);
// 从输入源文件容器中读取一个AVPacket数据包
// 该函数读出的包并不每次都是有效的,对于读出的包我们都应该进行相应的解码(视频解码/音频解码),
// 在返回值>=0时,循环调用该函数进行读取,循环调用之前请调用av_free_packet函数清理AVPacket
5 avcodec_decode_video2()
/**
* Decode the video frame of size avpkt->size from avpkt->data into picture.
* Some decoders may support multiple frames in a single AVPacket, such
* decoders would then just decode the first frame.
*
* @warning The input buffer must be FF_INPUT_BUFFER_PADDING_SIZE larger than
* the actual read bytes because some optimized bitstream readers read 32 or 64
* bits at once and could read over the end.
*
* @warning The end of the input buffer buf should be set to 0 to ensure that
* no overreading happens for damaged MPEG streams.
*
* @note You might have to align the input buffer avpkt->data.
* The alignment requirements depend on the CPU: on some CPUs it isn't
* necessary at all, on others it won't work at all if not aligned and on others
* it will work but it will have an impact on performance.
*
* In practice, avpkt->data should have 4 byte alignment at minimum.
*
* @note Some codecs have a delay between input and output, these need to be
* fed with avpkt->data=NULL, avpkt->size=0 at the end to return the remaining frames.
*
* @param avctx the codec context
* @param[out] picture The AVFrame in which the decoded video frame will be stored.
* Use avcodec_alloc_frame to get an AVFrame, the codec will
* allocate memory for the actual bitmap.
* with default get/release_buffer(), the decoder frees/reuses the bitmap as it sees fit.
* with overridden get/release_buffer() (needs CODEC_CAP_DR1) the user decides into what buffer the decoder
* decodes and the decoder tells the user once it does not need the data anymore,
* the user app can at this point free/reuse/keep the memory as it sees fit.
*
* @param[in] avpkt The input AVpacket containing the input buffer.
* You can create such packet with av_init_packet() and by then setting
* data and size, some decoders might in addition need other fields like
* flags&AV_PKT_FLAG_KEY. All decoders are designed to use the least
* fields possible.
* @param[in,out] got_picture_ptr Zero if no frame could be decompressed, otherwise, it is nonzero.
* @return On error a negative value is returned, otherwise the number of bytes
* used or zero if no frame could be decompressed.
*/
int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,
int *got_picture_ptr,
AVPacket *avpkt);
// 解码视频流AVPacket
// 使用av_read_frame读取媒体流后需要进行判断,如果为视频流则调用该函数解码
// 返回结果<0时失败,此时程序应该退出检查原因
// 返回>=0时正常,假设 读取包为:AVPacket vPacket 返回值为 int vLen; 每次解码正常时,对vPacket做
// 如下处理:
// vPacket.size -= vLen;
// vPacket.data += vLen;
// 如果 vPacket.size==0,则继续读下一流包,否则继续调度该方法进行解码,直到vPacket.size==0
// 返回 got_picture_ptr > 0 时,表示解码到了AVFrame *picture,其后可以对picture进程处理
6 avcodec_decode_audio3()
/**
* Decode the audio frame of size avpkt->size from avpkt->data into samples.
* Some decoders may support multiple frames in a single AVPacket, such
* decoders would then just decode the first frame. In this case,
* avcodec_decode_audio3 has to be called again with an AVPacket that contains
* the remaining data in order to decode the second frame etc.
* If no frame
* could be outputted, frame_size_ptr is zero. Otherwise, it is the
* decompressed frame size in bytes.
*
* @warning You must set frame_size_ptr to the allocated size of the
* output buffer before calling avcodec_decode_audio3().
*
* @warning The input buffer must be FF_INPUT_BUFFER_PADDING_SIZE larger than
* the actual read bytes because some optimized bitstream readers read 32 or 64
* bits at once and could read over the end.
*
* @warning The end of the input buffer avpkt->data should be set to 0 to ensure that
* no overreading happens for damaged MPEG streams.
*
* @note You might have to align the input buffer avpkt->data and output buffer
* samples. The alignment requirements depend on the CPU: On some CPUs it isn't
* necessary at all, on others it won't work at all if not aligned and on others
* it will work but it will have an impact on performance.
*
* In practice, avpkt->data should have 4 byte alignment at minimum and
* samples should be 16 byte aligned unless the CPU doesn't need it
* (AltiVec and SSE do).
*
* @param avctx the codec context
* @param[out] samples the output buffer, sample type in avctx->sample_fmt
* @param[in,out] frame_size_ptr the output buffer size in bytes
* @param[in] avpkt The input AVPacket containing the input buffer.
* You can create such packet with av_init_packet() and by then setting
* data and size, some decoders might in addition need other fields.
* All decoders are designed to use the least fields possible though.
* @return On error a negative value is returned, otherwise the number of bytes
* used or zero if no frame data was decompressed (used) from the input AVPacket.
*/
int avcodec_decode_audio3(AVCodecContext *avctx, int16_t *samples,
int *frame_size_ptr,
AVPacket *avpkt);
// 解码音频流AVPacket
// 使用av_read_frame读取媒体流后需要进行判断,如果为音频流则调用该函数解码
// 返回结果<0时失败,此时程序应该退出检查原因
// 返回>=0时正常,假设 读取包为:AVPacket vPacket 返回值为 int vLen; 每次解码正常时,对vPacket做
// 如下处理:
// vPacket.size -= vLen;
// vPacket.data += vLen;
// 如果 vPacket.size==0,则继续读下一流包,否则继续调度该方法进行解码,直到vPacket.size==0
转自：http://blog.chinaunix.net/uid/20718335/frmd/153034.html