man2/umask.2

   1 .\" Copyright (c) 2006, 2008, Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\" (A few fragments remain from an earlier (1992) version written in
   3 .\" 1992 by Drew Eckhardt <drew@cs.colorado.edu>.)
   4 .\"
   5 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   6 .\"
   7 .\" Modified by Michael Haardt <michael@moria.de>
   8 .\" Modified Sat Jul 24 12:51:53 1993 by Rik Faith <faith@cs.unc.edu>
   9 .\" Modified Tue Oct 22 22:39:04 1996 by Eric S. Raymond <esr@thyrsus.com>
  10 .\" Modified Thu May  1 06:05:54 UTC 1997 by Nicolás Lichtmaier
  11 .\"  <nick@debian.com> with Lars Wirzenius <liw@iki.fi> suggestion
  12 .\" 2006-05-13, mtk, substantial rewrite of description of 'mask'
  13 .\" 2008-01-09, mtk, a few rewrites and additions.
  14 .TH UMASK 2 2021-03-22 "Linux" "Linux Programmer's Manual"
  15 .SH NAME
  16 umask \- set file mode creation mask
  17 .SH LIBRARY
  18 Standard C library
  19 .RI ( libc ", " \-lc )
  20 .SH SYNOPSIS
  21 .nf
  22 .B #include <sys/stat.h>
  23 .PP
  24 .BI "mode_t umask(mode_t " mask );
  25 .fi
  26 .SH DESCRIPTION
  27 .BR umask ()
  28 sets the calling process's file mode creation mask (umask) to
  29 .I mask
  30 & 0777 (i.e., only the file permission bits of
  31 .I mask
  32 are used), and returns the previous value of the mask.
  33 .PP
  34 The umask is used by
  35 .BR open (2),
  36 .BR mkdir (2),
  37 and other system calls that create files
  38 .\" e.g., mkfifo(), creat(), mknod(), sem_open(), mq_open(), shm_open()
  39 .\" but NOT the System V IPC *get() calls
  40 to modify the permissions placed on newly created files or directories.
  41 Specifically, permissions in the umask are turned off from
  42 the
  43 .I mode
  44 argument to
  45 .BR open (2)
  46 and
  47 .BR mkdir (2).
  48 .PP
  49 Alternatively, if the parent directory has a default ACL (see
  50 .BR acl (5)),
  51 the umask is ignored, the default ACL is inherited,
  52 the permission bits are set based on the inherited ACL,
  53 and permission bits absent in the
  54 .I mode
  55 argument are turned off.
  56 For example, the following default ACL is equivalent to a umask of 022:
  57 .PP
  58 .in +4n
  59 .EX
  60 u::rwx,g::r-x,o::r-x
  61 .EE
  62 .in
  63 .PP
  64 Combining the effect of this default ACL with a
  65 .I mode
  66 argument of 0666 (rw-rw-rw-), the resulting file permissions would be 0644
  67 (rw-r--r--).
  68 .PP
  69 The constants that should be used to specify
  70 .I mask
  71 are described in
  72 .BR inode (7).
  73 .PP
  74 The typical default value for the process umask is
  75 .I S_IWGRP\ |\ S_IWOTH
  76 (octal 022).
  77 In the usual case where the
  78 .I mode
  79 argument to
  80 .BR open (2)
  81 is specified as:
  82 .PP
  83 .in +4n
  84 .EX
  85 S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH
  86 .EE
  87 .in
  88 .PP
  89 (octal 0666) when creating a new file, the permissions on the
  90 resulting file will be:
  91 .PP
  92 .in +4n
  93 .EX
  94 S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH
  95 .EE
  96 .in
  97 .PP
  98 (because 0666 & \(ti022 = 0644; i.e., rw\-r\-\-r\-\-).
  99 .SH RETURN VALUE
 100 This system call always succeeds and the previous value of the mask
 101 is returned.
 102 .SH CONFORMING TO
 103 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 104 .SH NOTES
 105 A child process created via
 106 .BR fork (2)
 107 inherits its parent's umask.
 108 The umask is left unchanged by
 109 .BR execve (2).
 110 .PP
 111 It is impossible to use
 112 .BR umask ()
 113 to fetch a process's umask without at the same time changing it.
 114 A second call to
 115 .BR umask ()
 116 would then be needed to restore the umask.
 117 The nonatomicity of these two steps provides the potential
 118 for races in multithreaded programs.
 119 .PP
 120 Since Linux 4.7, the umask of any process can be viewed via the
 121 .I Umask
 122 field of
 123 .IR /proc/ pid /status .
 124 Inspecting this field in
 125 .I /proc/self/status
 126 allows a process to retrieve its umask without at the same time changing it.
 127 .PP
 128 The umask setting also affects the permissions assigned to POSIX IPC objects
 129 .RB ( mq_open (3),
 130 .BR sem_open (3),
 131 .BR shm_open (3)),
 132 FIFOs
 133 .RB ( mkfifo (3)),
 134 and UNIX domain sockets
 135 .RB ( unix (7))
 136 created by the process.
 137 The umask does not affect the permissions assigned
 138 to System\ V IPC objects created by the process (using
 139 .BR msgget (2),
 140 .BR semget (2),
 141 .BR shmget (2)).
 142 .SH SEE ALSO
 143 .BR chmod (2),
 144 .BR mkdir (2),
 145 .BR open (2),
 146 .BR stat (2),
 147 .BR acl (5)