Merge #13 Add some tips to C-Allocators

modules/ROOT/pages/programming-languages/C-Allocators.adoc

@@ -43,6 +43,20 @@ compiler may assume that a comparison between the old and new
 pointer will always return false, so it is impossible to detect
 movement this way.
 
+On a related note, `realloc` frees the memory area if the new size is
+zero. If the size unintentionally becomes zero, as a result of
+unsigned integer wrap-around for instance, the following idiom causes
+a double-free.
+
+[source,c]
+----
+new_size = size + x; /* 'x' is a very large value and the result wraps around to zero */
+new_ptr = realloc(ptr, new_size);
+if (!new_ptr) {
+	free(ptr);
+}
+----
+
 ==== Handling Memory Allocation Errors
 
 Recovering from out-of-memory errors is often difficult or even
@@ -105,6 +119,30 @@ Otherwise, call `malloc`. When exiting the
 function, check if `malloc` had been called,
 and free the buffer as needed.
 
+If portability is not important in your program, an alternative way of
+automatic memory management is to leverage the `cleanup` attribute
+supported by the recent versions of GCC and Clang. If a local variable
+is declared with the attribute, the specified cleanup function will be
+called when the variable goes out of scope.
+
+[source,c]
+----
+static inline void freep(void *p) {
+        free(*(void**) p);
+}
+
+void somefunction(const char *param) {
+	if (strcmp(param, "do_something_complex") == 0) {
+		__attribute__((cleanup(freep))) char *ptr = NULL;
+
+		/* Allocate a temporary buffer */
+		ptr = malloc(size);
+
+		/* Do something on it, but do not need to manually call free() */
+	}
+}
+----
+
 [[sect-Defensive_Coding-C-Allocators-Arrays]]
 === Array Allocation
 
@@ -118,6 +156,12 @@ to allocate an array of `n` elements of type
 greater than `((size_t) -1) / sizeof(T)`. See
 <<sect-Defensive_Coding-C-Arithmetic>>.
 
+GNU libc provides a dedicated function `reallocarray` that allocates
+an array with those checks performed internally.  However, care must
+be taken if portability is important: while the interface originated
+in OpenBSD and has been adopted in many other platforms, NetBSD
+exposes an incompatible behavior with the same interface.
+
 [[sect-Defensive_Coding-C-Allocators-Custom]]
 === Custom Memory Allocators