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 2008-01-09 "Linux" "Linux Programmer's Manual"
  35 .SH NAME
  36 umask \- set file mode creation mask
  37 .SH SYNOPSIS
  38 .B #include <sys/types.h>
  39 .br
  40 .B #include <sys/stat.h>
  41 .sp
  42 .BI "mode_t umask(mode_t " mask );
  43 .SH DESCRIPTION
  44 .BR umask ()
  45 sets the calling process's file mode creation mask (umask) to
  46 .I mask
  47 & 0777 (i.e., only the file permission bits of
  48 .I mask
  49 are used), and returns the previous value of the mask.
  50
  51 The umask is used by
  52 .BR open (2),
  53 .BR mkdir (2),
  54 and other system calls that create files
  55 .\" e.g., mkfifo(), creat(), mknod(), sem_open(), mq_open(), shm_open()
  56 .\" but NOT the System V IPC *get() calls
  57 to modify the permissions placed on newly created files or directories.
  58 Specifically, permissions in the umask are turned off from
  59 the \fImode\fP argument to
  60 .BR open (2)
  61 and
  62 .BR mkdir (2).
  63
  64 The constants that should be used to specify
  65 .I mask
  66 are described under
  67 .BR stat (2).
  68
  69 The typical default value for the process umask is
  70 .I S_IWGRP\ |\ S_IWOTH
  71 (octal 022).
  72 In the usual case where the
  73 .I mode
  74 argument to
  75 .BR open (2)
  76 is specified as:
  77 .nf
  78
  79     S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH
  80
  81 .fi
  82 (octal 0666) when creating a new file, the permissions on the
  83 resulting file will be:
  84 .nf
  85
  86     S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH
  87
  88 .fi
  89 (because 0666 & ~022 = 0644; i.e., rw\-r\-\-r\-\-).
  90 .SH RETURN VALUE
  91 This system call always succeeds and the previous value of the mask
  92 is returned.
  93 .SH CONFORMING TO
  94 SVr4, 4.3BSD, POSIX.1-2001.
  95 .SH NOTES
  96 A child process created via
  97 .BR fork (2)
  98 inherits its parent's umask.
  99 The umask is left unchanged by
 100 .BR execve (2).
 101
 102 The umask setting also affects the permissions assigned to POSIX IPC objects
 103 .RB ( mq_open (3),
 104 .BR sem_open (3),
 105 .BR shm_open (3)),
 106 FIFOs
 107 .RB ( mkfifo (3)),
 108 and UNIX domain sockets
 109 .RB ( unix (7))
 110 created by the process.
 111 The umask does not affect the permissions assigned
 112 to System V IPC objects created by the process (using
 113 .BR msgget (2),
 114 .BR semget (2),
 115 .BR shmget (2)).
 116 .SH SEE ALSO
 117 .BR chmod (2),
 118 .BR mkdir (2),
 119 .BR open (2),
 120 .BR stat (2)
 121 .\" FIXME . eventually: .BR acl (5)