* unix/tclUnixCompat.c (PadBuffer): Fixed bug in calculation of

	  the increment needed to align the pointer, and added
	  documentation explaining why the macro is implemented as it is.


FossilOrigin-Name: ee4af820661e4226be8e089ace74953d8826118f

1 files changed, 11 insertions, 5 deletions

diff --git a/unix/tclUnixCompat.c b/unix/tclUnixCompat.c
index 37d6d05..d9deb8a 100644
--- a/unix/tclUnixCompat.c
+++ b/unix/tclUnixCompat.c
@@ -6,7 +6,7 @@
  * See the file "license.terms" for information on usage and redistribution
  * of this file, and for a DISCLAIMER OF ALL WARRANTIES.
  *
- * RCS: @(#) $Id: tclUnixCompat.c,v 1.7 2006/09/11 16:07:33 das Exp $
+ * RCS: @(#) $Id: tclUnixCompat.c,v 1.8 2006/09/12 21:59:53 andreas_kupries Exp $
  *
  */
 
@@ -19,12 +19,18 @@
 
 /*
  * Used to pad structures at size'd boundaries
+ *
+ * This macro assumes that the pointer 'buffer' was created from an
+ * aligned pointer by adding the 'length'. If this 'length' was not a
+ * multiple of the 'size' the result is unaligned and PadBuffer
+ * corrects both the pointer, _and_ the 'length'. The latter means
+ * that future increments of 'buffer' by 'length' stay aligned.
  */
 
-#define PadBuffer(buffer, length, size)         \
-    if (((length) % (size))) {                  \
-	(buffer) += ((length) % (size));        \
-	(length) += ((length) % (size));        \
+#define PadBuffer(buffer, length, size)             \
+    if (((length) % (size))) {                      \
+	(buffer) += ((size) - ((length) % (size))); \
+	(length) += ((size) - ((length) % (size))); \
     }
 
 /*