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
(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), 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.