note that even set -o noclobber can be coërced into dropping O_EXCL by a TOCTOU

mksh.1

@@ -1,4 +1,4 @@
-.\" $MirOS: src/bin/mksh/mksh.1,v 1.324 2013/11/17 22:24:06 tg Exp $
+.\" $MirOS: src/bin/mksh/mksh.1,v 1.325 2013/11/30 18:16:54 tg Exp $
 .\" $OpenBSD: ksh.1,v 1.148 2013/09/04 15:49:18 millert Exp $
 .\"-
 .\" Copyright © 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,
@@ -74,7 +74,7 @@
 .\" with -mandoc, it might implement .Mx itself, but we want to
 .\" use our own definition. And .Dd must come *first*, always.
 .\"
-.Dd $Mdocdate: November 17 2013 $
+.Dd $Mdocdate: November 30 2013 $
 .\"
 .\" Check which macro package we use, and do other -mdoc setup.
 .\"
@@ -4044,6 +4044,12 @@ Only used if job control is enabled
 .It Fl C \*(Ba Fl o Ic noclobber
 Prevent \*(Gt redirection from overwriting existing files.
 Instead, \*(Gt\*(Ba must be used to force an overwrite.
+Note that this is not safe to use for creation of temporary files or
+lockfiles due to a TOCTOU in a check allowing one to redirect output to
+.Pa /dev/null
+or other device files even in
+.Ic noclobber
+mode.
 .It Fl e \*(Ba Fl o Ic errexit
 Exit (after executing the
 .Dv ERR