avformat_open_input() 方法完成了媒体文件的打开和格式探测的功能

avformat_open_input

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int avformat_open_input(AVFormatContext **ps, const char *filename,
                        AVInputFormat *fmt, AVDictionary **options)
{
    AVFormatContext *s = *ps;
    int i, ret = 0;
    AVDictionary *tmp = NULL;
    AVDictionary *tmp2 = 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 either\n");
        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;

    av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));
    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)) {
        if (s->iformat->read_header2) {
            if (options)
                av_dict_copy(&tmp2, *options, 0);

            if ((ret = s->iformat->read_header2(s, &tmp2)) < 0)
                goto fail;
        } else if (s->iformat->read_header && (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)
            return AVERROR_INVALIDDATA;
    }

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

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

    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;
        av_dict_free(&tmp2);
    }
    *ps = s;
    return 0;

fail:
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);
    av_dict_free(&tmp);
    av_dict_free(&tmp2);
    if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
        avio_closep(&s->pb);
    avformat_free_context(s);
    *ps = NULL;
    return ret;
}

int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options) 第一个参数是封装格式的上下文，第二个参数是文件地址，第三个参数是数据的封装格式，第四个参数是配置？

接下来比较重要的是 init_input(s, filename, &tmp) ，在这里面完成了查找流媒体协议和解复用器的工作

init_input

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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;

    if (s->pb) {
        s->flags |= AVFMT_FLAG_CUSTOM_IO;
        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;
    }

    if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
        return score;

    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);
}

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，这里一开始的分数是 25 分

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#define AVPROBE_SCORE_RETRY (AVPROBE_SCORE_MAX/4)

#define AVPROBE_SCORE_MAX       100 ///< maximum score
  • 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
  • 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_format(NULL,…) 根据文件路径判断文件格式。这里特意把 av_probe_input_format() 的第 1 个参数写成 “NULL”，是为了强调这个时候实际上并没有给函数提供输入数据，此时仅仅通过文件路径推测 AVInputFormat
  • 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 avio_open2() 打开文件，然后调用 av_probe_input_buffer2() 推测 AVInputFormat

av_probe_input_buffer2

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int av_probe_input_buffer2(AVIOContext *pb, 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 < %u\n", 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 = '\0';
        }
    }
#if 0
    if (!*fmt && pb->av_class && av_opt_get(pb, "mime_type", AV_OPT_SEARCH_CHILDREN, &mime_type) >= 0 && mime_type) {
        if (!av_strcasecmp(mime_type, "audio/aacp")) {
            *fmt = av_find_input_format("aac");
        }
        av_freep(&mime_type);
    }
#endif

    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=%d\n",
                       (*fmt)->name, probe_size, score);
#if 0
            FILE *f = fopen("probestat.tmp", "ab");
            fprintf(f, "probe_size:%d format:%s score:%d filename:%s\n", 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;
}

AVIOContext *pb 用于读取数据的 AVIOContext，AVInputFormat **fmt 输出推测出来的 AVInputFormat，unsigned int offset 开始推测 AVInputFormat 的偏移量，unsigned int max_probe_size 用于推测格式的媒体数据的最大值

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（因为并不是一次性读取 max_probe_size 字节的媒体数据），调用 avio_read() 读取数据并且使用 av_probe_input_format2() 去推测文件格式：

av_probe_input_format2

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AVInputFormat *av_probe_input_format2(AVProbeData *pd, int is_opened, int *score_max)
{
    int score_ret;
    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;
}

av_probe_input_format3() 给出了一个分数 score_ret，然后进行比对：

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AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened,
                                      int *score_ret)
{
    AVProbeData lpd = *pd;
    AVInputFormat *fmt1 = NULL, *fmt;
    int score, score_max = 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;
    }

    fmt = NULL;
    while ((fmt1 = av_iformat_next(fmt1))) {
        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:%d\n", 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 type\n", fmt1->name, score, AVPROBE_SCORE_MIME);
                score = AVPROBE_SCORE_MIME;
            }
        }
        if (score > score_max) {
            score_max = score;
            fmt       = 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;
}

while ((fmt1 = av_iformat_next(fmt1))) 循环遍历 AVInputFormat，该方法中的 first_iformat 链表中的头节点，并根据以下规则确定 AVInputFormat 和输入媒体数据的匹配分数

  • 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（即 50 分）
  • 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（即 75 分）
  • 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

read_probe

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

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AVInputFormat ff_mov_demuxer = {
    .name           = "mov,mp4,m4a,3gp,3g2,mj2",
    .long_name      = NULL_IF_CONFIG_SMALL("QuickTime / MOV"),
    .priv_class     = &mov_class,
    .priv_data_size = sizeof(MOVContext),
    .extensions     = "mov,mp4,m4a,3gp,3g2,mj2",
    .read_probe     = mov_probe,
    .read_header    = mov_read_header,
    .read_packet    = mov_read_packet,
    .read_close     = mov_read_close,
    .read_seek      = mov_read_seek,
    .flags          = AVFMT_NO_BYTE_SEEK,
};

其中，read_probe() 函数对应的是 mov_probe() 函数：

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static int mov_probe(AVProbeData *p)
{
    int64_t offset;
    uint32_t tag;
    int score = 0;
    int moov_offset = -1;

    /* check file header */
    offset = 0;
    for (;;) {
        /* ignore invalid offset */
        if ((offset + 8) > (unsigned int)p->buf_size)
            break;
        tag = AV_RL32(p->buf + offset + 4);
        switch(tag) {
        /* check for obvious tags */
        case MKTAG('m','o','o','v'):
            moov_offset = offset + 4;
        case MKTAG('m','d','a','t'):
        case MKTAG('p','n','o','t'): /* detect movs with preview pics like ew.mov and april.mov */
        case MKTAG('u','d','t','a'): /* Packet Video PVAuthor adds this and a lot of more junk */
        case MKTAG('f','t','y','p'):
            if (AV_RB32(p->buf+offset) < 8 &&
                (AV_RB32(p->buf+offset) != 1 ||
                 offset + 12 > (unsigned int)p->buf_size ||
                 AV_RB64(p->buf+offset + 8) == 0)) {
                score = FFMAX(score, AVPROBE_SCORE_EXTENSION);
            } else if (tag == MKTAG('f','t','y','p') &&
                       (   AV_RL32(p->buf + offset + 8) == MKTAG('j','p','2',' ')
                        || AV_RL32(p->buf + offset + 8) == MKTAG('j','p','x',' ')
                    )) {
                score = FFMAX(score, 5);
            } else {
                score = AVPROBE_SCORE_MAX;
            }
            offset = FFMAX(4, AV_RB32(p->buf+offset)) + offset;
            break;
        /* those are more common words, so rate then a bit less */
        case MKTAG('e','d','i','w'): /* xdcam files have reverted first tags */
        case MKTAG('w','i','d','e'):
        case MKTAG('f','r','e','e'):
        case MKTAG('j','u','n','k'):
        case MKTAG('p','i','c','t'):
            score  = FFMAX(score, AVPROBE_SCORE_MAX - 5);
            offset = FFMAX(4, AV_RB32(p->buf+offset)) + offset;
            break;
        case MKTAG(0x82,0x82,0x7f,0x7d):
        case MKTAG('s','k','i','p'):
        case MKTAG('u','u','i','d'):
        case MKTAG('p','r','f','l'):
            /* if we only find those cause probedata is too small at least rate them */
            score  = FFMAX(score, AVPROBE_SCORE_EXTENSION);
            offset = FFMAX(4, AV_RB32(p->buf+offset)) + offset;
            break;
        default:
            offset = FFMAX(4, AV_RB32(p->buf+offset)) + offset;
        }
    }
    if(score > AVPROBE_SCORE_MAX - 50 && moov_offset != -1) {
        /* moov atom in the header - we should make sure that this is not a
         * MOV-packed MPEG-PS */
        offset = moov_offset;

        while(offset < (p->buf_size - 16)){ /* Sufficient space */
               /* We found an actual hdlr atom */
            if(AV_RL32(p->buf + offset     ) == MKTAG('h','d','l','r') &&
               AV_RL32(p->buf + offset +  8) == MKTAG('m','h','l','r') &&
               AV_RL32(p->buf + offset + 12) == MKTAG('M','P','E','G')){
                av_log(NULL, AV_LOG_WARNING, "Found media data tag MPEG indicating this is a MOV-packed MPEG-PS.\n");
                /* We found a media handler reference atom describing an
                 * MPEG-PS-in-MOV, return a
                 * low score to force expanding the probe window until
                 * mpegps_probe finds what it needs */
                return 5;
            }else
                /* Keep looking */
                offset+=2;
        }
    }

    return score;
}

av_match_name

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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;
}

用于比较两个格式的名称(不区分大小写)

av_match_ext

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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;
}

用于比较文件的后缀

io_open

init_input 中调用了 s->io_open ，实际上调用的就是 io_open_default ：

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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);
}

在 io_open_default() 中会调用 ffio_open_whitelist()：

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int ffio_open_whitelist(AVIOContext **s, const char *filename, int flags,
                         const AVIOInterruptCB *int_cb, AVDictionary **options,
                         const char *whitelist, const char *blacklist
                        )
{
    URLContext *h;
    int err;

    err = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);
    if (err < 0)
        return err;
    err = ffio_fdopen(s, h);
    if (err < 0) {
        ffurl_close(h);
        return err;
    }
    return 0;
}

接着跟下去：

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int ffurl_open_whitelist(URLContext **puc, const char *filename, int flags,
                         const AVIOInterruptCB *int_cb, AVDictionary **options,
                         const char *whitelist, const char* blacklist,
                         URLContext *parent)
{
    AVDictionary *tmp_opts = NULL;
    AVDictionaryEntry *e;
    int ret = ffurl_alloc(puc, filename, flags, int_cb);
    if (ret < 0)
        return ret;
    if (parent)
        av_opt_copy(*puc, parent);
    if (options &&
        (ret = av_opt_set_dict(*puc, options)) < 0)
        goto fail;
    if (options && (*puc)->prot->priv_data_class &&
        (ret = av_opt_set_dict((*puc)->priv_data, options)) < 0)
        goto fail;

    if (!options)
        options = &tmp_opts;

    av_assert0(!whitelist ||
               !(e=av_dict_get(*options, "protocol_whitelist", NULL, 0)) ||
               !strcmp(whitelist, e->value));
    av_assert0(!blacklist ||
               !(e=av_dict_get(*options, "protocol_blacklist", NULL, 0)) ||
               !strcmp(blacklist, e->value));

    if ((ret = av_dict_set(options, "protocol_whitelist", whitelist, 0)) < 0)
        goto fail;

    if ((ret = av_dict_set(options, "protocol_blacklist", blacklist, 0)) < 0)
        goto fail;

    if ((ret = av_opt_set_dict(*puc, options)) < 0)
        goto fail;

    ret = ffurl_connect(*puc, options);

    if (!ret)
        return 0;
fail:
    ffurl_close(*puc);
    *puc = NULL;
    return ret;
}

在 ffurl_alloc() 中调用了 url_find_protocol() 方法:

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int ffurl_alloc(URLContext **puc, const char *filename, int flags,
                const AVIOInterruptCB *int_cb)
{
    const URLProtocol *p = NULL;

    p = url_find_protocol(filename);
    if (p)
       return url_alloc_for_protocol(puc, p, filename, flags, int_cb);

    *puc = NULL;
    if (av_strstart(filename, "https:", NULL))
        av_log(NULL, AV_LOG_WARNING, "https protocol not found, recompile FFmpeg with "
                                     "openssl, gnutls "
                                     "or securetransport enabled.\n");
    return AVERROR_PROTOCOL_NOT_FOUND;
}

url_find_protocol

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#define URL_SCHEME_CHARS                        \
    "abcdefghijklmnopqrstuvwxyz"                \
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"                \
    "0123456789+-."

static const struct URLProtocol *url_find_protocol(const char *filename)
{
    const URLProtocol **protocols;
    char proto_str[128], proto_nested[128], *ptr;
    size_t proto_len = strspn(filename, URL_SCHEME_CHARS);
    int i;

    if (filename[proto_len] != ':' &&
        (strncmp(filename, "subfile,", 8) || !strchr(filename + proto_len + 1, ':')) ||
        is_dos_path(filename))
        strcpy(proto_str, "file");
    else
        av_strlcpy(proto_str, filename,
                   FFMIN(proto_len + 1, sizeof(proto_str)));

    av_strlcpy(proto_nested, proto_str, sizeof(proto_nested));
    if ((ptr = strchr(proto_nested, '+')))
        *ptr = '\0';

    protocols = ffurl_get_protocols(NULL, NULL);
    if (!protocols)
        return NULL;
    for (i = 0; protocols[i]; i++) {
            const URLProtocol *up = protocols[i];
        if (!strcmp(proto_str, up->name)) {
            av_freep(&protocols);
            return up;
        }
        if (up->flags & URL_PROTOCOL_FLAG_NESTED_SCHEME &&
            !strcmp(proto_nested, up->name)) {
            av_freep(&protocols);
            return up;
        }
    }
    av_freep(&protocols);

    return NULL;
}

ffurl_get_protocols 可以得到当前编译的 FFmpeg 支持的所有流媒体协议，通过 url 的 scheme 和 protocol->name 相比较，得到正确的 protocol

URLProtocol

url_find_protocol() 方法返回一个 URLProtocol 变量

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typedef struct URLProtocol {
    const char *name;
    int     (*url_open)( URLContext *h, const char *url, int flags);
    /**
     * This callback is to be used by protocols which open further nested
     * protocols. options are then to be passed to ffurl_open()/ffurl_connect()
     * for those nested protocols.
     */
    int     (*url_open2)(URLContext *h, const char *url, int flags, AVDictionary **options);
    int     (*url_accept)(URLContext *s, URLContext **c);
    int     (*url_handshake)(URLContext *c);

    /**
     * Read data from the protocol.
     * If data is immediately available (even less than size), EOF is
     * reached or an error occurs (including EINTR), return immediately.
     * Otherwise:
     * In non-blocking mode, return AVERROR(EAGAIN) immediately.
     * In blocking mode, wait for data/EOF/error with a short timeout (0.1s),
     * and return AVERROR(EAGAIN) on timeout.
     * Checking interrupt_callback, looping on EINTR and EAGAIN and until
     * enough data has been read is left to the calling function; see
     * retry_transfer_wrapper in avio.c.
     */
    int     (*url_read)( URLContext *h, unsigned char *buf, int size);
    int     (*url_write)(URLContext *h, const unsigned char *buf, int size);
    int64_t (*url_seek)( URLContext *h, int64_t pos, int whence);
    int     (*url_close)(URLContext *h);
    int (*url_read_pause)(URLContext *h, int pause);
    int64_t (*url_read_seek)(URLContext *h, int stream_index,
                             int64_t timestamp, int flags);
    int (*url_get_file_handle)(URLContext *h);
    int (*url_get_multi_file_handle)(URLContext *h, int **handles,
                                     int *numhandles);
    int (*url_get_short_seek)(URLContext *h);
    int (*url_shutdown)(URLContext *h, int flags);
    int priv_data_size;
    const AVClass *priv_data_class;
    int flags;
    int (*url_check)(URLContext *h, int mask);
    int (*url_open_dir)(URLContext *h);
    int (*url_read_dir)(URLContext *h, AVIODirEntry **next);
    int (*url_close_dir)(URLContext *h);
    int (*url_delete)(URLContext *h);
    int (*url_move)(URLContext *h_src, URLContext *h_dst);
    const char *default_whitelist;
} URLProtocol;

URLProtocol 功能就是完成各种输入协议的读写等操作，ijkplayer 和 ffmpeg 支持的协议也有很多：

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extern const URLProtocol ff_async_protocol;
extern const URLProtocol ff_bluray_protocol;
extern const URLProtocol ff_cache_protocol;
extern const URLProtocol ff_concat_protocol;
extern const URLProtocol ff_crypto_protocol;
extern const URLProtocol ff_data_protocol;
extern const URLProtocol ff_ffrtmpcrypt_protocol;
extern const URLProtocol ff_ffrtmphttp_protocol;
extern const URLProtocol ff_file_protocol;
extern const URLProtocol ff_ftp_protocol;
extern const URLProtocol ff_gopher_protocol;
extern const URLProtocol ff_hls_protocol;
extern const URLProtocol ff_http_protocol;
extern const URLProtocol ff_httpproxy_protocol;
extern const URLProtocol ff_https_protocol;
extern const URLProtocol ff_icecast_protocol;
extern const URLProtocol ff_ijkhttphook_protocol;
extern const URLProtocol ff_ijklongurl_protocol;
extern const URLProtocol ff_ijkmediadatasource_protocol;
extern const URLProtocol ff_ijksegment_protocol;
extern const URLProtocol ff_ijktcphook_protocol;
extern const URLProtocol ff_ijkio_protocol;
extern const URLProtocol ff_mmsh_protocol;
extern const URLProtocol ff_mmst_protocol;
extern const URLProtocol ff_md5_protocol;
extern const URLProtocol ff_pipe_protocol;
extern const URLProtocol ff_prompeg_protocol;
extern const URLProtocol ff_rtmp_protocol;
extern const URLProtocol ff_rtmpe_protocol;
extern const URLProtocol ff_rtmps_protocol;
extern const URLProtocol ff_rtmpt_protocol;
extern const URLProtocol ff_rtmpte_protocol;
extern const URLProtocol ff_rtmpts_protocol;
extern const URLProtocol ff_rtp_protocol;
extern const URLProtocol ff_sctp_protocol;
extern const URLProtocol ff_srtp_protocol;
extern const URLProtocol ff_subfile_protocol;
extern const URLProtocol ff_tee_protocol;
extern const URLProtocol ff_tcp_protocol;
extern const URLProtocol ff_tls_gnutls_protocol;
extern const URLProtocol ff_tls_schannel_protocol;
extern const URLProtocol ff_tls_securetransport_protocol;
extern const URLProtocol ff_tls_openssl_protocol;
extern const URLProtocol ff_udp_protocol;
extern const URLProtocol ff_udplite_protocol;
extern const URLProtocol ff_unix_protocol;
extern const URLProtocol ff_librtmp_protocol;
extern const URLProtocol ff_librtmpe_protocol;
extern const URLProtocol ff_librtmps_protocol;
extern const URLProtocol ff_librtmpt_protocol;
extern const URLProtocol ff_librtmpte_protocol;
extern const URLProtocol ff_libssh_protocol;
extern const URLProtocol ff_libsmbclient_protocol;

read_header

read_header() 也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

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AVInputFormat ff_mov_demuxer = {
    .name           = "mov,mp4,m4a,3gp,3g2,mj2",
    .long_name      = NULL_IF_CONFIG_SMALL("QuickTime / MOV"),
    .priv_class     = &mov_class,
    .priv_data_size = sizeof(MOVContext),
    .extensions     = "mov,mp4,m4a,3gp,3g2,mj2",
    .read_probe     = mov_probe,
    .read_header    = mov_read_header,
    .read_packet    = mov_read_packet,
    .read_close     = mov_read_close,
    .read_seek      = mov_read_seek,
    .flags          = AVFMT_NO_BYTE_SEEK,
};

read_header() 指向了 mov_read_header() 函数

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static int mov_read_header(AVFormatContext *s)
{
    MOVContext *mov = s->priv_data;
    AVIOContext *pb = s->pb;
    int j, err;
    MOVAtom atom = { AV_RL32("root") };
    int i;

    if (mov->decryption_key_len != 0 && mov->decryption_key_len != AES_CTR_KEY_SIZE) {
        av_log(s, AV_LOG_ERROR, "Invalid decryption key len %d expected %d\n",
            mov->decryption_key_len, AES_CTR_KEY_SIZE);
        return AVERROR(EINVAL);
    }

    mov->fc = s;
    mov->trak_index = -1;
    /* .mov and .mp4 aren't streamable anyway (only progressive download if moov is before mdat) */
    if (pb->seekable & AVIO_SEEKABLE_NORMAL)
        atom.size = avio_size(pb);
    else
        atom.size = INT64_MAX;

    /* check MOV header */
    do {
    if (mov->moov_retry)
        avio_seek(pb, 0, SEEK_SET);
    if ((err = mov_read_default(mov, pb, atom)) < 0) {
        av_log(s, AV_LOG_ERROR, "error reading header\n");
        mov_read_close(s);
        return err;
    }
    } while ((pb->seekable & AVIO_SEEKABLE_NORMAL) && !mov->found_moov && !mov->moov_retry++);
    if (!mov->found_moov) {
        av_log(s, AV_LOG_ERROR, "moov atom not found\n");
        mov_read_close(s);
        return AVERROR_INVALIDDATA;
    }
    av_log(mov->fc, AV_LOG_TRACE, "on_parse_exit_offset=%"PRId64"\n", avio_tell(pb));

    if (pb->seekable & AVIO_SEEKABLE_NORMAL) {
        if (mov->nb_chapter_tracks > 0 && !mov->ignore_chapters)
            mov_read_chapters(s);
        for (i = 0; i < s->nb_streams; i++)
            if (s->streams[i]->codecpar->codec_tag == AV_RL32("tmcd")) {
                mov_read_timecode_track(s, s->streams[i]);
            } else if (s->streams[i]->codecpar->codec_tag == AV_RL32("rtmd")) {
                mov_read_rtmd_track(s, s->streams[i]);
            }
    }

    /* copy timecode metadata from tmcd tracks to the related video streams */
    for (i = 0; i < s->nb_streams; i++) {
        AVStream *st = s->streams[i];
        MOVStreamContext *sc = st->priv_data;
        if (sc->timecode_track > 0) {
            AVDictionaryEntry *tcr;
            int tmcd_st_id = -1;

            for (j = 0; j < s->nb_streams; j++)
                if (s->streams[j]->id == sc->timecode_track)
                    tmcd_st_id = j;

            if (tmcd_st_id < 0 || tmcd_st_id == i)
                continue;
            tcr = av_dict_get(s->streams[tmcd_st_id]->metadata, "timecode", NULL, 0);
            if (tcr)
                av_dict_set(&st->metadata, "timecode", tcr->value, 0);
        }
    }
    export_orphan_timecode(s);

    for (i = 0; i < s->nb_streams; i++) {
        AVStream *st = s->streams[i];
        MOVStreamContext *sc = st->priv_data;
        fix_timescale(mov, sc);
        if(st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO && st->codecpar->codec_id == AV_CODEC_ID_AAC) {
            st->skip_samples = sc->start_pad;
        }
        if (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO && sc->nb_frames_for_fps > 0 && sc->duration_for_fps > 0)
            av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                      sc->time_scale*(int64_t)sc->nb_frames_for_fps, sc->duration_for_fps, INT_MAX);
        if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE) {
            if (st->codecpar->width <= 0 || st->codecpar->height <= 0) {
                st->codecpar->width  = sc->width;
                st->codecpar->height = sc->height;
            }
            if (st->codecpar->codec_id == AV_CODEC_ID_DVD_SUBTITLE) {
                if ((err = mov_rewrite_dvd_sub_extradata(st)) < 0)
                    return err;
            }
        }
        if (mov->handbrake_version &&
            mov->handbrake_version <= 1000000*0 + 1000*10 + 2 &&  // 0.10.2
            st->codecpar->codec_id == AV_CODEC_ID_MP3
        ) {
            av_log(s, AV_LOG_VERBOSE, "Forcing full parsing for mp3 stream\n");
            st->need_parsing = AVSTREAM_PARSE_FULL;
        }
    }

    if (mov->trex_data) {
        for (i = 0; i < s->nb_streams; i++) {
            AVStream *st = s->streams[i];
            MOVStreamContext *sc = st->priv_data;
            if (st->duration > 0) {
                if (sc->data_size > INT64_MAX / sc->time_scale / 8) {
                    av_log(s, AV_LOG_ERROR, "Overflow during bit rate calculation %"PRId64" * 8 * %d\n",
                           sc->data_size, sc->time_scale);
                    mov_read_close(s);
                    return AVERROR_INVALIDDATA;
                }
                st->codecpar->bit_rate = sc->data_size * 8 * sc->time_scale / st->duration;
            }
        }
    }

    if (mov->use_mfra_for > 0) {
        for (i = 0; i < s->nb_streams; i++) {
            AVStream *st = s->streams[i];
            MOVStreamContext *sc = st->priv_data;
            if (sc->duration_for_fps > 0) {
                if (sc->data_size > INT64_MAX / sc->time_scale / 8) {
                    av_log(s, AV_LOG_ERROR, "Overflow during bit rate calculation %"PRId64" * 8 * %d\n",
                           sc->data_size, sc->time_scale);
                    mov_read_close(s);
                    return AVERROR_INVALIDDATA;
                }
                st->codecpar->bit_rate = sc->data_size * 8 * sc->time_scale /
                    sc->duration_for_fps;
            }
        }
    }

    for (i = 0; i < mov->bitrates_count && i < s->nb_streams; i++) {
        if (mov->bitrates[i]) {
            s->streams[i]->codecpar->bit_rate = mov->bitrates[i];
        }
    }

    ff_rfps_calculate(s);

    for (i = 0; i < s->nb_streams; i++) {
        AVStream *st = s->streams[i];
        MOVStreamContext *sc = st->priv_data;

        switch (st->codecpar->codec_type) {
        case AVMEDIA_TYPE_AUDIO:
            err = ff_replaygain_export(st, s->metadata);
            if (err < 0) {
                mov_read_close(s);
                return err;
            }
            break;
        case AVMEDIA_TYPE_VIDEO:
            if (sc->display_matrix) {
                err = av_stream_add_side_data(st, AV_PKT_DATA_DISPLAYMATRIX, (uint8_t*)sc->display_matrix,
                                              sizeof(int32_t) * 9);
                if (err < 0)
                    return err;

                sc->display_matrix = NULL;
            }
            if (sc->stereo3d) {
                err = av_stream_add_side_data(st, AV_PKT_DATA_STEREO3D,
                                              (uint8_t *)sc->stereo3d,
                                              sizeof(*sc->stereo3d));
                if (err < 0)
                    return err;

                sc->stereo3d = NULL;
            }
            if (sc->spherical) {
                err = av_stream_add_side_data(st, AV_PKT_DATA_SPHERICAL,
                                              (uint8_t *)sc->spherical,
                                              sc->spherical_size);
                if (err < 0)
                    return err;

                sc->spherical = NULL;
            }
            if (sc->mastering) {
                err = av_stream_add_side_data(st, AV_PKT_DATA_MASTERING_DISPLAY_METADATA,
                                              (uint8_t *)sc->mastering,
                                              sizeof(*sc->mastering));
                if (err < 0)
                    return err;

                sc->mastering = NULL;
            }
            if (sc->coll) {
                err = av_stream_add_side_data(st, AV_PKT_DATA_CONTENT_LIGHT_LEVEL,
                                              (uint8_t *)sc->coll,
                                              sc->coll_size);
                if (err < 0)
                    return err;

                sc->coll = NULL;
            }
            break;
        }
    }
    ff_configure_buffers_for_index(s, AV_TIME_BASE);

    for (i = 0; i < mov->fragment_index_count; i++) {
        MOVFragmentIndex *idx = mov->fragment_index_data[i];
        for (j = 0; j < idx->item_count; j++)
            if (idx->items[j].moof_offset <= mov->fragment.moof_offset)
                idx->items[j].headers_read = 1;
    }

    return 0;
}

调用链

雷霄骅(图解FFMPEG打开媒体的函数avformat_open_input)

参考