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### 24.6 Registering custom attributes or pragmas

For analysis (or other) purposes it is useful to be able to add custom attributes or pragmas.

The `PLUGIN_ATTRIBUTES` callback is called during attribute registration. Use the `register_attribute` function to register custom attributes.

    /* Attribute handler callback */
    static tree
    handle_user_attribute (tree *node, tree name, tree args,
                           int flags, bool *no_add_attrs)
    {
      return NULL_TREE;
    }

    /* Attribute definition */
    static struct attribute_spec user_attr =
      { "user", 1, 1, false,  false, false, false, handle_user_attribute, NULL };

    /* Plugin callback called during attribute registration.
    Registered with register_callback (plugin_name, PLUGIN_ATTRIBUTES, register_attributes, NULL)
    */
    static void
    register_attributes (void *event_data, void *data)
    {
      warning (0, G_("Callback to register attributes"));
      register_attribute (&user_attr);
    }

The PLUGIN_PRAGMAS callback is called once during pragmas registration. Use the `c_register_pragma`, `c_register_pragma_with_data`, `c_register_pragma_with_expansion`, `c_register_pragma_with_expansion_and_data` functions to register custom pragmas and their handlers (which often want to call `pragma_lex`) from `c-family/c-pragma.h`.

    /* Plugin callback called during pragmas registration. Registered with
         register_callback (plugin_name, PLUGIN_PRAGMAS,
                            register_my_pragma, NULL);
    */
    static void
    register_my_pragma (void *event_data, void *data)
    {
      warning (0, G_("Callback to register pragmas"));
      c_register_pragma ("GCCPLUGIN", "sayhello", handle_pragma_sayhello);
    }

It is suggested to pass `"GCCPLUGIN"` (or a short name identifying your plugin) as the “space” argument of your pragma.

Pragmas registered with `c_register_pragma_with_expansion` or `c_register_pragma_with_expansion_and_data` support preprocessor expansions. For example:

    #define NUMBER 10
    #pragma GCCPLUGIN foothreshold (NUMBER)