从头用脚分析FFmpeg源码 --- avformat_open_input

本文所使用的是FFmpeg n4.4的源码，所有分析均来自博主瞎猜，如果有误，欢迎批评指正。

avformat_open_input 作用

打开一个input类型的AVFormatContext。

AVFormatContext中包含了对这个输入文件的基本信息的总和，比如开始时间，时长，码率，流和对应的流信息AVStream，除此之前还有读取文件的上下文AVIOContext， 打开的文件格式对应的AVInputFormat等。

而avformat_open_input需要做的就是根据输入的参数，初始化这些信息。

avformat_open_input 源码分析:

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;

	// 输入的ps可以已经指向一个AVFormatContext，这样在后续的过程中，就可以利用这些已经初始化过的参数进行处理，
	//FFmpeg源码中，也是先在外面先调用avformat_alloc_context，然后设置参数，然后再调用avformat_open_input的
    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);

	//AVFormatContext在为初始化之前，就已经被赋予了IO操作上下文AVIOContext，这样的话，调用avformat_close_input也不会自动关闭AVIOContext，需要让用户自己调用avio_closep或其他方法关闭流。
	//AVFMT_FLAG_CUSTOM_IO就是标识这种情况的
    if (s->pb) // must be before any goto fail
        s->flags |= AVFMT_FLAG_CUSTOM_IO;
	
	//这里是把所有设置的参数保存在AVFormatContext的AVClass*中，因为后续需要把AVFormatContext* 强转成为AVClass** 类型，
	//所以AVClass*必须在AVFormatContext结构体的头部，其它结构体也是这个思路。
    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

	// 这个函数比较重要，就是根据之前的输入，遍历所有的 解封装器，然后选出一个评分最高的，作为AVFormatContext的AVInputFormat。具体过程看下面的分析
    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 FF_API_DEMUXER_OPEN
    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
#else
    if (s->iformat->read_header)
#endif
        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) {
        av_log(s, AV_LOG_WARNING, "Discarding ID3 tags because more suitable tags were found.\n");
        av_dict_free(&s->internal->id3v2_meta);
    }

    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, skipping\n");
    }
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);

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

#if FF_API_DEMUXER_OPEN
    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->internal->data_offset)
#else
    if (s->pb && !s->internal->data_offset)
#endif
        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包含很多，包括根据输入参数选择不同的处理方式，看看AVFormatContext*是否已经初始化等等，还有对特殊格式，url的协议等处理过程，但是这些不是很关键，这里面最关键的是两个函数。

• init_input() 这个函数非常重要，AVFormatContext中大部分信息都是在这里面初始化的，包括根据输入的url找到对应的URLContext，打开IO流，从IO流中读取部分自己，然后探测格式，从而选择对应的AVInputFormat等等，这些操作都是在init_input()中完成的，总结来说init_input()创建了AVIOContext，和AVInputFormat。
• s->iformat->read_header() 这个函数是根据上面确定的AVInputFormat和AVIOContext读取头部数据，然后创建AVStream。

下面来分析一下init_input() 函数实现

init_input源码

/* 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 };
    //初识评分 25，（最大为100） 如果后面的评分小于这个值，就认为没有找到
    int score = AVPROBE_SCORE_RETRY;

    if (s->pb) {
        s->flags |= AVFMT_FLAG_CUSTOM_IO;
        // 如果已经打开了IO，就调用av_probe_input_buffer2
        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;
    }

	//这个时候 AVProbeData中的buffer为NULL，所以不能通过read_probe读取一定字节，然后判断，
	//故这里实际上是看这个输入文件的后缀名是不是那种没有实现read_probe格式的，如果是就返回
	//不过一般情况都是往下调用av_probe_input_buffer2
    if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
        return score;

	// 打开文件的回调，如果没有单独设置，就调用默认的回调（io_open_default）
    if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
        return ret;

    if (s->iformat)
        return 0;
    //通过调用io_open函数后，已经初始化IO，这时候读取头部信息，判断格式
    return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                 s, 0, s->format_probesize);
}

init_input 中，最关键的流程是，通过调用io_open()函数，初始化AVIOContext，然后再通过调用av_probe_input_buffer2()函数，找到适合的AVInputFormat。 当然，还有一些特殊情况是通过av_probe_input_format2找到AVInputFormat的，所以下面详细分析一下这三个函数的实现。

av_probe_input_format2源码

ff_const59 AVInputFormat *av_probe_input_format2(ff_const59 AVProbeData *pd, int is_opened, int *score_max)
{
    int score_ret;
    //调用av_probe_input_format3 选择出所有AVInputFormat中评分最大的AVInputFormat
    ff_const59 AVInputFormat *fmt = av_probe_input_format3(pd, is_opened, &score_ret);
    //评分如果比预期的差，就不选择这个
    if (score_ret > *score_max) {
        *score_max = score_ret;
        return fmt;
    } else
        return NULL;
}

av_probe_input_format3源码

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;

	//这里和调用avformat_open_input创建AVInputStream没关系，从avformat_open_input调用过来的lpd.buf_size都为0
	//看源码好像和codec有关，但目前不是很清楚，不知道为啥。
	//id3v2是音频文件中的metadata，尤其是mp3文件
    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;
    }

	// 遍历demuxer_list 解封装list，然后找其中最高分的AVInputFormat
	// demuxer_list是在编译期自动生成的一个list
    while ((fmt1 = av_demuxer_iterate(&i))) {
        if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
            continue;
        score = 0;
        // 从每个格式都有自己的标准，因此读取一定的字节，按照格式要求看看是不是和当前输入的文件匹配
        // 但是因为默认的buf_size为0，所以这里也就没有办法判断
        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;
                }
            }
            //如果没有read_probe，就通过后缀名来判断
        } 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       = (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;
}

以上是av_probe_input_format2()的实现，后续还会调用这个方法来确定输入的AVInputFormat类型，所以比较重要。

io_open_default源码

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 %s\n", 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

	//最终是调用到ffio_open_whitelist，这个方法
    return ffio_open_whitelist(pb, url, flags, &s->interrupt_callback, options, s->protocol_whitelist, s->protocol_blacklist);
}

上面就简单的进行了一下设置，关键的实现还是在ffio_open_whitelist()这个实现中。

ffio_open_whitelist源码

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;

    *s = NULL;

	// 根据filename初始化 URLContext
    err = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);
    if (err < 0)
        return err;
    // 根据URLContext 设置AVIOContext
    err = ffio_fdopen(s, h);
    if (err < 0) {
        ffurl_close(h);
        return err;
    }
    return 0;
}

ffio_open_whitelist主要就分为两步
第一步是调用ffurl_open_whitelist函数，根据filename找到 URLProtocol，然后创建对应的URLContext，并初步connect。
第二步是调用ffio_fdopen函数，根据上一步创建的URLContext 创建出AVIOContext。
主要还是在第一步，下面详细看一下这两个函数的具体实现。

ffurl_open_whitelist源码

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;
    // 这里根据filename找到URLProtocol并生成URLContext。
    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;

	//调用open方法，初步实现连接，不同协议，使用的方式不一样
    ret = ffurl_connect(*puc, options);

    if (!ret)
        return 0;
fail:
    ffurl_closep(puc);
    return ret;
}

ffurl_open_whitelist函数内也是就只有两步，第一步调用ffurl_alloc方法，初始化URLContext。第二步初始化完URLContext之后，调用ffurl_connect看看是否有对应的open方法，比如如果urlcontext是file类型，就用fopen等方法，如果是rtmp等类型，也用对应的方法，具体的实现都在URLProtocol中。
下面详细看一下这两步：

ffurl_alloc源码

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

	// 首先根据filename找到对应的URLProtocol
    p = url_find_protocol(filename);
    // 根据URLProtocol初始化URLContext
    if (p)
       return url_alloc_for_protocol(puc, p, filename, flags, int_cb);

    *puc = NULL;
    return AVERROR_PROTOCOL_NOT_FOUND;
}

下面看看是怎么找的URLProtocol

static const struct URLProtocol *url_find_protocol(const char *filename)
{
    const URLProtocol **protocols;
    //proto_strshi 具体的协议前缀，本地文件的话，用file
    char proto_str[128], proto_nested[128], *ptr;
    //首先根据strspn()函数查找字符串中第一个“非字母或数字”的字符的位置
    //一般来说都是 :  ,比如rtmp协议 rtmp://....  ，https协议  https://.... 
    size_t proto_len = strspn(filename, URL_SCHEME_CHARS);
    int i;

	//判断是不是本地file类型
    if (filename[proto_len] != ':' &&
    	//判断filename的开头不是"subfile,"， 且之后的字符串中没有':'
        (strncmp(filename, "subfile,", 8) || !strchr(filename + proto_len + 1, ':')) ||
        is_dos_path(filename)) // 这个感觉即使判断是不是在windows上 一般windows是 C://...
        strcpy(proto_str, "file");
    else
    	//判断是网络协议，非本地协议，就copy 非字母或数字之前的， 比如rtmp:// 就拷贝rtmp
        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';

	//从url_protocols数组中，获取所有URLProtocol
    protocols = ffurl_get_protocols(NULL, NULL);
    if (!protocols)
        return NULL;
    //然后对这些URLProtocol分别进行判断，看看是不是这个协议
    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);
    if (av_strstart(filename, "https:", NULL) || av_strstart(filename, "tls:", NULL))
        av_log(NULL, AV_LOG_WARNING, "https protocol not found, recompile FFmpeg with "
                                     "openssl, gnutls or securetransport enabled.\n");

    return NULL;
}

从上面的代码分析中，可以看出清楚的匹配流程，每种协议都有自己的URLProtocol，这样就可以根据filename找到对应的URLProtocol了。

找到URLProtocol之后，就是根据URLProtocol来初始化URLContext了，调用的url_alloc_for_protocol函数。

url_alloc_for_protocol源码

static int url_alloc_for_protocol(URLContext **puc, const URLProtocol *up,
                                  const char *filename, int flags,
                                  const AVIOInterruptCB *int_cb)
{
    URLContext *uc;
    int err;

#if CONFIG_NETWORK
    if (up->flags & URL_PROTOCOL_FLAG_NETWORK && !ff_network_init())
        return AVERROR(EIO);
#endif
	//用标识和相应的协议验证，如果要求只读，那么对应的协议必须实现url_read，其余同理
    if ((flags & AVIO_FLAG_READ) && !up->url_read) {
        av_log(NULL, AV_LOG_ERROR,
               "Impossible to open the '%s' protocol for reading\n", up->name);
        return AVERROR(EIO);
    }
    if ((flags & AVIO_FLAG_WRITE) && !up->url_write) {
        av_log(NULL, AV_LOG_ERROR,
               "Impossible to open the '%s' protocol for writing\n", up->name);
        return AVERROR(EIO);
    }
    uc = av_mallocz(sizeof(URLContext) + strlen(filename) + 1);
    if (!uc) {
        err = AVERROR(ENOMEM);
        goto fail;
    }
    //设置参数
    uc->av_class = &ffurl_context_class;
    uc->filename = (char *)&uc[1];
    strcpy(uc->filename, filename);
    uc->prot            = up;
    uc->flags           = flags;
    uc->is_streamed     = 0; /* default = not streamed */
    uc->max_packet_size = 0; /* default: stream file */
    if (up->priv_data_size) {
        uc->priv_data = av_mallocz(up->priv_data_size);
        if (!uc->priv_data) {
            err = AVERROR(ENOMEM);
            goto fail;
        }
        if (up->priv_data_class) {
            char *start;
            *(const AVClass **)uc->priv_data = up->priv_data_class;
            av_opt_set_defaults(uc->priv_data);
            if (av_strstart(uc->filename, up->name, (const char**)&start) && *start == ',') {
                int ret= 0;
                char *p= start;
                char sep= *++p;
                char *key, *val;
                p++;

                if (strcmp(up->name, "subfile"))
                    ret = AVERROR(EINVAL);

                while(ret >= 0 && (key= strchr(p, sep)) && p<key && (val = strchr(key+1, sep))){
                    *val= *key= 0;
                    if (strcmp(p, "start") && strcmp(p, "end")) {
                        ret = AVERROR_OPTION_NOT_FOUND;
                    } else
                        ret= av_opt_set(uc->priv_data, p, key+1, 0);
                    if (ret == AVERROR_OPTION_NOT_FOUND)
                        av_log(uc, AV_LOG_ERROR, "Key '%s' not found.\n", p);
                    *val= *key= sep;
                    p= val+1;
                }
                if(ret<0 || p!=key){
                    av_log(uc, AV_LOG_ERROR, "Error parsing options string %s\n", start);
                    av_freep(&uc->priv_data);
                    av_freep(&uc);
                    err = AVERROR(EINVAL);
                    goto fail;
                }
                memmove(start, key+1, strlen(key));
            }
        }
    }
    if (int_cb)
        uc->interrupt_callback = *int_cb;

    *puc = uc;
    return 0;
fail:
    *puc = NULL;
    if (uc)
        av_freep(&uc->priv_data);
    av_freep(&uc);
#if CONFIG_NETWORK
    if (up->flags & URL_PROTOCOL_FLAG_NETWORK)
        ff_network_close();
#endif
    return err;
}

初始化完成后，就可以得到URLContext变量了，接下来就需要回到ffurl_open_whitelist函数中，继续调用ffurl_connect函数。

ffurl_connect源码

int ffurl_connect(URLContext *uc, AVDictionary **options)
{
    int err;
    AVDictionary *tmp_opts = NULL;
    AVDictionaryEntry *e;

    if (!options)
        options = &tmp_opts;

    // Check that URLContext was initialized correctly and lists are matching if set
    av_assert0(!(e=av_dict_get(*options, "protocol_whitelist", NULL, 0)) ||
               (uc->protocol_whitelist && !strcmp(uc->protocol_whitelist, e->value)));
    av_assert0(!(e=av_dict_get(*options, "protocol_blacklist", NULL, 0)) ||
               (uc->protocol_blacklist && !strcmp(uc->protocol_blacklist, e->value)));

	//这里先对黑白名单进行判断，看看我们找到的协议是否在里面
    if (uc->protocol_whitelist && av_match_list(uc->prot->name, uc->protocol_whitelist, ',') <= 0) {
        av_log(uc, AV_LOG_ERROR, "Protocol '%s' not on whitelist '%s'!\n", uc->prot->name, uc->protocol_whitelist);
        return AVERROR(EINVAL);
    }

    if (uc->protocol_blacklist && av_match_list(uc->prot->name, uc->protocol_blacklist, ',') > 0) {
        av_log(uc, AV_LOG_ERROR, "Protocol '%s' on blacklist '%s'!\n", uc->prot->name, uc->protocol_blacklist);
        return AVERROR(EINVAL);
    }

    if (!uc->protocol_whitelist && uc->prot->default_whitelist) {
        av_log(uc, AV_LOG_DEBUG, "Setting default whitelist '%s'\n", uc->prot->default_whitelist);
        uc->protocol_whitelist = av_strdup(uc->prot->default_whitelist);
        if (!uc->protocol_whitelist) {
            return AVERROR(ENOMEM);
        }
    } else if (!uc->protocol_whitelist)
        av_log(uc, AV_LOG_DEBUG, "No default whitelist set\n"); // This should be an error once all declare a default whitelist

    if ((err = av_dict_set(options, "protocol_whitelist", uc->protocol_whitelist, 0)) < 0)
        return err;
    if ((err = av_dict_set(options, "protocol_blacklist", uc->protocol_blacklist, 0)) < 0)
        return err;

	//这里是关键，调用对应协议的url_open方法，现在测试的是本地文件，所以用到的是ff_file_protocol的uc->prot->url_open方法。
    err =
        uc->prot->url_open2 ? uc->prot->url_open2(uc,
                                                  uc->filename,
                                                  uc->flags,
                                                  options) :
        uc->prot->url_open(uc, uc->filename, uc->flags);

    av_dict_set(options, "protocol_whitelist", NULL, 0);
    av_dict_set(options, "protocol_blacklist", NULL, 0);

    if (err)
        return err;
    uc->is_connected = 1;
    /* We must be careful here as ffurl_seek() could be slow,
     * for example for http */
    if ((uc->flags & AVIO_FLAG_WRITE) || !strcmp(uc->prot->name, "file"))
        if (!uc->is_streamed && ffurl_seek(uc, 0, SEEK_SET) < 0)
            uc->is_streamed = 1;
    return 0;
}

看一下ff_file_protocol 中url_open()的一些实现

const URLProtocol ff_file_protocol = {
    .name                = "file",
    .url_open            = file_open, //这里是open方法
    .url_read            = file_read,
    .url_write           = file_write,
    .url_seek            = file_seek,
    .url_close           = file_close,
    .url_get_file_handle = file_get_handle,
    .url_check           = file_check,
    .url_delete          = file_delete,
    .url_move            = file_move,
    .priv_data_size      = sizeof(FileContext),
    .priv_data_class     = &file_class,
    .url_open_dir        = file_open_dir,
    .url_read_dir        = file_read_dir,
    .url_close_dir       = file_close_dir,
    .default_whitelist   = "file,crypto,data"
};

static int file_open(URLContext *h, const char *filename, int flags)
{
    FileContext *c = h->priv_data;
    int access;
    int fd;
    struct stat st;

    av_strstart(filename, "file:", &filename);

	//根据flags设置一下打开文件的权限类型
    if (flags & AVIO_FLAG_WRITE && flags & AVIO_FLAG_READ) {
        access = O_CREAT | O_RDWR;
        if (c->trunc)
            access |= O_TRUNC;
    } else if (flags & AVIO_FLAG_WRITE) {
        access = O_CREAT | O_WRONLY;
        if (c->trunc)
            access |= O_TRUNC;
    } else {
        access = O_RDONLY;
    }
#ifdef O_BINARY
    access |= O_BINARY;
#endif
	//打开文件
    fd = avpriv_open(filename, access, 0666);
    if (fd == -1)
        return AVERROR(errno);
    c->fd = fd;

    h->is_streamed = !fstat(fd, &st) && S_ISFIFO(st.st_mode);

    /* Buffer writes more than the default 32k to improve throughput especially
     * with networked file systems */
    if (!h->is_streamed && flags & AVIO_FLAG_WRITE)
        h->min_packet_size = h->max_packet_size = 262144;

    if (c->seekable >= 0)
        h->is_streamed = !c->seekable;

    return 0;
}

调用完成之后，我们就完成了通过ffurl_open_whitelist()函数，找到并打开URLContext的任务，接下来就回到ffio_open_whitelist()中，继续接下来的任务：调用ffio_fdopen()函数，通过URLContext初始化AVIOContext。

ffio_fdopen源码

int ffio_fdopen(AVIOContext **s, URLContext *h)
{
    uint8_t *buffer = NULL;
    int buffer_size, max_packet_size;

    max_packet_size = h->max_packet_size;
    if (max_packet_size) {
        buffer_size = max_packet_size; /* no need to bufferize more than one packet */
    } else {
        buffer_size = IO_BUFFER_SIZE;
    }
    if (!(h->flags & AVIO_FLAG_WRITE) && h->is_streamed) {
        if (buffer_size > INT_MAX/2)
            return AVERROR(EINVAL);
        buffer_size *= 2;
    }
    buffer = av_malloc(buffer_size);
    if (!buffer)
        return AVERROR(ENOMEM);

	//创建AVIOContext，其中ffurl_read，ffurl_write，ffurl_seek都是套了一层壳，增加容错，本质还是调用url_read， url_write，url_seek
    *s = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE, h,
                            (int (*)(void *, uint8_t *, int))  ffurl_read,
                            (int (*)(void *, uint8_t *, int))  ffurl_write,
                            (int64_t (*)(void *, int64_t, int))ffurl_seek);
    if (!*s)
        goto fail;

    (*s)->protocol_whitelist = av_strdup(h->protocol_whitelist);
    if (!(*s)->protocol_whitelist && h->protocol_whitelist) {
        avio_closep(s);
        goto fail;
    }
    (*s)->protocol_blacklist = av_strdup(h->protocol_blacklist);
    if (!(*s)->protocol_blacklist && h->protocol_blacklist) {
        avio_closep(s);
        goto fail;
    }
    (*s)->direct = h->flags & AVIO_FLAG_DIRECT;

    (*s)->seekable = h->is_streamed ? 0 : AVIO_SEEKABLE_NORMAL;
    (*s)->max_packet_size = max_packet_size;
    (*s)->min_packet_size = h->min_packet_size;
    if(h->prot) {
        (*s)->read_pause = (int (*)(void *, int))h->prot->url_read_pause;
        (*s)->read_seek  =
            (int64_t (*)(void *, int, int64_t, int))h->prot->url_read_seek;

        if (h->prot->url_read_seek)
            (*s)->seekable |= AVIO_SEEKABLE_TIME;
    }
    (*s)->short_seek_get = (int (*)(void *))ffurl_get_short_seek;
    (*s)->av_class = &ff_avio_class;
    return 0;
fail:
    av_freep(&buffer);
    return AVERROR(ENOMEM);
}

ffio_fdopen函数调用完，创建和打开AVIOContext的流程就完全完成了，下面不忘初心，回到init_input函数中，继续调用av_probe_input_buffer2函数检测数据类型，并生成对应的AVInputFormat。

av_probe_input_buffer2源码

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 < %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';
        }
    }

    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. */
        //读头部数据
        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. */
        // 丢丢转转又回到这个函数，这个函数上面分析过，只不过之前调用的时候，并没有读数据
        // 所以没发通过数据的头部信息来判断是否是对应格式，而这此读取了部分头部数据，可以凭此判断了。
        *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;
}

到此为止，算是简单的把init_input()函数过了一遍，得到了一个AVInputFormat，和一个AVIOContext。

下面要调用s->iformat->read_header创建AVStream，测试调用的是本地Mp4文件，我们也解析这种格式。 read_header会解析mp4的所有box，从中提取信息，并凭借这些创建AVStream。
mp4对应的AVInputFormat在 mov.c文件中

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,psp,m4b,ism,ismv,isma,f4v",
    .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 | AVFMT_SEEK_TO_PTS,
};

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");
            goto fail;
        }
    } 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");
        err = AVERROR_INVALIDDATA;
        goto fail;
    }
    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->internal->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)
                    goto fail;
            }
        }
        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);
                    err = AVERROR_INVALIDDATA;
                    goto fail;
                }
                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);
                    err = AVERROR_INVALIDDATA;
                    goto fail;
                }
                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)
                goto fail;
            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)
                    goto fail;

                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)
                    goto fail;

                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)
                    goto fail;

                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)
                    goto fail;

                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)
                    goto fail;

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

    for (i = 0; i < mov->frag_index.nb_items; i++)
        if (mov->frag_index.item[i].moof_offset <= mov->fragment.moof_offset)
            mov->frag_index.item[i].headers_read = 1;

    return 0;
fail:
    mov_read_close(s);
    return err;
}

完成这个后，基本就完成了avformat_open_input()流程。