2.9BSD/usr/man/man2/umask.2

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.TH UMASK 2
.SH NAME
umask \- set file creation mode mask
.SH SYNOPSIS
.B umask(complmode)
.SH DESCRIPTION
.I Umask
sets a mask used whenever a file is created by
.IR creat (2)
or
.IR mknod (2):
the actual mode (see
.IR chmod (2))
of the newly-created file is the logical AND
of the given mode and the complement of the argument.
Only the low-order 9 bits of the mask (the protection bits)
participate.
In other words, the mask shows the bits to be turned off
when files are created.
The mask is inherited by child processes.
The value is initially 0 (this is a system configuration
parameter).
.SH RETURN VALUE
The previous value of the mask is returned by the call.
.SH SEE ALSO
chmod(2), creat(2), mknod(2)
.SH ASSEMBLER
(umask = 60.)
.br
.B sys umask; complmode