FFMPEG内存操作（一） avio_reading.c 回调读取数据到内存解析

相关博客列表 ：

    FFMPEG内存操作（一） avio_reading.c 回调读取数据到内存解析 

    FFMPEG内存操作（二）从内存中读取数及数据格式的转换 

    FFmpeg内存操作（三）内存转码器

   在FFMPEG的官方例程中，有个avio_reading.c 的文件，他的主要功能是将音视频文件读取到内存，如果FFMPEG需要使用输入文件的数据，则直接从内存中调用。初学FFMPEG，给avio_reading.c 文件做了一个注释，如果不对，欢迎指正。

/*
 * 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; /* 文件中对应位置指针 */
    size_t size; ///< size left in the buffer /* 文件当前指针到末尾 */
};

/* 将文件中数据拷贝到缓冲区，同时文件指针位置偏移，数据大小改变 */
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
", 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
"
                "API example program to show how to read from a custom buffer "
                "accessed through AVIOContext.
", 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 : 输入文件的文件名
     * buffer : 文件开始地址
     * buffer_size : 文件大小
     * 类似于UNIX下的mmap函数所实现的功能，返回文件开始指针，文件大小
     * 经测试，并不耗内存，可视为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 是指经过文件映射之后的文件，并不是指需要缓存区 */
    bd.ptr  = buffer;
    bd.size = buffer_size;

    /* 初始化文件格式的结构体,就是分配内存 */
    if (!(fmt_ctx = avformat_alloc_context())) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    /* 分配内存, 可以自己设置缓冲大小,这里设置的是4K */
    avio_ctx_buffer = av_malloc(avio_ctx_buffer_size);
    if (!avio_ctx_buffer) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    /* avio_ctx_buffer是缓冲区，
     * avio_ctx_buffer 的初始地址赋值到 avio_ctx->buffer
     * avio_ctx_buffer_size是缓冲区大小 , 也是每次读取数据的大小
     * bd 是输入文件文件的映射文件
     * read_packet 回调函数,读取数据的功能 , 具体在什么情况下才会回调 ?
     */
    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 回调函数会在这里被调用, 它将输入文件的所有数据都先存入缓存中,
    * 如果后面有需要用到数据，那么它就从缓存中直接调用数据
    */
    ret = avformat_open_input(&fmt_ctx, NULL, NULL, NULL);
    if (ret < 0) {
        fprintf(stderr, "Could not open input
");
        goto end;
    }

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

    /* 输出基本信息 */
    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 */
    //应该就是av_free(&avio_ctx_buffer),但位置不对
    //两者有差异
    //printf("%p %p
",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
", av_err2str(ret));
        return 1;
    }

    return 0;
}

    这里定义了一个avio_ctx_buffer_size 变量表示每次从文件中读取到少数据到内存。在该例子中，FFMPEG是先把输入的音视频数据全部读取到内存，而并不是需要使用的时候才去读取。