使用libavfilter API时av_buffersink_get_frame始终返回EAGAIN

尝试通过libavfilter的C API结合libplacebo滤镜对图片进行缩放，但调用av_buffersink_get_frame函数始终返回EAGAIN，且filtered_frame中未填充任何数据。相关代码如下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <libgen.h>

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavfilter/avfilter.h>
#include <libavfilter/buffersrc.h>
#include <libavfilter/buffersink.h>
#include <libavutil/log.h>

int main(int argc, char **argv)
{
    if (argc != 2) {
        fprintf(stderr, "usage: %s <filename>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    const char *src_name = argv[1];
    const char *dst_name = basename(src_name);
    int ret = 0;

    const enum AVPixelFormat src_format = AV_PIX_FMT_YUV422P;

    av_log_set_level(AV_LOG_TRACE);

    AVFormatContext *fmt_ctx = avformat_alloc_context();
    if (fmt_ctx == NULL) {
        av_log(NULL, AV_LOG_TRACE, "avformat_alloc_context(): failed.\n");
        exit(EXIT_SUCCESS);
    }
    if ((ret = avformat_open_input(&fmt_ctx, src_name, NULL, NULL)) != 0) {
        av_log(NULL, AV_LOG_TRACE, "avformat_open_input(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }

    AVCodecContext *dec_ctx;
    AVCodec *dec;

    if ((ret = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, &dec, 0)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "av_find_best_stream(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }
    dec_ctx = avcodec_alloc_context3(dec);
    if (dec_ctx == NULL) {
        av_log(NULL, AV_LOG_TRACE, "avcodec_alloc_context3(): failed.\n");
        exit(EXIT_SUCCESS);
    }
    if ((ret = avcodec_open2(dec_ctx, dec, NULL)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "avcodec_open2(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }

    AVFrame *frame = av_frame_alloc();
    if (frame == NULL) {
        av_log(NULL, AV_LOG_TRACE, "av_frame_alloc(): failed.\n");
        exit(EXIT_SUCCESS);
    }
    AVPacket *packet = av_packet_alloc();
    if (packet == NULL) {
        av_log(NULL, AV_LOG_TRACE, "av_packet_alloc(): failed.\n");
        exit(EXIT_SUCCESS);
    }

    if ((ret = av_read_frame(fmt_ctx, packet)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "av_read_frame(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }
    ret = avcodec_send_packet(dec_ctx, packet);
    if (ret == AVERROR(EAGAIN)) {
        av_log(NULL, AV_LOG_TRACE, "avcodec_send_packet(): %s.\n", av_err2str(ret));
        avcodec_receive_frame(dec_ctx, frame);
        avcodec_send_packet(dec_ctx, packet);
    }
    
    if ((ret = avcodec_receive_frame(dec_ctx, frame)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "avcodec_receive_frame(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }
    av_packet_unref(packet);

    av_log(NULL, AV_LOG_TRACE, "filename: %s, w: %d, h: %d, fmt: %d\n", src_name, frame->width, frame->height, frame->format);

    AVFilterGraph *filter_graph = avfilter_graph_alloc();

    char buffersrc_args[512];
    snprintf(buffersrc_args, sizeof(buffersrc_args), "video_size=%dx%d:pix_fmt=%d:time_base=1/25", frame->width, frame->height, frame->format); 

    AVFilterContext *buffersrc_ctx;
    if ((ret = avfilter_graph_create_filter(&buffersrc_ctx, avfilter_get_by_name("buffer"), NULL, buffersrc_args, NULL, filter_graph)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "avfilter_graph_create_filter(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }

    char libplacebo_args[512];
    snprintf(libplacebo_args, sizeof(libplacebo_args), "format=yuv420p:colorspace=bt470bg:color_primaries=bt709:color_trc=iec61966-2-1:range=pc:w=(iw/2):h=(ih/2):downscaler=none:dithering=none");
    AVFilterContext *libplacebo_ctx;
    if ((ret = avfilter_graph_create_filter(&libplacebo_ctx, avfilter_get_by_name("libplacebo"), NULL, libplacebo_args, NULL, filter_graph)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "avfilter_graph_create_filter(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }

    AVFilterContext *buffersink_ctx;
    if ((ret = avfilter_graph_create_filter(&buffersink_ctx, avfilter_get_by_name("buffersink"), NULL, NULL, NULL, filter_graph)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "avfilter_graph_create_filter(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }

    if ((ret = avfilter_link(buffersrc_ctx, 0, libplacebo_ctx, 0)) != 0) {
        av_log(NULL, AV_LOG_TRACE, "avfilter_link(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }
    if ((ret = avfilter_link(libplacebo_ctx, 0, buffersink_ctx, 0)) != 0) {
        av_log(NULL, AV_LOG_TRACE, "avfilter_link(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }
    
    if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "avfilter_graph_config(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }

    AVFrame *filtered_frame = av_frame_alloc();
    if (filtered_frame == NULL) {
        av_log(NULL, AV_LOG_TRACE, "av_frame_alloc(): failed.\n");
        exit(EXIT_SUCCESS);
    }
    if ((ret = av_buffersrc_add_frame(buffersrc_ctx, frame)) < 0) {
        av_log(NULL, AV_LOG_TRACE, "av_buffersrc_add_frame(): %s.\n", av_err2str(ret));
        exit(EXIT_SUCCESS);
    }
    
    while (1) {
        int ret = av_buffersink_get_frame(buffersink_ctx, filtered_frame);
        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
            break;
        if (ret < 0) {
            av_log(NULL, AV_LOG_TRACE, "av_buffersink_get_frame(): %s.\n", av_err2str(ret));
            exit(EXIT_FAILURE);
        }    
    }

    av_log(NULL, AV_LOG_TRACE, "filename: %s, w: %d, h: %d, f: %d\n", dst_name, filtered_frame->width, filtered_frame->height, filtered_frame->format);

    AVCodecContext *enc_ctx;
    AVCodec *enc;
    AVPacket *enc_packet = av_packet_alloc();

    enc = avcodec_find_encoder_by_name("mjpeg");
    enc_ctx = avcodec_alloc_context3(enc);
    enc_ctx->width = filtered_frame->width;
    enc_ctx->height = filtered_frame->height;
    enc_ctx->bit_rate = dec_ctx->bit_rate * 1024;
    enc_ctx->time_base = (AVRational) {1, 25};
    enc_ctx->framerate = (AVRational) {25, 1};
    enc_ctx->pix_fmt = filtered_frame->format;
    enc_ctx->strict_std_compliance = FF_COMPLIANCE_EXPERIMENTAL;
    enc_ctx->compression_level = 0;
    enc_ctx->color_range = AVCOL_RANGE_JPEG;

    avcodec_open2(enc_ctx, enc, NULL);

    avcodec_send_frame(enc_ctx, filtered_frame);
    avcodec_receive_packet(enc_ctx, enc_packet);

    FILE *dst_file = fopen(dst_name, "wb");
    fwrite(enc_packet->data, 1, enc_packet->size, dst_file);
    fclose(dst_file);

    av_packet_unref(enc_packet);
    av_frame_free(&frame);
    av_frame_free(&filtered_frame);
    
    avfilter_graph_free(&filter_graph);

    avformat_close_input(&fmt_ctx);
    avformat_free_context(fmt_ctx);
    
    exit(EXIT_SUCCESS);
}

问题分析与解决方法 #

1. 未向滤镜链发送EOF信号 #

这是EAGAIN返回值的核心原因：当你把输入帧添加到buffersrc后，滤镜链默认认为后续还有输入，会一直等待而不处理当前帧。解决方法是添加完帧后，调用av_buffersrc_add_frame_flags传入AV_BUFFERSRC_FLAG_EOS，明确通知滤镜链输入结束：

// 添加完输入帧后发送EOF信号
if ((ret = av_buffersrc_add_frame_flags(buffersrc_ctx, NULL, AV_BUFFERSRC_FLAG_EOS)) < 0) {
    av_log(NULL, AV_LOG_ERROR, "av_buffersrc_add_frame_flags(EOS): %s.\n", av_err2str(ret));
    exit(EXIT_FAILURE);
}

2. 修正avcodec_send_packet的错误处理逻辑 #

原代码中avcodec_send_packet返回EAGAIN时的处理逻辑不严谨，正确流程应该是循环尝试接收帧，直到编码器不再返回EAGAIN，再重新发送数据包：

ret = avcodec_send_packet(dec_ctx, packet);
if (ret == AVERROR(EAGAIN)) {
    av_log(NULL, AV_LOG_TRACE, "avcodec_send_packet(): EAGAIN, trying to receive frame first.\n");
    while ((ret = avcodec_receive_frame(dec_ctx, frame)) == AVERROR(EAGAIN)) {
        continue;
    }
    if (ret >= 0) {
        ret = avcodec_send_packet(dec_ctx, packet);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "avcodec_send_packet() after receive: %s.\n", av_err2str(ret));
            exit(EXIT_FAILURE);
        }
    } else {
        av_log(NULL, AV_LOG_ERROR, "avcodec_receive_frame() during EAGAIN handling: %s.\n", av_err2str(ret));
        exit(EXIT_FAILURE);
    }
} else if (ret < 0) {
    av_log(NULL, AV_LOG_ERROR, "avcodec_send_packet(): %s.\n", av_err2str(ret));
    exit(EXIT_FAILURE);
}

3. 为buffersink指定输出像素格式 #

创建buffersink时，明确指定期望的输出格式，避免滤镜输出格式不兼容：

enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_YUV420P, AV_PIX_FMT_NONE };
AVFilterContext *buffersink_ctx;
if ((ret = avfilter_graph_create_filter(&buffersink_ctx, avfilter_get_by_name("buffersink"), NULL, NULL, NULL, filter_graph)) < 0) {
    av_log(NULL, AV_LOG_ERROR, "avfilter_graph_create_filter(buffersink): %s.\n", av_err2str(ret));
    exit(EXIT_FAILURE);
}
av_opt_set_int_list(buffersink_ctx, "pix_fmts", pix_fmts, AV_PIX_FMT_NONE, AV_OPT_SEARCH_CHILDREN);

4. 完善编码阶段的错误处理 #

原代码中avcodec_send_frame和avcodec_receive_packet未处理返回值，可能导致编码失败却无报错：

ret = avcodec_send_frame(enc_ctx, filtered_frame);
if (ret < 0) {
    av_log(NULL, AV_LOG_ERROR, "avcodec_send_frame(): %s.\n", av_err2str(ret));
    exit(EXIT_FAILURE);
}

while ((ret = avcodec_receive_packet(enc_ctx, enc_packet)) == AVERROR(EAGAIN)) {
    continue;
}
if (ret < 0) {
    av_log(NULL, AV_LOG_ERROR, "avcodec_receive_packet(): %s.\n", av_err2str(ret));
    exit(EXIT_FAILURE);
}

内容的提问来源于stack exchange，提问作者aculnaig