X-Git-Url: https://git.distorted.org.uk/u/mdw/putty/blobdiff_plain/9af6afe3b890f550fec7cdffde8ffcfc287dee55..HEAD:/puttymem.h

diff --git a/puttymem.h b/puttymem.h
index 06c294d9..941aded3 100644
--- a/puttymem.h
+++ b/puttymem.h
@@ -34,9 +34,19 @@ void safefree(void *);
  * possible, in favour of these type-casting macros which ensure
  * you don't mistakenly allocate enough space for one sort of
  * structure and assign it to a different sort of pointer.
+ *
+ * The nasty trick in sresize with sizeof arranges for the compiler,
+ * in passing, to type-check the expression ((type *)0 == (ptr)), i.e.
+ * to type-check that the input pointer is a pointer to the correct
+ * type. The construction sizeof(stuff) ? (b) : (b) looks like a
+ * violation of the first principle of safe macros, but in fact it's
+ * OK - although it _expands_ the macro parameter more than once, it
+ * only _evaluates_ it once, so it's still side-effect safe.
  */
 #define snew(type) ((type *)snmalloc(1, sizeof(type)))
 #define snewn(n, type) ((type *)snmalloc((n), sizeof(type)))
-#define sresize(ptr, n, type) ((type *)snrealloc((ptr), (n), sizeof(type)))
+#define sresize(ptr, n, type)                                           \
+    ((type *)snrealloc(sizeof((type *)0 == (ptr)) ? (ptr) : (ptr),      \
+                       (n), sizeof(type)))
 
 #endif