umask(2) — Linux manual page

umask(2)                   System Calls Manual                  umask(2)

NAME

       umask - set file mode creation mask

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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
       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--).

RETURN VALUE

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

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, SVr4, 4.3BSD.

NOTES

       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

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

COLOPHON

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Linux man-pages 6.9.1          2024-05-02                       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)

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