Gets the directory to use for temporary files.
On UNIX, this is taken from the
TMPDIR environment variable.
If the variable is not set,
used, as defined by the system C library. Failing that, a
hard-coded default of “/tmp” is returned.
On Windows, the
TEMP environment variable is used, with the
root directory of the Windows installation (eg: “C:") used
as a default.
The encoding of the returned string is system-defined. On Windows,
it is always UTF-8. The return value is never
NULL or the empty string.