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)