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Re: "lang_statement_union_type and its usage?"
- To: xinant at cognigine dot com (Xinan Tang)
- Subject: Re: "lang_statement_union_type and its usage?"
- From: Ian Lance Taylor <ian at zembu dot com>
- Date: 31 May 2001 12:10:22 -0700
- Cc: binutils at sources dot redhat dot com
- References: <3B169422.firstname.lastname@example.org>
email@example.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
> 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
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.