FFMPEG内存操作（一） avio_reading.c 回调读取数据到内存解析-优快云博客

本文介绍FFmpeg如何通过AVIOContext API从内存中读取音视频数据。使用自定义缓冲区并配合回调函数实现文件内容映射到内存，便于后续处理。文中详细解释了avio_reading.c例程的工作原理。

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。

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

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。