用C语言调用ffmpeg的api

以下是一些基本的C语言代码，用于调用FFmpeg API：

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavutil/imgutils.h>
#include <libswscale/swscale.h>

int main(int argc, char *argv[]) {
    AVFormatContext *format_ctx = NULL;
    AVCodecContext *codec_ctx = NULL;
    AVCodec *codec = NULL;
    AVFrame *frame = NULL;
    AVPacket packet;
    struct SwsContext *sws_ctx = NULL;

    // Open video file
    if (avformat_open_input(&format_ctx, "input.mp4", NULL, NULL) != 0) {
        fprintf(stderr, "Error: Could not open video file\n");
        return -1;
    }

    // Retrieve stream information
    if (avformat_find_stream_info(format_ctx, NULL) < 0) {
        fprintf(stderr, "Error: Could not retrieve stream information\n");
        return -1;
    }

    // Find video stream
    int video_stream_index = -1;
    for (int i = 0; i < format_ctx->nb_streams; i++) {
        if (format_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) {
            video_stream_index = i;
            break;
        }
    }

    if (video_stream_index == -1) {
        fprintf(stderr, "Error: Could not find video stream\n");
        return -1;
    }

    // Get video codec
    codec = avcodec_find_decoder(format_ctx->streams[video_stream_index]->codecpar->codec_id);
    if (codec == NULL) {
        fprintf(stderr, "Error: Could not find video codec\n");
        return -1;
    }

    // Allocate codec context
    codec_ctx = avcodec_alloc_context3(codec);
    if (codec_ctx == NULL) {
        fprintf(stderr, "Error: Could not allocate codec context\n");
        return -1;
    }

    // Copy codec parameters to codec context
    if (avcodec_parameters_to_context(codec_ctx, format_ctx->streams[video_stream_index]->codecpar) < 0) {
        fprintf(stderr, "Error: Could not copy codec parameters to codec context\n");
        return -1;
    }

    // Open codec
    if (avcodec_open2(codec_ctx, codec, NULL) < 0) {
        fprintf(stderr, "Error: Could not open codec\n");
        return -1;
    }

    // Allocate frame
    frame = av_frame_alloc();
    if (frame == NULL) {
        fprintf(stderr, "Error: Could not allocate frame\n");
        return -1;
    }

    // Allocate packet
    av_init_packet(&packet);

    // Initialize scaler
    sws_ctx = sws_getContext(codec_ctx->width, codec_ctx->height, codec_ctx->pix_fmt,
                             codec_ctx->width, codec_ctx->height, AV_PIX_FMT_RGB24,
                             SWS_BILINEAR, NULL, NULL, NULL);

    if (sws_ctx == NULL) {
        fprintf(stderr, "Error: Could not initialize scaler\n");
        return -1;
    }

    // Read frames from video file
    while (av_read_frame(format_ctx, &packet) >= 0) {
        if (packet.stream_index == video_stream_index) {
            // Decode video frame
            int ret = avcodec_send_packet(codec_ctx, &packet);
            if (ret < 0) {
                fprintf(stderr, "Error: Could not send packet for decoding\n");
                return -1;
            }

            while (ret >= 0) {
                ret = avcodec_receive_frame(codec_ctx, frame);
                if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
                    break;
                } else if (ret < 0) {
                    fprintf(stderr, "Error: Could not decode video frame\n");
                    return -1;
                }

                // Convert pixel format
                uint8_t *data[1] = {NULL};
                data[0] = (uint8_t *) frame->data[0];
                int linesize[1] = {0};
                linesize[0] = frame->linesize[0];
                sws_scale(sws_ctx, data, linesize, 0, codec_ctx->height,
                          frame->data, frame->linesize);

                // Process video frame
                // ...

            }
        }

        // Free packet
        av_packet_unref(&packet);
    }

    // Free scaler context
    sws_freeContext(sws_ctx);

    // Free frame
    av_frame_free(&frame);

    // Close codec
    avcodec_close(codec_ctx);

    // Free codec context
    avcodec_free_context(&codec_ctx);

    // Close video file
    avformat_close_input(&format_ctx);

    return 0;
}

这个代码段演示了如何打开视频文件，检索流信息，找到视频流，获取视频编解码器，分配编解码器上下文，打开编解码器，分配帧，初始化包，初始化缩放器，从视频文件中读取帧，解码帧，转换像素格式，处理帧，释放资源并关闭视频文件。这是一个基本的例子，可以根据需要进行修改和扩展。