[Python-Dev] ANSIfication problems: warnings in marshal.c

Tim Peters tim_one@email.msn.com
Sun, 23 Jul 2000 15:43:33 -0400

> w_byte is a macro that either calls C's putc, or w_more.
> putc takes an int, and all calls to w_byte make sure they're
> passing the right thing.
> however, w_more is defined to take a char, which results in
> warnings from MSVC5.
> quick fix: change w_more to take an integer too:
> ...

And I see you checked that in.  Good!  I later replaced part of it, via a
new mini-subsystem that may tick somebody off, so I'll explain that here so
people can yell at me now:

1. asserts are a fundamental tool of good C practice, but are very
   rarely used in Python source.  To encourage their use, I added an
   include of <assert.h> to Python.h, so that assert is always available.

2. Added a new Py_SAFE_DOWNCAST(VALUE, WIDE, NARROW) macro to pyport.h,
   which is also always available.  This casts VALUE from type WIDE to
   type NARROW, but assert-fails if info is lost due to the cast and
   Py_DEBUG is #define'd (when we're not in Py_DEBUG mode, it just
   expands to (NARROW)(VALUE)).  Note:  this kind of thing is much
   easier to do in C++, because of inline functions and being able to
   templatize those on the types involved (i.e., the compiler can figure
   out which specific checker to expand at compile-time, rather than
   making you redundantly-- and hence possibly incorrectly --name the
   types "by hand" again).

3. It's nice that eyeballing every call to marshal's w_byte allows one
   to infer that the derived calls to w_more are safe, but it's much
   better to let the compiler check that.  So I replaced the raw cast
   with an invocation of Py_SAFE_DOWNCAST.  Doesn't cost anything
   except in Py_DEBUG mode.