The
umask() routine sets the process's file mode creation mask to
numask and returns the previous value of the mask. The 9 low-order access permission bits of
numask are used by system calls, including
open(2),
mkdir(2),
mkfifo(2) and
mknod(2) to turn off corresponding bits requested in file mode. (See
chmod(2)). 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 owner only). Child processes inherit the mask of the calling process.