FFMPEG內存操作（一） avio_reading.c 回調讀取數據到內存解析

相關博客列表 ：

    FFMPEG內存操作（一） avio_reading.c 回調讀取數據到內存解析 

    FFMPEG內存操作（二）從內存中讀取數及數據格式的轉換 

    FFmpeg內存操作（三）內存轉碼器

   在FFMPEG的官方例程中，有個avio_reading.c 的文件，他的主要功能是將音視頻文件讀取到內存，如果FFMPEG需要使用輸入文件的數據，則直接從內存中調用。初學FFMPEG，給avio_reading.c 文件做了一個注釋，如果不對，歡迎指正。

 

/*
 * Copyright (c) 2014 Stefano Sabatini
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

/**
 * @file
 * libavformat AVIOContext API example.
 *
 * Make libavformat demuxer access media content through a custom
 * AVIOContext read callback.
 * @example avio_reading.c
 */

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavformat/avio.h>
#include <libavutil/file.h>

struct buffer_data {
    uint8_t *ptr; /* 文件中對應位置指針 */
    size_t size; ///< size left in the buffer /* 文件當前指針到末尾 */
};

/* 將文件中數據拷貝到緩沖區，同時文件指針位置偏移，數據大小改變 */
static int read_packet(voidvoid *opaque, uint8_t *buf, int buf_size)
{
    struct buffer_data *bd = (struct buffer_data *)opaque;
    buf_size = FFMIN(buf_size, bd->size);

    printf("ptr:%p size:%zu\n", bd->ptr, bd->size);

    /* copy internal buffer data to buf */
    memcpy(buf, bd->ptr, buf_size);
    bd->ptr  += buf_size;
    bd->size -= buf_size;

    return buf_size;
}

int main(int argc, charchar *argv[])
{
    AVFormatContext *fmt_ctx = NULL;
    AVIOContext *avio_ctx = NULL;
    uint8_t *buffer = NULL, *avio_ctx_buffer = NULL;
    size_t buffer_size, avio_ctx_buffer_size = 4096;
    charchar *input_filename = NULL;
    int ret = 0;
    struct buffer_data bd = { 0 };

    if (argc != 2) {
        fprintf(stderr, "usage: %s input_file\n"
                "API example program to show how to read from a custom buffer "
                "accessed through AVIOContext.\n", argv[0]);
        return 1;
    }
    input_filename = argv[1];

    /* register codecs and formats and other lavf/lavc components*/
    av_register_all();

    /* slurp file content into buffer */
    /* input_filename : 輸入文件的文件名
     * buffer : 文件開始地址
     * buffer_size : 文件大小
     * 類似於UNIX下的mmap函數所實現的功能，返回文件開始指針，文件大小
     * 經測試，並不耗內存，可視為UNIX下的文件映射
    */
    ret = av_file_map(input_filename, &buffer, &buffer_size, 0, NULL);
    if (ret < 0)
        goto end;

    /* fill opaque structure used by the AVIOContext read callback */
    /* bd 是指經過文件映射之后的文件，並不是指需要緩存區 */
    bd.ptr  = buffer;
    bd.size = buffer_size;

    /* 初始化文件格式的結構體,就是分配內存 */
    if (!(fmt_ctx = avformat_alloc_context())) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    /* 分配內存, 可以自己設置緩沖大小,這里設置的是4K */
    avio_ctx_buffer = av_malloc(avio_ctx_buffer_size);
    if (!avio_ctx_buffer) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    /* avio_ctx_buffer是緩沖區，
     * avio_ctx_buffer 的初始地址賦值到 avio_ctx->buffer
     * avio_ctx_buffer_size是緩沖區大小 , 也是每次讀取數據的大小
     * bd 是輸入文件文件的映射文件
     * read_packet 回調函數,讀取數據的功能 , 具體在什么情況下才會回調 ?
     */
    avio_ctx = avio_alloc_context(avio_ctx_buffer, avio_ctx_buffer_size,
                                  0, &bd, &read_packet, NULL, NULL);
    if (!avio_ctx) {
        ret = AVERROR(ENOMEM);
        goto end;
    }
    fmt_ctx->pb = avio_ctx;
    /* 配置初始化信息
    * read_packet 回調函數會在這里被調用, 它將輸入文件的所有數據都先存入緩存中,
    * 如果后面有需要用到數據，那么它就從緩存中直接調用數據
    */
    ret = avformat_open_input(&fmt_ctx, NULL, NULL, NULL);
    if (ret < 0) {
        fprintf(stderr, "Could not open input\n");
        goto end;
    }

    ret = avformat_find_stream_info(fmt_ctx, NULL);
    if (ret < 0) {
        fprintf(stderr, "Could not find stream information\n");
        goto end;
    }

    /* 輸出基本信息 */
    av_dump_format(fmt_ctx, 0, input_filename, 0);

end:
    avformat_close_input(&fmt_ctx);
    /* note: the internal buffer could have changed, and be != avio_ctx_buffer */
    //應該就是av_free(&avio_ctx_buffer),但位置不對
    //兩者有差異
    //printf("%p %p\n",avio_ctx_buffer,avio_ctx->buffer);
    if (avio_ctx) {
        av_freep(&avio_ctx->buffer);
        av_freep(&avio_ctx);
    }
    av_file_unmap(buffer, buffer_size);

    if (ret < 0) {
        fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));
        return 1;
    }

    return 0;
}

    這里定義了一個avio_ctx_buffer_size 變量表示每次從文件中讀取到少數據到內存。在該例子中，FFMPEG是先把輸入的音視頻數據全部讀取到內存，而並不是需要使用的時候才去讀取。