[petsc-dev] need c++ expert help

[Jed Brown]

"Smith, Barry F." <bsmith at mcs.anl.gov> writes:

>   There is always a C++ linker available so long as PETSc was built without --with-cxx=0 so just use it, why make a big deal out of it?

What is it called?  I would like to just use CXX, but we have PCC_LINKER
written out independently.  I don't know any circumstance in which
PCC_LINKER is different from PCC, but maybe there is.  We should get rid
of this duplication unless there is a specific circumstance in which
they are different.

$ grep mpicxx ompi/lib/petsc/conf/petscvariables                                                                                                              
CXX = /home/jed/usr/ccache/ompi/bin/mpicxx                                                                                                                                                    
CXXCPP = /home/jed/usr/ccache/ompi/bin/mpicxx -E

$ grep mpicc ompi/lib/petsc/conf/petscvariables                                                                                                              
SL_LINKER = /home/jed/usr/ccache/ompi/bin/mpicc                                                                                                                                               
PCC = /home/jed/usr/ccache/ompi/bin/mpicc
PCC_LINKER = /home/jed/usr/ccache/ompi/bin/mpicc
CC = /home/jed/usr/ccache/ompi/bin/mpicc
DYNAMICLINKER = /home/jed/usr/ccache/ompi/bin/mpicc
CPP = /home/jed/usr/ccache/ompi/bin/mpicc -E
LD_SHARED = /home/jed/usr/ccache/ompi/bin/mpicc


>    Barry
>
>> On Dec 31, 2017, at 5:58 PM, Jed Brown <jed at jedbrown.org> wrote:
>> 
>> "Smith, Barry F." <bsmith at mcs.anl.gov> writes:
>> 
>>>> On Dec 31, 2017, at 2:14 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>> 
>>>> Matthew Knepley <knepley at gmail.com> writes:
>>>> 
>>>>> On Sun, Dec 31, 2017 at 2:35 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>>> 
>>>>>> Matthew Knepley <knepley at gmail.com> writes:
>>>>>> 
>>>>>>> On Sun, Dec 31, 2017 at 1:55 PM, Jed Brown <jed at jedbrown.org> wrote:
>>>>>>> 
>>>>>>>> These look like linker errors and that build uses
>>>>>>>> --with-cxxlib-autodetect=0.  We either need a rule to *link* C++ using
>>>>>>>> CXX (i.e., mpicxx) or add LIBS=-lstdc++.
>>>>>>>> 
>>>>>>> 
>>>>>>> I thought we were doing this (linking C++ mains with CXX).
>>>>>> 
>>>>>> After compiling to the object file (ex3.o) we don't know what language
>>>>>> main was written in, and this doesn't solve the actual problem that C++
>>>>>> linking is required if any object (not just main) depends on C++.
>>>>>> 
>>>>> 
>>>>> My thinking here was the following:
>>>>> 
>>>>> 1) No C++ is allowed in PETSc, unless --with-clanguage=cxx, in which case
>>>>> the linker is C++
>>>>> 2) If C++ is in an external library, then that configure requires the C++
>>>>> library
>>>>> 3) The executable itself could be C++, in which case I proposed using the
>>>>> C++ linker explicitly
>>>>> 
>>>>> About not knowing which objects come with C++ main: I thought we did. Don't
>>>>> they go into a separate set?
>>>> 
>>>> Right, we do have that information for tests.  Note that a single test
>>>> executable can depend on multiple source files, and the one containing
>>>> main might not be C++ while another is.  I don't know if there are any
>>>> such instances in PETSc.
>>>> 
>>>> I can change the build rules for tests with C++ sources later today.
>>> 
>>>   Jed, are you indicating that you will resolve the original problem I reported? Where will you resolve it? The problem may come up in one of my branches.
>> 
>> I was going to put it in a new branch from 'master'.  There is
>> disgusting duplication in 'maint' that has since been fixed so I'd
>> rather not do it there.
>> 
>> Under what conditions is PCC_LINKER different from CC (or CXX when
>> clanguage=C++)?  I need to find a C++ linker and would prefer to use
>> $(CXX) rather than adding a new configure test.  I feel like the
>> generated petscvariables has an inordinate amount of duplication all
>> with non-standard names and no documentation.
>> 
>> The cheap way to fix your issue is to add LIBS=-lstdc++ to your
>> configure.