diff --git a/src/main/main.cc b/src/main/main.cc
index 97c17c486..9b949b1d9 100644
--- a/src/main/main.cc
+++ b/src/main/main.cc
@@ -40,6 +40,7 @@
 #include <fstream>                                     // for ofstream
 #include <iostream>                                    // for operator<<
 #include <memory>                                      // for unique_ptr
+#include <mutex>                                       // for lock_guard, mutex
 #include <ostream>                                     // for std::flush
 #include <string>                                      // for string
 
@@ -81,10 +82,12 @@ public:
     }
     void Send(const absl::LogEntry& entry) override
     {
+        std::lock_guard<std::mutex> lock(logfile_mutex);
         logfile << entry.text_message_with_prefix_and_newline() << std::flush;
     }
 
 private:
+    std::mutex logfile_mutex;
     std::ofstream logfile;
     std::string filename;
 };
diff --git a/tests/single_test_main.cc b/tests/single_test_main.cc
index 9a340de1b..9537a1f09 100644
--- a/tests/single_test_main.cc
+++ b/tests/single_test_main.cc
@@ -24,6 +24,7 @@
 #include <fstream>
 #include <iostream>
 #include <memory>
+#include <mutex>
 #include <ostream>
 #include <string>
 
@@ -61,10 +62,12 @@ public:
     }
     void Send(const absl::LogEntry& entry) override
     {
+        std::lock_guard<std::mutex> lock(logfile_mutex);
         logfile << entry.text_message_with_prefix_and_newline() << std::flush;
     }
 
 private:
+    std::mutex logfile_mutex;
     std::ofstream logfile;
 };
 
diff --git a/tests/system-tests/position_test.cc b/tests/system-tests/position_test.cc
index 0d30c1e8a..df90734ad 100644
--- a/tests/system-tests/position_test.cc
+++ b/tests/system-tests/position_test.cc
@@ -47,6 +47,7 @@
 #include <fstream>
 #include <iomanip>
 #include <memory>
+#include <mutex>
 #include <numeric>
 #include <ostream>
 #include <string>
@@ -89,10 +90,12 @@ public:
     }
     void Send(const absl::LogEntry& entry) override
     {
+        std::lock_guard<std::mutex> lock(logfile_mutex);
         logfile << entry.text_message_with_prefix_and_newline() << std::flush;
     }
 
 private:
+    std::mutex logfile_mutex;
     std::ofstream logfile;
 };
 #endif
diff --git a/tests/test_main.cc b/tests/test_main.cc
index b1ef7ea70..15528adf5 100644
--- a/tests/test_main.cc
+++ b/tests/test_main.cc
@@ -22,6 +22,7 @@
 #include <fstream>
 #include <iostream>
 #include <memory>
+#include <mutex>
 #include <ostream>
 #include <string>
 
@@ -63,10 +64,12 @@ public:
     }
     void Send(const absl::LogEntry& entry) override
     {
+        std::lock_guard<std::mutex> lock(logfile_mutex);
         logfile << entry.text_message_with_prefix_and_newline() << std::flush;
     }
 
 private:
+    std::mutex logfile_mutex;
     std::ofstream logfile;
     std::string filename;
 };
diff --git a/utils/front-end-cal/main.cc b/utils/front-end-cal/main.cc
index 47bd1822f..6ff908fe1 100644
--- a/utils/front-end-cal/main.cc
+++ b/utils/front-end-cal/main.cc
@@ -63,6 +63,7 @@
 #include <iostream>
 #include <map>
 #include <memory>
+#include <mutex>
 #include <stdexcept>  // for logic_error
 #include <string>
 #include <thread>
@@ -122,10 +123,12 @@ public:
     }
     void Send(const absl::LogEntry& entry) override
     {
+        std::lock_guard<std::mutex> lock(logfile_mutex);
         logfile << entry.text_message_with_prefix_and_newline() << std::flush;
     }
 
 private:
+    std::mutex logfile_mutex;
     std::ofstream logfile;
 };