Date: Wed, 23 Nov 2005 12:11:46 +0100 From: Stefan Farfeleder <stefan@fafoe.narf.at> To: Daniel Rudy <dr2867@pacbell.net> Cc: freebsd-hackers@freebsd.org Subject: Re: getdirentries_args and other kernel syscall structures Message-ID: <20051123111143.GE1281@wombat.fafoe.narf.at> In-Reply-To: <4383F0CA.2030609@pacbell.net> References: <4383F0CA.2030609@pacbell.net>
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On Tue, Nov 22, 2005 at 08:32:10PM -0800, Daniel Rudy wrote:
>
> Ok, I'va got a little question here. In the structure
> getdirentries_args, there seems to be duplicated fields that I'm not
> entirely sure what they do. Here's the definition of a structure
> verbatim from sys/sysproto.h:
>
> struct getdirentries_args {
> char fd_l_[PADL_(int)]; int fd; char fd_r_[PADR_(int)];
> char buf_l_[PADL_(char *)]; char * buf; char buf_r_[PADR_(char *)];
> char count_l_[PADL_(u_int)]; u_int count; char
> count_r_[PADR_(u_int)];
> char basep_l_[PADL_(long *)]; long * basep; char
> basep_r_[PADR_(long *)];
> };
>
> Now my question is what does the l and r variables do? It seems that
> they do something with padding the data based on the endian of the
> machine? I look through this header file, and I see all the structures
> have similar constructs. Is it something that can be safely ignored?
This file is automatically generated by makesyscalls.sh. The l and r
variables are a hack to correctly the member between them. One of PADL_
or PADR_ always evalutes to 0, the other one to the needed padding,
depending on the passed type. This is unfortunate because it relies on
the GCC extension to accept 0-sized arrays. I'd love to fix that but
couldn't come up with something that isn't very involved.
Stefan
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