diff --git a/src/cairo-atomic-private.h b/src/cairo-atomic-private.h
index 86e400f49..bf79b7a99 100644
--- a/src/cairo-atomic-private.h
+++ b/src/cairo-atomic-private.h
@@ -43,6 +43,8 @@
 #include "config.h"
 #endif
 
+/* The autoconf on OpenBSD 4.5 produces the malformed constant name
+ * SIZEOF_VOID__ rather than SIZEOF_VOID_P.  Work around that here. */
 #if !defined(SIZEOF_VOID_P) && defined(SIZEOF_VOID__)
 # define SIZEOF_VOID_P SIZEOF_VOID__
 #endif
diff --git a/util/cairo-trace/trace.c b/util/cairo-trace/trace.c
index c4fae7c0d..ac88e43e4 100644
--- a/util/cairo-trace/trace.c
+++ b/util/cairo-trace/trace.c
@@ -22,6 +22,8 @@
 #include "config.h"
 #endif
 
+/* The autoconf on OpenBSD 4.5 produces the malformed constant name
+ * SIZEOF_VOID__ rather than SIZEOF_VOID_P.  Work around that here. */
 #if !defined(SIZEOF_VOID_P) && defined(SIZEOF_VOID__)
 # define SIZEOF_VOID_P SIZEOF_VOID__
 #endif