Add start function for separating object allocation from thread initialization

Allows threads to started separately from when they are declared,
avoiding the need to dynamically allocate threads at runtime.

core/mbed-rtos/rtos/Thread.cpp

@@ -26,8 +26,22 @@
 
 namespace rtos {
 
+Thread::Thread() {
+    _tid = NULL;
+}
+
 Thread::Thread(void (*task)(void const *argument), void *argument,
         osPriority priority, uint32_t stack_size, unsigned char *stack_pointer) {
+    _tid = NULL;
+    start(task, argument, priority, stack_size, stack_pointer);
+}
+
+osStatus Thread::start(void (*task)(void const *argument), void *argument,
+        osPriority priority, uint32_t stack_size, unsigned char *stack_pointer) {
+    if (_tid != NULL) {
+        return osErrorResource;
+    }
+
 #ifdef __MBED_CMSIS_RTOS_CM
     _thread_def.pthread = task;
     _thread_def.tpriority = priority;
@@ -48,6 +62,11 @@ Thread::Thread(void (*task)(void const *argument), void *argument,
     }
 #endif
     _tid = osThreadCreate(&_thread_def, argument);
+    if (_tid == NULL) {
+        return osErrorResource;
+    }
+
+    return osOK;
 }
 
 osStatus Thread::terminate() {

core/mbed-rtos/rtos/Thread.h

@@ -30,6 +30,10 @@ namespace rtos {
 /** The Thread class allow defining, creating, and controlling thread functions in the system. */
 class Thread {
 public:
+    /** Allocate a new thread without starting execution
+    */
+    Thread();
+
     /** Create a new thread, and start it executing the specified function.
       @param   task           function to be executed by this thread.
       @param   argument       pointer that is passed to the thread function as start argument. (default: NULL).
@@ -42,6 +46,19 @@ public:
            uint32_t stack_size=DEFAULT_STACK_SIZE,
            unsigned char *stack_pointer=NULL);
 
+    /** Starts a thread executing the specified function.
+      @param   task           function to be executed by this thread.
+      @param   argument       pointer that is passed to the thread function as start argument. (default: NULL).
+      @param   priority       initial priority of the thread function. (default: osPriorityNormal).
+      @param   stack_size      stack size (in bytes) requirements for the thread function. (default: DEFAULT_STACK_SIZE).
+      @param   stack_pointer  pointer to the stack area to be used by this thread (default: NULL).
+      @return  status code that indicates the execution status of the function.
+    */
+    osStatus start(void (*task)(void const *argument), void *argument=NULL,
+                   osPriority priority=osPriorityNormal,
+                   uint32_t stack_size=DEFAULT_STACK_SIZE,
+                   unsigned char *stack_pointer=NULL);
+
     /** Terminate execution of a thread and remove it from Active Threads
       @return  status code that indicates the execution status of the function.
     */