UMASK(2) Linux Programmer's Manual UMASK(2)
umask - set file mode creation mask
mode_t umask(mode_t mask);
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.
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 (rw-r--r--).
The constants that should be used to specify mask are described in
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
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--).
This system call always succeeds and the previous value of the mask is
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
A child process created via fork(2) inherits its parent's umask. The
umask is left unchanged by execve(2).
It is impossible to use umask() to fetch a process'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 pro-
vides the potential for races in multithreaded programs.
Since Linux 4.7, the umask of any process can be viewed via the Umask
field of /proc/[pid]/status. Inspecting this field in /proc/self/sta-
tus allows a process to retrieve its umask without at the same time
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)).
chmod(2), mkdir(2), open(2), stat(2), acl(5)
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Linux 2017-09-15 UMASK(2)
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