[Xcb] c_client.py: enumerator values generation

Christian Linhart chris at DemoRecorder.com
Sat Mar 11 12:01:08 UTC 2017

Hi Evgeny and all,

Thank you for your patch, and for bringing this issue to our attention.
However I am pretty sure we have to solve this problem in a different way.


1. Your proposed change is in xcb/proto which is also used by generators
   for other programming languages.

   They might create wrong code because the negative number may
   really be interpreted in a wrong way in languages other than

   Language specific stuff has to go into xcb/libxcb for C/C++  
   ( or xcb/xpyb for Python, etc or whatever ).

2. I have a bad gut-feeling even for C/C++.
   I am pretty sure that there are compilers which don't
   interpret this negative number as the proper bit-pattern,
   i.e., a 32bit value with bit 31 set.

Here's an alternative proposal for a solution:

I think that the core problem is that we abuse enums for things
that they weren't designed to, when we use them for masks.   

So, instead of:

  typedef enum xcb_input_modifier_mask_t {
      XCB_INPUT_MODIFIER_MASK_ANY = 2147483648
  } xcb_input_modifier_mask_t;

we should use something like:  

  typedef uint32_t xcb_input_modifier_mask_t;
  static const xcb_input_modifier_mask_t XCB_INPUT_MODIFIER_MASK_ANY = 2147483648;

Instead of static const we also could use a macro if static const is not yet supported by all relevant compilers.
(for C++ we could use "enum class", but that's another story: we don't have a generator for C++ yet) 

Maybe we should output the constants in masks as hex, and use the proper suffix, so that we get a 
32-bit unsigned constant instead of a 64 bit signed constant:

  typedef uint32_t xcb_input_modifier_mask_t;
  static const xcb_input_modifier_mask_t XCB_INPUT_MODIFIER_MASK_ANY = 0x80000000U;

This would also make these constants somehow human readable.

What do you think?

Will switching from "enum" to "typedef" and "static const" cause any ABI/API issues?
I am not aware of any but maybe I overlook something.



This proposal would also allow us to define 64bit masks because we'd have control over the type, e.g.:

  typedef uint64_t xcb_foo_bar_mask_t;
  static const xcb_foo_bar_mask_t = XCB_FOO_BAR_BAZ 0x1ULL;
  static const xcb_foo_bar_mask_t = XCB_FOO_BAR_ABC 0x8000000000000000ULL;

On 2017-01-17 09:17, Litvinenko, Evgeny wrote:
> From 2a9446c38f7f1977b32f85dcf4857db8c676799c Mon Sep 17 00:00:00 2001
> From: Evgeny Litvinenko <evgeny.v.litvinenko at gmail.com>
> Date: Tue, 17 Jan 2017 09:32:27 +0300
> Subject: [PATCH] Convert enumerator values to the range of C 'int'
> This fixes warning like the following (when gcc runs with -pedantic-errors)
> In file included from xinput.c:14:0:
> xinput.h:3079:35: error: ISO C restricts enumerator values to range of 'int' [-Wpedantic]
>      XCB_INPUT_MODIFIER_MASK_ANY = 2147483648
>                                    ^
> In C99 standard (ISO/IEC 9899:1999)
> ...
> Enumeration specifiers
> The expression that defines the value of an enumeration constant
> shall be an integer constant expression
> that has a value representable as an int.
> ...
> ---
> You can use this patch or any parts of it
> as you consider it necessary.
>  xcbgen/xtypes.py | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> diff --git a/xcbgen/xtypes.py b/xcbgen/xtypes.py
> index b83b119..93db2ad 100644
> --- a/xcbgen/xtypes.py
> +++ b/xcbgen/xtypes.py
> @@ -3,6 +3,7 @@ This module contains the classes which represent XCB data types.
>  '''
>  from xcbgen.expr import Field, Expression
>  from xcbgen.align import Alignment, AlignmentLog
> +from ctypes import c_int
>  import __main__
>  verbose_align_log = False
> @@ -243,7 +244,7 @@ class Enum(SimpleType):
>              if value.tag == 'value':
>                  self.values.append((item.get('name'), value.text))
>              elif value.tag == 'bit':
> -                self.values.append((item.get('name'), '%u' % (1 << int(value.text, 0))))
> +                self.values.append((item.get('name'), '%i' % c_int(1 <<  int(value.text, 0)).value))
>                  self.bits.append((item.get('name'), value.text))
>      def resolve(self, module):
> --
> 2.11.0
> Thanks,
> Evgeny.
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