From: Mark Wooding <mdw@distorted.org.uk>
Date: Sat, 15 Aug 2015 15:14:32 +0000 (+0100)
Subject: lib/sod.h: New macro `SOD_INSTBASE' to find the allocated base address.
X-Git-Tag: 0.2.0~87
X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~mdw/git/sod/commitdiff_plain/45c534845b49ab429532df59efde578e7b959bdb

lib/sod.h: New macro `SOD_INSTBASE' to find the allocated base address.

This turns out to be useful, e.g., for freeing dynamically allocated
objects.  A destructor method might usefully say something like

	free(SOD_INSTBASE(me));

after actually releasing any other resources.
---

diff --git a/lib/sod.h b/lib/sod.h
index 12c7817..6a444e9 100644
--- a/lib/sod.h
+++ b/lib/sod.h
@@ -109,11 +109,8 @@ struct sod_chain {
  *
  * Use:		Finds an instance's base address given a pointer to one of
  *		its ichains, if you know precisely the instance's class and
- *		which chain you're pointing to.  If you don't, then (a)
- *
- *		  @(char *)(p) - (p)->_vt->_base@
- *
- *		will do the job just fine, and (b) you'll have the wrong
+ *		which chain you're pointing to.  If you don't, then (a) you
+ *		want @SOD_INSTBASE@ below, and (b) you'll have the wrong
  *		ilayout anyway.
  *
  *		This macro is not intended to be used directly outside of
@@ -126,6 +123,23 @@ struct sod_chain {
   ((struct cls##__ilayout *)						\
    ((char *)(p) - offsetof(struct cls##__ilayout, chead)))
 
+/* --- @SOD_INSTBASE@ --- *
+ *
+ * Arguments:	@p@ = pointer to an instance (i.e., the address of one of its
+ *			instance chains)
+ *
+ * Returns:	The base address of @p@'s instance layout.
+ *
+ * Use:		Finds the base address of an instance.  If you know the
+ *		dynamic class of the object then @SOD_ILAYOUT@ is faster.  If
+ *		you don't, this is the right macro, but your options for
+ *		doing something sensible with the result are limited, mostly
+ *		to simple memory management operations such as freeing or
+ *		zeroizing the instance structure.
+ */
+
+#define SOD_INSTBASE(p) ((void *)((char *)(p) - (p)->_vt->_base))
+
 /*----- Utility macros ----------------------------------------------------*/
 
 /* --- @SOD_CLASSOF@ --- *