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.
Portability issues: "umask" in perlport.