| .\" Copyright (c) 2006, 2008, Michael Kerrisk <mtk.manpages@gmail.com> |
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| .\" 1992 by Drew Eckhardt <drew@cs.colorado.edu>.) |
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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 Nicolรกs 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 2017-09-15 "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> |
| .PP |
| .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. |
| .PP |
| 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 |
| .I mode |
| argument to |
| .BR open (2) |
| and |
| .BR mkdir (2). |
| .PP |
| Alternatively, if the parent directory has a default ACL (see |
| .BR 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 |
| .I mode |
| argument are turned off. |
| For example, the following default ACL is equivalent to a umask of 022: |
| .PP |
| u::rwx,g::r-x,o::r-x |
| .PP |
| Combining the effect of this default ACL with a |
| .I mode |
| argument of 0666 (rw-rw-rw-), the resulting file permissions would be 0644 |
| (rw-r--r--). |
| .PP |
| The constants that should be used to specify |
| .I mask |
| are described in |
| .BR inode (7). |
| .PP |
| 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: |
| .PP |
| .in +4n |
| .EX |
| S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH |
| .EE |
| .in |
| .PP |
| (octal 0666) when creating a new file, the permissions on the |
| resulting file will be: |
| .PP |
| .in +4n |
| .EX |
| S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH |
| .EE |
| .in |
| .PP |
| (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 |
| POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. |
| .SH NOTES |
| A child process created via |
| .BR fork (2) |
| inherits its parent's umask. |
| The umask is left unchanged by |
| .BR execve (2). |
| .PP |
| It is impossible to use |
| .BR umask () |
| to fetch a process's umask without at the same time changing it. |
| A second call to |
| .BR umask () |
| would then be needed to restore the umask. |
| The nonatomicity of these two steps provides the potential |
| for races in multithreaded programs. |
| .PP |
| Since Linux 4.7, the umask of any process can be viewed via the |
| .I Umask |
| field of |
| .IR /proc/[pid]/status . |
| Inspecting this field in |
| .IR /proc/self/status |
| allows a process to retrieve its umask without at the same time changing it. |
| .PP |
| 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), |
| .BR acl (5) |