The fastcwd() function looks the same as getcwd(), but runs faster. It's also more dangerous because you might conceivably chdir() out of a directory that you can't chdir() back into.
The cwd() function looks the same as getcwd and fastgetcwd but is implemented using the most natural and safe form for the current architecture. For most systems it is identical to `pwd` (but without the trailing line terminator). It is recommended that cwd (or another *cwd() function) is used in all code to ensure portability.
If you ask to override your chdir() built-in function, then your PWD environment variable will be kept up to date. (See Overriding builtin functions.) Note that it will only be kept up to date it all packages which use chdir import it from Cwd.