使用C语言编写一个可运行的视频倒放demo

要实现视频倒放的功能，我们需要对视频帧进行逆序处理。在C语言中，我们可以使用FFmpeg库来进行视频处理。

以下是一个简单的视频倒放demo的代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <libavutil/avutil.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
#include <libswresample/swresample.h>

#define INBUF_SIZE 4096

int main(int argc, char **argv)
{
    AVFormatContext *formatCtx = NULL;
    AVCodecContext *codecCtx = NULL;
    AVCodec *codec = NULL;
    AVFrame *frame = NULL;
    AVPacket packet;
    int videoStreamIndex = -1;
    int frameFinished = 0;
    uint8_t inbuf[INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
    uint8_t *data = NULL;
    int dataSize = 0;
    int dataUsed = 0;
    int ret = 0;
    int i = 0;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <input file>\n", argv[0]);
        exit(1);
    }

    av_register_all();

    if (avformat_open_input(&formatCtx, argv[1], NULL, NULL) != 0) {
        fprintf(stderr, "Failed to open input file.\n");
        exit(1);
    }

    if (avformat_find_stream_info(formatCtx, NULL) < 0) {
        fprintf(stderr, "Failed to find stream information.\n");
        exit(1);
    }

    for (i = 0; i < formatCtx->nb_streams; i++) {
        if (formatCtx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
            videoStreamIndex = i;
            break;
        }
    }

    if (videoStreamIndex == -1) {
        fprintf(stderr, "Failed to find video stream.\n");
        exit(1);
    }

    codec = avcodec_find_decoder(formatCtx->streams[videoStreamIndex]->codecpar->codec_id);
    if (codec == NULL) {
        fprintf(stderr, "Failed to find decoder.\n");
        exit(1);
    }

    codecCtx = avcodec_alloc_context3(codec);
    if (codecCtx == NULL) {
        fprintf(stderr, "Failed to allocate codec context.\n");
        exit(1);
    }

    if (avcodec_parameters_to_context(codecCtx, formatCtx->streams[videoStreamIndex]->codecpar) < 0) {
        fprintf(stderr, "Failed to copy codec parameters to context.\n");
        exit(1);
    }

    if (avcodec_open2(codecCtx, codec, NULL) < 0) {
        fprintf(stderr, "Failed to open codec.\n");
        exit(1);
    }

    frame = av_frame_alloc();
    if (frame == NULL) {
        fprintf(stderr, "Failed to allocate frame.\n");
        exit(1);
    }

    while (av_read_frame(formatCtx, &packet) >= 0) {
        if (packet.stream_index == videoStreamIndex) {
            dataSize = packet.size;
            data = packet.data;
            while (dataSize > 0) {
                ret = av_parser_parse2(codecCtx->parser, codecCtx, &data, &dataSize, inbuf, INBUF_SIZE, AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);
                if (ret < 0) {
                    fprintf(stderr, "Error while parsing.\n");
                    exit(1);
                }
                dataUsed = ret;
                ret = avcodec_send_packet(codecCtx, &packet);
                if (ret < 0) {
                    fprintf(stderr, "Error sending a packet for decoding.\n");
                    exit(1);
                }
                while (ret >= 0) {
                    ret = avcodec_receive_frame(codecCtx, frame);
                    if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
                        break;
                    } else if (ret < 0) {
                        fprintf(stderr, "Error during decoding.\n");
                        exit(1);
                    }
                    frame->pts = av_frame_get_best_effort_timestamp(frame);
                    avcodec_send_frame(codecCtx, frame);
                    ret = avcodec_receive_packet(codecCtx, &packet);
                    if (ret < 0) {
                        fprintf(stderr, "Error during encoding.\n");
                        exit(1);
                    }
                    av_interleaved_write_frame(formatCtx, &packet);
                    av_packet_unref(&packet);
                }
                data += dataUsed;
                dataSize -= dataUsed;
            }
        }
        av_packet_unref(&packet);
    }

    avformat_close_input(&formatCtx);
    avcodec_free_context(&codecCtx);
    av_frame_free(&frame);

    return 0;
}

这个demo的主要逻辑是读取视频帧，对每一帧进行处理并写入文件。在处理过程中，我们使用了FFmpeg提供的avcodec和avformat库来进行解码和编码。具体的实现细节可以参考代码注释。