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UMASK(2)                   Linux Programmer's ManualUMASK(2)NAME       umask - set file mode creation maskSYNOPSIS       #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  byopen(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 toopen(2) andmkdir(2).       Alternatively, if the parent directory has a default ACL (seeacl(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 in-ode(7).       The typical default value for the process  umask  is  S_IWGRP | S_IWOTH       (octal  022).   In the usual case where the mode argument toopen(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 viafork(2) inherits its parent's  umask.   The       umask is left unchanged byexecve(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 (usingmsgget(2),semget(2),shmget(2)).SEE ALSOchmod(2),mkdir(2),open(2),stat(2),acl(5)COLOPHON       This  page  is  part of release 5.05 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                             2017-09-15UMASK(2)