Section (2) umask
umask — set file mode creation mask
#include <sys/types.h> #include <sys/stat.h>
umask() sets the calling
process_zsingle_quotesz_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:
Combining the effect of this default ACL with a
mode argument of 0666
(rw-rw-rw-), the resulting file permissions would be 0644
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
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−−).
It is impossible to use
umask() to fetch a process_zsingle_quotesz_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
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)).
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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 _zsingle_quotesz_mask_zsingle_quotesz_
2008-01-09, mtk, a few rewrites and additions.