Create FFmpeg 结构体学习(七)： AVIOContext 分析.md

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 AVIOContext是FFMPEG管理输入输出数据的结构体。下面我们来分析一下该结构体里重要变量的含义和作用。
 # 一、源码整理
 首先我们先看一下结构体AVIOContext的定义的结构体源码(位于libavformat/avio.h)：
 ```
 /**
 * 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 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 */
    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;
 ```
 # 二、AVIOContext 重点字段
 AVIOContext中有以下几个变量比较重要：
 ```
 unsigned char *buffer：缓存开始位置
 int buffer_size：缓存大小（默认32768）
 unsigned char *buf_ptr：当前指针读取到的位置
 unsigned char *buf_end：缓存结束的位置
 void *opaque：URLContext结构体
 ```
 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。
 比如说，文件协议中代码如下（file.c）：
 ```
 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,
 };
 ```
 libRTMP中代码如下（libRTMP.c）：
 ```
 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,
 };
 ```
 udp协议代码如下（udp.c）：
 ```
 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）：
 ```
 /* 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_PROTOCOL
 static 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_BINARY
    access |= O_BINARY;
 #endif
    fd = 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);
 }
 ```