Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask.
The Unix permissionrwxr-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). Theumask value is such a number representing disabled permissions bits. The permission (or "mode") values you passmkdir orsysopen are modified by your umask, so even if you tellsysopen to create a file with permissions0777, if your umask is0022, then the file will actually be created with permissions0755. If yourumask were0027 (group can't write; others can't read, write, or execute), then passingsysopen0666 would create a file with mode0640 (because0666 &~ 027 is0640).
Here's some advice: supply a creation mode of0666 for regular files (insysopen) and one of0777 for directories (inmkdir) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of022,027, or even the particularly antisocial mask of077. 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.
Ifumask(2) is not implemented on your system and you are trying to restrict access foryourself (i.e.,(EXPR & 0700) > 0), raises an exception. Ifumask(2) is not implemented and you are not trying to restrict access for yourself, returnsundef.
Remember that a umask is a number, usually given in octal; it isnot a string of octal digits. See alsooct, if all you have is a string.
Portability issues:"umask" in perlport.
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