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From: John Baldwin <jhb@FreeBSD.org>
To: des@des.no (Dag-Erling =?iso-8859-1?q?Sm=F8rgrav?=),
	alpha@freebsd.org
Date: Thu, 8 Jan 2004 15:42:49 -0500
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Subject: Re: structure padding
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On Thursday 08 January 2004 03:14 pm, Dag-Erling Sm=F8rgrav wrote:
> In -STABLE, struct kinfo_proc is currently defined as follows:
>
> struct kinfo_proc {
> 	struct	proc kp_proc;
> 	struct	eproc {
>                 /* ... */
> 		char	e_login[roundup(MAXLOGNAME, sizeof(long))];
> 		long	e_spare[2];
> 	} kp_eproc;
> };
>
> I want to add an e_sid field to hold the process's session ID:
>
> struct kinfo_proc {
> 	struct	proc kp_proc;
> 	struct	eproc {
>                 /* ... */
> 		char	e_login[roundup(MAXLOGNAME, sizeof(long))];
>                 pid_t   e_sid;
> 		long	e_spare[1];
> 	} kp_eproc;
> };
>
> However, a pid_t is an int, and sizeof int !=3D sizeof long on Alpha.
> I'm not sure what will happen: will alignment rules cause gcc to add
> four bytes of padding before e_spare, conserving the size of struct
> kinfo_proc, or will struct kinfo_proc shrink by four bytes?

Maybe:

	struct eproc {
		union {
			pid_t e_sid;
			long e_oldspare;
		}
		long	e_spare[1];

(I think gcc supports anonymous unions like that.)

In 6.0 you could remove the union hack and change the ABI, assuming that yo=
u=20
want to put this in 5.x as well.

=2D-=20
John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =3D  http://www.FreeBSD.org