Ubuntu Manpage:

   umask - set file mode creation mask ([original](http://manpages.ubuntu.com/manpages/xenial/man2/umask.2.html)) ([raw](?raw))

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NAME

   umask - set file mode creation mask

SYNOPSIS

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

   Alternatively, if the parent directory has a default ACL (see **[acl](../man5/acl.5.html)**(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](../man2/stat.2.html)**(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](../man2/open.2.html)**(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](../man2/fork.2.html)**(2)  inherits  its  parent's  umask.  The umask is left unchanged by
   **[execve](../man2/execve.2.html)**(2).

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

SEE ALSO

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

COLOPHON

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