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).
The constants that should be used to specify mask are described under
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
SVr4, 4.3BSD, POSIX.1-2001.
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)).
chmod(2), mkdir(2), open(2), stat(2), acl(5)
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