2 files changed, 37 insertions, 2 deletions
diff --git a/butl/diagnostics b/butl/diagnostics
index 56219ce..0c22337 100644
--- a/butl/diagnostics
+++ b/butl/diagnostics
@@ -21,10 +21,23 @@ namespace butl
   //
 
   // Diagnostics destination stream (std::cerr by default). Note that its
-  // modification is not MT-safe.
+  // modification is not MT-safe. Also note that concurrent writing to the
+  // stream from multiple threads can result in interleaved characters. To
+  // prevent this an object of diag_lock type (see below) must be created prior
+  // to write operation.
   //
   LIBBUTL_EXPORT extern std::ostream* diag_stream;
 
+  // Acquire the diagnostics exclusive access mutex in ctor, release in dtor.
+  // An object of the type must be created prior to writing to diag_stream (see
+  // above).
+  //
+  struct LIBBUTL_EXPORT diag_lock
+  {
+    diag_lock ();
+    ~diag_lock ();
+  };
+
   struct diag_record;
   template <typename> struct diag_prologue;
   template <typename> struct diag_mark;
diff --git a/butl/diagnostics.cxx b/butl/diagnostics.cxx
index 7dc8bd0..b692777 100644
--- a/butl/diagnostics.cxx
+++ b/butl/diagnostics.cxx
@@ -4,6 +4,7 @@
 
 #include <butl/diagnostics>
 
+#include <mutex>
 #include <iostream> // cerr
 
 using namespace std;
@@ -12,6 +13,18 @@ namespace butl
 {
   ostream* diag_stream = &cerr;
 
+  static mutex diag_mutex;
+
+  diag_lock::diag_lock ()
+  {
+    diag_mutex.lock ();
+  }
+
+  diag_lock::~diag_lock ()
+  {
+    diag_mutex.unlock ();
+  }
+
   void diag_record::
   flush () const
   {
@@ -20,7 +33,16 @@ namespace butl
       if (epilogue_ == nullptr)
       {
         os.put ('\n');
-        *diag_stream << os.str ();
+
+        {
+          diag_lock l;
+          *diag_stream << os.str ();
+        }
+
+        // We can endup flushing the result of several writes. The last one may
+        // possibly be incomplete, but that's not a problem as it will also be
+        // followed by the flush() call.
+        //
         diag_stream->flush ();
 
         empty_ = true;