diff --git a/src/ChangeLog b/src/ChangeLog
index b5e617b1f20..3bd4fc1f4c0 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,7 @@
 2011-04-09  Paul Eggert  <eggert@cs.ucla.edu>
 
+	* xdisp.c (vmessage): Use a better test for character truncation.
+
 	* charset.c (load_charset_map): <, not <=, for optimization,
 	and to avoid potential problems with integer overflow.
 	* chartab.c (sub_char_table_set_range, char_table_set_range): Likewise.
diff --git a/src/xdisp.c b/src/xdisp.c
index b1209b998e2..f37254419eb 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -8410,13 +8410,14 @@ vmessage (const char *m, va_list ap)
 	    {
 	      char *buf = FRAME_MESSAGE_BUF (f);
 	      size_t bufsize = FRAME_MESSAGE_BUF_SIZE (f);
-	      int len = vsnprintf (buf, bufsize, m, ap);
-	      if (len < 0)
-		len = 0;
+	      int len;
+
+	      memset (buf, 0, bufsize);
+	      len = vsnprintf (buf, bufsize, m, ap);
 
 	      /* Do any truncation at a character boundary.  */
-	      if (0 < bufsize && bufsize <= len)
-		for (len = bufsize - 1;
+	      if (! (0 <= len && len < bufsize))
+		for (len = strnlen (buf, bufsize);
 		     len && ! CHAR_HEAD_P (buf[len - 1]);
 		     len--)
 		  continue;
@@ -19495,7 +19496,7 @@ decode_mode_spec (struct window *w, register int c, int field_width,
 	    EMACS_INT limit = BUF_BEGV (b);
 	    EMACS_INT limit_byte = BUF_BEGV_BYTE (b);
 	    EMACS_INT position;
-	    EMACS_INT distance = 
+	    EMACS_INT distance =
 	      (height * 2 + 30) * line_number_display_limit_width;
 
 	    if (startpos - distance > limit)