"lang_statement_union_type and its usage?"

[Ian Lance Taylor]

xinant@cognigine.com (Xinan Tang) writes:

>   In ld/ldlang.h, lang_statement_union_type is defined as:
> 
> typedef union lang_statement_union
> {
>   lang_statement_header_type header;
>   union lang_statement_union *next;
>   ... /* Other stuffs */
> } lang_statement_union_type;
> 
> 
>   However, I saw in many places in `ldlang.c', the list is traversed as
> follows:
> 
>    for (; s != (lang_statement_union_type *) NULL; s = s->next)
>     {
>       switch (s->header.type)
>     ...
>     }
> 
>   The question is that the 'next' and 'header' fields are exclusive, how
> could they be used at the same time. Could some one inlight me why
> this usage is SAFE?
> 
>   My goal is to traverse the statement list at the linking time. I have
> difficulty to understand why the list is searched in this way.

All elements of lang_statement_union start with
  lang_statement_header_type header;
The first field in lang_statement_header_type is
  union lang_statement_union *next;

Therefore, in all cases, the first field in any instance of
lang_statement_union will be the next pointer.

This is a pretty common hack when implementing derived classes in C.

Ian