diff --git a/lib/jt9a.f90 b/lib/jt9a.f90
index 5ea7ac949..13765e9b1 100644
--- a/lib/jt9a.f90
+++ b/lib/jt9a.f90
@@ -16,7 +16,6 @@ subroutine jt9a()
 
   integer*2 id2a(180000)
   integer*1 attach_jt9
-!  integer*1 lock_jt9,unlock_jt9
   integer size_jt9
 ! Multiple instances:
   character*80 mykey
@@ -41,11 +40,14 @@ subroutine jt9a()
   call c_f_pointer(address_jt9(),shared_data)
 
 ! Wait here until GUI has set ss(2,1) to 1.0
-10 if(shared_data%ss(2,1).eq.999.0) then
+10 call lock_jt9()
+  if(shared_data%ss(2,1).eq.999.0) then
+     call unlock_jt9()
      i1=detach_jt9()
      go to 999
   endif
-   if(shared_data%ss(2,1).ne.1.0) then
+  if(shared_data%ss(2,1).ne.1.0) then
+     call unlock_jt9()
      call sleep_msec(msdelay)
      go to 10
   endif
@@ -53,11 +55,13 @@ subroutine jt9a()
 
   nbytes=size_jt9()
   if(nbytes.le.0) then
+     call unlock_jt9()
      print*,'jt9a: Shared memory mem_jt9 does not exist.'
      print*,"Must start 'jt9 -s <thekey>' from within WSJT-X."
      go to 999
   endif
   local_params=shared_data%params !save a copy because wsjtx carries on accessing  
+  call unlock_jt9()
   call flush(6)
   call timer('decoder ',0)
   if(local_params%nmode.eq.8 .and. local_params%ndiskdat) then
@@ -80,11 +84,14 @@ subroutine jt9a()
 
 
 ! Wait here until GUI routine decodeDone() has set ss(3,1) to 1.0
-100  if(shared_data%ss(3,1).ne.1.0) then
+100 call lock_jt9()
+  if(shared_data%ss(3,1).ne.1.0) then
+     call unlock_jt9()
      call sleep_msec(msdelay)
      go to 100
   endif
   shared_data%ss(3,1)=0.
+  call unlock_jt9()
   go to 10
   
 999 call timer('decoder ',101)
diff --git a/main.cpp b/main.cpp
index fc791adec..74174e378 100644
--- a/main.cpp
+++ b/main.cpp
@@ -364,7 +364,9 @@ int main(int argc, char *argv[])
               throw std::runtime_error {"Shared memory error"};
             }
           }
+          mem_jt9.lock ();
           memset(mem_jt9.data(),0,sizeof(struct dec_data)); //Zero all decoding params in shared memory
+          mem_jt9.unlock ();
 
           unsigned downSampleFactor;
           {
diff --git a/widgets/mainwindow.cpp b/widgets/mainwindow.cpp
index b4c59f79d..52394c8a5 100644
--- a/widgets/mainwindow.cpp
+++ b/widgets/mainwindow.cpp
@@ -3011,7 +3011,9 @@ void MainWindow::decode()                                       //decode()
         &narg[0],&m_TRperiod,&m_msg[0][0],
         dec_data.params.mycall,dec_data.params.hiscall,8000,12,12)));
   } else {
+    mem_jt9->lock ();
     memcpy(to, from, qMin(mem_jt9->size(), size));
+    mem_jt9->unlock ();
 
     auto now = QDateTime::currentDateTimeUtc();
     double tsec = fmod(double(now.toMSecsSinceEpoch()),86400000.0)/1000.0;
@@ -3072,11 +3074,13 @@ void::MainWindow::fast_decode_done()
 void MainWindow::to_jt9(qint32 n, qint32 istart, qint32 idone)
 {
   float ss0[3];
+  mem_jt9->lock ();
   memcpy(ss0,(char*)mem_jt9->data(),12);
   ss0[0]=n;
   if(istart>=0) ss0[1]=istart;
   if(idone>=0)  ss0[2]=idone;
   memcpy((char*)mem_jt9->data(),ss0,12);
+  mem_jt9->unlock ();
 }
 
 void MainWindow::decodeDone ()