If
umask(2)
is not implemented
on your system and you are trying to restrict access for yourself (i.e.,
(EXPR & 0700) > 0), produces a fatal error at run time. If
umask(2)
is not implemented and you are not trying to restrict access for yourself, returns
undef.
Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also oct, if all you have is a string.