[Python-Dev] Pervasive socket failures on Windows (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Fri Feb 10 22:00:46 CET 2006

Thomas Wouters wrote:

I doubt it will have problems on other platforms. As Tim said, FDSETSIZE is mandated by POSIX. Perhaps some platforms do allow larger sizes, by replacing the FD* macros with functions that dynamically grow whatever magic is the 'fdset' datatype. I sincerely doubt it's a common approach, though, and for them to be POSIX they would need to have FDSETSIZE set to some semi-sane value. So at worst, on those platforms (if any), we're reducing the number of sockets you can actually select() on, from some undefined platform maximum to whatever the platform claims is the maximum.

I think the Windows interpretation is actually well-designed: FD_SETSIZE shouldn't be the number of the largest descriptor, but instead be the maximum size of the set. So FD_SETSIZE is 64 on Windows, but you still can have much larger file descriptor numbers.

The implementation strategy of Windows is to use an array of integers, rather than the bit mask, and an index telling you how many slots have already been filled. With FD_SETSIZE being 64, the fd_set requires 256 bytes.

This strategy has a number of interesting implications:

In any case, POSIX makes it undefined what FD_SET does when the socket is larger than FD_SETSIZE, and apparently clearly expects an fd_set to be a bit mask.

Regards, Martin

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