[petsc-users] Why is PETSC_COMM_WORLD changed?

[Barry Smith]

> You can set PETSC_COMM_WORLD before PetscInitialize, but its value is
> not valid/useful before PetscInitialize.


  I have added text to the manual page for PETSC_COMM_WORLD to make this clear.


> On Dec 19, 2014, at 11:30 AM, Jed Brown <jed at jedbrown.org> wrote:
> 
> Florian Lindner <mailinglists at xgm.de> writes:
> 
>> Hello,
>> 
>> I have a piece of code that looks like that:
>> 
>>  // PETSC_COMM_WORLD = MPI_COMM_WORLD;
>>  PetscBool petscIsInitialized;
>>  PetscInitialized(&petscIsInitialized);
>> 
>>  if (not petscIsInitialized) {
>>    PetscErrorCode ierr;
>> 
>>    std::cout << "PETSC == WORLD: " << (PETSC_COMM_WORLD == MPI_COMM_WORLD) << std::endl;
>>    std::cout << "PETSC_COMM_WORLD: " << PETSC_COMM_WORLD << std::endl;
>>    std::cout << "Petsc before PetscInitializeNoArguments()"  << std::endl;
>>    ierr = PetscInitializeNoArguments(); CHKERRV(ierr);
>>    std::cout << "Petsc after PetscInitializeNoArguments()"  << std::endl;
>>  }
>> 
>> PETSC_COMM_WORLD is touched nowhere else in our source, I promise, having grepped through right now.
>> 
>> The code runs fine like that, but when I uncomment the first line it does not anymore.
>> 
>> As far as I know PETSC_COMM_WORLD equals to MPI_COMM_WORLD unless
>> changed, 
> 
> src/sys/objects/pinit.c:
> 
>  /* user may set this BEFORE calling PetscInitialize() */
>  MPI_Comm PETSC_COMM_WORLD = MPI_COMM_NULL;
> 
> (and later)
> 
>  if (PETSC_COMM_WORLD == MPI_COMM_NULL) PETSC_COMM_WORLD = MPI_COMM_WORLD;
> 
> You can set PETSC_COMM_WORLD before PetscInitialize, but its value is
> not valid/useful before PetscInitialize.
> 
>> but when I run it prints
>> 
>> PETSC == WORLD: 0
>> PETSC_COMM_WORLD: 67108864
>> 
>> The first line uncommented gives:
>> 
>> PETSC == WORLD: 1
>> PETSC_COMM_WORLD: 1140850688
>> 
>> (and an error in my program: Attempting to use an MPI routine before initializing MPICH)
>> 
>> Just trying to understand what's going on...
>> 
>> Thanks and have a nice weekend!
>> 
>> Florian