UMASK(2) Linux Programmer's Manual UMASK(2)
NAME
umask, umask2 - get and set file mode creation mask
SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
mode_t umask(mode_t mask);
#define _GNU_SOURCE
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
mode_t umask2(mode_t mask, int flags);
DESCRIPTION
umask() sets the calling process'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.
If flags is 0, then umask2() is the same as umask().
If flags is UMASK_GET_MASK then umask2() ignores the mask parameter and
returns the process's current umask. The process's current mask is not
modified in this case.
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:
u::rwx,g::r-x,o::r-x
Combining the effect of this default ACL with a mode argument of 0666
(rw-rw-rw-), the resulting file permissions would be 0644 (rw-r--r--).
The constants that should be used to specify mask are described under
stat(2).
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 as:
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--).
RETURN VALUE
The umask() system call always succeeds and the previous value of the
mask is returned.
The umask2() system call returns the process's current umask on suc‐
cess. On error it returns -1 and sets errno appropriately.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
A child process created via fork(2) inherits its parent's umask. The
umask is left unchanged by execve(2).
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)).
SEE ALSO
chmod(2), mkdir(2), open(2), stat(2), acl(5)
COLOPHON
This page is part of release 4.00 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2016-04-13 UMASK(2)
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