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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 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.
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 pro-
       vides 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/sta-
       tus 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 4.15 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)