[C++-sig] Bugs in the Boost.Python tutorial example

[Michael Rolle]

Hi,

I had a hard time building the example/tutorial project.  Some of this is, I believe, due to bugs in the distribution, and some is probably my lack of understanding.

Some of this I posted to the Boost interest group before I could join this (Python C++) SIG, hoping it would get forwarded on here, so there may be a duplication.

1. Calling bjam in the example/tutorial project failed to even start up the build system.  This was because the example/bootstrap.jam pointed to the wrong path for the build system root.  Instead of ../../../tools/build/v2, it should be ../../../tools/build/src/kernel.  When I changed this, bjam now got past the build system startup.

2. Building the project not only compiles hello.cpp, but it also builds a private copy of the Boost/Python library.  So I needed to supply the properties needed to correctly build this library (i.e., link=shared, address-mode=64).  Of course, I needed to supply those same properties anyway as part of creating the extension.

There's probably a way to change something so that the extension uses the library built in the Boost.Python's own project, or if I have obtained the libraries without having to build them, it would use these.  I don't know if you intended to have the tutorial example make its own copies, but it seems a waste of resources to do so.

3. The link for debug mode failed, saying that the .pdb file was in the wrong format (LNK1207).  This is a bug, due to an option in the link command '/IMPORTLIB:...hello_ext.pdb'.  So the linker is creating the .pdb file as an import library, then complaining that it's not a valid pdb file.  I changed '.pdb' to '.lib'.  I could also have removed this option entirely, since hello_ext.pyd doesn't have anything to export anyway.

4. Before figuring out that the link was the problem, I changed the /Z7 argument to /Zi in the compile command for hello.cpp.  I don't know if this was necessary, or if it was necessary to leave it in place.  For now, I just wanted to get it to build.  Without /Z7, the debug symbols go into example/tutorial/vc120.pdb.  I don't know if the linker found these or not.  When I try stepping into the extension, I'll know for sure.  Microsoft prefers that .pdb files be used for debug symbols rather than embedding them in the .obj files, so this might be the only real reason to make the change.

4. The link for both release mode failed with two undefined symbols, __imp_DecodePointer and __imp_EncodePointer, which are in kernel32.lib.  I tried adding kernel32.lib to the link's inputs.  But then it warned that there could be static constructors/destructors not getting called.  After much research on this topic, I found that the source of the problem was the /NOENTRY argument in the link command, which has the consequence that the automatic CRT initialization of the DLL doesn't occur.  So I remove the /NOENTRY and got not warnings, and I didn't need to add kernerl32.lib either.

5. A minor point.  The MACHINE:X64 is redundant.  The linker knows it's X64 because of the architecture of the input files and libraries.  Nothing wrong with it being there, but it's just clutter in the code.

6. Now bjam was successful in building whatever it wanted to build.  It said that hello.test passed.  Sounds great, I thought.  But I then went into Python and tried 'import hello_ext' and that failed.  So I have an issue with the test program passing, while the extension didn't actually work.

7. The problem was that bjam didn't put the hello_ext.pyd file in my Python's lib/site-packages folder.  It built the .pyd and .pdb files in the example/tutorial/bin/... staging area, and copied (ONLY) the .pyd file to example/tutorial.  So not only did the .pyd file get put in the wrong place, but the .pdb was left out.  If I am going to debug my extension (such as with Visual Studio's Python Tools), the .pdb file also needs to be in the lib/site-packages folder.  Without the .pdb file, the PTVS debugger will not set breakpoints or step into the extension code.

To summarize, the fixes I made in order to build the example and use it in Python were:
    * Change the build system path in examples/bootstrap.jam.
    * Change the /IMPORTLIB:....pdb to ....lib in the link command.  I could have removed it altogether.
    * Remove /NOENTRY from the link command.
    * Manually copy the example/tutorial/bin/.../hello_ext.(pyd,pdb) files to Python's lib/site-packages.

This cost me about a whole day's time, and I'm pretty resourceful.  And I actually want to build my own extensions in VS instead of bjam, linking to the Boost.Python DLL.  I started of by trying this, and my extension crashed.  So that's why I went about building the example with bjam, to at least see what a successful deployment looked like.  But I'll still have to figure out what went wrong with my original attempt; there may be additional bugs to report.

I suppose that a lot of people would have given up building the tutorial example and then given up on Boost.Python altogether.

Michael Rolle
(408) 313-8149
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