routine sets the process's file mode creation mask to
and returns the previous value of the mask.
The 9 low-order access permission bits of
are used by system calls, including
to turn off corresponding bits requested in the file mode
This clearing allows each user to restrict the default access to his files.
The default mask value is S_IWGRP|S_IWOTH (022, write access for the
Child processes inherit the mask of the calling process.
The previous value of the file mode mask is returned by the call.