diff --git a/libraries/AP_HAL_Linux/Semaphores.cpp b/libraries/AP_HAL_Linux/Semaphores.cpp
index 4fb6796640..0bcf98057c 100644
--- a/libraries/AP_HAL_Linux/Semaphores.cpp
+++ b/libraries/AP_HAL_Linux/Semaphores.cpp
@@ -6,6 +6,21 @@ extern const AP_HAL::HAL& hal;
 
 using namespace Linux;
 
+// construct a semaphore
+Semaphore::Semaphore()
+{
+    pthread_mutex_init(&_lock, nullptr);
+}
+
+// construct a recursive semaphore (allows a thread to take it more than once)
+Semaphore_Recursive::Semaphore_Recursive()
+{
+    pthread_mutexattr_t attr;
+    pthread_mutexattr_init(&attr);
+    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
+    pthread_mutex_init(&_lock, &attr);
+}
+
 bool Semaphore::give()
 {
     return pthread_mutex_unlock(&_lock) == 0;
@@ -33,3 +48,4 @@ bool Semaphore::take_nonblocking()
 {
     return pthread_mutex_trylock(&_lock) == 0;
 }
+
diff --git a/libraries/AP_HAL_Linux/Semaphores.h b/libraries/AP_HAL_Linux/Semaphores.h
index decec18e16..b43b547913 100644
--- a/libraries/AP_HAL_Linux/Semaphores.h
+++ b/libraries/AP_HAL_Linux/Semaphores.h
@@ -10,14 +10,17 @@ namespace Linux {
 
 class Semaphore : public AP_HAL::Semaphore {
 public:
-    Semaphore() {
-        pthread_mutex_init(&_lock, nullptr);
-    }
+    Semaphore();
     bool give();
     bool take(uint32_t timeout_ms);
     bool take_nonblocking();
-private:
+protected:
     pthread_mutex_t _lock;
 };
 
+class Semaphore_Recursive : public Semaphore {
+public:
+    Semaphore_Recursive();
+};
+    
 }