FFmpeg中的关键方法及结构体（二）avformat_open_input

本文主要是介绍FFmpeg中的关键方法及结构体（二）avformat_open_input，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

avformat_open_input

该方法声明在libavformat/avformat.h:2093

int avformat_open_input(AVFormatContext **ps, const char *url,
                        const AVInputFormat *fmt, AVDictionary **options);

方法实现位于libavformat/demux.c:207，该方法主要用来选择IO以及解复用组件

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

    if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);
    // 通过强转来获得一个FFFormatContext类型的指针
    si = ffformatcontext(s);
    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);
    }
    // 是否指定AVInputFormat，如果指定的话就使用指定的Format
    if (fmt)
        s->iformat = fmt;

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

    // 这个pb的类型为AVIOContext，pb不为null表示使用自定义的IO
    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;
　　 // 拷贝url
    if (!(s->url = av_strdup(filename ? filename : ""))) {
        ret = AVERROR(ENOMEM);
        goto fail;
    }
    // 调用init_input方法来初始化，主要做了两件事选择AVIOContext、选择AVInputFormat
    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 an AVIOContext */
    if (s->pb)
        ff_id3v2_read_dict(s->pb, &si->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);

    if (s->iformat->read_header)
        if ((ret = s->iformat->read_header(s)) < 0) {
            if (s->iformat->flags_internal & FF_FMT_INIT_CLEANUP)
                goto close;
            goto fail;
        }

    if (!s->metadata) {
        s->metadata    = si->id3v2_meta;
        si->id3v2_meta = NULL;
    } else if (si->id3v2_meta) {
        av_log(s, AV_LOG_WARNING, "Discarding ID3 tags because more suitable tags were found.\n");
        av_dict_free(&si->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 (s->pb && !si->data_offset)
        si->data_offset = avio_tell(s->pb);

    si->raw_packet_buffer_size = 0;

    update_stream_avctx(s);

    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;
}

init_input方法会调用AVFormatContext的io_open方法

static int init_input(AVFormatContext *s, const char *filename,
                      AVDictionary **options)
{
    int ret;
    // 创建一个AVProbeData
    AVProbeData pd = { filename, NULL, 0 };
    int score = AVPROBE_SCORE_RETRY;

    // 默认不使用自己的IO
    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;
    }

    // 如果我们指定了format，那么这里直接返回score
    if ((s->iformat && s->iformat->flags & AVFMT_NOFILE) ||
        (!s->iformat && (s->iformat = av_probe_input_format2(&pd, 0, &score))))
        return score;

    // 否则调用io_open，io_open的实现在libavformat/options.c中，pb的类型为AVIOContext
    if ((ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ | s->avio_flags, options)) < 0)
        return ret;

    if (s->iformat)
        return 0;
　　 // 调用av_probe_input_buffer2去探测IO类型
    return av_probe_input_buffer2(s->pb, &s->iformat, filename,
                                  s, 0, s->format_probesize);
}

io_open方法的实现位于libavformat/options.c，方法实现为io_open_default，用于创建并初始化AVFormatContext中的AVIOContext

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");
    // 这里的url就是前面的filename
    return ffio_open_whitelist(pb, url, flags, &s->interrupt_callback, options, s->protocol_whitelist, s->protocol_blacklist);
}

ffio_open_whitelist声明在libavformat/avio_internal.h:225，实现位于libavformat/aviobuf.c:1225

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;
　　// 调用该方法返回一个URLContext
    err = ffurl_open_whitelist(&h, filename, flags, int_cb, options, whitelist, blacklist, NULL);
    if (err < 0)
        return err;
    // 用URLContext来创建并初始化一个AVIOContext
    err = ffio_fdopen(s, h);
    if (err < 0) {
        ffurl_close(h);
        return err;
    }
    return 0;
}

ffurl_open_whitelist实现位于libavformat/avio.c:306

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;
    // 创建并且初始化一个URLContext对象
    int ret = ffurl_alloc(puc, filename, flags, int_cb);
    if (ret < 0)
        return ret;
    if (parent) {
        ret = av_opt_copy(*puc, parent);
        if (ret < 0)
            goto fail;
    }
    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ffurl_connect，打开url
    ret = ffurl_connect(*puc, options);

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

ffurl_alloc实现在libavformat/avio.c:293

int ffurl_alloc(URLContext **puc, const char *filename, int flags,
                const AVIOInterruptCB *int_cb)
{
    const URLProtocol *p = NULL;
    // 找到合适的protocol
    p = url_find_protocol(filename);
    // 利用找到的protocol来创建一个URLContext
    if (p)
       return url_alloc_for_protocol(puc, p, filename, flags, int_cb);

    *puc = NULL;
    return AVERROR_PROTOCOL_NOT_FOUND;
}

url_find_protocol实现位于libavformat/avio.c:251

static const struct URLProtocol *url_find_protocol(const char *filename)
{
    const URLProtocol **protocols;
    char proto_str[128], proto_nested[128], *ptr;
    // 这个方法比较有意思，是用于查找filename中第一个没有出现在URL_SCHEME_CHARS中的字符的下表，起始就是查找":"
    size_t proto_len = strspn(filename, URL_SCHEME_CHARS);
    int i;

    // 如果filename中没有":"，并且以subfile开头或者没有":"，或者是路径，protocol类型为file
    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';
 
    // 获取所有的protocal
    protocols = ffurl_get_protocols(NULL, NULL);
    if (!protocols)
        return NULL；
    // 遍历protocals列表，对比protocol name
    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;
        }
    }
    // 释放protocols指针
    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;
}

ffurl_get_protocols实现位于libavformat/protocols.c

const URLProtocol **ffurl_get_protocols(const char *whitelist,
                                        const char *blacklist)
{
    const URLProtocol **ret;
    int i, ret_idx = 0;
　　 // 开辟一块内存空间
    ret = av_calloc(FF_ARRAY_ELEMS(url_protocols), sizeof(*ret));
    if (!ret)
        return NULL;

    for (i = 0; url_protocols[i]; i++) {
        const URLProtocol *up = url_protocols[i];

        if (whitelist && *whitelist && !av_match_name(up->name, whitelist))
            continue;
        if (blacklist && *blacklist && av_match_name(up->name, blacklist))
            continue;
　　　　 // 将url_protocals中的地址拷贝到返回值中，它这里做拷贝主要是为了使用白名单和黑名单的筛选功能
        ret[ret_idx++] = up;
    }

    return ret;
}

url_protocols定义在libavformat/protocol_list.c，是一个二级指针，保存着各个protocol的地址

static const URLProtocol * const url_protocols[] = {
    &ff_async_protocol,
    &ff_cache_protocol,
    &ff_concat_protocol,
    &ff_concatf_protocol,
    &ff_crypto_protocol,
    &ff_data_protocol,
    &ff_ffrtmphttp_protocol,
    &ff_file_protocol,
    &ff_ftp_protocol,
    &ff_gopher_protocol,
    &ff_hls_protocol,
    &ff_http_protocol,
    &ff_httpproxy_protocol,
    &ff_icecast_protocol,
    &ff_mmsh_protocol,
    &ff_mmst_protocol,
    &ff_md5_protocol,
    &ff_pipe_protocol,
    &ff_prompeg_protocol,
    &ff_rtmp_protocol,
    &ff_rtmpt_protocol,
    &ff_rtp_protocol,
    &ff_srtp_protocol,
    &ff_subfile_protocol,
    &ff_tee_protocol,
    &ff_tcp_protocol,
    &ff_udp_protocol,
    &ff_udplite_protocol,
    &ff_unix_protocol,
    NULL };

下面以 ff_hls_protocol 和 ff_file_protocol举个例子

URLProtocol这个结构体中有很多函数指针，用于实现多态，在这里不同的protocol会为结构体中的函数指针赋不同的值

ff_hls_protocol 定义在libavformat/hlsproto.c:311

const URLProtocol ff_hls_protocol = {
    .name           = "hls",
    .url_open       = hls_open,
    .url_read       = hls_read,
    .url_close      = hls_close,
    .flags          = URL_PROTOCOL_FLAG_NESTED_SCHEME,
    .priv_data_size = sizeof(HLSContext),
};

ff_file_protocol定义在libavformat/file.c:357

const URLProtocol ff_file_protocol = {
    .name                = "file",
    .url_open            = file_open,
    .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"
};

再回到ffurl_alloc，找到合适的protocol之后，调用url_alloc_for_protocol来创建URLContext

url_alloc_for_protocol实现位于libavformat/avio.c:74

URLContext这个上下文保存有URL相关信息，并保存了获取到的URLProtocol

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
    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;
    }
    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;
}

再回到ffurl_open_whitelist中来，调用ffurl_alloc获取到URLContext之后会调用ffurl_connect来打开url

ffurl_connect实现位于libavformat/avio.c:166

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)));

    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;
　　
　　// 调用URLContext的prot url_open方法，如果有url_open2就调用，没有就调用url_open，该方法定义在URLProtocol中，不同的protocol会有不同的实现
    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;
    // 将URLContext中的连接flag置1
    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;
}

下面以ff_file_protocol的file_open为例子，这是用来打开本地文件的protocol

方法实现位于libavformat/file.c:208

static int file_open(URLContext *h, const char *filename, int flags)
{
    // URLContext中还有一个FileContext，用于保存和文件相关的内容
    FileContext *c = h->priv_data;
    int access;
    int fd;
    struct stat st;

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

    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
    // 调用系统函数open来打开文件，获取文件描述符
    fd = avpriv_open(filename, access, 0666);
    if (fd == -1)
        return AVERROR(errno);
    // 将文件描述符保存到FileContext中
    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;
}

我们再回到init_input中，调用io_open获取到AVIOContext之后，会再调用av_probe_input_buffer2来做探测

av_probe_input_buffer2实现位于libavformat/format.c:225，这里开始就是来探测demux的组件了，也就是AVInputFomat

int av_probe_input_buffer2(AVIOContext *pb, const 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. */
        if ((ret = av_reallocp(&buf, probe_size + AVPROBE_PADDING_SIZE)) < 0)
            goto fail;
        // 调用avio_read来读取buffer
        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;
        // 将读到的数据封装到AVProbeData中
        pd.buf_size = buf_offset - offset;
        pd.buf = &buf[offset];

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

        /* Guess file format. */
        // 调用该方法来创建一个最为合适的 AVInputFormat
        *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;
}

avio_read实现位于libavformat/aviobuf.c:641

int avio_read(AVIOContext *s, unsigned char *buf, int size)
{
    int len, size1;

    size1 = size;
    while (size > 0) {
        len = FFMIN(s->buf_end - s->buf_ptr, size);
        if (len == 0 || s->write_flag) {
            if((s->direct || size > s->buffer_size) && !s->update_checksum && s->read_packet) {
                // bypass the buffer and read data directly into buf
                // 调用AVIOContext的read_packet方法读取数据
                len = read_packet_wrapper(s, buf, size);
                if (len == AVERROR_EOF) {
                    /* do not modify buffer if EOF reached so that a seek back can
                    be done without rereading data */
                    s->eof_reached = 1;
                    break;
                } else if (len < 0) {
                    s->eof_reached = 1;
                    s->error= len;
                    break;
                } else {
                    s->pos += len;
                    ffiocontext(s)->bytes_read += len;
                    s->bytes_read = ffiocontext(s)->bytes_read;
                    size -= len;
                    buf += len;
                    // reset the buffer
                    s->buf_ptr = s->buffer;
                    s->buf_end = s->buffer/* + len*/;
                }
            } else {
                fill_buffer(s);
                len = s->buf_end - s->buf_ptr;
                if (len == 0)
                    break;
            }
        } else {
            memcpy(buf, s->buf_ptr, len);
            buf += len;
            s->buf_ptr += len;
            size -= len;
        }
    }
    if (size1 == size) {
        if (s->error)      return s->error;
        if (avio_feof(s))  return AVERROR_EOF;
    }
    return size1 - size;
}

av_probe_input_format2实现位于libavformat/format.c:128

主要内容就是调用了av_probe_input_format3

const AVInputFormat *av_probe_input_format2(const AVProbeData *pd,
                                            int is_opened, int *score_max)
{
    int score_ret;
    const 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 实现于 libavformat/format.c:207

const AVInputFormat *av_probe_input_format3(const AVProbeData *pd,
                                            int is_opened, int *score_ret)
{
    AVProbeData lpd = *pd;
    const AVInputFormat *fmt1 = NULL;
    const 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;

    // 这是是用来判断是否有id3信息
    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;
    }

    // 调用av_demuxer_iterate来遍历获取demuxer
    while ((fmt1 = av_demuxer_iterate(&i))) {
        if (fmt1->flags & AVFMT_EXPERIMENTAL)
            continue;
        if (!is_opened == !(fmt1->flags & AVFMT_NOFILE) && strcmp(fmt1->name, "image2"))
            continue;
        score = 0;
        if (fmt1->read_probe) {
            // 调用AVInputFormat的read_probe方法来探测并且获取分数
            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;
            }
        }
        // 记录下最高分数和对应的AVInputFormat
        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;
}

av_demuxer_iterate的实现位于libavformat/allformats.c:564，它和前面遍历获取URLContext时很类似

const AVInputFormat *av_demuxer_iterate(void **opaque)
{
    static const uintptr_t size = sizeof(demuxer_list)/sizeof(demuxer_list[0]) - 1;
    uintptr_t i = (uintptr_t)*opaque;
    const AVInputFormat *f = NULL;
    uintptr_t tmp;

    if (i < size) {
        // 从demuxer_list中获取指定项
        f = demuxer_list[i];
    } else if (tmp = atomic_load_explicit(&indev_list_intptr, memory_order_relaxed)) {
        const AVInputFormat *const *indev_list = (const AVInputFormat *const *)tmp;
        f = indev_list[i - size];
    }

    // 将索引加一
    if (f)
        *opaque = (void*)(i + 1);
    return f;
}

demuxer_list声明于libavformat/demuxer_list.c中，由于实在太长，这里就不贴代码了，这是一个二级指针，保存了各个demuxer（AVInputFormat）的地址。下面以ff_mpegts_demuxer为例子看看代码

ff_mpegts_demuxer声明在libavformat/mpegts.c:3422中

const AVInputFormat ff_mpegts_demuxer = {
    .name           = "mpegts",
    .long_name      = NULL_IF_CONFIG_SMALL("MPEG-TS (MPEG-2 Transport Stream)"),
    .priv_data_size = sizeof(MpegTSContext),
    .read_probe     = mpegts_probe,
    .read_header    = mpegts_read_header,
    .read_packet    = mpegts_read_packet,
    .read_close     = mpegts_read_close,
    .read_timestamp = mpegts_get_dts,
    .flags          = AVFMT_SHOW_IDS | AVFMT_TS_DISCONT,
    .priv_class     = &mpegts_class,
};

read_probe方法就是用来探测的

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