umask - set file mode creation mask
Standard C library (libc, -lc)
#include <sys/stat.h>
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:
u::rwx,g::r-x,o::r-xCombining 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 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 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--).
This system call always succeeds and the previous value of the mask is returned.
POSIX.1-2008.
POSIX.1-2001, 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 provides 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/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)).