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