Is there a cross-platform way to reliably find the stdout file descriptor with ctypes?

As so often when it comes to C, if you want compatibility, you'll have to go and look in the relevant standard. Since you mention windows, I guess you're not actually wanting the POSIX standard, but rather the C one.

C99 section 7,19,1 defines stdout to be a macro, and thus not a variable. That means there's no way you can rely on finding it using dlsym (which I assume in_dll uses). The actual expression could just as well be a function call or a fixed address. Perhaps not very likely, but it is possible...

As said in the comments, the fileno function is in turn defined by POSIX, not by C. C has no concept of file descriptors. I think you're better off assuming POSIX and just checking for the value 1, which it specifies.

If you're simply interested in making things work, rather than strict standards adherence (like me), you can find the "real" name of stdout by writing a simple C snippet:

echo -e '#include <stdio.h>\nFILE* mystdout = stdout;' > test.c
cpp test.c | tail

Gives you the output:

FILE* mystdout = __stdoutp;

This means that you also need to try ctypes.c_void_p.in_dll(libc, '__stdoutp') to cover the case of darwin.