Remove the broadcast and fix some comment

source/base/data_out_base.cc

@@ -7689,11 +7689,6 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
 #else
 
   const int myrank = Utilities::MPI::this_mpi_process(comm);
-  int rank;
-  MPI_Comm_size(comm, &rank);
-  MPI_Offset offset;
-  std::uint64_t size_placeholder;
-
   MPI_Info info;
   int ierr = MPI_Info_create(&info);
   AssertThrowMPI(ierr);
@@ -7711,16 +7706,20 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
   ierr = MPI_Info_free(&info);
   AssertThrowMPI(ierr);
 
+  // Define offset and header size so we can broadcast and send them
+  // to specific processors.
   unsigned int header_size;
+  MPI_Offset offset;
+  int size;
+  MPI_Comm_size(MPI_COMM_WORLD, &size);
 
   // write header
   if (myrank == 0)
     {
       std::stringstream ss;
       DataOutBase::write_vtu_header(ss, vtk_flags);
       header_size = ss.str().size();
-      // Write the header on rank 0 and automatically move the
-      // shared file pointer to the location after header;
+      // Write the header on rank 0 at the starting of a file, i.e., offset 0.
       ierr = MPI_File_write_at(
         fh, 0, ss.str().c_str(), header_size, MPI_CHAR, MPI_STATUS_IGNORE);
       AssertThrowMPI(ierr);
@@ -7737,9 +7736,6 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
     // the first piece written. But if nobody has any pieces to write (file is
     // empty), let processor 0 write their empty data, otherwise the vtk file is
     // invalid.
-
-
-
     std::stringstream ss;
     if (my_n_patches > 0 || (global_n_patches == 0 && myrank == 0))
       DataOutBase::write_vtu_main(patches,
@@ -7749,17 +7745,17 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
                                   ss);
 
     // using prefix sum to find specific offset to write at.
-
     const std::uint64_t size_on_proc = ss.str().size();
-    size_placeholder = size_on_proc;
     std::uint64_t prefix_sum = 0;
     ierr = MPI_Exscan(&size_on_proc, &prefix_sum, 1, MPI_CHAR, MPI_SUM, comm);
     AssertThrowMPI(ierr);
 
+    // Boradcasting the header size to all processor.
     ierr = MPI_Bcast(&header_size, 1, MPI_UNSIGNED, 0, comm);
     AssertThrowMPI(ierr);
 
-    offset = static_cast<MPI_Offset>(header_size) * sizeof(char) + prefix_sum;
+    // Locate specific offset for each processor.
+    offset = static_cast<MPI_Offset>(header_size) + prefix_sum;
 
     ierr = MPI_File_write_at_all(fh,
                                  offset,
@@ -7768,25 +7764,44 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
                                  MPI_CHAR,
                                  MPI_STATUS_IGNORE);
     AssertThrowMPI(ierr);
+
+    // Sending the size_on_proc and offset of last rank to rank 0 to write
+    // footer. So offset + size_on_proc is where the header should start
+    // writing.
+    if (myrank == size - 1)
+      {
+        ierr = MPI_Send(&size_on_proc, 1, MPI_UINT64_T, 0, 0, comm);
+        AssertThrowMPI(ierr);
+        ierr = MPI_Send(&offset, 1, MPI_UNSIGNED_LONG_LONG, 0, 0, comm);
+        AssertThrowMPI(ierr);
+      }
   }
 
   // write footer
-  if (myrank == rank - 1)
+  if (myrank == 0)
     {
       std::stringstream ss;
       DataOutBase::write_vtu_footer(ss);
       unsigned int footer_size = ss.str().size();
 
-      ierr = MPI_Bcast(&offset, 1, MPI_UNSIGNED_LONG_LONG, rank - 1, comm);
+      // Receiving size_of_proc;
+      std::uint64_t size_on_proc;
+      ierr = MPI_Recv(
+        &size_on_proc, 1, MPI_UINT64_T, size - 1, 0, comm, MPI_STATUS_IGNORE);
       AssertThrowMPI(ierr);
 
-      // std::uint64_t size_on_proc;
-      // ierr = MPI_Bcast(&size_on_proc, 1, MPI_INT64_T, rank-1, comm);
-      ierr = MPI_Bcast(&size_placeholder, 1, MPI_INT64_T, rank - 1, comm);
+      // Receiving Offset;
+      ierr = MPI_Recv(&offset,
+                      1,
+                      MPI_UNSIGNED_LONG_LONG,
+                      size - 1,
+                      0,
+                      comm,
+                      MPI_STATUS_IGNORE);
+      AssertThrowMPI(ierr);
 
-      offset =
-        offset + static_cast<MPI_Offset>(size_placeholder) * sizeof(char);
-      // offset += size_on_proc;
+      // Calculating offset to write footer.
+      offset = offset + static_cast<MPI_Offset>(size_on_proc);
 
       // Writing Footer.
       ierr = MPI_File_write_at(