From 157320427d0cc7627e68bb41189c41102ebf79d9 Mon Sep 17 00:00:00 2001
From: yxpw <yxpw@no-reply.github.com>
Date: Wed, 13 May 2026 11:42:31 +0800
Subject: [PATCH] fix: fix MPI race condition in permittivity file output

Move root rank check before opening the output file to prevent multiple
MPI processes from simultaneously creating/truncating the file. This
fixes intermittent null bytes appearing at the beginning of
permittivity.dat when running with multiple processes.
---
 src/functions/optics/permittivity.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/functions/optics/permittivity.cpp b/src/functions/optics/permittivity.cpp
index 8642ed9..e055a2d 100644
--- a/src/functions/optics/permittivity.cpp
+++ b/src/functions/optics/permittivity.cpp
@@ -92,13 +92,13 @@ void CalcOptPermit::at_k(FracCoord k_frac) {
 
 
 void CalcOptPermit::save_data() {
+    // Only the root rank output data!
+    if (!UtilsMPI::i_am_root_rank()) return;
     // Check and prepare
     UtilsMisc::check_output_directory_is_ok();
     std::ofstream outputFile(PERMITTIVITY_OUT);
     // Check if the file open with no error
     if (!outputFile) UtilsMPI::Abort() << "Fail to create the output file.";
-    // Only the root rank output data!
-    if (!UtilsMPI::i_am_root_rank()) return;
     // Write the data to file
     for (size_t i_w=0; i_w<omega_num; ++i_w) {
         outputFile << omega_list[i_w] << "    " << global_optics_values[i_w].real()  << "    " << global_optics_values[i_w].imag() << std::endl;