From: deal <deal@0785d39b-7218-0410-832d-ea1e28bc413d>
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<dim>::vector_value_list (const std::vector<Point<dim> > &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 <int dim>
 unsigned int
 ElasticProblem<dim>::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<dim>::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;
 }