[Python-Dev] How to force export of a particular symbol from python.exe?

[Mark Dickinson]

Hi all,

I'm having some trouble making some bits of the Python core code
available to extension modules.  Specifically,  I'm trying to add a
function 'Py_force_to_memory' to Python/pymath.c and then use
it (via a macro) from Modules/cmathmodule.c. But importing of
the cmath module fails with a 'Symbol not found' error.  The
function is declared with a 'PyAPI_FUNC' in Python/pymath.h.

Here's the relevant portion of the make output:

*** WARNING: renaming "cmath" since importing it failed:
dlopen(build/lib.macosx-10.3-i386-2.7/cmath.so, 2): Symbol not found:
_Py_force_to_memory
  Referenced from:
/Users/dickinsm/python_source/branches/trunk/build/lib.macosx-10.3-i386-2.7/cmath.so
  Expected in: dynamic lookup

This is a non-debug trunk build, on OS X (10.5.5), with all
the defaults.  I'm using Apple's standard toolchain (gcc 4.0.1,
Darwin linker).  The patch I'm building with can be seen at:

http://bugs.python.org/issue4575

(It's the first of the two patches there, called 'force_to_memory.patch'.)

I think I understand the cause of this problem;  I just don't know how
to fix it.  The cause seems to be that none of the symbols in pymath.o
is used in the Python executable;  they're used only in the extension
modules.  So while the '_Py_force_to_memory' symbol appears in
libpython2.7.a, it doesn't appear in the python.exe executable;  hence
the above error.

If I move the definition of Py_force_to_memory from Python/pymath.c
to Objects/floatobject.c then everything works as expected.

Questions:

(1) Is this an OS X only problem?

(2) Is there an easy way to force a particular symbol (or all the
symbols from a particular object file) to be exported in the Python
executable, so that it's available to a dynamically loaded extension
module?

I've found the -u option to gcc, but this doesn't seem like a
particularly portable solution.  Of course, if this problem exists
only on OS X, then the solution doesn't need to be portable.

Thanks,

Mark