try the following approach: use rb_sourcefile to get the current file…

… as C string and run all checks on that. Only if all checks are successful, get the current file as VALUE from profile frames.

ext/datadog_cov/datadog_cov.c

@@ -106,24 +106,7 @@ static void dd_cov_update_coverage(rb_event_flag_t event, VALUE data, VALUE self
   struct dd_cov_data *dd_cov_data;
   TypedData_Get_Struct(data, struct dd_cov_data, &dd_cov_data_type, dd_cov_data);
 
-  int captured_frames = rb_profile_frames(
-      0 /* stack starting depth */,
-      1, /* number of stack frames to return */
-      dd_cov_data->frame_buffer,
-      NULL);
-
-  if (captured_frames == 0)
-  {
-    return;
-  }
-
-  VALUE filename = rb_profile_frame_path(dd_cov_data->frame_buffer[0]);
-  if (filename == Qnil)
-  {
-    return;
-  }
-
-  const char *c_filename = RSTRING_PTR(filename);
+  const char *c_filename = rb_sourcefile();
 
   // skip if we cover the same file again
   uintptr_t current_filename_ptr = (uintptr_t)c_filename;
@@ -134,7 +117,7 @@ static void dd_cov_update_coverage(rb_event_flag_t event, VALUE data, VALUE self
   dd_cov_data->last_filename_ptr = current_filename_ptr;
 
   // if given filename is not located under the root, we skip it
-  if (dd_cov_data->root_len == 0 || strncmp(dd_cov_data->root, c_filename, dd_cov_data->root_len) != 0)
+  if (strncmp(dd_cov_data->root, c_filename, dd_cov_data->root_len) != 0)
   {
     return;
   }
@@ -146,6 +129,23 @@ static void dd_cov_update_coverage(rb_event_flag_t event, VALUE data, VALUE self
     return;
   }
 
+  int captured_frames = rb_profile_frames(
+      0 /* stack starting depth */,
+      1, /* number of stack frames to return */
+      dd_cov_data->frame_buffer,
+      NULL);
+
+  if (captured_frames == 0)
+  {
+    return;
+  }
+
+  VALUE filename = rb_profile_frame_path(dd_cov_data->frame_buffer[0]);
+  if (filename == Qnil)
+  {
+    return;
+  }
+
   rb_hash_aset(dd_cov_data->coverage, filename, Qtrue);
 }
 
@@ -154,8 +154,14 @@ static VALUE dd_cov_start(VALUE self)
   // get current thread
   VALUE thval = rb_thread_current();
 
-  // add event hook
-  rb_thread_add_event_hook(thval, dd_cov_update_coverage, RUBY_EVENT_LINE, self);
+  struct dd_cov_data *dd_cov_data;
+  TypedData_Get_Struct(self, struct dd_cov_data, &dd_cov_data_type, dd_cov_data);
+
+  if (dd_cov_data->root_len != 0)
+  {
+    // add event hook
+    rb_thread_add_event_hook(thval, dd_cov_update_coverage, RUBY_EVENT_LINE, self);
+  }
 
   return self;
 }