FFmpeg 結構體學習(七)： AVIOContext 分析

在上文FFmpeg 結構體學習(六)： AVCodecContext 分析咱們學習了AVCodec結構體的相關內容。本文，咱們將講述一下AVIOContext。

AVIOContext是FFMPEG管理輸入輸出數據的結構體。下面咱們來分析一下該結構體裏重要變量的含義和做用。

1、源碼整理

首先咱們先看一下結構體AVIOContext的定義的結構體源碼(位於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;

2、AVIOContext 重點字段

AVIOContext中有如下幾個變量比較重要：

unsigned char *buffer：緩存開始位置

int buffer_size：緩存大小（默認32768）

unsigned char *buf_ptr：當前指針讀取到的位置

unsigned char *buf_end：緩存結束的位置

void *opaque：URLContext結構體

在解碼的狀況下，buffer用於存儲ffmpeg讀入的數據。例如打開一個視頻文件的時候，先把數據從硬盤讀入buffer，而後在送給解碼器用於解碼。

其中opaque指向了URLContext。注意，這個結構體並不在FFMPEG提供的頭文件中，而是在FFMPEG的源代碼中。從FFMPEG源代碼中翻出的定義以下所示：

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結構體中還有一個結構體URLProtocol。注：每種協議（rtp，rtmp，file等）對應一個URLProtocol。這個結構體也不在FFMPEG提供的頭文件中。從FFMPEG源代碼中翻出其的定義：

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;

在這個結構體中，除了一些回調函數接口以外，有一個變量const char *name，該變量存儲了協議的名稱。每一種輸入協議都對應這樣一個結構體。

好比說，文件協議中代碼以下（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中代碼以下（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協議代碼以下（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,
};

等號右邊的函數是完成具體讀寫功能的函數。能夠看一下file協議的幾個函數（其實就是讀文件，寫文件這樣的操做）（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);
}