[Xcb] [PATCH proto 1/2] Add sizeof tag to struct

[Peter Hutterer]

On Sun, Dec 23, 2012 at 03:29:23PM +0100, Daniel Martin wrote:
> If a struct provides all necessary informations to calculate it's
> size, i.e. an explicit length field, this sizeof tag makes it possible
> to reference to the length field with an expression (known from lists).
> This makes further iterations over these structs very easy, as we don't
> have to sum up all field lengths, lists and may add padding.
> 
> structs with such a length field can be found in the XInput extension.
> For example the device classes (xXIButtonInfo, xXIKeyInfo, ...):
>   http://cgit.freedesktop.org/xorg/proto/inputproto/tree/XI2proto.h#n136
> 
> Signed-off-by: Daniel Martin <consume.noise at gmail.com>

I don't know enough about xcb's internals but the approach seems sensible.
two typos pointed out below.

Cheers,
   Peter

> ---
>  doc/xml-xcb.txt  | 12 +++++++++++-
>  src/xcb.xsd      |  7 +++++++
>  xcbgen/xtypes.py | 25 +++++++++++++++++++++++++
>  3 files changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/doc/xml-xcb.txt b/doc/xml-xcb.txt
> index 15121be..91ffdeb 100644
> --- a/doc/xml-xcb.txt
> +++ b/doc/xml-xcb.txt
> @@ -61,7 +61,10 @@ Top-Level Elements
>    root node.  Note that types from xproto are automatically available, without
>    explicitly importing them.
>  
> -<struct name="identifier" [packed="true"]>structure contents</struct>
> +<struct name="identifier" [packed="true"]>
> +  structure contents
> +  [<sizeof>expression</sizeof>]
> +</struct>
>  
>    This element represents a data structure.  The name attribute gives the name
>    of the structure.  The content represents the fields of the structure, and
> @@ -72,6 +75,13 @@ Top-Level Elements
>    part of the data structure is not a multiple of 4 and it mustn't be
>    aligned to the next 4byte boundary.
>  
> +  If the optional sizeof element is present it provides a simple way to
> +  compute the struct size at runtime. This is usefull if the struct itself

useful

> +  contains a length field referencing the whole struct. With that given it's
> +  not necessary to sum up all fields, lists and pads of the struct.
> +  DO NOT use it the work around alignmend issues. Use the packed attribute

alignment

> +  for that.
> +
>  <union name="identifier">structure contents</union>
>  
>    This element represents a union of data types, which can hold one value of
> diff --git a/src/xcb.xsd b/src/xcb.xsd
> index bcad558..98111be 100644
> --- a/src/xcb.xsd
> +++ b/src/xcb.xsd
> @@ -188,6 +188,12 @@ authorization from the authors.
>      </xsd:choice>
>    </xsd:group>
>  
> +  <xsd:element name="sizeof">
> +    <xsd:complexType>
> +      <xsd:group ref="expression" minOccurs="1" maxOccurs="1" />
> +    </xsd:complexType>
> +  </xsd:element>
> +
>    <!-- Type for a structure -->
>    <xsd:complexType name="struct">
>      <xsd:sequence>
> @@ -195,6 +201,7 @@ authorization from the authors.
>        <xsd:choice minOccurs="0" maxOccurs="1">
>          <xsd:element ref="switch" />
>        </xsd:choice>
> +      <xsd:element ref="sizeof" minOccurs="0" maxOccurs="1" />
>      </xsd:sequence>
>      <xsd:attribute name="name" type="xsd:string" use="required" />
>      <xsd:attribute name="packed" type="xsd:boolean" use="optional" />
> diff --git a/xcbgen/xtypes.py b/xcbgen/xtypes.py
> index e6d15c8..a799d2e 100644
> --- a/xcbgen/xtypes.py
> +++ b/xcbgen/xtypes.py
> @@ -464,6 +464,31 @@ class Struct(ComplexType):
>      def __init__(self, name, elt):
>          ComplexType.__init__(self, name, elt)
>          self.packed = elt.get('packed')
> +        self.sizeof_expr = None
> +
> +        for child in list(elt):
> +            if child.tag == 'sizeof':
> +                self.sizeof_expr = Expression(child[0], self)
> +
> +    def resolve(self, module):
> +        if self.resolved:
> +            return
> +
> +        ComplexType.resolve(self, module)
> +        if self.sizeof_expr != None:
> +            self.sizeof_expr.resolve(module, self)
> +
> +    def calc_size(self):
> +        if self.sizeof_expr != None:
> +            pass
> +        else:
> +            ComplexType.calc_size(self)
> +
> +    def fixed_size(self):
> +        if self.sizeof_expr != None:
> +            return False
> +        else:
> +            return ComplexType.fixed_size(self)
>  
>      out = __main__.output['struct']
>  
> -- 
> 1.8.0.2
>