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

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(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 ig‐
       nored, 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 re‐
       sulting 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.

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

NOTES
       A child process created via fork(2) inherits its parent's umask.  The umask  is  left  un‐
       changed by execve(2).

       It is impossible to use umask() to fetch a process's umask without at the same time chang‐
       ing 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 pro‐
       grams.

       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 re‐
       trieve 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
       This  page  is  part of release 4.16 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 https://www.kernel.org/doc/man-pages/.

Linux                                       2017-09-15                                   UMASK(2)