The umask is used by open(2) to set initial file permissions on a
newly-created file. Specifically, permissions in the umask are turned off from
the mode argument to open(2) (so, for example, the common umask
default value of 022 results in new files being created with permissions 0666
& ~022 = 0644 = rw-r--r-- in the usual case where the mode is
specified as 0666).