add fcn product_overflow()

src/misc/alloc.c

@@ -47,7 +47,6 @@ extern mutexType allocMutex;
  */
 
 /* New implementation of tmalloc, it uses calloc and does not call memset()  */
-
 void *
 tmalloc(size_t num)
 {
@@ -148,6 +147,45 @@ txfree(const void *ptr)
 #elif defined SHARED_MODULE
   mutex_unlock(&allocMutex);
 #endif
-}
+} /* end of function txfree */
 
-#endif
+
+/* This function returns the product of a and b if it does not overflow.
+ *
+ * Return codes
+ * 0: No overflow
+ * 1: overflow
+ */
+static inline int product_overflow(size_t a, size_t b, size_t *p_n)
+{
+    /* Some overflow conditions:
+     * a == SIZE_MAX and b > 1
+     * a > 1 and b == SIZE_MAX
+     * a * b < a
+     * a * b < b
+     */
+    if ((a == SIZE_MAX && b > 1) || (a > 1 && b == SIZE_MAX)) {
+        return +1;
+    }
+
+    const size_t n = a * b;
+    if (n < a || n < b) {
+        return +1;
+    }
+
+    *p_n = n;
+    return 0;
+} /* end of function product_overflow */
+
+
+/* Print error related to allocating a product that cannot fit in a
+ * size_t and exit. This function does not return. */
+static void overflow_error(size_t num, size_t size)
+{
+    (void) fprintf(stderr, "Cannot allocate %zu X %zu bytes: "
+            "Product exceeds largest size_t = %zu.\n",
+            num, size, SIZE_MAX);
+    controlled_exit(EXIT_FAILURE);
+} /* end of function overflow_error */
+
+#endif /* #ifndef HAVE_LIBGC */