BITD the one "fd sanitizer" I ever encountered was "try using the code on VxWorks" which at the time was "posix inspired" at best - fds actually were pointers, so effectively random and not small integers. It didn't catch enough things to be worth the trouble, but it did clean up some network code (ISTR I was working on SNTP and Kerberos v4 and Kerberized FTP when I ran into this...)