From: deal Date: Thu, 20 May 2004 17:55:37 +0000 (+0000) Subject: Work around a nasty compiler problem... X-Git-Tag: v8.0.0~15158 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1e7103f92e5ae8c25fb0d83555d187d8a8ec4dd9;p=dealii.git Work around a nasty compiler problem... git-svn-id: https://svn.dealii.org/trunk@9271 0785d39b-7218-0410-832d-ea1e28bc413d --- diff --git a/deal.II/examples/step-17/step-17.cc b/deal.II/examples/step-17/step-17.cc index cb6527273b..357060eaee 100644 --- a/deal.II/examples/step-17/step-17.cc +++ b/deal.II/examples/step-17/step-17.cc @@ -282,12 +282,46 @@ void RightHandSide::vector_value_list (const std::vector > &poin // total number of processes and the rank of // the present process within the // communicator, respectively. + // + // Unfortunately, we have to work + // around an oddity in the way PETSc + // and some gcc versions interact. If + // we use PETSc's MPI dummy + // implementation, it expands the + // calls to the two MPI functions + // basically as ``(n_jobs=1, 0)'', + // i.e. it assigns the number one to + // the variable holding the number of + // jobs, and then uses the comma + // operator to let the entire + // expression have the value + // zero. The latter is important, + // since ``MPI_Comm_size'' returns an + // error code that we may want to + // check (we don't here, but one + // could in principle), and the trick + // with the comma operator makes sure + // that both the number of jobs is + // correctly assigned, and the return + // value is zero. Unfortunately, if + // some recent versions of gcc detect + // that the comma expression just + // stands by itself, i.e. the result + // is not assigned to another + // variable, then they warn + // ``right-hand operand of comma has + // no effect''. This unwanted side + // effect can be suppressed by + // casting the result of the entire + // expression to type ``void'' -- not + // beautiful, but helps calming down + // unwarranted compiler warnings... template unsigned int ElasticProblem::get_n_mpi_processes (const MPI_Comm &mpi_communicator) { int n_jobs; - MPI_Comm_size (mpi_communicator, &n_jobs); + (void)MPI_Comm_size (mpi_communicator, &n_jobs); return n_jobs; } @@ -299,7 +333,7 @@ unsigned int ElasticProblem::get_this_mpi_process (const MPI_Comm &mpi_communicator) { int rank; - MPI_Comm_rank (mpi_communicator, &rank); + (void)MPI_Comm_rank (mpi_communicator, &rank); return rank; }