diff --git a/library/common.h b/library/common.h
index eb159a7..68af840 100644
--- a/library/common.h
+++ b/library/common.h
@@ -65,6 +65,44 @@
 #define MBEDTLS_TEST_HOOK_TEST_ASSERT(TEST)
 #endif /* defined(MBEDTLS_TEST_HOOKS) */
 
+/** \def ARRAY_LENGTH
+ * Return the number of elements of a static or stack array.
+ *
+ * \param array         A value of array (not pointer) type.
+ *
+ * \return The number of elements of the array.
+ */
+/* A correct implementation of ARRAY_LENGTH, but which silently gives
+ * a nonsensical result if called with a pointer rather than an array. */
+#define ARRAY_LENGTH_UNSAFE(array)            \
+    (sizeof(array) / sizeof(*(array)))
+
+#if defined(__GNUC__)
+/* Test if arg and &(arg)[0] have the same type. This is true if arg is
+ * an array but not if it's a pointer. */
+#define IS_ARRAY_NOT_POINTER(arg)                                     \
+    (!__builtin_types_compatible_p(__typeof__(arg),                \
+                                   __typeof__(&(arg)[0])))
+/* A compile-time constant with the value 0. If `const_expr` is not a
+ * compile-time constant with a nonzero value, cause a compile-time error. */
+#define STATIC_ASSERT_EXPR(const_expr)                                \
+    (0 && sizeof(struct { unsigned int STATIC_ASSERT : 1 - 2 * !(const_expr); }))
+
+/* Return the scalar value `value` (possibly promoted). This is a compile-time
+ * constant if `value` is. `condition` must be a compile-time constant.
+ * If `condition` is false, arrange to cause a compile-time error. */
+#define STATIC_ASSERT_THEN_RETURN(condition, value)   \
+    (STATIC_ASSERT_EXPR(condition) ? 0 : (value))
+
+#define ARRAY_LENGTH(array)                                           \
+    (STATIC_ASSERT_THEN_RETURN(IS_ARRAY_NOT_POINTER(array),         \
+                               ARRAY_LENGTH_UNSAFE(array)))
+
+#else
+/* If we aren't sure the compiler supports our non-standard tricks,
+ * fall back to the unsafe implementation. */
+#define ARRAY_LENGTH(array) ARRAY_LENGTH_UNSAFE(array)
+#endif
 /** Allow library to access its structs' private members.
  *
  * Although structs defined in header files are publicly available,