From owner-freebsd-current  Thu Oct  5  4: 4: 1 2000
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Date: Thu, 5 Oct 2000 22:03:47 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Jeffrey Hsu <hsu@FreeBSD.ORG>
Cc: current@FreeBSD.ORG
Subject: Re: #include <struct.h> in <sys/queue.h>
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On Tue, 3 Oct 2000, Jeffrey Hsu wrote:

>   > I think <struct.h> should have been removed when offsetof() became
>   > standard.
> 
> At last count, offsetof() is defined in 11 different places in the kernel:
>     alpha/alpha/machdep.c, line 234 
>...

Some other places include <stddef.h>.

> If we unify it in one header file, I'd be happy to use that for STAILQ_LAST().

<machine/ansi.h> is a good place for it now.

>   > strbase() is now used in <sys/queue.h>.  It is
>   > easy to implement directly using offsetof().  Unfortunately, if it is
>   > implemented using offsetof(), then <sys/queue.h> will depend on
>   > <stddef.h>.
> 
> So, what's the correct solution?

I'm currently using the following.  This avoids a bug in fldoff(): casting
to int is wrong and causes warnings on alphas.  I use a cast to __uintptr_t
since casting to size_t is normally right (offsetof() depends on it), and
__uintptr_t is normally equivalent to size_t and is easier to get at.

---
diff -c2 queue.h~ queue.h
*** queue.h~	Fri Aug  4 21:38:55 2000
--- queue.h	Thu Oct  5 21:22:51 2000
***************
*** 38,43 ****
  #define	_SYS_QUEUE_H_
  
- #include <struct.h>
- 
  /*
   * This file defines five types of data structures: singly-linked lists,
--- 38,41 ----
***************
*** 224,231 ****
  } while (0)
  
  #define	STAILQ_LAST(head, type, field)					\
! 	(STAILQ_EMPTY(head) ?						\
! 		NULL :							\
! 		strbase(type, (head)->stqh_last, field))
  
  #define	STAILQ_NEXT(elm, field)	((elm)->field.stqe_next)
--- 222,239 ----
  } while (0)
  
+ /*
+  * XXX auxiliary macros for STAILQ_LAST() adapted from offsetof() in
+  * <stddef.h> and strbase() in <struct.h>.
+  */
+ #include <machine/ansi.h>
+ 
+ #define	__offsetof(type, member)					\
+ 	((__uintptr_t)(&((type *)0)->member))
+ #define	__strbase(name, addr, field)					\
+ 	((struct name *)((char *)(addr) - __offsetof(struct name, field)))
+ 
  #define	STAILQ_LAST(head, type, field)					\
! 	(STAILQ_EMPTY(head) ? NULL :					\
! 	    __strbase(type, (head)->stqh_last, field))
  
  #define	STAILQ_NEXT(elm, field)	((elm)->field.stqe_next)
---

Bruce



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