umask(2) - Linux manual page (original) (raw)

umask(2) System Calls Manual umask(2)

NAME top

   umask - set file mode creation mask

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <sys/stat.h>**

   **mode_t umask(mode_t** _mask_**);**

DESCRIPTION top

   **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)](../man2/open.2.html), [mkdir(2)](../man2/mkdir.2.html), 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)](../man2/open.2.html) and
   [mkdir(2)](../man2/mkdir.2.html).

   Alternatively, if the parent directory has a default ACL (see
   [acl(5)](../man5/acl.5.html)), 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)](../man7/inode.7.html).

   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)](../man2/open.2.html) 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 top

   This system call always succeeds and the previous value of the
   mask is returned.

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001, SVr4, 4.3BSD.

NOTES top

   A child process created via [fork(2)](../man2/fork.2.html) inherits its parent's umask.
   The umask is left unchanged by [execve(2)](../man2/execve.2.html).

   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)](../man3/mq%5Fopen.3.html), [sem_open(3)](../man3/sem%5Fopen.3.html), [shm_open(3)](../man3/shm%5Fopen.3.html)), FIFOs
   ([mkfifo(3)](../man3/mkfifo.3.html)), and UNIX domain sockets ([unix(7)](../man7/unix.7.html)) created by the
   process.  The umask does not affect the permissions assigned to
   System V IPC objects created by the process (using [msgget(2)](../man2/msgget.2.html),
   [semget(2)](../man2/semget.2.html), [shmget(2)](../man2/shmget.2.html)).

SEE ALSO top

   [chmod(2)](../man2/chmod.2.html), [mkdir(2)](../man2/mkdir.2.html), [open(2)](../man2/open.2.html), [stat(2)](../man2/stat.2.html), [acl(5)](../man5/acl.5.html)

COLOPHON top

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   man-pages@man7.org

Linux man-pages 6.10 2024-07-23 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)