ffmpeg(2) 关于AVFMT_NOFILE

转自：http://blog.youkuaiyun.com/xiruanliuwei/article/details/20325467

在学习ffmpeg源码时，常常看到一个Flag：AVFMT_NOFILE，这个Flag非常重要，其在avformat.h中定义：#define AVFMT_NOFILE        0x0001

在整个工程中搜索AVFMT_NOFILE，能够看到ff_alsa_demuxer、ff_bktr_demuxer、ff_dshow_demuxer、ff_dv1394_demuxer、ff_fbdev_demuxer、ff_image2_demuxer等部分

AVInputFormat类型的变量，其结构体成员flags的值被设置为AVFMT_NOFILE，当然，不是只有AVInputFormat类型的变量，部分AVOutputFormat类型的变量，其结构体成员

flags的值也被设置为AVFMT_NOFILE，查看AVFMT_NOFILE的搜索结果，对其稍微详细的解释在avdevice.h中：

/**
 * @defgroup lavd Special devices muxing/demuxing library
 * @{
 * Libavdevice is a complementary library to @ref libavf "libavformat". It
 * provides various "special" platform-specific muxers and demuxers, e.g. for
 * grabbing devices, audio capture and playback etc. As a consequence, the
 * (de)muxers in libavdevice are of the AVFMT_NOFILE type (they use their own
 * I/O functions). The filename passed to avformat_open_input() often does not
 * refer to an actually existing file, but has some special device-specific
 * meaning - e.g. for x11grab it is the display name.
 *
 * To use libavdevice, simply call avdevice_register_all() to register all
 * compiled muxers and demuxers. They all use standard libavformat API.
 * @}
 */

libavdevice目录中的内容是libavformat目录中内容的补充，libavdevice提供的是各种特殊平台专用的muxers和demuxers，例如grabbing devices、audio capture、playback等，

因此，libavdevice中的muxers和demuxers，都是AVFMT_NOFILE类型的，这些muxers和demuxers都使用它们自己的I/O函数，传递给avformat_open_input()函数的函数filename，

通常不是指一个实际存在的文件，而是根据特定的设备确定，例如，x11grab，就是display name？显示器名字？

在AVFormatContext结构体中：

    /**
     * I/O context.
     *
     * decoding: either set by the user before avformat_open_input() (then
     * the user must close it manually) or set by avformat_open_input().
     * encoding: set by the user.
     *
     * Do NOT set this field if AVFMT_NOFILE flag is set in
     * iformat/oformat.flags. In such a case, the (de)muxer will handle
     * I/O in some other way and this field will be NULL.
     */
    AVIOContext *pb;

AVIOContext表示字节流输入/输出的上下文，在muxers和demuxers的数据成员flags有设置AVFMT_NOFILE时，这个成员变量pb就不需要设置，因为muxers和demuxers会

使用其它的方式处理输入/输出。

    /**
     * Custom interrupt callbacks for the I/O layer.
     *
     * decoding: set by the user before avformat_open_input().
     * encoding: set by the user before avformat_write_header()
     * (mainly useful for AVFMT_NOFILE formats). The callback
     * should also be passed to avio_open2() if it's used to
     * open the file.
     */
    AVIOInterruptCB interrupt_callback;

AVIOInterruptCB在cflags有设置AVFMT_NOFILE时使用，传递给函数avio_open2使用。

/**
 * Callback for checking whether to abort blocking functions.
 * AVERROR_EXIT is returned in this case by the interrupted
 * function. During blocking operations, callback is called with
 * opaque as parameter. If the callback returns 1, the
 * blocking operation will be aborted.
 *
 * No members can be added to this struct without a major bump, if
 * new elements have been added after this struct in AVFormatContext
 * or AVIOContext.
 */
typedef struct AVIOInterruptCB 
{
    int (*callback)(void*);
    void *opaque;
} AVIOInterruptCB;

另外，就是：

/**
 * Guess the file format.
 *
 * @param is_opened Whether the file is already opened; determines whether
 *                  demuxers with or without AVFMT_NOFILE are probed.
 */
AVInputFormat *av_probe_input_format(AVProbeData *pd, int is_opened);


/**
 * Guess the file format.
 *
 * @param is_opened Whether the file is already opened; determines whether
 *                  demuxers with or without AVFMT_NOFILE are probed.
 * @param score_max A probe score larger that this is required to accept a
 *                  detection, the variable is set to the actual detection
 *                  score afterwards.
 *                  If the score is <= AVPROBE_SCORE_MAX / 4 it is recommended
 *                  to retry with a larger probe buffer.
 */
AVInputFormat *av_probe_input_format2(AVProbeData *pd, int is_opened, int *score_max);

参数is_opened，与muxers和demuxers有关系。

关于avio_open2：

/**
 * Create and initialize a AVIOContext for accessing the
 * resource indicated by url.
 * @note When the resource indicated by url has been opened in
 * read+write mode, the AVIOContext can be used only for writing.
 *
 * @param s Used to return the pointer to the created AVIOContext.
 * In case of failure the pointed to value is set to NULL.
 * @param flags flags which control how the resource indicated by url
 * is to be opened
 * @param int_cb an interrupt callback to be used at the protocols level
 * @param options  A dictionary filled with protocol-private options. On return
 * this parameter will be destroyed and replaced with a dict containing options
 * that were not found. May be NULL.
 * @return >= 0 in case of success, a negative value corresponding to an
 * AVERROR code in case of failure
 */
int avio_open2(AVIOContext **s, const char *url, int flags, const AVIOInterruptCB *int_cb, AVDictionary **options);

参数const AVIOInterruptCB *int_cb，经过层层调用，最终在函数url_alloc_for_protocol中，赋值给新分配的URLContext类型变量的数据成员AVIOInterruptCB interrupt_callback;

static int url_alloc_for_protocol(URLContext **puc, struct URLProtocol *up,
                                  const char *filename, int flags,
                                  const AVIOInterruptCB *int_cb)
{
    URLContext *uc;

   ...

   uc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1);

   ...

       if (int_cb)
        uc->interrupt_callback = *int_cb;

   ...

接下来要做的事情，就是理清楚，在有AVFMT_NOFILE和没有AVFMT_NOFILE的两种情况下，avformat input open分别的流程，以及二者之间有什么差别？

另外，还有AVIOInterruptCB使用的问题。