Name

umask — set file mode creation mask

Synopsis

#include <sys/stat.h>
mode_t umask( mode_t mask);
 

DESCRIPTION

umask() sets the calling process's file mode creation mask (umask) to mask & 0777 (i.e., only the file permission bits of mask are used), and returns the previous value of the mask.

The umask is used by open(2), mkdir(2), and other system calls that create files to modify the permissions placed on newly created files or directories. Specifically, permissions in the umask are turned off from the mode argument to open(2) and mkdir(2).

Alternatively, if the parent directory has a default ACL (see acl(5)), the umask is ignored, the default ACL is inherited, the permission bits are set based on the inherited ACL, and permission bits absent in the mode argument are turned off. For example, the following default ACL is equivalent to a umask of 022:

u::rwx,g::r-x,o::r-x

Combining the effect of this default ACL with a mode argument of 0666 (rw-rw-rw-), the resulting file permissions would be 0644 (rw-r--r--).

The constants that should be used to specify mask are described in inode(7).

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

S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH

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

S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH

(because 0666 & ~022 = 0644; i.e., rw−r−−r−−).

RETURN VALUE

This system call always succeeds and the previous value of the mask is returned.

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES

A child process created via fork(2) inherits its parent's umask. The umask is left unchanged by execve(2).

It is impossible to use umask() to fetch a process's umask without at the same time changing it. A second call to umask() would then be needed to restore the umask. The nonatomicity of these two steps provides the potential for races in multithreaded programs.

Since Linux 4.7, the umask of any process can be viewed via the Umask field of /proc/[pid]/status. Inspecting this field in /proc/self/status allows a process to retrieve its umask without at the same time changing it.

The umask setting also affects the permissions assigned to POSIX IPC objects (mq_open(3), sem_open(3), shm_open(3)), FIFOs (mkfifo(3)), and UNIX domain sockets (unix(7)) created by the process. The umask does not affect the permissions assigned to System V IPC objects created by the process (using msgget(2), semget(2), shmget(2)).

SEE ALSO

chmod(2), mkdir(2), open(2), stat(2), acl(5)

COLOPHON

This page is part of release 5.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  Copyright (c) 2006, 2008, Michael Kerrisk <mtk.manpagesgmail.com>
(A few fragments remain from an earlier (1992) version written in
1992 by Drew Eckhardt <drewcs.colorado.edu>.)

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

Modified by Michael Haardt <michaelmoria.de>
Modified Sat Jul 24 12:51:53 1993 by Rik Faith <faithcs.unc.edu>
Modified Tue Oct 22 22:39:04 1996 by Eric S. Raymond <esrthyrsus.com>
Modified Thu May  1 06:05:54 UTC 1997 by Nicolás Lichtmaier
 <nickdebian.com> with Lars Wirzenius <liwiki.fi> suggestion
2006-05-13, mtk, substantial rewrite of description of 'mask'
2008-01-09, mtk, a few rewrites and additions.