diff --git a/exercise4/wave_2d_parallel.c b/exercise4/wave_2d_parallel.c
index 6e3442e..841fe97 100644
--- a/exercise4/wave_2d_parallel.c
+++ b/exercise4/wave_2d_parallel.c
@@ -74,39 +74,44 @@ void move_buffer_window(void) {
 }
 
 // TASK: T8
-// Save the present time step in a numbered file under 'data/'
 void domain_save(int_t step) {
   // BEGIN: T8
   char filename[256];
 
   // Ensure output directory exists (ignore error if it already exists)
-  if (mkdir("data", 0755) != 0 && errno != EEXIST) {
-    perror("mkdir data");
-    exit(EXIT_FAILURE);
+  if (world_rank == ROOT) {
+    if (mkdir("data", 0755) != 0 && errno != EEXIST) {
+      perror("mkdir data");
+      exit(EXIT_FAILURE);
+    }
   }
 
+  MPI_Barrier(MPI_COMM_WORLD);
+
   snprintf(filename, sizeof(filename), "data/%05" PRId64 ".dat", step);
 
-  FILE *out = fopen(filename, "wb");
-  if (out == NULL) {
+  MPI_File fh;
+  int rc = MPI_File_open(cartesian_comm, filename, MPI_MODE_CREATE | MPI_MODE_WRONLY, MPI_INFO_NULL, &fh);
+  if (rc != MPI_SUCCESS) {
     perror("fopen output file");
     fprintf(stderr, "Failed to open '%s' for writing.\n", filename);
     exit(EXIT_FAILURE);
   }
 
-  for (int_t i = 0; i < M; ++i) {
-    size_t written = fwrite(&U(i, 0), sizeof(real_t), (size_t)N, out);
-    if (written != (size_t)N) {
+  MPI_Status status;
+  for (int_t i = 0; i < local_M; i++) {
+    MPI_Offset offset = ((coordinates[0] * local_M + i) * N // global row position
+                         + coordinates[1] * local_N) *
+                        sizeof(real_t);
+    rc = MPI_File_write_at(fh, offset, &U(i, 0), local_N, MPI_DOUBLE, &status);
+    if (rc != MPI_SUCCESS) {
       perror("fwrite");
-      fclose(out);
+      MPI_File_close(&fh);
       exit(EXIT_FAILURE);
     }
   }
 
-  if (fclose(out) != 0) {
-    perror("fclose");
-    exit(EXIT_FAILURE);
-  }
+  MPI_File_close(&fh);
   // END: T8
 }