注：写了一系列的结构体的分析的文章，在这里列一个列表：

FFMPEG结构体分析：AVFrame
FFMPEG结构体分析：AVFormatContext
FFMPEG结构体分析：AVCodecContext
FFMPEG结构体分析：AVIOContext
FFMPEG结构体分析：AVCodec
FFMPEG结构体分析：AVStream
FFMPEG结构体分析：AVPacket

FFMPEG有几个最重要的结构体，包含了解协议，解封装，解码操作，此前已经进行过分析：

FFMPEG中最关键的结构体之间的关系

在此不再详述，其中AVIOContext是FFMPEG管理输入输出数据的结构体。本文将会详细分析一下该结构体里每个变量的含义和作用。

首先看一下结构体的定义（位于avio.h）：

/* 雷霄骅* 中国传媒大学/数字电视技术* leixiaohua1020@126.com**//*** 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 {/*** A class for private options.** If this AVIOContext is created by avio_open2(), av_class is set and* passes the options down to protocols.** If this AVIOContext is manually allocated, then av_class may be set by* the caller.** warning -- this field can be NULL, be sure to not pass this AVIOContext* to any av_opt_* functions in that case.*/AVClass *av_class;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 thanbuffer+buffer_size if the read function returnedless data than requested, e.g. for streams whereno 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 */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;/*** max filesize, used to limit allocations* This field is internal to libavformat and access from outside is not allowed.*/int64_t maxsize;
} AVIOContext;

AVIOContext中有以下几个变量比较重要：

在解码的情况下，buffer用于存储ffmpeg读入的数据。例如打开一个视频文件的时候，先把数据从硬盘读入buffer，然后在送给解码器用于解码。

其中opaque指向了URLContext。注意，这个结构体并不在FFMPEG提供的头文件中，而是在FFMPEG的源代码中。从FFMPEG源代码中翻出的定义如下所示：

typedef struct URLContext {const AVClass *av_class; ///< information for av_log(). Set by url_open().struct URLProtocol *prot;int flags;int is_streamed;  /**< true if streamed (no seek possible), default = false */int max_packet_size;  /**< if non zero, the stream is packetized with this max packet size */void *priv_data;char *filename; /**< specified URL */int is_connected;AVIOInterruptCB interrupt_callback;
} URLContext;

URLContext结构体中还有一个结构体URLProtocol。注：每种协议（rtp，rtmp，file等）对应一个URLProtocol。这个结构体也不在FFMPEG提供的头文件中。从FFMPEG源代码中翻出其的定义：

typedef struct URLProtocol {const char *name;int (*url_open)(URLContext *h, const char *url, int flags);int (*url_read)(URLContext *h, unsigned char *buf, int size);int (*url_write)(URLContext *h, const unsigned char *buf, int size);int64_t (*url_seek)(URLContext *h, int64_t pos, int whence);int (*url_close)(URLContext *h);struct URLProtocol *next;int (*url_read_pause)(URLContext *h, int pause);int64_t (*url_read_seek)(URLContext *h, int stream_index,int64_t timestamp, int flags);int (*url_get_file_handle)(URLContext *h);int priv_data_size;const AVClass *priv_data_class;int flags;int (*url_check)(URLContext *h, int mask);
} URLProtocol;

在这个结构体中，除了一些回调函数接口之外，有一个变量const char *name，该变量存储了协议的名称。每一种输入协议都对应这样一个结构体。

URLProtocol ff_file_protocol = {.name                = "file",.url_open            = file_open,.url_read            = file_read,.url_write           = file_write,.url_seek            = file_seek,.url_close           = file_close,.url_get_file_handle = file_get_handle,.url_check           = file_check,
};

URLProtocol ff_rtmp_protocol = {.name                = "rtmp",.url_open            = rtmp_open,.url_read            = rtmp_read,.url_write           = rtmp_write,.url_close           = rtmp_close,.url_read_pause      = rtmp_read_pause,.url_read_seek       = rtmp_read_seek,.url_get_file_handle = rtmp_get_file_handle,.priv_data_size      = sizeof(RTMP),.flags               = URL_PROTOCOL_FLAG_NETWORK,
};

URLProtocol ff_udp_protocol = {.name                = "udp",.url_open            = udp_open,.url_read            = udp_read,.url_write           = udp_write,.url_close           = udp_close,.url_get_file_handle = udp_get_file_handle,.priv_data_size      = sizeof(UDPContext),.flags               = URL_PROTOCOL_FLAG_NETWORK,
};

等号右边的函数是完成具体读写功能的函数。可以看一下file协议的几个函数（其实就是读文件，写文件这样的操作）（file.c）：

/**雷霄骅*leixiaohua1020@126.com*中国传媒大学/数字电视技术*/
/* standard file protocol */static int file_read(URLContext *h, unsigned char *buf, int size)
{int fd = (intptr_t) h->priv_data;int r = read(fd, buf, size);return (-1 == r)?AVERROR(errno):r;
}static int file_write(URLContext *h, const unsigned char *buf, int size)
{int fd = (intptr_t) h->priv_data;int r = write(fd, buf, size);return (-1 == r)?AVERROR(errno):r;
}static int file_get_handle(URLContext *h)
{return (intptr_t) h->priv_data;
}static int file_check(URLContext *h, int mask)
{struct stat st;int ret = stat(h->filename, &st);if (ret < 0)return AVERROR(errno);ret |= st.st_mode&S_IRUSR ? mask&AVIO_FLAG_READ  : 0;ret |= st.st_mode&S_IWUSR ? mask&AVIO_FLAG_WRITE : 0;return ret;
}#if CONFIG_FILE_PROTOCOLstatic int file_open(URLContext *h, const char *filename, int flags)
{int access;int fd;av_strstart(filename, "file:", &filename);if (flags & AVIO_FLAG_WRITE && flags & AVIO_FLAG_READ) {access = O_CREAT | O_TRUNC | O_RDWR;} else if (flags & AVIO_FLAG_WRITE) {access = O_CREAT | O_TRUNC | O_WRONLY;} else {access = O_RDONLY;}
#ifdef O_BINARYaccess |= O_BINARY;
#endiffd = open(filename, access, 0666);if (fd == -1)return AVERROR(errno);h->priv_data = (void *) (intptr_t) fd;return 0;
}/* XXX: use llseek */
static int64_t file_seek(URLContext *h, int64_t pos, int whence)
{int fd = (intptr_t) h->priv_data;if (whence == AVSEEK_SIZE) {struct stat st;int ret = fstat(fd, &st);return ret < 0 ? AVERROR(errno) : st.st_size;}return lseek(fd, pos, whence);
}static int file_close(URLContext *h)
{int fd = (intptr_t) h->priv_data;return close(fd);
}