Sets the default temp directory.
A pathname or
set-temp-directory sets the default temp directory, that is the directory that get-temp-directory returns, and which is also used by create-temp-file and open-temp-file.
temp-dir must be either a pathname of a suitable directory, or
nil, which means use the default. The default is what the Operating System returns for a temp directory.
set-temp-directory affects the global setting, that is all threads, and it is not thread-safe. If you need to call it, do that during start up. When you want to use temp files not in the default temp directory, you should call open-temp-file or create-temp-file with a suitable directory argument.
LispWorks User Guide and Reference Manual - 13 Feb 2015