structure padding

[John Baldwin]

On Thursday 08 January 2004 03:14 pm, Dag-Erling Smørgrav 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 != 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 you 
want to put this in 5.x as well.

-- 
John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org