diff --git a/drivers/RawSerial.cpp b/drivers/RawSerial.cpp
index b473fddae4..1424c6c9af 100644
--- a/drivers/RawSerial.cpp
+++ b/drivers/RawSerial.cpp
@@ -63,9 +63,16 @@ int RawSerial::puts(const char *str)
 // length is above a certain threshold, otherwise we use just the stack.
 int RawSerial::printf(const char *format, ...)
 {
-    lock();
     std::va_list arg;
     va_start(arg, format);
+    int len = this->vprintf(format, arg);
+    va_end(arg);
+    return len;
+}
+
+int RawSerial::vprintf(const char *format, std::va_list arg)
+{
+    lock();
     // ARMCC microlib does not properly handle a size of 0.
     // As a workaround supply a dummy buffer with a size of 1.
     char dummy_buf[1];
@@ -80,7 +87,6 @@ int RawSerial::printf(const char *format, ...)
         puts(temp);
         delete[] temp;
     }
-    va_end(arg);
     unlock();
     return len;
 }
diff --git a/drivers/RawSerial.h b/drivers/RawSerial.h
index 4841eb9038..0e6c9c7c8f 100644
--- a/drivers/RawSerial.h
+++ b/drivers/RawSerial.h
@@ -25,6 +25,7 @@
 #include "drivers/SerialBase.h"
 #include "hal/serial_api.h"
 #include "platform/NonCopyable.h"
+#include <cstdarg>
 
 namespace mbed {
 /** \addtogroup drivers */
@@ -89,6 +90,7 @@ public:
     int puts(const char *str);
 
     int printf(const char *format, ...) MBED_PRINTF_METHOD(1, 2);
+    int vprintf(const char *format, std::va_list arg);
 
 #if !(DOXYGEN_ONLY)
 protected: