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);
+}
+```
+