avcodec_send_packet函数阻塞

用ffmpeg4.1.4开发一个播放器，解码过程如下，在每个函数前设置标志，测试发现程序阻塞在avcodec_send_packet函数。

while(true){av_read_frameavcodec_send_packetavcodec_receive_frameav_packet_unref
}

解释如下：

avcodec_send_packet 和 avcodec_receive_frame 的工作原理是，avcodec_send_packet 向解码器发送压缩数据包，而 avcodec_receive_frame 从解码器接收解码后的帧。解码器内部有一个缓冲区，用于存储解码过程中间的数据。如果缓冲区已满（即没有足够的空间来存储新的数据包），avcodec_send_packet 就会阻塞，直到有足够的空间。

我用的是处理器是rk3288，后台打印信息如下：

[h264_rkmpp @ 0xa5795e20] Received a frame.
[h264_rkmpp @ 0xa5795e20] Wrote 1615 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Received a frame.
[h264_rkmpp @ 0xa5795e20] Wrote 2107 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Received a frame.
[h264_rkmpp @ 0xa5795e20] Wrote 2064 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Received a frame.
[h264_rkmpp @ 0xa5795e20] Wrote 1504 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Received a frame.
[h264_rkmpp @ 0xa5795e20] Wrote 2588 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Received a frame.
[h264_rkmpp @ 0xa5795e20] Wrote 1642 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Received a frame.
[h264_rkmpp @ 0xa5795e20] Wrote 1721 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Received a frame.
[h264_rkmpp @ 0xa5795e20] Wrote 1437 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 2307 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1427 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1638 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 2303 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1342 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 2193 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1567 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 2247 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1396 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 2375 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1520 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1663 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 2273 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1288 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 2340 bytes to decoder
[h264_rkmpp @ 0xa5795e20] Wrote 1399 bytes to decoder

那avcodec_send_packe函数为什么会“get a frame”呢？源码如下，猜测是阻塞到这里

int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt)
{AVCodecInternal *avci = avctx->internal;int ret;if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))return AVERROR(EINVAL);if (avctx->internal->draining)return AVERROR_EOF;if (avpkt && !avpkt->size && avpkt->data)return AVERROR(EINVAL);av_packet_unref(avci->buffer_pkt);if (avpkt && (avpkt->data || avpkt->side_data_elems)) {ret = av_packet_ref(avci->buffer_pkt, avpkt);if (ret < 0)return ret;}ret = av_bsf_send_packet(avci->filter.bsfs[0], avci->buffer_pkt);if (ret < 0) {av_packet_unref(avci->buffer_pkt);return ret;}if (!avci->buffer_frame->buf[0]) {ret = decode_receive_frame_internal(avctx, avci->buffer_frame);if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)return ret;}return 0;
}static int decode_simple_receive_frame(AVCodecContext *avctx, AVFrame *frame)
{int ret;while (!frame->buf[0]) {ret = decode_simple_internal(avctx, frame);if (ret < 0)return ret;}return 0;
}

解决方法：

1、在调用 avcodec_send_packet 之前，可以先调用 avcodec_receive_frame，即使你不确定是否有帧可以接收。这有助于清理缓冲区。

2、在while循环中等待avcodec_receive_frame

    ret = avcodec_send_packet(codec_ctx, packet); while (ret >= 0) {AVFrame *frame = av_frame_alloc();if (!frame) {// 处理内存分配错误return AVERROR(ENOMEM);}ret = avcodec_receive_frame(codec_ctx, frame);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {// 没有更多帧可以接收，退出循环av_frame_free(&frame);break;} else if (ret < 0) {// 处理接收帧错误av_frame_free(&frame);return ret;}}

3、正常情况下向解码器发送一包数据会收到一个解码后的帧，因此增加一个检测机制，当多次发送数据包但没有收到解码帧后则重置解码器。

void reset_decoder(AVCodecContext* codecContext) {// 刷新解码器缓冲区avcodec_flush_buffers(codecContext);// 关闭解码器avcodec_close(codecContext);// 重新打开解码器if (avcodec_open2(codecContext, codecContext->codec, nullptr) < 0) {std::cerr << "Could not re-open codec" << std::endl;}
}