函数：int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);
头文件：Avcodec.h
官方解释：
/** * Initialize the AVCodecContext to use the given AVCodec. Prior to using this * function the context has to be allocated with avcodec_alloc_context3(). * * The functions avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(), * avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for * retrieving a codec. * * @warning This function is not thread safe! * * @note Always call this function before using decoding routines (such as * @ref avcodec_receive_frame()). * * @code * avcodec_register_all(); * av_dict_set(&opts, "b", "2.5M", 0); * codec = avcodec_find_decoder(AV_CODEC_ID_H264); * if (!codec) * exit(1); * * context = avcodec_alloc_context3(codec); * * if (avcodec_open2(context, codec, opts) < 0) * exit(1); * @endcode * * @param avctx The context to initialize. * @param codec The codec to open this context for. If a non-NULL codec has been * previously passed to avcodec_alloc_context3() or * for this context, then this parameter MUST be either NULL or * equal to the previously passed codec. * @param options A dictionary filled with AVCodecContext and codec-private options. * On return this object will be filled with options that were not found. * * @return zero on success, a negative value on error * @see avcodec_alloc_context3(), avcodec_find_decoder(), avcodec_find_encoder(), * av_dict_set(), av_opt_find(). */
问题：AVCodecContext和AVCodec两个结构体都赋值的前提下，该函数在编码时莫名其妙，返回负数。究其原因主要是由于AVCodecContext中赋值的不匹配，主要设置参数：codec_type、sample_fmt、channel_layout、sample_rate、channels。本人总结了PCM->OPUS和PCM转MP3参数的设置，如下表所示：
1、PCM转OPUS的参数设置：
AVMEDIA_TYPE_AUDIO
codec_type
AV_SAMPLE_FMT_S16
sample_fmt
AV_CH_LAYOUT_STEREO
channel_layout
48000
sample_rate
2
channels
2、PCM转MP3
AVMEDIA_TYPE_AUDIO
codec_type
AV_SAMPLE_FMT_S16P
sample_fmt
AV_CH_LAYOUT_STEREO
channel_layout
44100
sample_rate
2
channels
