int ffurl_alloc(URLContext **puc, const char *filename, int flags,
                const AVIOInterruptCB *int_cb)
{
    URLProtocol *up = NULL;
    char proto_str[128], proto_nested[128], *ptr;
    size_t proto_len = strspn(filename, URL_SCHEME_CHARS);

    if (filename[proto_len] != ':' || 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';

    while (up = ffurl_protocol_next(up)) {
        if (!strcmp(proto_str, up->name))
            return url_alloc_for_protocol(puc, up, filename, flags, int_cb);
        if (up->flags & URL_PROTOCOL_FLAG_NESTED_SCHEME &&
            !strcmp(proto_nested, up->name))
            return url_alloc_for_protocol(puc, up, filename, flags, int_cb);
    }
    *puc = NULL;
    return AVERROR_PROTOCOL_NOT_FOUND;
}