For Windows use wcslen(widename) and wcsrchr(widename, L'\\'). The latter returns a wchar_t *. The Windows code also needs to look for forward slash.

You could separate the Windows and POSIX checks using a boolean. For example:

if (S_ISREG(fdfstat.st_mode)) {
    int trailing_slash = 0;
#ifdef MS_WINDOWS
    size_t len = wcslen(widename);
    if (len && (widename[len - 1] == L'\\' || widename[len - 1] == L'/')) {
        trailing_slash = 1;
    }
#else
    size_t len = strlen(name);
    if (len && name[len - 1] == '/') {
        trailing_slash = 1;
    }
#endif
    if (trailing_slash) {
        errno = ENOTDIR;
        PyErr_SetFromErrnoWithFilenameObject(PyExc_OSError, nameobj);
        goto error;
    }
}

widename and name can be NULL at this point.

I cannot imagine how, at this point (as the first call when they must be defined should have returned the error THEN), but yes, since you could conceiveable get to this code without having to call open() - because it was already opened.

will prefix it all that name || widename respectively, is not NULL. If they are, they must have already passed successfully.

@aixtools, Python's open function accepts an integer FD instead of a filename. See my comment above. There's no need to check name and widename separately. Just check the common fd value.

tomorrow then. But if there is a fd already - should I care? assume the file was opened by an extern module. If that has accepted trailing slash, I, as python, no longer care. I, python, only care when I am doing an open() using a (wide)name.

You've misunderstood me. The fd variable is not self->fd. Please re-read the function from the top. It's assigned as fd = _PyLong_AsInt(nameobj). If Python open was called with an integer fd, then this variable will be 0 or greater. If it's less than 0, and we've made it this far, then we know that open was called with a filename, not an fd. This is all you have to test. You do not have to check whether name or widename is NULL.

Change this condition to fd < 0 && S_ISREG(fdfstat.st_mode) to ignore opening a file descriptor, so we know widename / name aren't NULL.

a) If fd < 0 I do not think we are even in this block. See lines 432-449
+432 Py_BEGIN_ALLOW_THREADS
+433 fstat_result = _Py_fstat_noraise(self->fd, &fdfstat);
+434 Py_END_ALLOW_THREADS
+435 if (fstat_result < 0) {
+436 /* Tolerate fstat() errors other than EBADF. See Issue #25717, where
+437 an anonymous file on a Virtual Box shared folder filesystem would
+438 raise ENOENT. */
+439 #ifdef MS_WINDOWS
+440 if (GetLastError() == ERROR_INVALID_HANDLE) {
+441 PyErr_SetFromWindowsErr(0);
+442 #else
+443 if (errno == EBADF) {
+444 PyErr_SetFromErrno(PyExc_OSError);
+445 #endif
+446 goto error;
+447 }
+448 }
+449 else {
b) afaik, if fd<0 the open() call did not succeed and the old logic will report "something"