Return the result of invoking the strlen simprocedure on `str_addr`.

def _sim_strlen(self, str_addr):
        """
        Return the result of invoking the strlen simprocedure on `str_addr`.
        """

        from .. import SIM_PROCEDURES
        strlen = SIM_PROCEDURES['libc']['strlen']

        return self.inline_call(strlen, str_addr).ret_expr