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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)).