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umask(2) System Calls Manual umask(2) 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-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 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)).
chmod(2), mkdir(2), open(2), stat(2), acl(5)
This page is part of the man-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which is not part of the original manual page), send a mail to [email protected] Linux man-pages 6.15 2025-05-17 umask(2) Pages that refer to this page: git-init(1), systemd-mount(1), clone(2), mkdir(2), mknod(2), open(2), spu_create(2), syscalls(2), unshare(2), fopen(3), mkfifo(3), mkstemp(3), mode_t(3type), shm_open(3), proc_pid_status(5), systemd.exec(5), pthreads(7), signal-safety(7), unix(7)
| HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. | |