[Patches] [ python-Patches-1324762 ] Compiling and linking main() with C++ compiler

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Patches item #1324762, was opened at 2005-10-12 13:45
Message generated for change (Comment added) made by cludwig
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Category: Build
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Christoph Ludwig (cludwig)
Assigned to: Nobody/Anonymous (nobody)
Summary: Compiling and linking main() with C++ compiler

Initial Comment:
The attached patch proposes a resolution to the discussion 
started in 
<url:http://thread.gmane.org/gmane.comp.python.devel/69651> 
regarding the compiler (C vs. C++) used to compile python's 
main() and to link the executable.  
 
The patch contains the following changes: 
 
1) The configure option --with-cxx is renamed 
--with-cxx-main. This was done to avoid surprising the user 
by the changed meaning. Furthermore, it is now possible 
that CXX has a different value than provided by 
--with-cxx-main, so the old name would have been 
confusing. 
 
2) The compiler used to translate python's main() function is 
stored in the configure / Makefile variable MAINCC. By 
default, MAINCC=$(CC). If --with-cxx-main is given (without 
an appended compiler name), then MAINCC=$(CXX). If 
--with-cxx-main=<compiler> is on the configure command 
line, then MAINCC=<compiler>. Additionally, configure sets 
CXX=<compiler> unless CXX was already set on the 
configure command line. 
 
3) The command used to link the python executable is (as 
before) stored in LINKCC. By default, LINKCC='$(PURIFY) 
$(MAINCC)', i.e. the linker front-end is the compiler used to 
translate main(). If necessary, LINKCC can be set on the 
configure command line in which case it won't be altered. 
 
4) If CXX is not set by the user (on the command line or via 
--with-cxx-main), then configure tries several likely C++ 
compiler names. CXX is assigned the first name that refers 
to a callable program in the system. (CXX is set even if 
python is built with a C compiler only, so distutils can build 
C++ extensions.)  
 
5) Modules/ccpython.cc is no longer used and can be 
removed. 
 
 

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>Comment By: Christoph Ludwig (cludwig)
Date: 2005-11-25 11:51

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distutils behaves the same way in Python 2.4.1, as I  
mentioned  
<url:http://article.gmane.org/gmane.comp.python.devel/69817>.  
My patch does not address this problem at all. (It should  
be fixed in distutils, but I did not the time to look into  
it yet.)   
 
I am surprised that, on your machine, CC=gcc and CXX=c++. I 
will look into this next week. 

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Comment By: Jack Jansen (jackjansen)
Date: 2005-11-25 00:26

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Well, as I commented on this patch and you quickly followed my suggestions I 
felt obliged to test your fix, but I'm not sure about the outcome.

I built a C++ extension on MacOSX 10.4, gcc 4, framework Python. The good 
news is that it worked fine, everything built and worked as before. BUT: both 
with and without your mods my C++ modules are compiled with "gcc", not "g
++" or "c++". Linking is done with "c++", in both cases. I looked at distutils 
and it seems that it could indeed be the case that CXX is only used for linking 
and never for compilation, but I'm not 100% sure.

Additionally, the Makefile has
CC=             gcc
CXX=            c++
which is technically fine on my machine (g++ == c++), but may not work 
always.

Maybe someone who has a machine with both cc/c++ and gcc/g++ installed, 
and preferrably incompatible compilers, could give this patch a try too?

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Comment By: Christoph Ludwig (cludwig)
Date: 2005-11-10 19:42

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I am going to upload a revision of my patch that addresses 
jackjansen's comment: It sets CXX to g++ or c++ if CC is gcc or 
cc, respectively. Additionally, it writes a warning if configure had to 
"guess" the C++ compiler and tells the user how to override this 
guess. 
 
The change is in lin with jackjansen's second suggestion. It is 
pretty straight forward and avoids fragile configure magic. 
 

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Comment By: Jack Jansen (jackjansen)
Date: 2005-11-08 23:51

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One question: is step 4 a wise idea? Picking a random C++ compiler if 
multiple are available may result in picking a vendor C++ when the user 
has specified CC=gcc, for example.

OTOH, actually doing the configure magic to determine that the selected 
C++ works together with the c-compiler selected for Python may be 
overkill too.

Maybe try only standard combinations cc/c++ gcc/g++ and otherwise 
require --with{out}-cxx?

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