[Libav-user] How can I get the pix_fmt info from AVFrame objects?

[B Aleck]

AVFrame* frame = av_frame_alloc();
avcodec_receive_frame(codec_ctx, frame);

Assuming that the operation succeeds, I found frame->format is always zero
What's the format field used for? Only for outputs?
And how can I get pix_fmt of the frame?