Implement variable length structure allocator

* This patch implements a memory allocator to obtain memory for
  structures whose last member is a variable length array.  C99 refers
  to these variable length objects as structs containing flexible
  array members.
* Fixed macro parentheses per Eric Blake

src/util/memory.c

@@ -164,6 +164,46 @@ int virReallocN(void *ptrptr, size_t size, size_t count)
     return 0;
 }
 
+/**
+ * Vir_Alloc_Var:
+ * @ptrptr: pointer to hold address of allocated memory
+ * @struct_size: size of initial struct
+ * @element_size: size of array elements
+ * @count: number of array elements to allocate
+ *
+ * Allocate struct_size bytes plus an array of 'count' elements, each
+ * of size element_size.  This sort of allocation is useful for
+ * receiving the data of certain ioctls and other APIs which return a
+ * struct in which the last element is an array of undefined length.
+ * The caller of this type of API is expected to know the length of
+ * the array that will be returned and allocate a suitable buffer to
+ * contain the returned data.  C99 refers to these variable length
+ * objects as structs containing flexible array members.
+ *
+ * Returns -1 on failure, 0 on success
+ */
+int virAllocVar(void *ptrptr, size_t struct_size, size_t element_size, size_t count)
+{
+    size_t alloc_size = 0;
+
+#if TEST_OOM
+    if (virAllocTestFail())
+        return -1;
+#endif
+
+    if (VIR_ALLOC_VAR_OVERSIZED(struct_size, count, element_size)) {
+        errno = ENOMEM;
+        return -1;
+    }
+
+    alloc_size = struct_size + (element_size * count);
+    *(void **)ptrptr = calloc(1, alloc_size);
+    if (*(void **)ptrptr == NULL)
+        return -1;
+    return 0;
+}
+
+
 /**
  * virFree:
  * @ptrptr: pointer to pointer for address of memory to be freed

src/util/memory.h

@@ -48,6 +48,10 @@
 int virAlloc(void *ptrptr, size_t size) ATTRIBUTE_RETURN_CHECK;
 int virAllocN(void *ptrptr, size_t size, size_t count) ATTRIBUTE_RETURN_CHECK;
 int virReallocN(void *ptrptr, size_t size, size_t count) ATTRIBUTE_RETURN_CHECK;
+int virAllocVar(void *ptrptr,
+                size_t struct_size,
+                size_t element_size,
+                size_t count) ATTRIBUTE_RETURN_CHECK;
 void virFree(void *ptrptr);
 
 /**
@@ -88,6 +92,37 @@ void virFree(void *ptrptr);
  */
 # define VIR_REALLOC_N(ptr, count) virReallocN(&(ptr), sizeof(*(ptr)), (count))
 
+/*
+ * VIR_ALLOC_VAR_OVERSIZED:
+ * @M: size of base structure
+ * @N: number of array elements in trailing array
+ * @S: size of trailing array elements
+ *
+ * Check to make sure that the requested allocation will not cause
+ * arithmetic overflow in the allocation size.  The check is
+ * essentially the same as that in gnulib's xalloc_oversized.
+ */
+#define VIR_ALLOC_VAR_OVERSIZED(M, N, S) ((((size_t)-1) - (M)) / (S) < (N))
+
+/**
+ * VIR_ALLOC_VAR:
+ * @ptr: pointer to hold address of allocated memory
+ * @type: element type of trailing array
+ * @count: number of array elements to allocate
+ *
+ * Allocate sizeof(*ptr) bytes plus an array of 'count' elements, each
+ * sizeof('type').  This sort of allocation is useful for receiving
+ * the data of certain ioctls and other APIs which return a struct in
+ * which the last element is an array of undefined length.  The caller
+ * of this type of API is expected to know the length of the array
+ * that will be returned and allocate a suitable buffer to contain the
+ * returned data.  C99 refers to these variable length objects as
+ * structs containing flexible array members.
+
+ * Returns -1 on failure, 0 on success
+ */
+#define VIR_ALLOC_VAR(ptr, type, count) virAllocVar(&(ptr), sizeof(*(ptr)), sizeof(type), (count))
+
 /**
  * VIR_FREE:
  * @ptr: pointer holding address to be freed