The umask() function shall set the process' file mode creation mask to
cmask and return the previous value of the mask. Only the file
permission bits of cmask (see <sys/stat.h>) are used; the
meaning of the other bits is implementation-defined.
The process' file mode creation mask is used during open(),
creat(), mkdir(), and mkfifo() to turn off permission
bits in the mode argument supplied. Bit positions that are set in
cmask are cleared in the mode of the created file.
The file permission bits in the value returned by umask() shall be the
previous value of the file mode creation mask. The state of any other bits in
that value is unspecified, except that a subsequent call to umask()
with the returned value as cmask shall leave the state of the mask the
same as its state before the first call, including any unspecified use of
Unsigned argument and return types for umask() were proposed. The return
type and the argument were both changed to mode_t.
Historical implementations have made use of additional bits in cmask for
their implementation-defined purposes. The addition of the text that the
meaning of other bits of the field is implementation-defined permits these
implementations to conform to this volume of
IEEE Std 1003.1-2001.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html