umask (2)
Leading comments
Copyright (c) 2006, 2008, Michael Kerrisk <mtk.manpages@gmail.com> (A few fragments remain from an earlier (1992) version written in 1992 by Drew Eckhardt <drew@cs.colorado.edu>.) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provi...
NAME
umask - set file mode creation maskSYNOPSIS
#include <sys/types.h>#include <sys/stat.h>
mode_t umask(mode_t mask);
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.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
This system call always succeeds and the previous value of the mask is returned.CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.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)).