(define-gboxed-opaque error "GError"
  :export t
  :type-initializer "g_error_get_type"
  :alloc (cl:error "GError cannot be created from the Lisp side."))  

The glib:error structure contains information about an error that has occurred. This boxed type is exported for use in signal handlers or callback functions, that take a g:error instance as an argument. There are no exported functions for g:error instances. Errors from C functions either create and signal an error condition, or the error is ignored and a suitable value is returned from the Lisp function.