ffmpeg源码分析：avformat_open_input()一、avformat_alloc_context()二、init_input()三、read_header()

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我是靠谱客的博主要减肥豌豆，这篇文章主要介绍ffmpeg源码分析：avformat_open_input()一、avformat_alloc_context()二、init_input()三、read_header()，现在分享给大家，希望可以做个参考。

一、avformat_alloc_context()

二、init_input()

2.1 av_probe_input_format2()

2.2 av_probe_input_buffer2()

2.3 io_open

三、read_header()

本文简单分析FFmpeg中一个常用的函数：avformat_open_input()。该函数用于打开多媒体数据并且获得一些相关的信息。它的声明位于libavformatavformat.h，如下所示。

复制代码

/**
 * Open an input stream and read the header. The codecs are not opened.
 * The stream must be closed with avformat_close_input().
 *
 * @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
 *           May be a pointer to NULL, in which case an AVFormatContext is allocated by this
 *           function and written into ps.
 *           Note that a user-supplied AVFormatContext will be freed on failure.
 * @param url URL of the stream to open.
 * @param fmt If non-NULL, this parameter forces a specific input format.
 *            Otherwise the format is autodetected.
 * @param options  A dictionary filled with AVFormatContext and demuxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return 0 on success, a negative AVERROR on failure.
 *
 * @note If you want to use custom IO, preallocate the format context and set its pb field.
 */
int avformat_open_input(AVFormatContext **ps, const char *url, ff_const59 AVInputFormat *fmt, AVDictionary **options);

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/**
 * Open an input stream and read the header. The codecs are not opened.
 * The stream must be closed with avformat_close_input().
 *
 * @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
 *           May be a pointer to NULL, in which case an AVFormatContext is allocated by this
 *           function and written into ps.
 *           Note that a user-supplied AVFormatContext will be freed on failure.
 * @param url URL of the stream to open.
 * @param fmt If non-NULL, this parameter forces a specific input format.
 *            Otherwise the format is autodetected.
 * @param options  A dictionary filled with AVFormatContext and demuxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return 0 on success, a negative AVERROR on failure.
 *
 * @note If you want to use custom IO, preallocate the format context and set its pb field.
 */
int avformat_open_input(AVFormatContext **ps, const char *url, ff_const59 AVInputFormat *fmt, AVDictionary **options);

@ ps:函数调用成功之后处理过的AVFormatContext结构体。
@ file：打开的视音频流的URL。
@ fmt：强制指定AVFormatContext中AVInputFormat的。这个参数一般情况下可以设置为NULL，这样FFmpeg可以自动检测AVInputFormat。
@ dictionay：附加的一些选项，一般情况下可以设置为NULL。

下面看一下的定义，位于libavformatutils.c中，如下所示。

复制代码

int avformat_open_input(AVFormatContext **ps, const char *filename,
                        ff_const59 AVInputFormat *fmt, AVDictionary **options)
{
    AVFormatContext *s = *ps;
    int i, ret = 0;
    AVDictionary *tmp = NULL;
    ID3v2ExtraMeta *id3v2_extra_meta = NULL;

if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);
    if (!s->av_class) {
        av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL eithern");
        return AVERROR(EINVAL);
    }
    if (fmt)
        s->iformat = fmt;

if (options)
        av_dict_copy(&tmp, *options, 0);

if (s->pb) // must be before any goto fail
        s->flags |= AVFMT_FLAG_CUSTOM_IO;

if ((ret = av_opt_set_dict(s, &tmp)) < 0)
        goto fail;

if (!(s->url = av_strdup(filename ? filename : ""))) {
        ret = AVERROR(ENOMEM);
        goto fail;
    }

#if FF_API_FORMAT_FILENAME
FF_DISABLE_DEPRECATION_WARNINGS
    av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if ((ret = init_input(s, filename, &tmp)) < 0)
        goto fail;
    s->probe_score = ret;

if (!s->protocol_whitelist && s->pb && s->pb->protocol_whitelist) {
        s->protocol_whitelist = av_strdup(s->pb->protocol_whitelist);
        if (!s->protocol_whitelist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

if (!s->protocol_blacklist && s->pb && s->pb->protocol_blacklist) {
        s->protocol_blacklist = av_strdup(s->pb->protocol_blacklist);
        if (!s->protocol_blacklist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

if (s->format_whitelist && av_match_list(s->iformat->name, s->format_whitelist, ',') <= 0) {
        av_log(s, AV_LOG_ERROR, "Format not on whitelist '%s'n", s->format_whitelist);
        ret = AVERROR(EINVAL);
        goto fail;
    }

avio_skip(s->pb, s->skip_initial_bytes);

/* Check filename in case an image number is expected. */
    if (s->iformat->flags & AVFMT_NEEDNUMBER) {
        if (!av_filename_number_test(filename)) {
            ret = AVERROR(EINVAL);
            goto fail;
        }
    }

s->duration = s->start_time = AV_NOPTS_VALUE;

/* Allocate private data. */
    if (s->iformat->priv_data_size > 0) {
        if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        if (s->iformat->priv_class) {
            *(const AVClass **) s->priv_data = s->iformat->priv_class;
            av_opt_set_defaults(s->priv_data);
            if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
                goto fail;
        }
    }

/* e.g. AVFMT_NOFILE formats will not have a AVIOContext */
    if (s->pb)
        ff_id3v2_read_dict(s->pb, &s->internal->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);

if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
        if ((ret = s->iformat->read_header(s)) < 0)
            goto fail;

if (!s->metadata) {
        s->metadata = s->internal->id3v2_meta;
        s->internal->id3v2_meta = NULL;
    } else if (s->internal->id3v2_meta) {
        int level = AV_LOG_WARNING;
        if (s->error_recognition & AV_EF_COMPLIANT)
            level = AV_LOG_ERROR;
        av_log(s, level, "Discarding ID3 tags because more suitable tags were found.n");
        av_dict_free(&s->internal->id3v2_meta);
        if (s->error_recognition & AV_EF_EXPLODE) {
            ret = AVERROR_INVALIDDATA;
            goto close;
        }
    }

if (id3v2_extra_meta) {
        if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
            !strcmp(s->iformat->name, "tta") || !strcmp(s->iformat->name, "wav")) {
            if ((ret = ff_id3v2_parse_apic(s, &id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_chapters(s, &id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_priv(s, &id3v2_extra_meta)) < 0)
                goto close;
        } else
            av_log(s, AV_LOG_DEBUG, "demuxer does not support additional id3 data, skippingn");
    }
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);

if ((ret = avformat_queue_attached_pictures(s)) < 0)
        goto close;

if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->internal->data_offset)
        s->internal->data_offset = avio_tell(s->pb);

s->internal->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;

update_stream_avctx(s);

for (i = 0; i < s->nb_streams; i++)
        s->streams[i]->internal->orig_codec_id = s->streams[i]->codecpar->codec_id;

if (options) {
        av_dict_free(options);
        *options = tmp;
    }
    *ps = s;
    return 0;

close:
    if (s->iformat->read_close)
        s->iformat->read_close(s);
fail:
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);
    av_dict_free(&tmp);
    if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
        avio_closep(&s->pb);
    avformat_free_context(s);
    *ps = NULL;
    return ret;
}

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int avformat_open_input(AVFormatContext **ps, const char *filename,
                        ff_const59 AVInputFormat *fmt, AVDictionary **options)
{
    AVFormatContext *s = *ps;
    int i, ret = 0;
    AVDictionary *tmp = NULL;
    ID3v2ExtraMeta *id3v2_extra_meta = NULL;

    if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);
    if (!s->av_class) {
        av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL eithern");
        return AVERROR(EINVAL);
    }
    if (fmt)
        s->iformat = fmt;

    if (options)
        av_dict_copy(&tmp, *options, 0);

    if (s->pb) // must be before any goto fail
        s->flags |= AVFMT_FLAG_CUSTOM_IO;

    if ((ret = av_opt_set_dict(s, &tmp)) < 0)
        goto fail;

    if (!(s->url = av_strdup(filename ? filename : ""))) {
        ret = AVERROR(ENOMEM);
        goto fail;
    }

#if FF_API_FORMAT_FILENAME
FF_DISABLE_DEPRECATION_WARNINGS
    av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if ((ret = init_input(s, filename, &tmp)) < 0)
        goto fail;
    s->probe_score = ret;

    if (!s->protocol_whitelist && s->pb && s->pb->protocol_whitelist) {
        s->protocol_whitelist = av_strdup(s->pb->protocol_whitelist);
        if (!s->protocol_whitelist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (!s->protocol_blacklist && s->pb && s->pb->protocol_blacklist) {
        s->protocol_blacklist = av_strdup(s->pb->protocol_blacklist);
        if (!s->protocol_blacklist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (s->format_whitelist && av_match_list(s->iformat->name, s->format_whitelist, ',') <= 0) {
        av_log(s, AV_LOG_ERROR, "Format not on whitelist '%s'n", s->format_whitelist);
        ret = AVERROR(EINVAL);
        goto fail;
    }

    avio_skip(s->pb, s->skip_initial_bytes);

    /* Check filename in case an image number is expected. */
    if (s->iformat->flags & AVFMT_NEEDNUMBER) {
        if (!av_filename_number_test(filename)) {
            ret = AVERROR(EINVAL);
            goto fail;
        }
    }

    s->duration = s->start_time = AV_NOPTS_VALUE;

    /* Allocate private data. */
    if (s->iformat->priv_data_size > 0) {
        if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        if (s->iformat->priv_class) {
            *(const AVClass **) s->priv_data = s->iformat->priv_class;
            av_opt_set_defaults(s->priv_data);
            if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
                goto fail;
        }
    }

    /* e.g. AVFMT_NOFILE formats will not have a AVIOContext */
    if (s->pb)
        ff_id3v2_read_dict(s->pb, &s->internal->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);


    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
        if ((ret = s->iformat->read_header(s)) < 0)
            goto fail;

    if (!s->metadata) {
        s->metadata = s->internal->id3v2_meta;
        s->internal->id3v2_meta = NULL;
    } else if (s->internal->id3v2_meta) {
        int level = AV_LOG_WARNING;
        if (s->error_recognition & AV_EF_COMPLIANT)
            level = AV_LOG_ERROR;
        av_log(s, level, "Discarding ID3 tags because more suitable tags were found.n");
        av_dict_free(&s->internal->id3v2_meta);
        if (s->error_recognition & AV_EF_EXPLODE) {
            ret = AVERROR_INVALIDDATA;
            goto close;
        }
    }

    if (id3v2_extra_meta) {
        if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
            !strcmp(s->iformat->name, "tta") || !strcmp(s->iformat->name, "wav")) {
            if ((ret = ff_id3v2_parse_apic(s, &id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_chapters(s, &id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_priv(s, &id3v2_extra_meta)) < 0)
                goto close;
        } else
            av_log(s, AV_LOG_DEBUG, "demuxer does not support additional id3 data, skippingn");
    }
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);

    if ((ret = avformat_queue_attached_pictures(s)) < 0)
        goto close;

    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->internal->data_offset)
        s->internal->data_offset = avio_tell(s->pb);

    s->internal->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;

    update_stream_avctx(s);

    for (i = 0; i < s->nb_streams; i++)
        s->streams[i]->internal->orig_codec_id = s->streams[i]->codecpar->codec_id;

    if (options) {
        av_dict_free(options);
        *options = tmp;
    }
    *ps = s;
    return 0;

close:
    if (s->iformat->read_close)
        s->iformat->read_close(s);
fail:
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);
    av_dict_free(&tmp);
    if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
        avio_closep(&s->pb);
    avformat_free_context(s);
    *ps = NULL;
    return ret;
}

avformat_open_input()源代码比较长，一部分是一些容错代码，比如说如果发现传入的AVFormatContext指针没有初始化过，就调用avformat_alloc_context()初始化该结构体；还有一部分是针对一些格式做的特殊处理，比如id3v2信息的处理等等。有关上述两种信息不再详细分析，在这里只选择它关键的两个函数进行分析：
(1) avformat_alloc_context:申请并简单初始化AVFormatContext结构体
(2) init_input()：绝大部分初始化工作都是在这里做的。
(3) s->iformat->read_header()：读取多媒体数据文件头，根据视音频流创建相应的AVStream。

一、avformat_alloc_context()

这个再前面已经总结过，现在《ffmpeg结构体（2）之AVFormatContext及其相关函数》不在做赘述。

二、init_input()

复制代码

/* Open input file and probe the format if necessary. */
static int init_input(AVFormatContext *s, const char *filename,
                      AVDictionary **options)
{
    int ret;
    AVProbeData pd = { filename, NULL, 0 };
    int score = AVPROBE_SCORE_RETRY;

/*情况1： 使用了自定义的AVIOContext,AVIOContext结构体pb存在 */
    if (s->pb) {
        s->flags |= AVFMT_FLAG_CUSTOM_IO;
		/* 如果指定了AVInputFormat *iformat，直接返回。没有指定，执行下面
		*av_probe_input_buffer2函数.*/
        if (!s->iformat)
            return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                         s, 0, s->format_probesize);
        else if (s->iformat->flags & AVFMT_NOFILE)
            av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense and "
                                      "will be ignored with AVFMT_NOFILE format.n");
        return 0;
    }

/*情况2：自定义AVIOContext结构体pb不存在，且指定了AVInputFormat *iformat，直接返回。
	*没有指定iformat，则调用av_probe_input_format2函数进行推测匹配*/
    if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
        return score;
	/* 情况3：没有指定iformat，且av_probe_input_format2函数推断不出来 */
    if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
        return ret;

if (s->iformat)
        return 0;
    return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                 s, 0, s->format_probesize);
}

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/* Open input file and probe the format if necessary. */
static int init_input(AVFormatContext *s, const char *filename,
                      AVDictionary **options)
{
    int ret;
    AVProbeData pd = { filename, NULL, 0 };
    int score = AVPROBE_SCORE_RETRY;

	/*情况1： 使用了自定义的AVIOContext,AVIOContext结构体pb存在 */
    if (s->pb) {
        s->flags |= AVFMT_FLAG_CUSTOM_IO;
		/* 如果指定了AVInputFormat *iformat，直接返回。没有指定，执行下面
		*av_probe_input_buffer2函数.*/
        if (!s->iformat)
            return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                         s, 0, s->format_probesize);
        else if (s->iformat->flags & AVFMT_NOFILE)
            av_log(s, AV_LOG_WARNING, "Custom AVIOContext makes no sense and "
                                      "will be ignored with AVFMT_NOFILE format.n");
        return 0;
    }

	/*情况2：自定义AVIOContext结构体pb不存在，且指定了AVInputFormat *iformat，直接返回。
	*没有指定iformat，则调用av_probe_input_format2函数进行推测匹配*/
    if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
        return score;
	/* 情况3：没有指定iformat，且av_probe_input_format2函数推断不出来 */
    if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
        return ret;

    if (s->iformat)
        return 0;
    return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                 s, 0, s->format_probesize);
}

这个函数在短短的几行代码中包含了好几个return，因此逻辑还是有点复杂的，我们可以梳理一下：
在函数的开头的score变量是一个判决AVInputFormat的分数的门限值，如果最后得到的AVInputFormat的分数低于该门限值，就认为没有找到合适的AVInputFormat。FFmpeg内部判断封装格式的原理实际上是对每种AVInputFormat给出一个分数，满分是100分，越有可能正确的AVInputFormat给出的分数就越高。最后选择分数最高的AVInputFormat作为推测结果。score的值是一个宏定义AVPROBE_SCORE_RETRY，我们可以看一下它的定义：

复制代码

1
  #define AVPROBE_SCORE_RETRY (AVPROBE_SCORE_MAX/4)

其中AVPROBE_SCORE_MAX是score的最大值，取值是100：

复制代码

1
#define AVPROBE_SCORE_MAX       100 ///< maximum score

由此我们可以得出score取值是25，即如果推测后得到的最佳AVInputFormat的分值低于25，就认为没有找到合适的AVInputFormat。
整个函数的逻辑大体如下：
（1）当使用了自定义的AVIOContext的时候（AVFormatContext中的AVIOContext不为空，即s->pb!=NULL），如果指定了AVInputFormat就直接返回，如果没有指定就调用av_probe_input_buffer2()推测AVInputFormat。这一情况出现的不算很多，但是当我们从内存中读取数据的时候（需要初始化自定义的AVIOContext），就会执行这一步骤。
（2）在更一般的情况下，如果已经指定了AVInputFormat，就直接返回；如果没有指定AVInputFormat，就调用av_probe_input_format2(NULL,…)根据文件路径判断文件格式。
（3）如果发现通过文件路径判断不出来文件格式，那么就需要打开文件探测文件格式了，这个时候会首先调用AVFormatContext中的io_open()打开文件，然后调用av_probe_input_buffer2()推测AVInputFormat。

2.1 av_probe_input_format2()

av_probe_input_format2()是一个API函数，声明位于libavformatavformat.h，如下所示。

复制代码

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/**
 * Guess the file format.
 *
 * @param pd        data to be probed
 * @param is_opened Whether the file is already opened; determines whether
 *                  demuxers with or without AVFMT_NOFILE are probed.
 * @param score_max A probe score larger that this is required to accept a
 *                  detection, the variable is set to the actual detection
 *                  score afterwards.
 *                  If the score is <= AVPROBE_SCORE_MAX / 4 it is recommended
 *                  to retry with a larger probe buffer.
 */
ff_const59 AVInputFormat *av_probe_input_format2(ff_const59 AVProbeData *pd, int is_opened, int *score_max);

该函数用于根据输入数据查找合适的AVInputFormat。参数含义如下所示：
@ pd：存储输入数据信息的AVProbeData结构体。
@ is_opened：文件是否打开。
@ score_max：判决AVInputFormat的门限值。只有某格式判决分数大于该门限值的时候，函数才会返回该封装格式，否则返回NULL。
该函数中涉及到一个结构体AVProbeData，用于存储输入文件的一些信息，它的定义如下所示。

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/**
 * This structure contains the data a format has to probe a file.
 */
typedef struct AVProbeData {
    const char *filename;
    unsigned char *buf; /**< Buffer must have AVPROBE_PADDING_SIZE of extra allocated bytes filled with zero. */
    int buf_size;       /**< Size of buf except extra allocated bytes */
    const char *mime_type; /**< mime_type, when known. */
} AVProbeData;

av_probe_input_format2()函数的定义位于libavformatformat.c，如下所示。

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ff_const59 AVInputFormat *av_probe_input_format2(ff_const59 AVProbeData *pd, int is_opened, int *score_max)
{
    int score_ret;
    ff_const59 AVInputFormat *fmt = av_probe_input_format3(pd, is_opened, &score_ret);
    if (score_ret > *score_max) {
        *score_max = score_ret;
        return fmt;
    } else
        return NULL;
}

从函数中可以看出，av_probe_input_format2()调用了av_probe_input_format3()，并且增加了一个判断，当av_probe_input_format3()返回的分数大于score_max的时候，才会返回AVInputFormat，否则返回NULL。
下面我们看一下av_probe_input_format3()。

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ff_const59 AVInputFormat *av_probe_input_format3(ff_const59 AVProbeData *pd, int is_opened,
                                      int *score_ret)
{
    AVProbeData lpd = *pd;
    const AVInputFormat *fmt1 = NULL;
    ff_const59 AVInputFormat *fmt = NULL;
    int score, score_max = 0;
    void *i = 0;
    const static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];
    enum nodat {
        NO_ID3,
        ID3_ALMOST_GREATER_PROBE,
        ID3_GREATER_PROBE,
        ID3_GREATER_MAX_PROBE,
    } nodat = NO_ID3;

if (!lpd.buf)
        lpd.buf = (unsigned char *) zerobuffer;

if (lpd.buf_size > 10 && ff_id3v2_match(lpd.buf, ID3v2_DEFAULT_MAGIC)) {
        int id3len = ff_id3v2_tag_len(lpd.buf);
        if (lpd.buf_size > id3len + 16) {
            if (lpd.buf_size < 2LL*id3len + 16)
                nodat = ID3_ALMOST_GREATER_PROBE;
            lpd.buf      += id3len;
            lpd.buf_size -= id3len;
        } else if (id3len >= PROBE_BUF_MAX) {
            nodat = ID3_GREATER_MAX_PROBE;
        } else
            nodat = ID3_GREATER_PROBE;
    }

while ((fmt1 = av_demuxer_iterate(&i))) {
        if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
            continue;
        score = 0;
        if (fmt1->read_probe) {
            score = fmt1->read_probe(&lpd);
            if (score)
                av_log(NULL, AV_LOG_TRACE, "Probing %s score:%d size:%dn", fmt1->name, score, lpd.buf_size);
            if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) {
                switch (nodat) {
                case NO_ID3:
                    score = FFMAX(score, 1);
                    break;
                case ID3_GREATER_PROBE:
                case ID3_ALMOST_GREATER_PROBE:
                    score = FFMAX(score, AVPROBE_SCORE_EXTENSION / 2 - 1);
                    break;
                case ID3_GREATER_MAX_PROBE:
                    score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
                    break;
                }
            }
        } else if (fmt1->extensions) {
            if (av_match_ext(lpd.filename, fmt1->extensions))
                score = AVPROBE_SCORE_EXTENSION;
        }
		
        if (av_match_name(lpd.mime_type, fmt1->mime_type)) {
            if (AVPROBE_SCORE_MIME > score) {
                av_log(NULL, AV_LOG_DEBUG, "Probing %s score:%d increased to %d due to MIME typen", fmt1->name, score, AVPROBE_SCORE_MIME);
                score = AVPROBE_SCORE_MIME;
            }
        }
        if (score > score_max) {
            score_max = score;
            fmt       = (AVInputFormat*)fmt1;
        } else if (score == score_max)
            fmt = NULL;
    }
    if (nodat == ID3_GREATER_PROBE)
        score_max = FFMIN(AVPROBE_SCORE_EXTENSION / 2 - 1, score_max);
    *score_ret = score_max;

return fmt;
}

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ff_const59 AVInputFormat *av_probe_input_format3(ff_const59 AVProbeData *pd, int is_opened,
                                      int *score_ret)
{
    AVProbeData lpd = *pd;
    const AVInputFormat *fmt1 = NULL;
    ff_const59 AVInputFormat *fmt = NULL;
    int score, score_max = 0;
    void *i = 0;
    const static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];
    enum nodat {
        NO_ID3,
        ID3_ALMOST_GREATER_PROBE,
        ID3_GREATER_PROBE,
        ID3_GREATER_MAX_PROBE,
    } nodat = NO_ID3;

    if (!lpd.buf)
        lpd.buf = (unsigned char *) zerobuffer;

    if (lpd.buf_size > 10 && ff_id3v2_match(lpd.buf, ID3v2_DEFAULT_MAGIC)) {
        int id3len = ff_id3v2_tag_len(lpd.buf);
        if (lpd.buf_size > id3len + 16) {
            if (lpd.buf_size < 2LL*id3len + 16)
                nodat = ID3_ALMOST_GREATER_PROBE;
            lpd.buf      += id3len;
            lpd.buf_size -= id3len;
        } else if (id3len >= PROBE_BUF_MAX) {
            nodat = ID3_GREATER_MAX_PROBE;
        } else
            nodat = ID3_GREATER_PROBE;
    }

    while ((fmt1 = av_demuxer_iterate(&i))) {
        if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
            continue;
        score = 0;
        if (fmt1->read_probe) {
            score = fmt1->read_probe(&lpd);
            if (score)
                av_log(NULL, AV_LOG_TRACE, "Probing %s score:%d size:%dn", fmt1->name, score, lpd.buf_size);
            if (fmt1->extensions && av_match_ext(lpd.filename, fmt1->extensions)) {
                switch (nodat) {
                case NO_ID3:
                    score = FFMAX(score, 1);
                    break;
                case ID3_GREATER_PROBE:
                case ID3_ALMOST_GREATER_PROBE:
                    score = FFMAX(score, AVPROBE_SCORE_EXTENSION / 2 - 1);
                    break;
                case ID3_GREATER_MAX_PROBE:
                    score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
                    break;
                }
            }
        } else if (fmt1->extensions) {
            if (av_match_ext(lpd.filename, fmt1->extensions))
                score = AVPROBE_SCORE_EXTENSION;
        }
		
        if (av_match_name(lpd.mime_type, fmt1->mime_type)) {
            if (AVPROBE_SCORE_MIME > score) {
                av_log(NULL, AV_LOG_DEBUG, "Probing %s score:%d increased to %d due to MIME typen", fmt1->name, score, AVPROBE_SCORE_MIME);
                score = AVPROBE_SCORE_MIME;
            }
        }
        if (score > score_max) {
            score_max = score;
            fmt       = (AVInputFormat*)fmt1;
        } else if (score == score_max)
            fmt = NULL;
    }
    if (nodat == ID3_GREATER_PROBE)
        score_max = FFMIN(AVPROBE_SCORE_EXTENSION / 2 - 1, score_max);
    *score_ret = score_max;

    return fmt;
}

av_probe_input_format3()根据输入数据查找合适的AVInputFormat。输入的数据位于AVProbeData中。前文已经提到过，AVProbeData定义如下。

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/**
 * This structure contains the data a format has to probe a file.
 */
typedef struct AVProbeData {
    const char *filename;
    unsigned char *buf; /**< Buffer must have AVPROBE_PADDING_SIZE of extra allocated bytes filled with zero. */
    int buf_size;       /**< Size of buf except extra allocated bytes */
    const char *mime_type; /**< mime_type, when known. */
} AVProbeData;

@ filename:文件路径;
@ buf:存储用于推测AVInputFormat的媒体数据，其中buf可以为空，但是其后面无论如何都需要填充AVPROBE_PADDING_SIZE个0（AVPROBE_PADDING_SIZE取值为32，即32个0）。
@ mime_type: 保存媒体的类型
该函数最主要的部分是一个循环。该循环调用av_iformat_next()遍历FFmpeg中所有的AVInputFormat，并根据以下规则确定AVInputFormat和输入媒体数据的匹配分数（score，反应匹配程度）：
（1）如果AVInputFormat中包含read_probe()，就调用read_probe()函数获取匹配分数（这一方法如果结果匹配的话，一般会获得AVPROBE_SCORE_MAX的分值，即100分）。如果不包含该函数，就使用av_match_ext()函数比较输入媒体的扩展名和AVInputFormat的扩展名是否匹配，如果匹配的话，设定匹配分数为AVPROBE_SCORE_EXTENSION（AVPROBE_SCORE_EXTENSION取值为50，即50分）。
（2）使用av_match_name()比较输入媒体的mime_type和AVInputFormat的mime_type，如果匹配的话，设定匹配分数为AVPROBE_SCORE_MIME（AVPROBE_SCORE_MIME取值为75，即75分）。
（3）如果该AVInputFormat的匹配分数大于此前的最大匹配分数，则记录当前的匹配分数为最大匹配分数，并且记录当前的AVInputFormat为最佳匹配的AVInputFormat。

上述过程中涉及到以下几个知识点：

2.1.1 AVInputFormat->read_probe()

AVInputFormat中包含read_probe()是用于获得匹配函数的函数指针，不同的封装格式包含不同的实现函数。例如，FLV封装格式的AVInputFormat模块定义（位于libavformatflvdec.c）如下所示。

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AVInputFormat ff_flv_demuxer = {
    .name           = "flv",
    .long_name      = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),
    .priv_data_size = sizeof(FLVContext),
    .read_probe     = flv_probe,
    .read_header    = flv_read_header,
    .read_packet    = flv_read_packet,
    .read_seek      = flv_read_seek,
    .read_close     = flv_read_close,
    .extensions     = "flv",
    .priv_class     = &flv_class,
};

其中，read_probe()函数对应的是flv_probe()函数。我们可以看一下flv_probe()函数的定义：

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static int flv_probe(const AVProbeData *p)
{
    return probe(p, 0);
}

可见flv_probe()调用了一个probe()函数。probe()函数的定义如下。

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static int probe(const AVProbeData *p, int live)
{
    const uint8_t *d = p->buf;
    unsigned offset = AV_RB32(d + 5);

    if (d[0] == 'F' &&
        d[1] == 'L' &&
        d[2] == 'V' &&
        d[3] < 5 && d[5] == 0 &&
        offset + 100 < p->buf_size &&
        offset > 8) {
        int is_live = !memcmp(d + offset + 40, "NGINX RTMP", 10);

        if (live == is_live)
            return AVPROBE_SCORE_MAX;
    }
    return 0;
}

从probe()函数我们可以看出，该函数做了如下工作：
（1）获得第6至第9字节的数据（对应Headersize字段）并且做大小端转换，然后存入offset变量。之所以要进行大小端转换是因为FLV是以“大端”方式存储数据，而操作系统是以“小端”方式存储数据，这一转换主要通过AV_RB32()函数实现。AV_RB32()是一个宏定义，其对应的函数是av_bswap32()。
（2）检查开头3个字符（Signature）是否为“FLV”。
（3）第4个字节（Version）小于5。
（4）第6个字节（Headersize的第1个字节？）为0。
（5）offset取值大于8。

2.1.2 av_match_name()

av_match_name()是一个API函数，声明位于libavutilavstring.h，如下所示。

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/**
 * Match instances of a name in a comma-separated list of names.
 * List entries are checked from the start to the end of the names list,
 * the first match ends further processing. If an entry prefixed with '-'
 * matches, then 0 is returned. The "ALL" list entry is considered to
 * match all names.
 *
 * @param name  Name to look for.
 * @param names List of names.
 * @return 1 on match, 0 otherwise.
 */
int av_match_name(const char *name, const char *names);

av_match_name()用于比较两个格式的名称。简单地说就是比较字符串。注意该函数的字符串是不区分大小写的：字符都转换为小写进行比较。

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int av_match_name(const char *name, const char *names)
{
    const char *p;
    int len, namelen;

    if (!name || !names)
        return 0;

    namelen = strlen(name);
    while (*names) {
        int negate = '-' == *names;
        p = strchr(names, ',');
        if (!p)
            p = names + strlen(names);
        names += negate;
        len = FFMAX(p - names, namelen);
        if (!av_strncasecmp(name, names, len) || !strncmp("ALL", names, FFMAX(3, p - names)))
            return !negate;
        names = p + (*p == ',');
    }
    return 0;
}

上述函数还有一点需要注意，其中使用了一个while()循环，用于搜索“,”。这是因为FFmpeg中有些格式是对应多种格式名称的，例如MKV格式的解复用器（Demuxer）的定义如下。

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AVInputFormat ff_matroska_demuxer = {
    .name           = "matroska,webm",
    .long_name      = NULL_IF_CONFIG_SMALL("Matroska / WebM"),
    .extensions     = "mkv,mk3d,mka,mks",
    .priv_data_size = sizeof(MatroskaDemuxContext),
    .read_probe     = matroska_probe,
    .read_header    = matroska_read_header,
    .read_packet    = matroska_read_packet,
    .read_close     = matroska_read_close,
    .read_seek      = matroska_read_seek,
    .mime_type      = "audio/webm,audio/x-matroska,video/webm,video/x-matroska"
};

从代码可以看出，ff_matroska_demuxer中的name字段对应“matroska,webm”，mime_type字段对应“audio/webm,audio/x-matroska,video/webm,video/x-matroska”。av_match_name()函数对于这样的字符串，会把它按照“,”截断成一个个的名称，然后一一进行比较。

2.1.3 av_match_ext()

av_match_ext()是一个API函数，声明位于libavformatavformat.h（注意位置和av_match_name()不一样），如下所示。

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/**
 * Return a positive value if the given filename has one of the given
 * extensions, 0 otherwise.
 *
 * @param filename   file name to check against the given extensions
 * @param extensions a comma-separated list of filename extensions
 */
int av_match_ext(const char *filename, const char *extensions);

av_match_ext()用于比较文件的后缀。该函数首先通过反向查找的方式找到输入文件名中的“.”，就可以通过获取“.”后面的字符串来得到该文件的后缀。然后调用av_match_name()，采用和比较格式名称的方法比较两个后缀。

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/**
 * @file
 * Format register and lookup
 */
int av_match_ext(const char *filename, const char *extensions)
{
    const char *ext;

    if (!filename)
        return 0;

    ext = strrchr(filename, '.');
    if (ext)
        return av_match_name(ext + 1, extensions);
    return 0;
}

2.2 av_probe_input_buffer2()

av_probe_input_buffer2()是一个API函数，它根据输入的媒体数据推测该媒体数据的AVInputFormat，声明位于libavformatavformat.h，如下所示。

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/**
 * Guess the file format.
 *
 * @param is_opened Whether the file is already opened; determines whether
 *                  demuxers with or without AVFMT_NOFILE are probed.
 * @param score_ret The score of the best detection.
 */
ff_const59 AVInputFormat *av_probe_input_format3(ff_const59 AVProbeData *pd, int is_opened, int *score_ret);

从函数声明中可以看出，av_probe_input_format3()和av_probe_input_format2()的区别是函数的第3个参数不同：av_probe_input_format2()是一个分数的门限值，而av_probe_input_format3()是一个探测后的最匹配的格式的分数值。
av_probe_input_format3()的定义位于libavformatformat.c，如下所示。

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/**
 * Probe a bytestream to determine the input format. Each time a probe returns
 * with a score that is too low, the probe buffer size is increased and another
 * attempt is made. When the maximum probe size is reached, the input format
 * with the highest score is returned.
 *
 * @param pb the bytestream to probe
 * @param fmt the input format is put here
 * @param url the url of the stream
 * @param logctx the log context
 * @param offset the offset within the bytestream to probe from
 * @param max_probe_size the maximum probe buffer size (zero for default)
 * @return the score in case of success, a negative value corresponding to an
 *         the maximal score is AVPROBE_SCORE_MAX
 * AVERROR code otherwise
 */
int av_probe_input_buffer2(AVIOContext *pb, ff_const59 AVInputFormat **fmt,
                           const char *url, void *logctx,
                           unsigned int offset, unsigned int max_probe_size);

av_probe_input_buffer2()参数的含义如下所示：
@ pb：用于读取数据的AVIOContext。
@ fmt：输出推测出来的AVInputFormat。
@ filename：输入媒体的路径。
@ logctx：日志（没有研究过）。
@ offset：开始推测AVInputFormat的偏移量。
@ max_probe_size：用于推测格式的媒体数据的最大值。
返回推测后的得到的AVInputFormat的匹配分数。
av_probe_input_buffer2()的定义位于libavformatformat.c，如下所示。

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int av_probe_input_buffer2(AVIOContext *pb, ff_const59 AVInputFormat **fmt,
                          const char *filename, void *logctx,
                          unsigned int offset, unsigned int max_probe_size)
{
    AVProbeData pd = { filename ? filename : "" };
    uint8_t *buf = NULL;
    int ret = 0, probe_size, buf_offset = 0;
    int score = 0;
    int ret2;

if (!max_probe_size)
        max_probe_size = PROBE_BUF_MAX;
    else if (max_probe_size < PROBE_BUF_MIN) {
        av_log(logctx, AV_LOG_ERROR,
               "Specified probe size value %u cannot be < %un", max_probe_size, PROBE_BUF_MIN);
        return AVERROR(EINVAL);
    }

if (offset >= max_probe_size)
        return AVERROR(EINVAL);

if (pb->av_class) {
        uint8_t *mime_type_opt = NULL;
        char *semi;
        av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type_opt);
        pd.mime_type = (const char *)mime_type_opt;
        semi = pd.mime_type ? strchr(pd.mime_type, ';') : NULL;
        if (semi) {
            *semi = '';
        }
    }

for (probe_size = PROBE_BUF_MIN; probe_size <= max_probe_size && !*fmt;
         probe_size = FFMIN(probe_size << 1,
                            FFMAX(max_probe_size, probe_size + 1))) {
        score = probe_size < max_probe_size ? AVPROBE_SCORE_RETRY : 0;

/* Read probe data. */
        if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0)
            goto fail;
        if ((ret = avio_read(pb, buf + buf_offset,
                             probe_size - buf_offset)) < 0) {
            /* Fail if error was not end of file, otherwise, lower score. */
            if (ret != AVERROR_EOF)
                goto fail;

score = 0;
            ret   = 0;          /* error was end of file, nothing read */
        }
        buf_offset += ret;
        if (buf_offset < offset)
            continue;
        pd.buf_size = buf_offset - offset;
        pd.buf = &buf[offset];

memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);

/* Guess file format. */
        *fmt = av_probe_input_format2(&pd, 1, &score);
        if (*fmt) {
            /* This can only be true in the last iteration. */
            if (score <= AVPROBE_SCORE_RETRY) {
                av_log(logctx, AV_LOG_WARNING,
                       "Format %s detected only with low score of %d, "
                       "misdetection possible!n", (*fmt)->name, score);
            } else
                av_log(logctx, AV_LOG_DEBUG,
                       "Format %s probed with size=%d and score=%dn",
                       (*fmt)->name, probe_size, score);
#if 0
            FILE *f = fopen("probestat.tmp", "ab");
            fprintf(f, "probe_size:%d format:%s score:%d filename:%sn", probe_size, (*fmt)->name, score, filename);
            fclose(f);
#endif
        }
    }

if (!*fmt)
        ret = AVERROR_INVALIDDATA;

fail:
    /* Rewind. Reuse probe buffer to avoid seeking. */
    ret2 = ffio_rewind_with_probe_data(pb, &buf, buf_offset);
    if (ret >= 0)
        ret = ret2;

av_freep(&pd.mime_type);
    return ret < 0 ? ret : score;
}

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int av_probe_input_buffer2(AVIOContext *pb, ff_const59 AVInputFormat **fmt,
                          const char *filename, void *logctx,
                          unsigned int offset, unsigned int max_probe_size)
{
    AVProbeData pd = { filename ? filename : "" };
    uint8_t *buf = NULL;
    int ret = 0, probe_size, buf_offset = 0;
    int score = 0;
    int ret2;

    if (!max_probe_size)
        max_probe_size = PROBE_BUF_MAX;
    else if (max_probe_size < PROBE_BUF_MIN) {
        av_log(logctx, AV_LOG_ERROR,
               "Specified probe size value %u cannot be < %un", max_probe_size, PROBE_BUF_MIN);
        return AVERROR(EINVAL);
    }

    if (offset >= max_probe_size)
        return AVERROR(EINVAL);

    if (pb->av_class) {
        uint8_t *mime_type_opt = NULL;
        char *semi;
        av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type_opt);
        pd.mime_type = (const char *)mime_type_opt;
        semi = pd.mime_type ? strchr(pd.mime_type, ';') : NULL;
        if (semi) {
            *semi = '';
        }
    }

    for (probe_size = PROBE_BUF_MIN; probe_size <= max_probe_size && !*fmt;
         probe_size = FFMIN(probe_size << 1,
                            FFMAX(max_probe_size, probe_size + 1))) {
        score = probe_size < max_probe_size ? AVPROBE_SCORE_RETRY : 0;

        /* Read probe data. */
        if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0)
            goto fail;
        if ((ret = avio_read(pb, buf + buf_offset,
                             probe_size - buf_offset)) < 0) {
            /* Fail if error was not end of file, otherwise, lower score. */
            if (ret != AVERROR_EOF)
                goto fail;

            score = 0;
            ret   = 0;          /* error was end of file, nothing read */
        }
        buf_offset += ret;
        if (buf_offset < offset)
            continue;
        pd.buf_size = buf_offset - offset;
        pd.buf = &buf[offset];

        memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);

        /* Guess file format. */
        *fmt = av_probe_input_format2(&pd, 1, &score);
        if (*fmt) {
            /* This can only be true in the last iteration. */
            if (score <= AVPROBE_SCORE_RETRY) {
                av_log(logctx, AV_LOG_WARNING,
                       "Format %s detected only with low score of %d, "
                       "misdetection possible!n", (*fmt)->name, score);
            } else
                av_log(logctx, AV_LOG_DEBUG,
                       "Format %s probed with size=%d and score=%dn",
                       (*fmt)->name, probe_size, score);
#if 0
            FILE *f = fopen("probestat.tmp", "ab");
            fprintf(f, "probe_size:%d format:%s score:%d filename:%sn", probe_size, (*fmt)->name, score, filename);
            fclose(f);
#endif
        }
    }

    if (!*fmt)
        ret = AVERROR_INVALIDDATA;

fail:
    /* Rewind. Reuse probe buffer to avoid seeking. */
    ret2 = ffio_rewind_with_probe_data(pb, &buf, buf_offset);
    if (ret >= 0)
        ret = ret2;

    av_freep(&pd.mime_type);
    return ret < 0 ? ret : score;
}

av_probe_input_buffer2()首先需要确定用于推测格式的媒体数据的最大值max_probe_size。max_probe_size默认为PROBE_BUF_MAX（PROBE_BUF_MAX取值为1 << 20，即1048576Byte，大约1MB）。
在确定了max_probe_size之后，函数就会进入到一个循环中，调用avio_read()读取数据并且使用av_probe_input_format2()（该函数前文已经记录过）推测文件格式。
肯定有人会奇怪这里为什么要使用一个循环，而不是只运行一次？其实这个循环是一个逐渐增加输入媒体数据量的过程。 av_probe_input_buffer2()并不是一次性读取max_probe_size字节的媒体数据，我个人感觉可能是因为这样做不是很经济，因为推测大部分媒体格式根本用不到1MB这么多的媒体数据。因此函数中使用一个probe_size存储需要读取的字节数，并且随着循环次数的增加逐渐增加这个值。函数首先从PROBE_BUF_MIN（取值为2048）个字节开始读取，如果通过这些数据已经可以推测出AVInputFormat，那么就可以直接退出循环了（参考for循环的判断条件“!*fmt”）；如果没有推测出来，就增加probe_size的量为过去的2倍（参考for循环的表达式“probe_size << 1”），继续推测AVInputFormat；如果一直读取到max_probe_size字节的数据依然没能确定AVInputFormat，则会退出循环并且返回错误信息。

2.3 io_open

在申请AVFormatContext的借口中会设置io_open。avformat_alloc_context()->avformat_get_context_defaults()。
libavformatoptions.c

复制代码

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static void avformat_get_context_defaults(AVFormatContext *s)
{
    memset(s, 0, sizeof(AVFormatContext));

    s->av_class = &av_format_context_class;

    s->io_open  = io_open_default;
    s->io_close = io_close_default;

    av_opt_set_defaults(s);
}

由上述代码可以看出io_open被赋值为io_open_default()。
libavformatoptions.c

复制代码

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static int io_open_default(AVFormatContext *s, AVIOContext **pb,
                           const char *url, int flags, AVDictionary **options)
{
    int loglevel;

    if (!strcmp(url, s->url) ||
        s->iformat && !strcmp(s->iformat->name, "image2") ||
        s->oformat && !strcmp(s->oformat->name, "image2")
    ) {
        loglevel = AV_LOG_DEBUG;
    } else
        loglevel = AV_LOG_INFO;

    av_log(s, loglevel, "Opening '%s' for %sn", url, flags & AVIO_FLAG_WRITE ? "writing" : "reading");

#if FF_API_OLD_OPEN_CALLBACKS
FF_DISABLE_DEPRECATION_WARNINGS
    if (s->open_cb)
        return s->open_cb(s, pb, url, flags, &s->interrupt_callback, options);
FF_ENABLE_DEPRECATION_WARNINGS
#endif

    return ffio_open_whitelist(pb, url, flags, &s->interrupt_callback, options, s->protocol_whitelist, s->protocol_blacklist);
}

io_open_default最后调用了ffio_open_whitelist()函数，而ffio_open_whitelist函数我们在分析avio_open2()是否分析过，详见xxx。

三、read_header()

在调用完init_input()完成基本的初始化并且推测得到相应的AVInputFormat之后，avformat_open_input()会调用AVInputFormat的read_header()方法读取媒体文件的文件头并且完成相关的初始化工作。read_header()是一个位于AVInputFormat结构体中的一个函数指针，对于不同的封装格式，会调用不同的read_header()的实现函数。举个例子，当输入视频的封装格式为FLV的时候，会调用FLV的AVInputFormat中的read_header()。FLV的AVInputFormat定义位于libavformatflvdec.c文件中，如下所示。

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AVInputFormat ff_flv_demuxer = {
    .name           = "flv",
    .long_name      = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),
    .priv_data_size = sizeof(FLVContext),
    .read_probe     = flv_probe,
    .read_header    = flv_read_header,
    .read_packet    = flv_read_packet,
    .read_seek      = flv_read_seek,
    .read_close     = flv_read_close,
    .extensions     = "flv",
    .priv_class     = &flv_class,
};

可以看出read_header()指向了flv_read_header()函数。flv_read_header()的实现同样位于libavformatflvdec.c文件中，如下所示。

复制代码

static int flv_read_header(AVFormatContext *s)
{
    int flags;
    FLVContext *flv = s->priv_data;
    int offset;
    int pre_tag_size = 0;

/* Actual FLV data at 0xe40000 in KUX file */
    if(!strcmp(s->iformat->name, "kux"))
        avio_skip(s->pb, 0xe40000);

avio_skip(s->pb, 4);
    flags = avio_r8(s->pb);

flv->missing_streams = flags & (FLV_HEADER_FLAG_HASVIDEO | FLV_HEADER_FLAG_HASAUDIO);

s->ctx_flags |= AVFMTCTX_NOHEADER;

offset = avio_rb32(s->pb);
    avio_seek(s->pb, offset, SEEK_SET);

/* Annex E. The FLV File Format
     * E.3 TheFLVFileBody
     *     Field               Type    Comment
     *     PreviousTagSize0    UI32    Always 0
     * */
    pre_tag_size = avio_rb32(s->pb);
    if (pre_tag_size) {
        av_log(s, AV_LOG_WARNING, "Read FLV header error, input file is not a standard flv format, first PreviousTagSize0 always is 0n");
    }

s->start_time = 0;
    flv->sum_flv_tag_size = 0;
    flv->last_keyframe_stream_index = -1;

return 0;
}

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static int flv_read_header(AVFormatContext *s)
{
    int flags;
    FLVContext *flv = s->priv_data;
    int offset;
    int pre_tag_size = 0;

    /* Actual FLV data at 0xe40000 in KUX file */
    if(!strcmp(s->iformat->name, "kux"))
        avio_skip(s->pb, 0xe40000);

    avio_skip(s->pb, 4);
    flags = avio_r8(s->pb);

    flv->missing_streams = flags & (FLV_HEADER_FLAG_HASVIDEO | FLV_HEADER_FLAG_HASAUDIO);

    s->ctx_flags |= AVFMTCTX_NOHEADER;

    offset = avio_rb32(s->pb);
    avio_seek(s->pb, offset, SEEK_SET);

    /* Annex E. The FLV File Format
     * E.3 TheFLVFileBody
     *     Field               Type    Comment
     *     PreviousTagSize0    UI32    Always 0
     * */
    pre_tag_size = avio_rb32(s->pb);
    if (pre_tag_size) {
        av_log(s, AV_LOG_WARNING, "Read FLV header error, input file is not a standard flv format, first PreviousTagSize0 always is 0n");
    }

    s->start_time = 0;
    flv->sum_flv_tag_size = 0;
    flv->last_keyframe_stream_index = -1;

    return 0;
}

可以看出，函数读取了FLV的文件头并且判断其中是否包含视频流和音频流。如果包含视频流或者音频流，就会设置标记位，在后面适当的时候调用create_stream创建视频流和音频流。

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flv->missing_streams = flags & (FLV_HEADER_FLAG_HASVIDEO | FLV_HEADER_FLAG_HASAUDIO);

create_stream()函数定义也位于libavformatflvdec.c中，如下所示。

复制代码

static AVStream *create_stream(AVFormatContext *s, int codec_type)
{
    FLVContext *flv   = s->priv_data;
    AVStream *st = avformat_new_stream(s, NULL);
    if (!st)
        return NULL;
    st->codecpar->codec_type = codec_type;
    if (s->nb_streams>=3 ||(   s->nb_streams==2
                           && s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
                           && s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
                           && s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_DATA
                           && s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_DATA))
        s->ctx_flags &= ~AVFMTCTX_NOHEADER;
    if (codec_type == AVMEDIA_TYPE_AUDIO) {
        st->codecpar->bit_rate = flv->audio_bit_rate;
        flv->missing_streams &= ~FLV_HEADER_FLAG_HASAUDIO;
    }
    if (codec_type == AVMEDIA_TYPE_VIDEO) {
        st->codecpar->bit_rate = flv->video_bit_rate;
        flv->missing_streams &= ~FLV_HEADER_FLAG_HASVIDEO;
        st->avg_frame_rate = flv->framerate;
    }

avpriv_set_pts_info(st, 32, 1, 1000); /* 32 bit pts in ms */
    flv->last_keyframe_stream_index = s->nb_streams - 1;
    add_keyframes_index(s);
    return st;
}

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static AVStream *create_stream(AVFormatContext *s, int codec_type)
{
    FLVContext *flv   = s->priv_data;
    AVStream *st = avformat_new_stream(s, NULL);
    if (!st)
        return NULL;
    st->codecpar->codec_type = codec_type;
    if (s->nb_streams>=3 ||(   s->nb_streams==2
                           && s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
                           && s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
                           && s->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_DATA
                           && s->streams[1]->codecpar->codec_type != AVMEDIA_TYPE_DATA))
        s->ctx_flags &= ~AVFMTCTX_NOHEADER;
    if (codec_type == AVMEDIA_TYPE_AUDIO) {
        st->codecpar->bit_rate = flv->audio_bit_rate;
        flv->missing_streams &= ~FLV_HEADER_FLAG_HASAUDIO;
    }
    if (codec_type == AVMEDIA_TYPE_VIDEO) {
        st->codecpar->bit_rate = flv->video_bit_rate;
        flv->missing_streams &= ~FLV_HEADER_FLAG_HASVIDEO;
        st->avg_frame_rate = flv->framerate;
    }


    avpriv_set_pts_info(st, 32, 1, 1000); /* 32 bit pts in ms */
    flv->last_keyframe_stream_index = s->nb_streams - 1;
    add_keyframes_index(s);
    return st;
}