Index: head/lib/libc/gen/errlst.c
===================================================================
--- head/lib/libc/gen/errlst.c
+++ head/lib/libc/gen/errlst.c
@@ -33,9 +33,12 @@
 __SCCSID("@(#)errlst.c	8.2 (Berkeley) 11/16/93");
 __FBSDID("$FreeBSD$");
 
+#include <errno.h>
 #include <stdio.h>
 #include "errlst.h"
 
+const char __uprefix[] = "Unknown error";
+
 const char *const sys_errlist[] = {
 	"No error: 0",				/*  0 - ENOERROR */
 	"Operation not permitted",		/*  1 - EPERM */
@@ -155,8 +158,67 @@
 	"Not permitted in capability mode",	/* 94 - ECAPMODE */
 	"State not recoverable",		/* 95 - ENOTRECOVERABLE */
 	"Previous owner died",			/* 96 - EOWNERDEAD */
+
+/*
+ * Reserved space in sys_errlist, take the next slot for a next error code.
+ * Reserve prevents the array size from changing for some time.
+ */
+	__uprefix,				/* 97 */
+	__uprefix,				/* 98 */
+	__uprefix,				/* 99 */
+	__uprefix,				/* 100 */
+	__uprefix,				/* 101 */
+	__uprefix,				/* 102 */
+	__uprefix,				/* 103 */
+	__uprefix,				/* 104 */
+	__uprefix,				/* 105 */
+	__uprefix,				/* 106 */
+	__uprefix,				/* 107 */
+	__uprefix,				/* 108 */
+	__uprefix,				/* 109 */
+	__uprefix,				/* 110 */
+	__uprefix,				/* 111 */
+	__uprefix,				/* 112 */
+	__uprefix,				/* 113 */
+	__uprefix,				/* 114 */
+	__uprefix,				/* 115 */
+	__uprefix,				/* 116 */
+	__uprefix,				/* 117 */
+	__uprefix,				/* 118 */
+	__uprefix,				/* 119 */
+	__uprefix,				/* 120 */
+	__uprefix,				/* 121 */
+	__uprefix,				/* 122 */
+	__uprefix,				/* 123 */
+	__uprefix,				/* 124 */
+	__uprefix,				/* 125 */
+	__uprefix,				/* 126 */
+	__uprefix,				/* 127 */
+	__uprefix,				/* 128 */
+	__uprefix,				/* 129 */
+	__uprefix,				/* 130 */
+	__uprefix,				/* 131 */
+	__uprefix,				/* 132 */
+	__uprefix,				/* 133 */
+	__uprefix,				/* 134 */
+	__uprefix,				/* 135 */
+	__uprefix,				/* 136 */
+	__uprefix,				/* 137 */
+	__uprefix,				/* 138 */
+	__uprefix,				/* 139 */
+	__uprefix,				/* 140 */
+	__uprefix,				/* 141 */
+	__uprefix,				/* 142 */
+	__uprefix,				/* 143 */
+	__uprefix,				/* 144 */
+	__uprefix,				/* 145 */
+	__uprefix,				/* 146 */
+	__uprefix,				/* 147 */
+	__uprefix,				/* 148 */
+	__uprefix,				/* 149 */
+	__uprefix,				/* 150 */
 };
-const int sys_nerr = sizeof(sys_errlist) / sizeof(sys_errlist[0]);
+const int sys_nerr = ELAST + 1;
 
 #ifdef PIC
 __strong_reference(sys_errlist, __hidden_sys_errlist);
Index: head/lib/libc/include/errlst.h
===================================================================
--- head/lib/libc/include/errlst.h
+++ head/lib/libc/include/errlst.h
@@ -42,4 +42,6 @@
 #define __hidden_sys_nerr sys_nerr
 #endif
 
+extern const char __uprefix[] __hidden;
+
 #endif /* __ERRLST_H__ */
Index: head/lib/libc/string/strerror.c
===================================================================
--- head/lib/libc/string/strerror.c
+++ head/lib/libc/string/strerror.c
@@ -46,15 +46,12 @@
 
 #include "errlst.h"
 
-#define	UPREFIX		"Unknown error"
-
 /*
- * Define a buffer size big enough to describe a 64-bit signed integer
- * converted to ASCII decimal (19 bytes), with an optional leading sign
- * (1 byte); finally, we get the prefix, delimiter (": ") and a trailing
- * NUL from UPREFIX.
+ * Define buffer big enough to contain delimiter (": ", 2 bytes),
+ * 64-bit signed integer converted to ASCII decimal (19 bytes) with
+ * optional leading sign (1 byte), and a trailing NUL.
  */
-#define	EBUFSIZE	(20 + 2 + sizeof(UPREFIX))
+#define	EBUFSIZE	(2 + 19 + 1 + 1)
 
 /*
  * Doing this by hand instead of linking with stdio(3) avoids bloat for
@@ -94,9 +91,9 @@
 	if (errnum < 0 || errnum >= __hidden_sys_nerr) {
 		errstr(errnum,
 #if defined(NLS)
-			catgets(catd, 1, 0xffff, UPREFIX),
+			catgets(catd, 1, 0xffff, __uprefix),
 #else
-			UPREFIX,
+		        __uprefix,
 #endif
 			strerrbuf, buflen);
 		retval = EINVAL;