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FreeBSD 11.0 - man page for umask (freebsd section 2)

UMASK(2)						      BSD System Calls Manual							  UMASK(2)

NAME
umask -- set file creation mode mask
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/stat.h> mode_t umask(mode_t numask);
DESCRIPTION
The umask() routine sets the process's file mode creation mask to numask and returns the previous value of the mask. The 9 low-order access permission bits of numask are used by system calls, including open(2), mkdir(2), and mkfifo(2), to turn off corresponding bits requested in file mode. (See chmod(2)). This clearing allows each user to restrict the default access to his files. The default mask value is S_IWGRP|S_IWOTH (022, write access for the owner only). Child processes inherit the mask of the calling process.
RETURN VALUES
The previous value of the file mode mask is returned by the call.
ERRORS
The umask() system call is always successful.
SEE ALSO
chmod(2), mkfifo(2), mknod(2), open(2)
STANDARDS
The umask() system call is expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1'').
HISTORY
The umask() function appeared in Version 7 AT&T UNIX.
BSD
June 4, 1993 BSD