printf "fails" in extension

[Alex Martelli]

Robert Ferrell wrote:

> I'm trying to "extend" Python by writing a bit of C code.  I followed
> the example in the Python documentation, and almost everything works
> fine.  I can build a DLL, fire up an interactive Python session and
> import my new module. Methods work almost as expected.
> 
> The only problem I'm having is that I can't print to the screen from
> the C code.  Printing to a file works fine, but neither printf nor
> fprintf(stdout,...) puts anything onto the screen.
> 
> I'm working in Windows2K, using Python2.3.  Is there some trick to get
> this to work?  I'm wondering if this is a compiler issue (I'm building
> the DLL using an Absoft compiler).  I googled around looking for
> relevant information, but I didn't find any.  Perhaps I didn't have
> the right key words?

This may well be the case.  Python doesn't necessarily have much
to do with it: it IS a question of EXE and DLL on Windows needing
to share the _same_ stdio infrastructure -- basically, the same
instance of the same runtime libraries.  Tricky enough when you
build EXE and DLL with the same compiler, because even then you
need to ensure they're using a version of the C runtime that lives
_in yet another DLL_ -- and it must be the same, not e.g. one
optimized and the other built for debug instead -- any static
linking of the runtime on either or both sides and you're hosed.
Even trickier with different compilers, unless one is specifically
built to use the C runtime DLL's from the other -- mingw32 being
an example (it's built to use MSVCRT.DLL, Microsoft's DLL version
of the C runtime libraries).

This is a well-known issue among experts of C compilers on
Windows -- particularly ones who have ever had the totally
harrowing experience of having to mix code built by multiple
compilers, but not exclusively, because even using e.g MSVC++ 6
throughout is still tricky due to the possibility of different
C runtime library instances being used by different modules
(in the Windows sense: each EXE or DLL is a module).  How to
manage to google for it may be a different issue, particularly
as the workaround (if any) will depend on your "Absoft
compiler".  I can however suggest to skip Python in your
searches, because it's not in the picture: what you need to
find out is, how (if at all) is it possible to use your
compiler to write a DLL which, used from a MSVC++ - made EXE,
is able to do normal output to stdout or stderr.


Alex