FFmpegsample 分析：demux_decode_audio.c


#include <libavutil/samplefmt.h>
#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>
#include <libavutil/log.h>static AVFormatContext *fmt_ctx = NULL;
static AVCodecContext *audio_dec_ctx = NULL;
static AVStream *audio_stream = NULL;
static const char *src_filename = NULL;
static const char *audio_dst_filename = NULL;
static FILE *audio_dst_file = NULL;static int audio_stream_idx = -1;
static AVFrame *frame = NULL;
static AVPacket pkt;
static int audio_frame_count = 0;/* Enable or disable frame reference counting. You are not supposed to support* both paths in your application but pick the one most appropriate to your* needs. Look for the use of refcount in this example to see what are the* differences of API usage between them. */static int open_codec_context(int *stream_idx,AVCodecContext **dec_ctx, AVFormatContext *fmt_ctx, enum AVMediaType type)
{int ret, stream_index;AVStream *st;AVCodec *dec = NULL;AVDictionary *opts = NULL;ret = av_find_best_stream(fmt_ctx, type, -1, -1, NULL, 0);if (ret < 0) {fprintf(stderr, "Could not find %s stream in input file '%s'\n",av_get_media_type_string(type), src_filename);return ret;} else {stream_index = ret;st = fmt_ctx->streams[stream_index];/* find decoder for the stream */dec = avcodec_find_decoder(st->codecpar->codec_id);if (!dec) {fprintf(stderr, "Failed to find %s codec\n",av_get_media_type_string(type));return AVERROR(EINVAL);}/* Allocate a codec context for the decoder */*dec_ctx = avcodec_alloc_context3(dec);if (!*dec_ctx) {fprintf(stderr, "Failed to allocate the %s codec context\n",av_get_media_type_string(type));return AVERROR(ENOMEM);}/* Copy codec parameters from input stream to output codec context */if ((ret = avcodec_parameters_to_context(*dec_ctx, st->codecpar)) < 0) {fprintf(stderr, "Failed to copy %s codec parameters to decoder context\n",av_get_media_type_string(type));return ret;}/* Init the decoders, with or without reference counting */if ((ret = avcodec_open2(*dec_ctx, dec, &opts)) < 0) {fprintf(stderr, "Failed to open %s codec\n",av_get_media_type_string(type));return ret;}*stream_idx = stream_index;}return 0;
}static int get_format_from_sample_fmt(const char **fmt,enum AVSampleFormat sample_fmt)
{int i;struct sample_fmt_entry {enum AVSampleFormat sample_fmt; const char *fmt_be, *fmt_le;} sample_fmt_entries[] = {{ AV_SAMPLE_FMT_U8,  "u8",    "u8"    },{ AV_SAMPLE_FMT_S16, "s16be", "s16le" },{ AV_SAMPLE_FMT_S32, "s32be", "s32le" },{ AV_SAMPLE_FMT_FLT, "f32be", "f32le" },{ AV_SAMPLE_FMT_DBL, "f64be", "f64le" },};*fmt = NULL;for (i = 0; i < FF_ARRAY_ELEMS(sample_fmt_entries); i++) {struct sample_fmt_entry *entry = &sample_fmt_entries[i];if (sample_fmt == entry->sample_fmt) {*fmt = AV_NE(entry->fmt_be, entry->fmt_le);return 0;}}fprintf(stderr,"sample format %s is not supported as output format\n",av_get_sample_fmt_name(sample_fmt));return -1;
}static void decode(AVCodecContext *dec_ctx, AVPacket *pkt, AVFrame *frame,FILE *outfile)
{int i, ch;int ret, data_size;/* send the packet with the compressed data to the decoder */ret = avcodec_send_packet(dec_ctx, pkt);if (ret < 0) {fprintf(stderr, "Error submitting the packet to the decoder\n");exit(1);}/* read all the output frames (in general there may be any number of them */while (ret >= 0) {ret = avcodec_receive_frame(dec_ctx, frame);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)return;else if (ret < 0) {fprintf(stderr, "Error during decoding\n");exit(1);}data_size = av_get_bytes_per_sample(dec_ctx->sample_fmt);if (data_size < 0) {/* This should not occur, checking just for paranoia */fprintf(stderr, "Failed to calculate data size\n");exit(1);}for (i = 0; i < frame->nb_samples; i++)for (ch = 0; ch < dec_ctx->channels; ch++)fwrite(frame->data[ch] + data_size*i, 1, data_size, outfile);}
}void print_usage(void)
{fprintf(stderr, "usage: demuxer_decode_audio input_file audio_output_file\n");exit(1);
}int main (int argc, char **argv)
{int ret = 0, got_frame;if (argc != 3) {print_usage();}src_filename = argv[1];audio_dst_filename = argv[2];/* open input file, and allocate format context */if (avformat_open_input(&fmt_ctx, src_filename, NULL, NULL) < 0) {fprintf(stderr, "Could not open source file %s\n", src_filename);exit(1);}/* retrieve stream information */if (avformat_find_stream_info(fmt_ctx, NULL) < 0) {fprintf(stderr, "Could not find stream information\n");exit(1);}if (open_codec_context(&audio_stream_idx, &audio_dec_ctx, fmt_ctx, AVMEDIA_TYPE_AUDIO) >= 0) {audio_stream = fmt_ctx->streams[audio_stream_idx];audio_dst_file = fopen(audio_dst_filename, "wb");if (!audio_dst_file) {fprintf(stderr, "Could not open destination file %s\n", audio_dst_filename);ret = 1;goto end;}}/* dump input information to stderr */av_dump_format(fmt_ctx, 0, src_filename, 0);if (!audio_stream) {fprintf(stderr, "Could not find audio or video stream in the input, aborting\n");ret = 1;goto end;}frame = av_frame_alloc();if (!frame) {fprintf(stderr, "Could not allocate frame\n");ret = AVERROR(ENOMEM);goto end;}/* initialize packet, set data to NULL, let the demuxer fill it */av_init_packet(&pkt);pkt.data = NULL;pkt.size = 0;if (audio_stream)printf("Demuxing audio from file '%s' into '%s'\n", src_filename, audio_dst_filename);/* read frames from the file */while (av_read_frame(fmt_ctx, &pkt) >= 0) {decode(audio_dec_ctx, &pkt, frame, audio_dst_file);}/* flush cached frames */pkt.data = NULL;pkt.size = 0;decode(audio_dec_ctx, &pkt, frame, audio_dst_file);printf("Demuxing decode succeeded.\n");if (audio_stream) {enum AVSampleFormat sfmt = audio_dec_ctx->sample_fmt;int n_channels = audio_dec_ctx->channels;const char *fmt;if (av_sample_fmt_is_planar(sfmt)) {const char *packed = av_get_sample_fmt_name(sfmt);printf("Warning: the sample format the decoder produced is planar ""(%s). This example will output the first channel only.\n",packed ? packed : "?");sfmt = av_get_packed_sample_fmt(sfmt);}if ((ret = get_format_from_sample_fmt(&fmt, sfmt)) < 0)goto end;printf("Play the output audio file with the command:\n""ffplay -f %s -ac %d -ar %d %s\n",fmt, n_channels, audio_dec_ctx->sample_rate,audio_dst_filename);}end:avcodec_free_context(&audio_dec_ctx);avformat_close_input(&fmt_ctx);if (audio_dst_file)fclose(audio_dst_file);av_frame_free(&frame);return ret < 0;
}

编译

gcc -o demuxer_decode_audio demuxer_decode_audio.c  `pkg-config --cflags --libs libavformat libavutil libavcodec`

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FFmpegsample 分析：demux_decode_audio.c

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