Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask.
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.