umask(2) — Linux manual page


UMASK(2)                  Linux Programmer's Manual                 UMASK(2)

NAME         top

       umask - set file mode creation mask

SYNOPSIS         top

       #include <sys/types.h>
       #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), 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:


       Combining the effect of this default ACL with a mode argument of 0666
       (rw-rw-rw-), the resulting file permissions would be 0644 (rw-

       The constants that should be used to specify mask are described in

       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:


       (octal 0666) when creating a new file, the permissions on the resultā€
       ing 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.

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES         top

       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)).

SEE ALSO         top

       chmod(2), mkdir(2), open(2), stat(2), acl(5)

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-08-13                         UMASK(2)

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