diff --git a/nanomodbus.c b/nanomodbus.c
index 42cb2ed..6d3ddd7 100644
--- a/nanomodbus.c
+++ b/nanomodbus.c
@@ -1575,6 +1575,11 @@ nmbs_error nmbs_server_poll(nmbs_t* nmbs) {
 
     return NMBS_ERROR_NONE;
 }
+
+void nmbs_set_callbacks_arg(nmbs_t* nmbs, void* arg)
+{
+    nmbs->callbacks.arg = arg;
+}
 #endif
 
 
diff --git a/nanomodbus.h b/nanomodbus.h
index b6eb79d..00331dd 100644
--- a/nanomodbus.h
+++ b/nanomodbus.h
@@ -195,7 +195,7 @@ typedef struct nmbs_callbacks {
 #endif
 #endif
 
-    char _nonempty;    // Struct may become empty, which is undefined behavior
+    void* arg;    // User data, will be passed to functions above
 } nmbs_callbacks;
 
 
@@ -272,6 +272,12 @@ nmbs_error nmbs_server_create(nmbs_t* nmbs, uint8_t address_rtu, const nmbs_plat
  * @return NMBS_ERROR_NONE if successful, other errors otherwise.
  */
 nmbs_error nmbs_server_poll(nmbs_t* nmbs);
+
+/** Set the pointer to user data argument passed to server request callbacks.
+ * @param  nmbs pointer to the nmbs_t instance
+ * @param  arg user data argument
+ */
+void nmbs_set_callbacks_arg(nmbs_t* nmbs, void* arg);
 #endif
 
 #ifndef NMBS_CLIENT_DISABLED