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Undefined calling conventions in Python.h

Hi there,

This is my first post over here and I hope someone can give me some
guidance.

I'm trying to embed Python into a Visual C++ 2008 application and I'm
getting linker problems. I've compiled a DLL of the Python source code
using the pythoncode VC++ project in the PCbuild folder of the source
download and this works 100% without any warnings etc. I've done this
in Debug and Release mode without any problems.

When I include python_install_ path\include\Py thon.h in my application
(with the linking setup correctly to the .lib and .dll files generated
by myself) it builds fine. However as soon as I try to call any Python
function (Py_Exit for example) I get linker errors as shown below:

1>application.o bj : error LNK2031: unable to generate p/invoke for
"extern "C" void __clrcall Py_Exit(int)" (?Py_Exit@@$$J0 YMXH@Z);
calling convention missing in metadata
1>frmPythonInte rface.obj : error LNK2031: unable to generate p/invoke
for "extern "C" void __clrcall Py_Exit(int)" (?Py_Exit@@$$J0 YMXH@Z);
calling convention missing in metadata

I'm probably missing something but I can't find any calling convention
details in Python.h or the other headers included in this file. In my
VC++ DLL project I've specified the calling convention as __stdcall. I
know the __clrcall naming convention has to do with managed code in VC+
+ but thats as far as my knowledge on that goes. Should I define the
calling conventions manually? Or is there a setting wrong somewhere in
my application project?

If anybody can give some guidance it would be greatly appreciated.

Thanks
Jaco

Jul 23 '08 #1
16 3763
Jaco Naude wrote:
1>application.o bj : error LNK2031: unable to generate p/invoke for
"extern "C" void __clrcall Py_Exit(int)" (?Py_Exit@@$$J0 YMXH@Z);
calling convention missing in metadata
1>frmPythonInte rface.obj : error LNK2031: unable to generate p/invoke
for "extern "C" void __clrcall Py_Exit(int)" (?Py_Exit@@$$J0 YMXH@Z);
calling convention missing in metadata

I'm probably missing something but I can't find any calling convention
details in Python.h or the other headers included in this file.
the precence of name mangling indicates that your compiler doesn't
understand that Python's a C library, and therefore uses C calling
conventions. have you managed to override the defaults in some odd
way?

</F>

Jul 23 '08 #2
On Jul 23, 9:59*am, Fredrik Lundh <fred...@python ware.comwrote:
Jaco Naude wrote:
1>application.o bj : error LNK2031: unable to generate p/invoke for
"extern "C" void __clrcall Py_Exit(int)" (?Py_Exit@@$$J0 YMXH@Z);
calling convention missing in metadata
1>frmPythonInte rface.obj : error LNK2031: unable to generate p/invoke
for "extern "C" void __clrcall Py_Exit(int)" (?Py_Exit@@$$J0 YMXH@Z);
calling convention missing in metadata
I'm probably missing something but I can't find any calling convention
details in Python.h or the other headers included in this file.

the precence of name mangling indicates that your compiler doesn't
understand that Python's a C library, and therefore uses C calling
conventions. *have you managed to override the defaults in some odd
way?

</F>
good point. I agree that the problem is probably due to name mangling.
I'm not sure how its possible to tell the application that the DLL is
a C dll? I've looked at the DLL using Dependency Walker and the
functions in the DLL are clean (Thus no name mangling used).

How do I tell Visual C++ that the DLL is a C dll? I thought it should
be in Python.h but this line appears at the top of the file:

/* Since this is a "meta-include" file, no #ifdef __cplusplus / extern
"C" { */

Thanks for the help
Jaco
Jul 23 '08 #3
Jaco Naude wrote:
good point. I agree that the problem is probably due to name mangling.
I'm not sure how its possible to tell the application that the DLL is
a C dll? I've looked at the DLL using Dependency Walker and the
functions in the DLL are clean (Thus no name mangling used).

How do I tell Visual C++ that the DLL is a C dll? I thought it should
be in Python.h but this line appears at the top of the file:

/* Since this is a "meta-include" file, no #ifdef __cplusplus / extern
"C" { */
All the individual includes are wrapped in the usual

#ifdef __cplusplus
extern "C" {
#endif

stuff, so the compiler should default to C bindings, without you having
to do anything. Unfortunately, I haven't used VS 2008, but I find it
hard to believe that they've messed this up completely, and a quick
googling indicates that people are using it with Python without trouble.

Maybe there's some override in your project settings caused by some
other parts of your project? (or an #undef __cplusplus somewhere, perhaps?)

</F>

Jul 23 '08 #4
On Jul 23, 12:16*pm, Fredrik Lundh <fred...@python ware.comwrote:
Jaco Naude wrote:
good point. I agree that the problem is probably due to name mangling.
I'm not sure how its possible to tell the application that the DLL is
a C dll? I've looked at the DLL using Dependency Walker and the
functions in the DLL are clean (Thus no name mangling used).
How do I tell Visual C++ that the DLL is a C dll? I thought it should
be in Python.h but this line appears at the top of the file:
/* Since this is a "meta-include" file, no #ifdef __cplusplus / extern
"C" { */

All the individual includes are wrapped in the usual

* * *#ifdef __cplusplus
* * *extern "C" {
* * *#endif

stuff, so the compiler should default to C bindings, without you having
to do anything. *Unfortunately, I haven't used VS 2008, but I find it
hard to believe that they've messed this up completely, and a quick
googling indicates that people are using it with Python without trouble.

Maybe there's some override in your project settings caused by some
other parts of your project? *(or an #undef __cplusplus somewhere, perhaps?)

</F>
I've figured out that the names in the DLL are not mangled by looking
into the DLL using dependancy walker. I also figured out that the
problem is on the application's side and not on the dll's side.

What Visual C++ is doing is that it is looking for mangled names since
it does not know the DLL contains C functions. I've managed to work
around this by declaring the Python functions as follows before using
them in the C++ application side:

extern "C"
{
void Py_Initialize(v oid);
}

This seems to work and the C++ application side is not looking for
mangled names any more. Is this the right way of doing it? It seems
unnecessary to have to declare each Python function you want to use
using the extern "C" way as shown above.

It is probably more of a C++ question it turns out, but I would think
that someone in the Python group would use the Python DLL in C++. The
documentation also suggest that there is no extra work needed when
using C++ rather than C.

Thanks for the help so far.
Jaco
Jul 23 '08 #5
Jaco Naude wrote:
What Visual C++ is doing is that it is looking for mangled names since
it does not know the DLL contains C functions. I've managed to work
around this by declaring the Python functions as follows before using
them in the C++ application side:

extern "C"
{
void Py_Initialize(v oid);
}

This seems to work and the C++ application side is not looking for
mangled names any more. Is this the right way of doing it? It seems
unnecessary to have to declare each Python function you want to use
using the extern "C" way as shown above.
Eh, are you saying that you're not including the Python.h file? Because
it does exactly that, for each and every public function in the C API.
It is probably more of a C++ question it turns out, but I would think
that someone in the Python group would use the Python DLL in C++. The
documentation also suggest that there is no extra work needed when
using C++ rather than C.
Oh, but I do that all the time, without doing any extra work. Both
embedding Python in C++ programs and existing it with C++ extensions.
And I'm definitely not alone.

Here's an actual session, using the version of Visual Studio I happen to
have on this machine (2003, I think) from the command line:
more test.cc
#include "Python.h"
#include <iostream>

main()
{
Py_Initialize() ;
PyRun_SimpleStr ing("print 'hello'\n");
Py_Finalize();

std::cout << "world\n";
}
cl cl -EHsc -MD -I \python25\inclu de test.cc \python25\libs\ python25.lib
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.10.3077
for 80x86
Copyright (C) Microsoft Corporation 1984-2002. All rights reserved.

....
test
hello
world

If you cannot get the same console program to work in your compiler
setup, something's wrong with your configuration.

</F>

Jul 23 '08 #6
Fredrik Lundh wrote:
cl cl -EHsc -MD -I \python25\inclu de test.cc \python25\libs\ python25.lib
cut and paste error there: the "cl cl" should be just one "cl", of course.

and just for the record/google, if I

1) don't include the header file, I get

test.cc(5) : error C3861: 'Py_Initialize' : identifier not found, even
with argument-dependent lookup

2) attempt to undef the __cplusplus macro, I get

test.cc(1) : warning C4117: macro name '__cplusplus' is reserved,
'#undef' ignored

3) cut and paste declarations from the header files to my own file
instead of including the files, ignoring the extern "C" part, I get

test.obj : error LNK2019: unresolved external symbol "int __cdecl
Py_Finalize(voi d)" (?Py_Finalize@@ YAHXZ) referenced in function _main

which looks pretty similar to the errors posted earlier.

</F>

Jul 23 '08 #7
On Jul 23, 1:10*pm, Fredrik Lundh <fred...@python ware.comwrote:
Jaco Naude wrote:
What Visual C++ is doing is that it is looking for mangled names since
it does not know the DLL contains C functions. I've managed to work
around this by declaring the Python functions as follows before using
them in the C++ application side:
extern "C"
{
* * void Py_Initialize(v oid);
}
This seems to work and the C++ application side is not looking for
mangled names any more. Is this the right way of doing it? It seems
unnecessary to have to declare each Python function you want to use
using the extern "C" way as shown above.

Eh, are you saying that you're not including the Python.h file? *Because
it does exactly that, for each and every public function in the C API.
It is probably more of a C++ question it turns out, but I would think
that someone in the Python group would use the Python DLL in C++. The
documentation also suggest that there is no extra work needed when
using C++ rather than C.

Oh, but I do that all the time, without doing any extra work. *Both
embedding Python in C++ programs and existing it with C++ extensions.
And I'm definitely not alone.

Here's an actual session, using the version of Visual Studio I happen to
have on this machine (2003, I think) from the command line:

*more test.cc

#include "Python.h"
#include <iostream>

main()
{
* * *Py_Initialize( );
* * *PyRun_SimpleSt ring("print 'hello'\n");
* * *Py_Finalize();

* * *std::cout << "world\n";

}

*cl cl -EHsc -MD -I \python25\inclu de test.cc \python25\libs\ python25..lib
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.10.3077
for 80x86
Copyright (C) Microsoft Corporation 1984-2002. All rights reserved.

...

*test
hello
world

If you cannot get the same console program to work in your compiler
setup, something's wrong with your configuration.

</F>
Ok that's probably good news, although it points out that there is
something wrong with my configuration since that does not work. I
would rather sort out the problem that having to defined each function
with a extern "C" command.

That said, let me double check something which might be causing
problems since you will be familiar with this. Which Python.h file do
you include when including the DLL in your programs? The one in the
source distribution of the one in the installation distribution? I've
been including the one in the installation distribution all along.
When I try to include the one in the source distribution it gets up to
the point where it looks for the following include:

#include "pyconfig.h "

This file is not in the same directory as the Python.h file in the
source distribution. Because of this I just used Python.h in the
installation distribution.

Thanks again,
Jaco

Jul 23 '08 #8
Jaco Naude wrote:
That said, let me double check something which might be causing
problems since you will be familiar with this. Which Python.h file do
you include when including the DLL in your programs? The one in the
source distribution of the one in the installation distribution? I've
been including the one in the installation distribution all along.
When I try to include the one in the source distribution it gets up to
the point where it looks for the following include:

#include "pyconfig.h "

This file is not in the same directory as the Python.h file in the
source distribution. Because of this I just used Python.h in the
installation distribution.
that's just two copies of the same file, as far as I know.

the "pyconfig.h " file contains platform-specific information; it's
generated from pyconfig.h.in on Unix-style systems, and copied from the
PC directory on Windows.

</F>

Jul 23 '08 #9
On Jul 23, 1:50*pm, Jaco Naude <naude.j...@gma il.comwrote:
On Jul 23, 1:10*pm, Fredrik Lundh <fred...@python ware.comwrote:
Jaco Naude wrote:
What Visual C++ is doing is that it is looking for mangled names since
it does not know the DLL contains C functions. I've managed to work
around this by declaring the Python functions as follows before using
them in the C++ application side:
extern "C"
{
* * void Py_Initialize(v oid);
}
This seems to work and the C++ application side is not looking for
mangled names any more. Is this the right way of doing it? It seems
unnecessary to have to declare each Python function you want to use
using the extern "C" way as shown above.
Eh, are you saying that you're not including the Python.h file? *Because
it does exactly that, for each and every public function in the C API.
It is probably more of a C++ question it turns out, but I would think
that someone in the Python group would use the Python DLL in C++. The
documentation also suggest that there is no extra work needed when
using C++ rather than C.
Oh, but I do that all the time, without doing any extra work. *Both
embedding Python in C++ programs and existing it with C++ extensions.
And I'm definitely not alone.
Here's an actual session, using the version of Visual Studio I happen to
have on this machine (2003, I think) from the command line:
*more test.cc
#include "Python.h"
#include <iostream>
main()
{
* * *Py_Initialize( );
* * *PyRun_SimpleSt ring("print 'hello'\n");
* * *Py_Finalize();
* * *std::cout << "world\n";
}
*cl cl -EHsc -MD -I \python25\inclu de test.cc \python25\libs\ python25.lib
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.10.3077
for 80x86
Copyright (C) Microsoft Corporation 1984-2002. All rights reserved.
...
*test
hello
world
If you cannot get the same console program to work in your compiler
setup, something's wrong with your configuration.
</F>

Ok that's probably good news, although it points out that there is
something wrong with my configuration since that does not work. I
would rather sort out the problem that having to defined each function
with a extern "C" command.

That said, let me double check something which might be causing
problems since you will be familiar with this. Which Python.h file do
you include when including the DLL in your programs? The one in the
source distribution of the one in the installation distribution? I've
been including the one in the installation distribution all along.
When I try to include the one in the source distribution it gets up to
the point where it looks for the following include:

#include "pyconfig.h "

This file is not in the same directory as the Python.h file in the
source distribution. Because of this I just used Python.h in the
installation distribution.

Thanks again,
Jaco
I only saw your last message after posting my previous one. Ok, so the
problem is definitely with the include file Python.h. As I said in my
last post, I might be including the wrong one. I would appreciate it
if you can tell me which one it the correct one to use: The one in the
source distribution or the one in the release distribution
(installation done from the .msi file).

Thanks
Jaco
Jul 23 '08 #10

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