FFMPEG-AVFormatContext结构详解

最新推荐文章于 2025-05-25 20:03:41 发布

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最新推荐文章于 2025-05-25 20:03:41 发布 · 4.6k 阅读

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本文深入探讨FFMPEG中的AVFormatContext结构体，通过图形化方式帮助新手理解其在FFMPEG中的核心地位。文章将逐个解析结构体的成员，并在代码注释中提供详细解释。

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FFMPEG的结构体看的让人找不到北，尤其对接触FFMPEG的新手来说，在接下来的一段时间内，我将用图形化的方式来讲解他们之间的关系，先从AVFormatContext说起吧，这个结构体像个root类，

直接上图：

每个成员的解释将在代码中注释讲解

/**
 * Format I/O 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(AVFormatContext) must not be used outside libav*, use
 * avformat_alloc_context() to create an AVFormatContext.
 */
typedef struct AVFormatContext {
    /**
     * A class for logging and @ref avoptions. Set by avformat_alloc_context().
     * Exports (de)muxer private options if they exist.
     */
    const AVClass *av_class;

    /**
     * The input container format.
     *
     * Demuxing only, set by avformat_open_input().
     */
    struct AVInputFormat *iformat;

    /**
     * The output container format.
     *
     * Muxing only, must be set by the caller before avformat_write_header().
     */
    struct AVOutputFormat *oformat;

    /**
     * Format private data. This is an AVOptions-enabled struct
     * if and only if iformat/oformat.priv_class is not NULL.
     *
     * - muxing: set by avformat_write_header()
     * - demuxing: set by avformat_open_input()
     */
    void *priv_data;

    /**
     * I/O context.
     * <span style="color:#cc33cc;">输入数据的缓存</span>
     * - demuxing: either set by the user before avformat_open_input() (then
     *             the user must close it manually) or set by avformat_open_input().
     * - muxing: set by the user before avformat_write_header(). The caller must
     *           take care of closing / freeing the IO context.
     *
     * 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;

    /* stream info */
    /**
     * Flags signalling stream properties. A combination of AVFMTCTX_*.
     * Set by libavformat.
     */
    int ctx_flags;

    /**
     * Number of elements in AVFormatContext.streams.
     * <span style="color:#cc33cc;">音视频流的个数</span>
     * Set by avformat_new_stream(), must not be modified by any other code.
     */
    unsigned int nb_streams;
    /**
     * A list of all streams in the file. New streams are created with
     * avformat_new_stream().
     * <span style="color: rgb(204, 51, 204); font-family: Arial, Helvetica, sans-serif;">音视频流</span>
     * - demuxing: streams are created by libavformat in avformat_open_input().
     *             If AVFMTCTX_NOHEADER is set in ctx_flags, then new streams may also
     *             appear in av_read_frame().
     * - muxing: streams are created by the user before avformat_write_header().
     *
     * Freed by libavformat in avformat_free_context().
     */
    AVStream **streams;

    /**
     * input or output filename
     *
     * - demuxing: set by avformat_open_input()
     * - muxing: may be set by the caller before avformat_write_header()
     */
    char filename[1024];

    /**
     * Position of the first frame of the component, in
     * AV_TIME_BASE fractional seconds. NEVER set this value directly:
     * It is deduced from the AVStream values.
     * <span style="color:#cc33cc;">开始时间 单位为微妙 10^-6</span>
     * Demuxing only, set by libavformat.
     */
    int64_t start_time;

    /**
     * Durati