【FFmpeg 系列】编解码（六）_decoder (codec none) not found for input stream #0

1、FFmpeg 对 H264 进行编码

• API：

<!-- 编码步骤 -->
// 查找编码器
avcodec_find_encoder_by_name()
// 设置编码参数，并打开编码器
avcodec_open2()
// 编码
avcodec_encode_video2()
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• 示例：

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

#include <libavcodec/avcodec.h>
#include <libavutil/opt.h>
#include <libavutil/imgutils.h>

int main(int argc, char **argv) {
    const char *filename, *codec_name;
    const AVCodec *codec;
    AVCodecContext *c = NULL;
    int i, ret, x, y, got_output;
    FILE *f;
    AVFrame *frame;
    AVPacket pkt;
    uint8_t endcode[] = {0, 0, 1, 0xb7};

    if (argc <= 2) {
        fprintf(stderr, "Usage: %s <output file> <codec name>\n", argv[0]);
        exit(0);
    }
    filename = argv[1];
    codec_name = argv[2];

    avcodec_register_all();

    /* find the mpeg1video encoder */
    codec = avcodec_find_encoder_by_name(codec_name);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }

    c = avcodec_alloc_context3(codec);
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }

    /* put sample parameters */
    c->bit_rate = 400000;
    /* resolution must be a multiple of two */
    c->width = 352;
    c->height = 288;
    /* frames per second */
    c->time_base = (AVRational) {1, 25};
    c->framerate = (AVRational) {25, 1};

    /* emit one intra frame every ten frames
     * check frame pict_type before passing frame
     * to encoder, if frame->pict_type is AV_PICTURE_TYPE_I
     * then gop_size is ignored and the output of encoder
     * will always be I frame irrespective to gop_size
     */
    c->gop_size = 10;
    c->max_b_frames = 1;
    c->pix_fmt = AV_PIX_FMT_YUV420P;

    if (codec->id == AV_CODEC_ID_H264)
        av_opt_set(c->priv_data, "preset", "slow", 0);

    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    f = fopen(filename, "wb");
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }

    frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }
    frame->format = c->pix_fmt;
    frame->width = c->width;
    frame->height = c->height;

    ret = av_frame_get_buffer(frame, 32);
    if (ret < 0) {
        fprintf(stderr, "Could not allocate the video frame data\n");
        exit(1);
    }

    /* encode 1 second of video */
    for (i = 0; i < 25; i++) {
        av_init_packet(&pkt);
        pkt.data = NULL;    // packet data will be allocated by the encoder
        pkt.size = 0;

        fflush(stdout);

        /* make sure the frame data is writable */
        ret = av_frame_make_writable(frame);
        if (ret < 0)
            exit(1);

        /* prepare a dummy image */
        /* Y */
        for (y = 0; y < c->height; y++) {
            for (x = 0; x < c->width; x++) {
                frame->data[0][y * frame->linesize[0] + x] = x + y + i * 3;
            }
        }

        /* Cb and Cr */
        for (y = 0; y < c->height / 2; y++) {
            for (x = 0; x < c->width / 2; x++) {
                frame->data[1][y * frame->linesize[1] + x] = 128 + y + i * 2;
                frame->data[2][y * frame->linesize[2] + x] = 64 + x + i * 5;
            }
        }

        frame->pts = i;

        /* encode the image */
        ret = avcodec_encode_video2(c, &pkt, frame, &got_output);
        if (ret < 0) {
            fprintf(stderr, "Error encoding frame\n");
            exit(1);
        }

        if (got_output) {
            printf("Write frame %3d (size=%5d)\n", i, pkt.size);
            fwrite(pkt.data, 1, pkt.size, f);
            av_packet_unref(&pkt);
        }
    }

    /* get the delayed frames */
    for (got_output = 1; got_output; i++) {
        fflush(stdout);

        ret = avcodec_encode_video2(c, &pkt, NULL, &got_output);
        if (ret < 0) {
            fprintf(stderr, "Error encoding frame\n");
            exit(1);
        }

        if (got_output) {
            printf("Write frame %3d (size=%5d)\n", i, pkt.size);
            fwrite(pkt.data, 1, pkt.size, f);
            av_packet_unref(&pkt);
        }
    }

    /* add sequence end code to have a real MPEG file */
    fwrite(endcode, 1, sizeof(endcode), f);
    fclose(f);

    avcodec_free_context(&c);
    av_frame_free(&frame);

    return 0;
}
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• 编译与运行：

clang -g -o 9_ff_encode_video 9_ff_encode_video.c `pkg-config --cflags --libs libavutil libavcodec`

// 执行编译后的结果：
./9_ff_encode_video ./files/encode.h264 libx264
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2、FFmpeg 对 H264 进行解码

• API：

<!-- 文文件 -->
// 添加头文件
libavcodec/avcodec.h

<!-- 常见的数据结构 -->
// 编码器结构体
AVCodec
// 编码器上下文
AVCodecContext
// 解码后的帧
AVFrame

<!-- 结构体内存的分配与释放 -->
// 分配空间
av_frame_alloc()
// 释放空间
av_frame_free()
// 编码器分配的上下文
avcodec_alloc_context3()
// 编码器分配的上下文释放
avcodec_free_context()

<!-- 解码步骤 -->
// 查找解码器
avcodec_find_decoder()
// 打开解码器
avcodec_open2()
// 解码
avcodec_decode_video2()
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• 示例：

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

#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libswscale/swscale.h>

#define INBUF_SIZE 4096

#define WORD uint16_t
#define DWORD uint32_t
#define LONG int32_t

#pragma pack(2)
typedef struct tagBITMAPFILEHEADER {
    WORD bfType;
    DWORD bfSize;
    WORD bfReserved1;
    WORD bfReserved2;
    DWORD bfOffBits;
} BITMAPFILEHEADER, *PBITMAPFILEHEADER;

typedef struct tagBITMAPINFOHEADER {
    DWORD biSize;
    LONG biWidth;
    LONG biHeight;
    WORD biPlanes;
    WORD biBitCount;
    DWORD biCompression;
    DWORD biSizeImage;
    LONG biXPelsPerMeter;
    LONG biYPelsPerMeter;
    DWORD biClrUsed;
    DWORD biClrImportant;
} BITMAPINFOHEADER, *PBITMAPINFOHEADER;

void saveBMP(struct SwsContext *img_convert_ctx, AVFrame *frame, char *filename) {
    //1 先进行转换,  YUV420=>RGB24:
    int w = frame->width;
    int h = frame->height;


    int numBytes = avpicture_get_size(AV_PIX_FMT_BGR24, w, h);
    uint8_t *buffer = (uint8_t *) av_malloc(numBytes * sizeof(uint8_t));


    AVFrame *pFrameRGB = av_frame_alloc();
    /* buffer is going to be written to rawvideo file, no alignment */
    /*
    if (av_image_alloc(pFrameRGB->data, pFrameRGB->linesize,
                              w, h, AV_PIX_FMT_BGR24, pix_fmt, 1) < 0) {
        fprintf(stderr, "Could not allocate destination image\n");
        exit(1);
    }
    */
    avpicture_fill((AVPicture *) pFrameRGB, buffer, AV_PIX_FMT_BGR24, w, h);

    sws_scale(img_convert_ctx, frame->data, frame->linesize,
              0, h, pFrameRGB->data, pFrameRGB->linesize);

    //2 构造 BITMAPINFOHEADER
    BITMAPINFOHEADER header;
    header.biSize = sizeof(BITMAPINFOHEADER);


    header.biWidth = w;
    header.biHeight = h * (-1);
    header.biBitCount = 24;
    header.biCompression = 0;
    header.biSizeImage = 0;
    header.biClrImportant = 0;
    header.biClrUsed = 0;
    header.biXPelsPerMeter = 0;
    header.biYPelsPerMeter = 0;
    header.biPlanes = 1;

    //3 构造文件头
    BITMAPFILEHEADER bmpFileHeader = {0,};
    //HANDLE hFile = NULL;
    DWORD dwTotalWriten = 0;
    DWORD dwWriten;

    bmpFileHeader.bfType = 0x4d42; //'BM';
    bmpFileHeader.bfSize = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) + numBytes;
    bmpFileHeader.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

    FILE *pf = fopen(filename, "wb");
    fwrite(&bmpFileHeader, sizeof(BITMAPFILEHEADER), 1, pf);
    fwrite(&header, sizeof(BITMAPINFOHEADER), 1, pf);
    fwrite(pFrameRGB->data[0], 1, numBytes, pf);
    fclose(pf);


    //释放资源
    //av_free(buffer);
    av_freep(&pFrameRGB[0]);
    av_free(pFrameRGB);
}

static void pgm_save(unsigned char *buf, int wrap, int xsize, int ysize,
                     char *filename) {
    FILE *f;
    int i;

    f = fopen(filename, "w");
    fprintf(f, "P5\n%d %d\n%d\n", xsize, ysize, 255);
    for (i = 0; i < ysize; i++)
        fwrite(buf + i * wrap, 1, xsize, f);
    fclose(f);
}

static int decode_write_frame(const char *outfilename, AVCodecContext *avctx,
                              struct SwsContext *img_convert_ctx, AVFrame *frame, int *frame_count, AVPacket *pkt,
                              int last) {
    int len, got_frame;
    char buf[1024];

    len = avcodec_decode_video2(avctx, frame, &got_frame, pkt);
    if (len < 0) {
        fprintf(stderr, "Error while decoding frame %d\n", *frame_count);
        return len;
    }
    if (got_frame) {
        printf("Saving %sframe %3d\n", last ? "last " : "", *frame_count);
        fflush(stdout);

        /* the picture is allocated by the decoder, no need to free it */
        snprintf(buf, sizeof(buf), "%s-%d.bmp", outfilename, *frame_count);

        /*
        pgm_save(frame->data[0], frame->linesize[0],
                 frame->width, frame->height, buf);
        */

        saveBMP(img_convert_ctx, frame, buf);

        (*frame_count)++;
    }
    return 0;
}

int main(int argc, char **argv) {
    int ret;

    FILE *f;

    const char *filename, *outfilename;

    AVFormatContext *fmt_ctx = NULL;

    const AVCodec *codec;
    AVCodecContext *c = NULL;

    AVStream *st = NULL;
    int stream_index;

    int frame_count;
    AVFrame *frame;

    struct SwsContext *img_convert_ctx;

    //uint8_t inbuf[INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
    AVPacket avpkt;

    if (argc <= 2) {
        fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);
        exit(0);
    }
    filename = argv[1];
    outfilename = argv[2];

    /* register all formats and codecs */
    av_register_all();

    /* open input file, and allocate format context */
    if (avformat_open_input(&fmt_ctx, filename, NULL, NULL) < 0) {
        fprintf(stderr, "Could not open source file %s\n", 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);
    }

    /* dump input information to stderr */
    av_dump_format(fmt_ctx, 0, filename, 0);

    av_init_packet(&avpkt);

    /* set end of buffer to 0 (this ensures that no overreading happens for damaged MPEG streams) */
    //memset(inbuf + INBUF_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);
    //

    ret = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if (ret < 0) {
        fprintf(stderr, "Could not find %s stream in input file '%s'\n",
                av_get_media_type_string(AVMEDIA_TYPE_VIDEO), filename);
        return ret;
    }

    stream_index = ret;
    st = fmt_ctx->streams[stream_index];

    /* find decoder for the stream */
    codec = avcodec_find_decoder(st->codecpar->codec_id);
    if (!codec) {
        fprintf(stderr, "Failed to find %s codec\n",
                av_get_media_type_string(AVMEDIA_TYPE_VIDEO));
        return AVERROR(EINVAL);
    }


    /* find the MPEG-1 video decoder */
    /*
    codec = avcodec_find_decoder(AV_CODEC_ID_MPEG1VIDEO);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }
    */

    c = avcodec_alloc_context3(NULL);
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }

    /* Copy codec parameters from input stream to output codec context */
    if ((ret = avcodec_parameters_to_context(c, st->codecpar)) < 0) {
        fprintf(stderr, "Failed to copy %s codec parameters to decoder context\n",
                av_get_media_type_string(AVMEDIA_TYPE_VIDEO));
        return ret;
    }


    /*
    if (codec->capabilities & AV_CODEC_CAP_TRUNCATED)
        c->flags |= AV_CODEC_FLAG_TRUNCATED; // we do not send complete frames
    */

    /* For some codecs, such as msmpeg4 and mpeg4, width and height
       MUST be initialized there because this information is not
       available in the bitstream. */

    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    /*
    f = fopen(filename, "rb");
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }
    */

    img_convert_ctx = sws_getContext(c->width, c->height,
                                     c->pix_fmt,
                                     c->width, c->height,
                                     AV_PIX_FMT_BGR24,
                                     SWS_BICUBIC, NULL, NULL, NULL);

    if (img_convert_ctx == NULL) {
        fprintf(stderr, "Cannot initialize the conversion context\n");
        exit(1);
    }

    frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }

    frame_count = 0;
    while (av_read_frame(fmt_ctx, &avpkt) >= 0) {
        /*
        avpkt.size = fread(inbuf, 1, INBUF_SIZE, f);
        if (avpkt.size == 0)
            break;
        */

        /* NOTE1: some codecs are stream based (mpegvideo, mpegaudio)
           and this is the only method to use them because you cannot
           know the compressed data size before analysing it.

           BUT some other codecs (msmpeg4, mpeg4) are inherently frame
           based, so you must call them with all the data for one
           frame exactly. You must also initialize 'width' and
           'height' before initializing them. */

        /* NOTE2: some codecs allow the raw parameters (frame size,
           sample rate) to be changed at any frame. We handle this, so
           you should also take care of it */

        /* here, we use a stream based decoder (mpeg1video), so we
           feed decoder and see if it could decode a frame */
        //avpkt.data = inbuf;
        //while (avpkt.size > 0)
        if (avpkt.stream_index == stream_index) {
            if (decode_write_frame(outfilename, c, img_convert_ctx, frame, &frame_count, &avpkt, 0) < 0)
                exit(1);
        }

        av_packet_unref(&avpkt);
    }

    /* Some codecs, such as MPEG, transmit the I- and P-frame with a
       latency of one frame. You must do the following to have a
       chance to get the last frame of the video. */
    avpkt.data = NULL;
    avpkt.size = 0;
    decode_write_frame(outfilename, c, img_convert_ctx, frame, &frame_count, &avpkt, 1);

    fclose(f);

    avformat_close_input(&fmt_ctx);

    sws_freeContext(img_convert_ctx);
    avcodec_free_context(&c);
    av_frame_free(&frame);

    return 0;
}
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• 编译与运行：

clang -g -o 10_ff_decode_video 10_ff_decode_video.c `pkg-config --cflags --libs libavformat libavcodec libswscale`

// 执行编译后的结果：(注意 images 需存在)
./10_ff_decode_video ./files/sample.h264 ./files/images/image
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3、FFmpeg 对 AAC 进行编码

• API：

<!-- 编码步骤（与视频相同） -->
// 查找编码器
avcodec_find_encoder_by_name()
// 设置编码参数，并打开编码器
avcodec_open2()
// 编码
avcodec_encodec_audio2()
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• 示例：

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

#include <libavcodec/avcodec.h>

#include <libavutil/channel_layout.h>
#include <libavutil/common.h>
#include <libavutil/frame.h>
#include <libavutil/samplefmt.h>

/* check that a given sample format is supported by the encoder */
static int check_sample_fmt(const AVCodec *codec, enum AVSampleFormat sample_fmt) {
    const enum AVSampleFormat *p = codec->sample_fmts;

    while (*p != AV_SAMPLE_FMT_NONE) {
        if (*p == sample_fmt)
            return 1;
        p++;
    }
    return 0;
}

/* just pick the highest supported samplerate */
static int select_sample_rate(const AVCodec *codec) {
    const int *p;
    int best_samplerate = 0;

    if (!codec->supported_samplerates)
        return 44100;

    p = codec->supported_samplerates;
    while (*p) {
        if (!best_samplerate || abs(44100 - *p) < abs(44100 - best_samplerate))
            best_samplerate = *p;
        p++;
    }
    return best_samplerate;
}

/* select layout with the highest channel count */
static int select_channel_layout(const AVCodec *codec) {
    const uint64_t *p;
    uint64_t best_ch_layout = 0;
    int best_nb_channels = 0;

    if (!codec->channel_layouts)
        return AV_CH_LAYOUT_STEREO;

    p = codec->channel_layouts;
    while (*p) {
        int nb_channels = av_get_channel_layout_nb_channels(*p);

        if (nb_channels > best_nb_channels) {
            best_ch_layout = *p;
            best_nb_channels = nb_channels;
        }
        p++;
    }
    return best_ch_layout;
}

int main(int argc, char **argv) {
    const char *filename;
    const AVCodec *codec;
    AVCodecContext *c = NULL;
    AVFrame *frame;
    AVPacket pkt;
    int i, j, k, ret, got_output;
    FILE *f;
    uint16_t *samples;
    float t, tincr;

    if (argc <= 1) {
        fprintf(stderr, "Usage: %s <output file>\n", argv[0]);
        return 0;
    }
    filename = argv[1];

    /* register all the codecs */
    avcodec_register_all();

    /* find the MP2 encoder */
    codec = avcodec_find_encoder(AV_CODEC_ID_MP2);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }

    c = avcodec_alloc_context3(codec);
    if (!c) {
        fprintf(stderr, "Could not allocate audio codec context\n");
        exit(1);
    }

    /* put sample parameters */
    c->bit_rate = 64000;

    /* check that the encoder supports s16 pcm input */
    c->sample_fmt = AV_SAMPLE_FMT_S16;
    if (!check_sample_fmt(codec, c->sample_fmt)) {
        fprintf(stderr, "Encoder does not support sample format %s",
                av_get_sample_fmt_name(c->sample_fmt));
        exit(1);
    }

    /* select other audio parameters supported by the encoder */
    c->sample_rate = select_sample_rate(codec);
    c->channel_layout = select_channel_layout(codec);
    c->channels = av_get_channel_layout_nb_channels(c->channel_layout);

    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    f = fopen(filename, "wb");
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }

    /* frame containing input raw audio */
    frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Could not allocate audio frame\n");
        exit(1);
    }

    frame->nb_samples = c->frame_size;
    frame->format = c->sample_fmt;
    frame->channel_layout = c->channel_layout;

    /* allocate the data buffers */
    ret = av_frame_get_buffer(frame, 0);
    if (ret < 0) {
        fprintf(stderr, "Could not allocate audio data buffers\n");
        exit(1);
    }

    /* encode a single tone sound */
    t = 0;
    tincr = 2 * M_PI * 440.0 / c->sample_rate;
    for (i = 0; i < 200; i++) {
        av_init_packet(&pkt);
        pkt.data = NULL; // packet data will be allocated by the encoder
        pkt.size = 0;

        /* make sure the frame is writable -- makes a copy if the encoder
         * kept a reference internally */
        ret = av_frame_make_writable(frame);
        if (ret < 0)
            exit(1);
        samples = (uint16_t *) frame->data[0];

        for (j = 0; j < c->frame_size; j++) {
            samples[2 * j] = (int) (sin(t) * 10000);

            for (k = 1; k < c->channels; k++)
                samples[2 * j + k] = samples[2 * j];
            t += tincr;
        }
        /* encode the samples */
        ret = avcodec_encode_audio2(c, &pkt, frame, &got_output);
        if (ret < 0) {
            fprintf(stderr, "Error encoding audio frame\n");
            exit(1);
        }
        if (got_output) {
            fwrite(pkt.data, 1, pkt.size, f);
            av_packet_unref(&pkt);
        }
    }

    /* get the delayed frames */
    for (got_output = 1; got_output; i++) {
        ret = avcodec_encode_audio2(c, &pkt, NULL, &got_output);
        if (ret < 0) {
            fprintf(stderr, "Error encoding frame\n");
            exit(1);
        }

        if (got_output) {
            fwrite(pkt.data, 1, pkt.size, f);
            av_packet_unref(&pkt);
        }
    }
    fclose(f);

    av_frame_free(&frame);
    avcodec_free_context(&c);

    return 0;
}
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• 编译与运行：

clang -g -o 11_ff_encode_audio 11_ff_encode_audio.c `pkg-config --cflags --libs libavcodec libavutil`

// 执行编译后的结果：
./11_ff_encode_audio ./files/encode.aac
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4、FFmpeg 对 AAC 进行解码

• API：

<!-- 编码步骤（与视频相同） -->
// 查找编码器
avcodec_find_encoder_by_name()
// 设置编码参数，并打开编码器
avcodec_open2()
// 编码
avcodec_encodec_audio2()
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• 示例：

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

#include <libavutil/frame.h>
#include <libavutil/mem.h>

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswresample/swresample.h>

#define AUDIO_INBUF_SIZE     20480
#define AUDIO_REFILL_THRESH  4096
#define MAX_AUDIO_FRAME_SIZE 192000

int main(int argc, char **argv) {
    int i, ret;

    //error message
    int err_code;
    char errors[1024];

    int audiostream_index = -1;

    //codec parameters
    int len = 0;
    const AVCodec *codec = NULL;
    AVCodecContext *c = NULL;
    AVFormatContext *pFormatCtx = NULL;

    //file parameters
    FILE *f, *outfile;
    const char *outfilename, *filename;

    uint8_t inbuf[AUDIO_INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];

    AVPacket avpkt;
    AVFrame *decoded_frame = NULL;

    if (argc <= 2) {
        fprintf(stderr, "Usage: %s <input file> <output file>\n", argv[0]);
        exit(0);
    }

    filename = argv[1];
    outfilename = argv[2];

    av_init_packet(&avpkt);
    decoded_frame = av_frame_alloc();
    if (!decodced_frame) {
        fprintf(stderr, "Could not allocate audio frame\n");
        exit(1);
    }

    /* open input file, and allocate format context */
    if ((err_code = avformat_open_input(&pFormatCtx, filename, NULL, NULL)) < 0) {
        av_strerror(err_code, errors, 1024);
        fprintf(stderr, "Could not open source file %s, %d(%s)\n", filename, err_code, errors);
        return -1;
    }

    //Retrieve stream information
    if (avformat_find_stream_info(pFormatCtx, NULL) < 0)
        return -1; // Couldn't find stream information

    //Dump information about file onto standard error
    av_dump_format(pFormatCtx, 0, filename, 0);

    //get index of audio stream
    for (i = 0; i < pFormatCtx->nb_streams; i++) {
        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
            audiostream_index = i;
        }
    }

    c = avcodec_alloc_context3(NULL);
    if (!c) {
        fprintf(stderr, "Could not allocate audio codec context\n");
        exit(1);
    }

    ret = avcodec_parameters_to_context(c, pFormatCtx->streams[audiostream_index]->codecpar);
    if (ret < 0) {
        return -1;
    }

    codec = avcodec_find_decoder(c->codec_id);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }

    //Out Audio Param
    uint64_t out_channel_layout = AV_CH_LAYOUT_STEREO;

    //AAC:1024  MP3:1152
    int out_nb_samples = c->frame_size;
    int out_sample_rate = 44100;
    int out_channels = av_get_channel_layout_nb_channels(out_channel_layout);

    //Out Buffer Size
    int out_buffer_size = av_samples_get_buffer_size(NULL,
                                                     out_channels,
                                                     out_nb_samples,
                                                     AV_SAMPLE_FMT_S16,
                                                     1);

    uint8_t *out_buffer = (uint8_t *) av_malloc(MAX_AUDIO_FRAME_SIZE * 2);
    int64_t in_channel_layout = av_get_default_channel_layout(c->channels);

    struct SwrContext *audio_convert_ctx;
    audio_convert_ctx = swr_alloc();
    audio_convert_ctx = swr_alloc_set_opts(audio_convert_ctx,
                                           out_channel_layout,
                                           AV_SAMPLE_FMT_S16,
                                           out_sample_rate,
                                           in_channel_layout,
                                           c->sample_fmt,
                                           c->sample_rate,
                                           0,
                                           NULL);
    swr_init(audio_convert_ctx);

    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    outfile = fopen(outfilename, "wb");
    if (!outfile) {
        av_free(c);
        exit(1);
    }

    while (1) {

        int ret;
        int i, ch;
        int got_frame = 0;

        ret = av_read_frame(pFormatCtx, &avpkt);
        if (ret == AVERROR(EAGAIN)) {
            av_usleep(100);
            continue;
        } else if (ret < 0) {
            break;
        }

        //如果不是音频则跳过
        if (avpkt.stream_index != audiostream_index) {
            av_packet_unref(&avpkt);
            continue;
        }

        len = avcodec_decode_audio2(c, decoded_frame, &got_frame, &avpkt);
        if (len < 0) {
            av_strerror(len, errors, 1024);
            fprintf(stderr, "Error while decoding, err_code:%d, err:%s\n", len, errors);
            exit(1);
        }
        if (got_frame) {
            /* if a frame has been decoded, output it */
            int data_size = av_get_bytes_per_sample(c->sample_fmt);
            if (data_size < 0) {
                /* This should not occur, checking just for paranoia */
                fprintf(stderr, "Failed to calculate data size\n");
                exit(1);
            }
            swr_convert(audio_convert_ctx,
                        &out_buffer,
                        MAX_AUDIO_FRAME_SIZE,
                        (const uint8_t **) decoded_frame->data,
                        decoded_frame->nb_samples);

            fwrite(out_buffer, 1, out_buffer_size, outfile);

        }
        avpkt.size -= len;
        avpkt.data += len;
        avpkt.dts = avpkt.pts = AV_NOPTS_VALUE;
    }

    fclose(outfile);

    avcodec_free_context(&c);
    av_frame_free(&decoded_frame);

    return 0;
}
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• 编译与运行：

clang -g -o 12_ff_decode_audio 12_ff_decode_audio.c `pkg-config --cflags --libs libavutil libavcodec libavformat libswresample`

// 执行编译后的结果：
./12_ff_decode_audio ./files/sample.mp4 ./files/decode.aac
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5、为什么编码后的视频少了几帧数据？

思路：
在没有新的要编码的数据给编码器之后，在编码器的缓冲区中还有一些数据没有编码，需要将在编码器缓冲区中的数据强制编码器输出出来。
点拨：
应在编码之后再次调用avcodec_encode_video2方法，并将输入数据设置为NULL。如下所示：
ret = avcodec_encode_video2(c, &pkt, NULL, &got_output);
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6、视频抽取图片

描述：
视频抽取图片时，指定了每秒抽取一张图片，那么这张图片是随机的一帧，还是优规律的呢？
思路：
要理解这个问题你要清楚视频帧排列的时间戳是多少？
点拨：
抽取图片时，会根据时间戳的变化抽取出第一张时间戳发生变化的图片。如果第2秒的第一帧的时间戳是1004毫秒，那么你取出的第二张图片的时间是1040毫秒。
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