Some documentation.

docs/contracts/libraries.rst

@@ -208,6 +208,48 @@ Restrictions for libraries in comparison to contracts:
 
 (These might be lifted at a later point.)
 
+.. _library-selectors:
+
+Function Signatures and Selectors in Libraries
+==============================================
+
+While external calls to public or external library functions are possible, the calling convention for such calls
+is considered to be internal to Solidity and not the same as specified for the regular :ref:`contract ABI<ABI>`.
+External library functions support more argument types than external contract functions, for example recursive structs
+and storage pointers. For that reason, the function signatures used to compute the 4-byte selector are computed
+following an internal naming schema and arguments of types not supported in the contract ABI use an internal encoding.
+
+The following identifiers are used for the types in the signatures:
+
+ - Value types, non-storage ``string`` and non-storage ``bytes`` use the same identifiers as in the contract ABI.
+ - Non-storage array types follow the same convention as in the contract ABI, i.e. ``<type>[]`` for dynamic arrays and
+   ``<type>[M]`` for fixed-size arrays of ``M`` elements.
+ - Non-storage structs are referred to by their fully qualified name, i.e. ``C.S`` for ``contract C { struct S { ... } }``.
+ - Storage pointer types use the type identifier of their corresponding non-storage type, but append a single space
+   followed by ``storage`` to it.
+
+The argument encoding is the same as for the regular contract ABI, except for storage pointers, which are encoded as a
+``uint256`` value referring to the storage slot to which they point.
+
+Similarly to the contract ABI, the selector consists of the first four bytes of the Keccak256-hash of the signature.
+Its value can be obtained from Solidity using the ``.selector`` member as follows:
+
+::
+
+    pragma solidity >0.5.13 <0.7.0;
+
+    library L {
+        function f(uint256) external {}
+    }
+
+    contract C {
+        function g() public pure returns (bytes4) {
+            return L.f.selector;
+        }
+    }
+
+
+
 .. _call-protection:
 
 Call Protection For Libraries