Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask.
The Unix permission rwxr-x--- is represented as three sets of three bits, or three octal digits: 0750 (the leading 0 indicates octal and isn't one of the digits). The umask value is such a number representing disabled permissions bits. The permission (or "mode") values you pass mkdir or sysopen are modified by your umask, so even if you tell sysopen to create a file with permissions 0777, if your umask is 0022, then the file will actually be created with permissions 0755. If your umask were 0027 (group can't write; others can't read, write, or execute), then passing sysopen 0666 would create a file with mode 0640 (because 0666 &~ 027 is 0640).
Here's some advice: supply a creation mode of 0666 for regular files (in sysopen) and one of 0777 for directories (in mkdir) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better left to the user. The exception to this is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on.
If umask(2) is not implemented on your system and you are trying to restrict access for yourself (i.e., (EXPR & 0700) > 0), raises an exception. 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.