1 files changed, 44 insertions, 0 deletions
diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index 9b39dc2..dbec1cd 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -2360,6 +2360,50 @@ evaluated, so any side effects of the expression will be discarded.
 
 Query for this feature with ``__has_builtin(__builtin_assume)``.
 
+``__builtin_offsetof``
+----------------------
+
+``__builtin_offsetof`` is used to implement the ``offsetof`` macro, which
+calculates the offset (in bytes) to a given member of the given type.
+
+**Syntax**:
+
+.. code-block:: c++
+
+    __builtin_offsetof(type-name, member-designator)
+
+**Example of Use**:
+
+.. code-block:: c++
+
+  struct S {
+    char c;
+    int i;
+    struct T {
+      float f[2];
+    } t;
+  };
+
+  const int offset_to_i = __builtin_offsetof(struct S, i);
+  const int ext1 = __builtin_offsetof(struct U { int i; }, i); // C extension
+  const int ext2 = __builtin_offsetof(struct S, t.f[1]); // C & C++ extension
+
+**Description**:
+
+This builtin is usable in an integer constant expression which returns a value
+of type ``size_t``. The value returned is the offset in bytes to the subobject
+designated by the member-designator from the beginning of an object of type
+``type-name``. Clang extends the required standard functionality in a few ways:
+
+* In C language modes, the first argument may be the definition of a new type.
+  Any type declared this way is scoped to the nearest scope containing the call
+  to the builtin.
+* The second argument may be a member-designator designated by a series of
+  member access expressions using the dot (``.``) operator or array subscript
+  expressions.
+
+Query for this feature with ``__has_builtin(__builtin_offsetof)``.
+
 ``__builtin_call_with_static_chain``
 ------------------------------------