[issue12880] ctypes: clearly document how structure bit fields are allocated

[Meador Inge]

Meador Inge <meadori at gmail.com> added the comment:

On Tue, Oct 4, 2011 at 10:21 AM, Vlad Riscutia <report at bugs.python.org> wrote:

> First, I'm saying "toying with the underlying buffer" because none of the bugs are actual issues of the form "I created this bitfield
> structure with Python, passed it to C function but C structure was different". That would be a bitfield bug. All of these bugs are people
> setting raw memory to some bytes, then looking at bitfield members and not seeing what they expect.

Please qualify "all" instead of generalizing.  I can point to two
issues (issue11990 "I'm generating python code from real c code.",
issue12945 "We have raw data packages from some tools. These packages
contains bitfields, arrays, simple data and so on.") where C
code or raw data was, in fact, involved and the reporters just don't
understand what layout algorithm is being used.  They may not need
to know the specifics of the algorithm, but they *do* need to know if
it matches the compiler they are using to do interop or the one
that generated the raw data.

The reason that we are seeing folks cast raw memory into a cyptes
bitfield structure is because they do not understand how the structure
layout algorithm works and are trying to figure it out via these
examples.

> Second, one of your review comments is: "GCC is used for most Unix systems and Microsoft VC++ is used on Windows.". This is not
> how ctypes works. Ctypes implements the bitfield allocation algorithm itself, it doesn't use the compiler with which it is built. Basically
> it says #ifdef WIN32 - allocate like VC++ - #else - allocate like GCC. So it doesn't really matter with which compiler you are building
> Python. It will still do GCC style allocation on Solaris.

I understand how it works.  This quote is taken somewhat out of
context as the preceding sentence is important.  Perhaps saying GCC-
style and VC++-style would have been more clear.  The reason that I
mentioned the compiler used to build Python is that it is an easy
reference point and more times than not the bitfield allocation and
layout *do* match that of the compiler used to build the interpreter.
Anyway, I am fine with dropping the "used to build the Python
interpreter" and going with something similar to what you originally
had.

Also, in general, the compiler used to build the ctypes extension
*does* matter.  Look in 'cfield.c' where all of the native alignments
are
computed at compile time.  These alignments affect the structure
layout and are defined by the compiler building the ctypes extension.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue12880>
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