freeing the memory from build_string

[Jarek Czekalski]

I was trying to google for Elisp memory management, but found nothing.

I don't understand in what way the memory is freed, that is allocated by 
build_string. The use case is in keyboard.c, cmd_error_internal.

    call3 (Vcommand_error_function, data,
       context ? build_string (context) : empty_unibyte_string,
       Vsignaling_function);

Here "context" is a "const char *" function argument, passed to 
cmd_error_internal.

The function that receives the lisp string context is a user function, 
usually not used. What makes as sure, that the memory allocated for the 
string inside lisp object will ever be returned?

Thanks,
Jarek