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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 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(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(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 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

   This  page  is  part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.



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