refactor mstimer callback handling (#400)

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

src/bacnet/basic/sys/mstimer.c

@@ -22,6 +22,60 @@
 #include <stdint.h>
 #include "bacnet/basic/sys/mstimer.h"
 
+/* callback data head of list */
+static struct mstimer_callback_data_t *Callback_Head;
+
+/**
+ * Handles an interrupt from a hardware millisecond timer
+ */
+void mstimer_callback_handler(void)
+{
+    struct mstimer_callback_data_t *cb;
+
+    cb = (struct mstimer_callback_data_t *)Callback_Head;
+    while (cb) {
+        if (mstimer_expired(&cb->timer)) {
+            cb->callback();
+            if (mstimer_interval(&cb->timer) > 0) {
+                mstimer_reset(&cb->timer);
+            }
+        }
+        cb = cb->next;
+    }
+}
+
+/**
+ * Configures and enables a repeating callback function
+ *
+ * @param new_cb - pointer to #mstimer_callback_data_t
+ * @param callback - pointer to a #timer_callback_function function
+ * @param milliseconds - how often to call the function
+ */
+void mstimer_callback(struct mstimer_callback_data_t *new_cb,
+    mstimer_callback_function callback,
+    unsigned long milliseconds)
+{
+    struct mstimer_callback_data_t *cb;
+
+    if (new_cb) {
+        new_cb->callback = callback;
+        mstimer_set(&new_cb->timer, milliseconds);
+    }
+    if (Callback_Head) {
+        cb = (struct mstimer_callback_data_t *)Callback_Head;
+        while (cb) {
+            if (!cb->next) {
+                cb->next = new_cb;
+                break;
+            } else {
+                cb = cb->next;
+            }
+        }
+    } else {
+        Callback_Head = new_cb;
+    }
+}
+
 /**
  * @brief Set a timer for a time sometime in the future
  *

src/bacnet/basic/sys/mstimer.h

@@ -35,8 +35,9 @@ struct mstimer {
   unsigned long interval;
 };
 
+/* optional callback function form */
 typedef void (*mstimer_callback_function) (void);
-/* callback data structure */
+/* optional callback data structure */
 struct mstimer_callback_data_t;
 struct mstimer_callback_data_t {
     struct mstimer timer;
@@ -62,15 +63,18 @@ BACNET_STACK_EXPORT
 unsigned long mstimer_elapsed(struct mstimer *t);
 BACNET_STACK_EXPORT
 unsigned long mstimer_interval(struct mstimer *t);
-/* HAL implementation */
-BACNET_STACK_EXPORT
-unsigned long mstimer_now(void);
+/* optional callback timer support for embedded systems */
 BACNET_STACK_EXPORT
 void mstimer_callback(
     struct mstimer_callback_data_t *cb,
     mstimer_callback_function callback,
     unsigned long milliseconds);
 BACNET_STACK_EXPORT
+void mstimer_callback_handler(void);
+/* HAL implementation */
+BACNET_STACK_EXPORT
+unsigned long mstimer_now(void);
+BACNET_STACK_EXPORT
 void mstimer_init(void);
 
 #ifdef __cplusplus