(VERIFY_EXPR): Define.

(X2REALLOC): New macro, to make using x2realloc a little safer.

lib/xalloc.h

@@ -1,7 +1,7 @@
 /* xalloc.h -- malloc with out-of-memory checking
 
    Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
-   1999, 2000, 2003, 2004 Free Software Foundation, Inc.
+   1999, 2000, 2003, 2004, 2005 Free Software Foundation, Inc.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -56,6 +56,15 @@ void *x2nrealloc (void *p, size_t *pn, size_t s);
 void *xmemdup (void const *p, size_t s);
 char *xstrdup (char const *str);
 
+# define VERIFY_EXPR(assertion) \
+    (void)((struct {char a[(assertion) ? 1 : -1]; } *) 0)
+
+/* Using x2realloc (when appropriate) usually makes your code more
+   readable than using x2nrealloc, but it also makes it so your
+   code will malfunction if sizeof (*P) ever becomes 2 or greater.
+   So use this macro instead of using x2realloc directly.  */
+# define X2REALLOC(P, PN) (VERIFY_EXPR (sizeof(*P) == 1), x2realloc (P, PN))
+
 /* Return 1 if an array of N objects, each of size S, cannot exist due
    to size arithmetic overflow.  S must be positive and N must be
    nonnegative.  This is a macro, not an inline function, so that it