[petsc-dev] need c++ expert help

[Smith, Barry F.]

  I think it is just CXX.  The PCC business is the thing that magically changes from CC to CXX depending on --with-clanguage being set. But CXX is always the C++ linker.

  Barry


> On Dec 31, 2017, at 6:18 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> "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.