small fix to get rid of some compiler warnings

also added very basic documentation for xkb

doc/xkb_internals

@@ -0,0 +1,49 @@
+
+XKB introduces several uncommon data structures:
+ - switch allows conditional inclusion of fields
+ - several complex objects intermix variable and fixed size fields
+ - lists with a variable number of variable size objects
+
+To handle these objects, a number of new functions is generated:
+ - _serialize() turns a structured object into a byte stream, 
+   (re)ordering or including fields according to the protocol
+ - _unserialize() rewrites data from a buffer into a structured object
+ - _unpack() expands a buffer representing a switch object into 
+   a special structured type, all flags needed to resolve the switch
+   expression have to given as parameters
+ - _sizeof() calculates the size of a serialized object, often by calling
+   _unserialize()/_unpack() internally
+
+The new structured data type for switch is special as it contains fixed 
+and variable size fields. Variable size fields can be accessed via pointers. 
+
+If switch appears in a request, an additional set of request helpers is 
+generated with the suffix _aux or _aux_(un)checked. While the 'common'
+request functions require that switch has been serialized before, the _aux
+variants take the structured data type. They are especially designed to 
+replace certain functions in xcb-util/aux. 
+
+Accessors for switch members need two parameters, where the first is usually
+a pointer to the respective request or reply structure, while the second 
+is a pointer to the unpacked switch data structure. 
+
+Functions from the serialize family that take a double pointer can allocate 
+memory on their own, which is useful if the size of a buffer has to be 
+calculated depending on the data within. These functions call malloc() when 
+the double pointer is given as the address of a pointer that has been 
+initialized to 0. It is the responsibility of the user to free any allocated
+memory. 
+
+Intermixed variable and fixed size fields are an important special case in XKB. 
+The current implementation resolves the issue by reordering the fields before
+sending them on the wire as well as before returning a reply. That means that 
+these objects look like 'common' XCB data types and they can be accessed as such 
+(i.e. fixed size fields directly via the structured type and variable size fields
+via accessors/iterators).  
+
+In case a list with variable size elements needs to be accessed, it is necessary 
+to use iterators. The iterator functions take care of determining the actual 
+object size for each element automatically. 
+
+A small and preliminary set of auxiliary functions is available in xkb_util.c 
+in the check_xkb module.  

doc/xkb_issues

@@ -0,0 +1,38 @@
+
+There are a number of problematic special cases in XKB. The issues 
+mentioned here are at most partly resolved.
+ 
+1. The are several XxxDoodad structures defined in xkb.xml. They are used
+   in a few lists, but in a rather special way: 
+   The struct "CommonDoodad" is supposed to be a rather generic data type, 
+   combining the most basic Doodad fields that are common in all these structures. 
+   All Doodads are encapsulated in a union type simply called "Doodad". 
+   Now this union is used in subsequent list definitions, aiming at a kind of 
+   'polymorphism': From inspection of the protocol and Xlib, the Doodads are to 
+   be discriminated based on their type field.
+   However the special meaning of the type field is not encoded in the protocol. 
+   Furthermore the TextDoodad and the LogoDoodad are variable size types due to
+   some fields of type CountedString16, thereby turning the union into a 
+   possibly variable size type as well. 
+   However, for lists with variable size elements, special sizeof functions are 
+   required. These cannot be autogenerated as it cannot be referred which 
+   Doodad type to use for the union. 
+   Therefore, the Doodad type structures are unsupported at the moment. 
+
+2. There are still some bugs in xkb.xml: Either certain fields are missing
+   that are required by the protocol, or Xlib simply has another understanding
+   of the protocol. 
+
+3. The interface for accessors should be reviewed. 
+
+4. Currently some bitcases carry 'name' attributes. These could be avoided if 
+   the data within would consist of a singe struct field only. 
+
+5. switch could get a 'fixed_size' attribute, so when rewriting valueparam to switch, 
+   an uint32_t * pointer could be used instead of void *. 
+
+6. The automatic inclusion of padding requires some complicated coding in the 
+   generator. This is errorprone and could be avoided if all padding is explicitly
+   given in the protocol definition. For variable size fields that require padding, 
+   the pad tag could get a 'fieldref' attribute. That way padding could be handled
+   a lot easier in the autogenerator. 

src/c_client.py

@@ -1293,11 +1293,13 @@ def _c_iterator(self, name):
                 _c('    xcb_generic_iterator_t child;')
                 _c('    child.data = (%s *)(((char *)R) + %s(R));', 
                    self.c_type, self.c_sizeof_name)
+                _c('    i->index = (char *) child.data - (char *) i->data;')
             else:
                 _c('    xcb_generic_iterator_t child = %s;', _c_iterator_get_end(self.last_varsized_field, 'R'))
+                _c('    i->index = child.index;')
             _c('    --i->rem;')
             _c('    i->data = (%s *) child.data;', self.c_type)
-            _c('    i->index = child.index;')
+
     else:
         _c('    --i->rem;')
         _c('    ++i->data;')
@@ -1937,7 +1939,7 @@ def _c_request_helper(self, name, cookie_type, void, regular, aux=False):
     _c('    %s xcb_out;', self.c_type)
     if self.var_followed_by_fixed_fields:
         _c('    /* in the protocol description, variable size fields are followed by fixed size fields */')
-        _c('    char *xcb_aux = 0;')
+        _c('    void *xcb_aux = 0;')
         
 
     for idx, f in enumerate(serial_fields):