int url_open(URLContext **puc, const char *filename, int flags)
{
    URLProtocol *up;
    const char *p;
    char proto_str[128], *q;

    p = filename;
    q = proto_str;
    while (*p != '\0' && *p != ':') {
        /* protocols can only contain alphabetic chars */
        if (!isalpha(*p))
            goto file_proto;
        if ((q - proto_str) < sizeof(proto_str) - 1)
            *q++ = *p;
        p++;
    }
    /* if the protocol has length 1, we consider it is a dos drive */
    if (*p == '\0' || (q - proto_str) <= 1) {
    file_proto:
        strcpy(proto_str, "file");
    } else {
        *q = '\0';
    }

    up = first_protocol;
    while (up != NULL) {
        if (!strcmp(proto_str, up->name))
            return url_open_protocol (puc, up, filename, flags);
        up = up->next;
    }
    *puc = NULL;
    return AVERROR(ENOENT);
}