Date: Sun, 3 Mar 2002 11:05:00 -0500 From: Brian T.Schellenberger <bts@babbleon.org> To: conrads@cox.net, freebsd-hackers@FreeBSD.ORG Subject: Re: A few questions about a few includes Message-ID: <20020303160501.6D87EBA03@i8k.babbleon.org> In-Reply-To: <XFMail.20020303091938.conrads@cox.net> References: <XFMail.20020303091938.conrads@cox.net>
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On Sunday 03 March 2002 10:19 am, Conrad Sabatier wrote:
> Am I just completely stupid, or do we have a few things that could use a
> little cleaning up in /usr/include as well as in the man page for kvm_*?
>
> System: FreeBSD 4.5-STABLE
>
> 2) If compiling with the -pedantic switch, one might see something like
> this:
>
> In file included from main.c:151:
> /usr/include/sys/proc.h:108: warning: ANSI C forbids zero-size array
> `ar_args'
>
> In <sys/proc.h>:
>
> /*
> * pargs, used to hold a copy of the command line, if it had a sane
> * length
> */
> struct pargs {
> u_int ar_ref; /* Reference count */
> u_int ar_length; /* Length */
> u_char ar_args[0]; /* Arguments */
> };
>
> This does indeed seem to make little or no sense. Could someone explain
> this? Is ar_args supposed to be a pointer or what?
I can explain this one; it's a moderately-common C programming technique.
This strucuture will be allocated dynamically. I could be declared like
struct pargs { ... u_char *ar_args; }
and allocated like
pargs = malloc(sizeof(struct pargs));
pargs->ar_args = malloc(args_size);
but it's a lot more efficient to declare it as above and allocate it as
pargs = malloc(sizeof(struct pargs) + args_+size);
since pointers & arrays are equivalent, except that array data is at the
location of the name rather than having a pointer to the data at that
location, a [0] array has exactly the right semantics for this. You can
declare a [1] array for this to make ANSI happy, but then you have to
subtract in the allocation and besides that the [0] is "self-documenting" for
those who are familir with the technique--after all, there isn't anything
*else* you can do with a [0]-sized array.
> 3) Furthermore, on including <sys/user.h>, one then sees this:
>
> In file included from /usr/include/sys/user.h:40,
> from main.c:153:
> /usr/include/machine/pcb.h:90: warning: struct has no members
>
> In <machine/pcb.h>:
>
> /*
> * The pcb is augmented with machine-dependent additional data for
> * core dumps. For the i386: ???
> */
> struct md_coredump {
> };
>
> Nowhere under /usr/include is a more complete definition of md_coredump to
> be found. This looks awfully "kludgy" to me.
A little guesswork here, but this seems pretty self-explanatory to me, too.
The md_coredump{} is a structure that includes the *extra* information
(beyond what's already in the portable structure) for coredumps on a given
architecture. In the case of the 386, there is no useful extra information,
so the structure is empty, but since there's a structure someplace that has
an instance of struct md_coredump, it must be declared. Since there's no
useful content, it is declared as being empty.
Seems pretty elegant to me.
If I'm off-base here, somebody please jump in!
>
> I do hope someone can shed some light here. Thanks.
[Your point #1 seems valid to me, though.]
--
Brian T. Schellenberger . . . . . . . bts@wnt.sas.com (work)
Brian, the man from Babble-On . . . . bts@babbleon.org (personal)
ME --> http://www.babbleon.org
http://www.eff.org <-- GOOD GUYS --> http://www.programming-freedom.org
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