r11824@catbus: nickm | 2007-02-16 13:16:47 -0500

Move all struct-offset-manipulation macros into util.h, and use them consistently.  Because there are days when "SUBTYPE_P(handle, subtype, _base)" is just easier to read and write than "(basetp*)(((handle) - STRUCT_OFFSET(subtype, _base))".


svn:r9592

src/common/compat.c

@@ -174,8 +174,7 @@ tor_mmap_file(const char *filename)
 void
 tor_munmap_file(tor_mmap_t *handle)
 {
-  tor_mmap_impl_t *h = (tor_mmap_impl_t*)
-    (((char*)handle) - STRUCT_OFFSET(tor_mmap_impl_t, base));
+  tor_mmap_impl_t *h = SUBTYPE_P(handle, tor_mmap_impl_t, base);
   munmap((char*)h->base.data, h->mapping_size);
   tor_free(h);
 }
@@ -244,8 +243,7 @@ tor_mmap_file(const char *filename)
 void
 tor_munmap_file(tor_mmap_t *handle)
 {
-  win_mmap_t *h = (win_mmap_t*)
-    (((char*)handle) - STRUCT_OFFSET(win_mmap_t, base));
+  win_mmap_t *h = SUBTYPE_P(handle, win_mmap_t, base);
   if (handle->data)
     /* This is an ugly cast, but without it, "data" in struct tor_mmap_t would
        have to be redefined as non-const. */

src/common/util.h

@@ -110,6 +110,29 @@ extern int dmalloc_free(const char *file, const int line, void *pnt,
    ((off_t) (((char*)&((tp*)0)->member)-(char*)0))
 #endif
 
+/** Macro: yield a pointer to the field at position <b>off</b> within the
+ * structure <b>st</b>.  Example:
+ * <pre>
+ *   struct a { int foo; int bar; } x;
+ *   off_t bar_offset = STRUCT_OFFSET(struct a, bar);
+ *   int *bar_p = STRUCT_VAR_P(&x, bar_offset);
+ *   *bar_p = 3;
+ * </pre>
+ */
+#define STRUCT_VAR_P(st, off) ((void*) ( ((char*)(st)) + (off) ) )
+
+/** Macro: yield a pointer to an enclosing structure given a pointer to
+ * a substructure at offset <b>off</b>. Example:
+ * <pre>
+ *   struct base { ... };
+ *   struct subtype { int x; struct base b; } x;
+ *   struct base *bp = &x.base;
+ *   struct *sp = SUBTYPE_P(bp, struct subtype, b);
+ * </pre>
+ */
+#define SUBTYPE_P(p, subtype, basemember) \
+  ((void*) ( ((char*)(p)) - STRUCT_OFFSET(subtype, basemember) ))
+
 /* String manipulation */
 
 /** Allowable characters in a hexadecimal string. */