static BOOL get_unix_path(ANSI_STRING *unix_name, const char *filename)
{
    UNICODE_STRING dospathW, ntpathW;
    ANSI_STRING dospath;
    NTSTATUS status;

    RtlInitAnsiString(&dospath, filename);

    if (RtlAnsiStringToUnicodeString(&dospathW, &dospath, TRUE))
        return FALSE;

    if (!RtlDosPathNameToNtPathName_U(dospathW.Buffer, &ntpathW, NULL, NULL))
    {
        RtlFreeUnicodeString(&dospathW);
        return FALSE;
    }

    status = wine_nt_to_unix_file_name(&ntpathW, unix_name, FILE_OPEN, FALSE);

    RtlFreeUnicodeString(&ntpathW);
    RtlFreeUnicodeString(&dospathW);
    return !status;
}


Usage:
    ANSI_STRING unix_name;

    if (!get_unix_path(&unix_name, pszFileName))
        return ... // handle error

    // unix_name.Buffer contains unix path

    RtlFreeAnsiString(&unix_name);