diff --git a/check.t b/check.t
index 4d6dd06..6521361 100644
--- a/check.t
+++ b/check.t
@@ -1,4 +1,4 @@
-# $MirOS: src/bin/mksh/check.t,v 1.682 2015/03/01 15:23:03 tg Exp $
+# $MirOS: src/bin/mksh/check.t,v 1.683 2015/03/07 20:46:26 tg Exp $
 # -*- mode: sh -*-
 #-
 # Copyright © 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
@@ -30,7 +30,7 @@
 # (2013/12/02 20:39:44) http://openbsd.cs.toronto.edu/cgi-bin/cvsweb/src/regress/bin/ksh/?sortby=date
 
 expected-stdout:
-	@(#)MIRBSD KSH R50 2015/03/02
+	@(#)MIRBSD KSH R50 2015/03/07
 description:
 	Check version of shell.
 stdin:
@@ -39,7 +39,7 @@ name: KSH_VERSION
 category: shell:legacy-no
 ---
 expected-stdout:
-	@(#)LEGACY KSH R50 2015/03/02
+	@(#)LEGACY KSH R50 2015/03/07
 description:
 	Check version of legacy shell.
 stdin:
diff --git a/histrap.c b/histrap.c
index dcb3b2d..316f72f 100644
--- a/histrap.c
+++ b/histrap.c
@@ -27,7 +27,7 @@
 #include <sys/file.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/histrap.c,v 1.139 2015/02/06 10:09:06 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/histrap.c,v 1.140 2015/03/07 20:46:28 tg Exp $");
 
 Trap sigtraps[NSIG + 1];
 static struct sigaction Sigact_ign;
@@ -563,7 +563,7 @@ sethistfile(const char *name)
 		return;
 
 	/* if the name is the same as the name we have */
-	if (hname && strcmp(hname, name) == 0)
+	if (hname && name && !strcmp(hname, name))
 		return;
 
 	/*
@@ -581,7 +581,8 @@ sethistfile(const char *name)
 		hist_source->line = 0;
 	}
 
-	hist_init(hist_source);
+	if (name)
+		hist_init(hist_source);
 }
 #endif
 
@@ -712,8 +713,10 @@ hist_init(Source *s)
 	hist_source = s;
 
 #if HAVE_PERSISTENT_HISTORY
-	if ((hname = str_val(global("HISTFILE"))) == NULL)
+	if (((hname = str_val(global("HISTFILE"))) == NULL) || !*hname) {
+		hname = NULL;
 		return;
+	}
 	strdupx(hname, hname, APERM);
 	hs = hist_init_first;
 
diff --git a/mksh.1 b/mksh.1
index 8badbc4..b5d9cb8 100644
--- a/mksh.1
+++ b/mksh.1
@@ -1,4 +1,4 @@
-.\" $MirOS: src/bin/mksh/mksh.1,v 1.351 2015/03/05 13:30:57 tg Exp $
+.\" $MirOS: src/bin/mksh/mksh.1,v 1.352 2015/03/07 20:46:29 tg Exp $
 .\" $OpenBSD: ksh.1,v 1.156 2015/01/16 15:32:32 schwarze 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: March 5 2015 $
+.Dd $Mdocdate: March 7 2015 $
 .\"
 .\" Check which macro package we use, and do other -mdoc setup.
 .\"
@@ -1862,7 +1862,8 @@ See
 below for more information.
 .It Ev HISTFILE
 The name of the file used to store command history.
-When assigned to, history is loaded from the specified file.
+When assigned to or unset, the file is opened, history is truncated
+then loaded from the file; subsequent new lines are appended.
 Also, several invocations of the shell will share history if their
 .Ev HISTFILE
 parameters all point to the same file.
@@ -1870,7 +1871,7 @@ parameters all point to the same file.
 .Sy Note :
 If
 .Ev HISTFILE
-isn't set, no history file is used.
+is unset or empty, no history file is used.
 This is different from
 .At
 .Nm ksh .
@@ -6500,8 +6501,20 @@ is immediately printed on suspension (but not later after an
 $ /bin/sleep 666 && echo fubar
 .Ed
 .Pp
+The truncation process involved when changing
+.Ev HISTFILE
+does not free old history entries (leaks memory) and leaks
+old entries into the new history if their line numbers are
+not overwritten by same-numer entries from the persistent
+history file; truncating the on-disc file to
+.Ev HISTSIZE
+lines has always been broken and prone to history file corruption
+when multiple shells are accessing the file; the rollover process
+for the in-memory portion of the history is slow, should use
+.Xr memmove 3 .
+.Pp
 This document attempts to describe
-.Nm mksh\ R50e
+.Nm mksh\ R50f
 and up,
 .\" with vendor patches from insert-your-name-here,
 compiled without any options impacting functionality, such as
diff --git a/sh.h b/sh.h
index f6fd3a3..adf3792 100644
--- a/sh.h
+++ b/sh.h
@@ -169,9 +169,9 @@
 #endif
 
 #ifdef EXTERN
-__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.714 2015/03/01 15:23:05 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.715 2015/03/07 20:46:30 tg Exp $");
 #endif
-#define MKSH_VERSION "R50 2015/03/02"
+#define MKSH_VERSION "R50 2015/03/07"
 
 /* arithmetic types: C implementation */
 #if !HAVE_CAN_INTTYPES
diff --git a/var.c b/var.c
index 18a1633..395496b 100644
--- a/var.c
+++ b/var.c
@@ -28,7 +28,7 @@
 #include <sys/sysctl.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/var.c,v 1.186 2015/02/06 10:56:49 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/var.c,v 1.187 2015/03/07 20:46:31 tg Exp $");
 
 /*-
  * Variables
@@ -1351,6 +1351,11 @@ unsetspec(struct tbl *vp)
 	 */
 
 	switch (special(vp->name)) {
+#if HAVE_PERSISTENT_HISTORY
+	case V_HISTFILE:
+		sethistfile(NULL);
+		return;
+#endif
 	case V_IFS:
 		setctypes(TC_IFSWS, C_IFS);
 		ifs0 = ' ';