zero size structs with extern (C) attribute

Author: WalterBright

spec/struct.dd

@@ -169,7 +169,9 @@ $(H2 $(LNAME2 struct_layout, Struct Layout))
     the first field and the start of the object.
     )
 
-    $(P Structs with no fields of non-zero size (aka $(I Empty Structs)) have a size of one byte.)
+    $(P A struct with no fields of non-zero size is an $(I Empty Struct).
+    An Empty Struct with a $(DDSUBLINK spec/attribute, linkage, Linkage Attribute) of `extern (C)` has a size of zero.
+    Otherwise, its size is one byte.)
 
     $(P Non-static $(RELATIVE_LINK2 nested, function-nested D structs), which access the context of
     their enclosing scope, have an extra field.
@@ -179,20 +181,22 @@ $(H2 $(LNAME2 struct_layout, Struct Layout))
     $(OL
     $(LI The default layout of the fields of a struct is an exact
     match with the $(I associated C compiler).)
-    $(LI g++ and clang++ differ in how empty structs are handled. Both return `1` from `sizeof`,
+    $(LI g++ and clang++ differ in how Empty Structs are handled. Both return `1` from `sizeof`,
     however, clang++ does not push them onto the parameter stack while g++ does. This is a
     binary incompatibility between g++ and clang++.
     dmd follows clang++ behavior for OSX and FreeBSD, and g++ behavior for Linux and other
     Posix platforms.
     )
-    $(LI clang and gcc both return `0` from `sizeof` for empty structs. Using `extern "C++"`
-    in clang++ and g++ does not cause them to conform to the behavior of their respective C compilers.)
+    $(LI C compilers normally evaluate `sizeof` applied to Empty Structs as `0`, although it
+    is undefined behavior according to the C11 Standard.
+    C++ compilers normally evaluate the size as `1`, even if `extern "C"` is applied.
+    )
     ))
 
     $(UNDEFINED_BEHAVIOR
     $(OL
     $(LI The padding data can be accessed, but its contents are undefined.)
-    $(LI Do not pass or return structs with no fields of non-zero size to `extern (C)` functions.
+    $(LI Do not pass or return Empty Structs to `extern (C)` functions.
     According to C11 6.7.2.1p8 this is undefined behavior.)
     ))
 
@@ -202,8 +206,9 @@ $(H2 $(LNAME2 struct_layout, Struct Layout))
     attributes to describe an exact match. Using a $(DDSUBLINK spec/version, static-assert, Static Assert)
     to ensure the result is as expected.)
     $(LI Although the contents of the padding are often zero, do not rely on that.)
-    $(LI Avoid using empty structs when interfacing with C and C++ code.)
-    $(LI Avoid using empty structs as parameters or arguments to variadic functions.)
+    $(LI Avoid using Empty Structs when interfacing with C++ code that declares them with `extern "C"`.
+    Add a one byte dummy field to those structs.)
+    $(LI Avoid using Empty Structs as parameters or arguments to variadic functions.)
     ))
 
 $(H2 $(LNAME2 POD, Plain Old Data))