add functions insturmentation example

Chapter02/instrument_functions.cpp

@@ -0,0 +1,32 @@
+#include <cstdio>
+
+// Stub DWT_CYCCNT register
+#define DWT_CYCCNT 100
+
+extern "C"
+{
+    __attribute__((no_instrument_function)) void __cyg_profile_func_enter(void *this_fn, void *call_site)
+    {
+
+        printf("entry, %p, %d\n", this_fn, DWT_CYCCNT);
+    }
+
+    __attribute__((no_instrument_function)) void __cyg_profile_func_exit(void *this_fn, void *call_site)
+    {
+
+        printf("entry, %p, %d\n", this_fn, DWT_CYCCNT);
+    }
+}
+
+void fun()
+{
+    printf("Hi from fun!\n");
+}
+
+int main()
+{
+    fun();
+    printf("Hi from main!\n");
+
+    return 0;
+}