[issue11920] ctypes: Strange bitfield structure sizing issue
Steve Thompson
report at bugs.python.org
Mon Apr 25 23:21:58 CEST 2011
Steve Thompson <steve.f.thompson at gmail.com> added the comment:
So, knowing there's a potential cross platform inconsistency here, is there
a proposed way to deal with this that doesn't involve modifying the real c
code I'm interfacing with? That's not always an option.
On Mon, Apr 25, 2011 at 2:49 PM, Santoso Wijaya <report at bugs.python.org>wrote:
>
> Santoso Wijaya <santoso.wijaya at gmail.com> added the comment:
>
> What compilers were used to build your Python distro and the native
> structure?
>
> I found out in _ctypes/cfield.c (lns. 76-95):
>
> if (bitsize /* this is a bitfield request */
> && *pfield_size /* we have a bitfield open */
> #ifdef MS_WIN32
> /* MSVC, GCC with -mms-bitfields */
> && dict->size * 8 == *pfield_size
> #else
> /* GCC */
> && dict->size * 8 <= *pfield_size
> #endif
> && (*pbitofs + bitsize) <= *pfield_size) {
> /* continue bit field */
> fieldtype = CONT_BITFIELD;
> #ifndef MS_WIN32
> } else if (bitsize /* this is a bitfield request */
> && *pfield_size /* we have a bitfield open */
> && dict->size * 8 >= *pfield_size
> && (*pbitofs + bitsize) <= dict->size * 8) {
> /* expand bit field */
> fieldtype = EXPAND_BITFIELD;
> #endif
>
> So the allocation of the extra byte for the structure seems to depend on
> Python's compiler. To make sure, I compiled a native structure using MSVC:
>
> #pragma pack(1)
> typedef struct _struct1
> {
> UINT8 first : 1;
> UINT8 second : 1;
> UINT8 third : 1;
> UINT8 fourth : 1;
> UINT8 fifth : 1;
> UINT16 pad : 11;
> } struct1;
>
> And I got the same value (sizeof == 3).
>
> ----------
>
> _______________________________________
> Python tracker <report at bugs.python.org>
> <http://bugs.python.org/issue11920>
> _______________________________________
>
----------
Added file: http://bugs.python.org/file21777/unnamed
_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11920>
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-------------- next part --------------
So, knowing there's a potential cross platform inconsistency here, is there a proposed way to deal with this that doesn't involve modifying the real c code I'm interfacing with? Â That's not always an option.<br>
<br><div class="gmail_quote">On Mon, Apr 25, 2011 at 2:49 PM, Santoso Wijaya <span dir="ltr"><<a href="mailto:report at bugs.python.org">report at bugs.python.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<br>
Santoso Wijaya <<a href="mailto:santoso.wijaya at gmail.com">santoso.wijaya at gmail.com</a>> added the comment:<br>
<br>
What compilers were used to build your Python distro and the native structure?<br>
<br>
I found out in _ctypes/cfield.c (lns. 76-95):<br>
<br>
  if (bitsize /* this is a bitfield request */<br>
    && *pfield_size /* we have a bitfield open */<br>
#ifdef MS_WIN32<br>
    /* MSVC, GCC with -mms-bitfields */<br>
    && dict->size * 8 == *pfield_size<br>
#else<br>
    /* GCC */<br>
    && dict->size * 8 <= *pfield_size<br>
#endif<br>
    && (*pbitofs + bitsize) <= *pfield_size) {<br>
    /* continue bit field */<br>
    fieldtype = CONT_BITFIELD;<br>
#ifndef MS_WIN32<br>
  } else if (bitsize /* this is a bitfield request */<br>
    && *pfield_size /* we have a bitfield open */<br>
    && dict->size * 8 >= *pfield_size<br>
    && (*pbitofs + bitsize) <= dict->size * 8) {<br>
    /* expand bit field */<br>
    fieldtype = EXPAND_BITFIELD;<br>
#endif<br>
<br>
So the allocation of the extra byte for the structure seems to depend on Python's compiler. To make sure, I compiled a native structure using MSVC:<br>
<br>
#pragma pack(1)<br>
typedef struct _struct1<br>
{<br>
  UINT8 first  : 1;<br>
  UINT8 second  : 1;<br>
  UINT8 third  : 1;<br>
  UINT8 fourth  : 1;<br>
  UINT8 fifth  : 1;<br>
  UINT16 pad   : 11;<br>
} struct1;<br>
<br>
And I got the same value (sizeof == 3).<br>
<br>
----------<br>
<div><div></div><div class="h5"><br>
_______________________________________<br>
Python tracker <<a href="mailto:report at bugs.python.org">report at bugs.python.org</a>><br>
<<a href="http://bugs.python.org/issue11920" target="_blank">http://bugs.python.org/issue11920</a>><br>
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</div></div></blockquote></div><br>
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