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
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 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--).
This system call always succeeds and the previous value of the mask is returned.
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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Linux 2008-01-09 UMASK(2)