man2/umask.2

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.\" Copyright (c) 2006, 2008, Michael Kerrisk (mtk.manpages@gmail.com)
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.\" Modified by Michael Haardt <michael@moria.de>
.\" Modified Sat Jul 24 12:51:53 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Tue Oct 22 22:39:04 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Thu May  1 06:05:54 UTC 1997 by Nicolás Lichtmaier
.\"  <nick@debian.com> with Lars Wirzenius <liw@iki.fi> suggestion
.\" 2006-05-13, mtk, substantial rewrite of description of 'mask'
.\" 2008-01-09, mtk, a few rewrites and additions.
.TH UMASK 2 2008-01-09 "Linux" "Linux Programmer's Manual"
.SH NAME
umask \- set file mode creation mask
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/stat.h>
.sp
.BI "mode_t umask(mode_t " mask );
.SH DESCRIPTION
.BR umask ()
sets the calling process's file mode creation mask (umask) to
.I mask
& 0777 (i.e., only the file permission bits of
.I mask
are used), and returns the previous value of the mask.

The umask is used by
.BR open (2),
.BR mkdir (2),
and other system calls that create files
.\" e.g., mkfifo(), creat(), mknod(), sem_open(), mq_open(), shm_open()
.\" but NOT the System V IPC *get() calls
to modify the permissions placed on newly created files or directories.
Specifically, permissions in the umask are turned off from
the \fImode\fP argument to
.BR open (2)
and
.BR mkdir (2).

The constants that should be used to specify
.I mask
are described under
.BR stat (2).

The typical default value for the process umask is
.I S_IWGRP\ |\ S_IWOTH
(octal 022).
In the usual case where the
.I mode
argument to
.BR open (2)
is specified as:
.nf

    S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH

.fi
(octal 0666) when creating a new file, the permissions on the
resulting file will be:
.nf

    S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH

.fi
(because 0666 & ~022 = 0644; i.e., rw\-r\-\-r\-\-).
.SH "RETURN VALUE"
This system call always succeeds and the previous value of the mask
is returned.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
.SH NOTES
A child process created via
.BR fork (2)
inherits its parent's umask.
The umask is left unchanged by
.BR execve (2).

The umask setting also affects the permissions assigned to POSIX IPC objects
.RB ( mq_open (3),
.BR sem_open (3),
.BR shm_open (3)),
FIFOs
.RB ( mkfifo (3)),
and UNIX domain sockets
.RB ( unix (7))
created by the process.
The umask does not affect the permissions assigned
to System V IPC objects created by the process (using
.BR msgget (2),
.BR semget (2),
.BR shmget (2)).
.SH "SEE ALSO"
.BR chmod (2),
.BR mkdir (2),
.BR open (2),
.BR stat (2)
.\" FIXME . eventually: .BR acl (5)