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Title: Compiling and linking main() with C++ compiler
Type: Stage:
Components: Build Versions: Python 2.5
Status: closed Resolution: accepted
Dependencies: Superseder:
Assigned To: Nosy List: cludwig, jackjansen, jholg, loewis
Priority: normal Keywords: patch

Created on 2005-10-12 11:45 by cludwig, last changed 2022-04-11 14:56 by admin. This issue is now closed.

File name Uploaded Description Edit
cxx-main.patch cludwig, 2005-10-12 11:45 Proposed resolution to url:>
cxx-main.patch2 cludwig, 2005-11-10 18:42 incorporates jackjansen's suggestion
cxx-main.patch3 cludwig, 2005-11-30 18:07 avoids CC=gcc and CXX=c++
Messages (9)
msg48849 - (view) Author: Christoph Ludwig (cludwig) Date: 2005-10-12 11:45
The attached patch proposes a resolution to the discussion 
started in 
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 
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/ is no longer used and can be 
msg48850 - (view) Author: Jack Jansen (jackjansen) * (Python committer) Date: 2005-11-08 22:51
Logged In: YES 

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?
msg48851 - (view) Author: Christoph Ludwig (cludwig) Date: 2005-11-10 18:42
Logged In: YES 

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 
The change is in lin with jackjansen's second suggestion. It is 
pretty straight forward and avoids fragile configure magic. 
msg48852 - (view) Author: Jack Jansen (jackjansen) * (Python committer) Date: 2005-11-24 23:26
Logged In: YES 

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 

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?
msg48853 - (view) Author: Christoph Ludwig (cludwig) Date: 2005-11-25 10:51
Logged In: YES 

distutils behaves the same way in Python 2.4.1, as I  
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. 
msg48854 - (view) Author: Christoph Ludwig (cludwig) Date: 2005-11-30 18:07
Logged In: YES 

My second patch was indeed broken since it referred to $CC  
before the call to AC_PROG_CC in That caused  
CC=gcc and CXX=c++ if neither --with-cxx-main nor CC was  
given on the command line.   
I am going to upload a third version (cxx-main.patch3) that 
fixes this problem by moving the code that evaluates 
--with-cxx-main below AC_PROG_CC.  
To repeat what I wrote in my last comment: This patch does  
not address the issue that distutils calls $CC in order to  
compile C++ source files and calls $CXX in the linking step  
only. Howevr, the patch tries to ensure that CXX is always  
set to a sensible value in the Makefile - even if Python is  
built exclusively with $CC. Thus, distutils has a fighting  
chance to do the right thing (whatever that is) when  
building C++ extensions since it uses the value of CXX  
found in Python's Makefile.  
msg48855 - (view) Author: Holger Joukl (jholg) Date: 2006-04-10 14:28
Logged In: YES 

I can confirm cxx-main.patch3 fixes build problems with gcc
3.4.4 on Solaris 8, enabling me to _not_ link to libstdc++.
I also think the patched README text is clearer than the
Just my +1 for this patch.
msg48856 - (view) Author: Holger Joukl (jholg) Date: 2006-04-10 14:32
Logged In: YES 

Forgot to mention the python version: I used it for both
2.4.2 and 2.4.3.
>I can confirm cxx-main.patch3 fixes build problems with gcc
>3.4.4 on Solaris 8, enabling me to _not_ link to libstdc++.
msg48857 - (view) Author: Martin v. Löwis (loewis) * (Python committer) Date: 2006-04-14 14:36
Logged In: YES 

Thanks for the patch, committed as 45387. I modified it
slightly, dropping the PROG_CXX_WORKS part, as selection of
the C++ compiler is now entirely in the hands of the user.
Date User Action Args
2022-04-11 14:56:13adminsetgithub: 42471
2005-10-12 11:45:22cludwigcreate