fixing work

source/base/data_out_base.cc

@@ -7689,6 +7689,10 @@ 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);
@@ -7718,7 +7722,7 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
       // Write the header on rank 0 and automatically move the
       // shared file pointer to the location after header;
       ierr = MPI_File_write_at(
-        fh, ss.str().c_str(), header_size, MPI_CHAR, MPI_STATUS_IGNORE);
+        fh, 0, ss.str().c_str(), header_size, MPI_CHAR, MPI_STATUS_IGNORE);
       AssertThrowMPI(ierr);
     }
 
@@ -7733,6 +7737,9 @@ 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,
@@ -7741,19 +7748,48 @@ DataOutInterface<dim, spacedim>::write_vtu_in_parallel(
                                   vtk_flags,
                                   ss);
 
-    ierr = MPI_File_write_at_all_c(
-      fh, ss.str().c_str(), ss.str().size(), MPI_CHAR, MPI_STATUS_IGNORE);
+    //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);
+
+    ierr = MPI_Bcast(&header_size, 1, MPI_UNSIGNED, 0, comm);
+    AssertThrowMPI(ierr);
+
+    offset = static_cast<MPI_Offset>(header_size) * sizeof( char ) + prefix_sum;
+
+    ierr = MPI_File_write_at_all(
+      fh, offset, ss.str().c_str(), ss.str().size(), MPI_CHAR, MPI_STATUS_IGNORE);
     AssertThrowMPI(ierr);
   }
 
   // write footer
-  if (myrank == 0)
+  if (myrank == rank-1)
     {
       std::stringstream ss;
       DataOutBase::write_vtu_footer(ss);
       unsigned int footer_size = ss.str().size();
-      ierr = MPI_File_write_at(
-        fh, ss.str().c_str(), footer_size, MPI_CHAR, MPI_STATUS_IGNORE);
+
+      ierr = MPI_Bcast(&offset, 1, MPI_UNSIGNED_LONG_LONG, rank-1,comm);
+      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);
+
+      offset = offset + static_cast<MPI_Offset>(size_placeholder) * sizeof( char );
+      // offset += size_on_proc;
+
+      // Writing Footer.
+      ierr = MPI_File_write_at(fh, offset, ss.str().c_str(), footer_size, MPI_CHAR, MPI_STATUS_IGNORE);
       AssertThrowMPI(ierr);
     }
   ierr = MPI_File_close(&fh);