Date: Mon, 4 Mar 2002 10:29:18 +0100 (CET) From: Harti Brandt <brandt@fokus.gmd.de> To: Erik Trulsson <ertr1013@student.uu.se> Cc: Ian <freebsd@damnhippie.dyndns.org>, freebsd-hackers <freebsd-hackers@FreeBSD.ORG> Subject: Re: A few questions about a few includes Message-ID: <20020304102750.O74223-100000@beagle.fokus.gmd.de> In-Reply-To: <20020303180029.GA56041@student.uu.se>
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On Sun, 3 Mar 2002, Erik Trulsson wrote:
ET>On Sun, Mar 03, 2002 at 10:27:17AM -0700, Ian wrote:
ET>> >
ET>> > In <sys/proc.h>:
ET>> >
ET>> > /*
ET>> > * pargs, used to hold a copy of the command line, if it had a sane
ET>> > * length
ET>> > */
ET>> > struct pargs {
ET>> > u_int ar_ref; /* Reference count */
ET>> > u_int ar_length; /* Length */
ET>> > u_char ar_args[0]; /* Arguments */
ET>> > };
ET>> >
ET>> > This does indeed seem to make little or no sense. Could someone explain
ET>> > this? Is ar_args supposed to be a pointer or what?
ET>>
ET>> This is a common technique for defining a structure which is some
ET>> descriptive information about an array of objects is followed by an
ET>> open-ended array of those objects. (In this case the "objects" are
ET>> characters.) The ar_args member of the structure gives a name to that
ET>> location in the structure without reserving any space (and thus when the
ET>> technique is used, there can only ever be one [0] member and it must be at
ET>> the end of the structure). You access the open-ended array of objects just
ET>> as you would any other array embedded within a structure, E.G.
ET>> instance->ar_args[n].
ET>>
ET>> Not all compilers support defining zero-length arrays like this. And that's
ET>> a pity; it's an incredibly useful technique, and the alternatives to it are
ET>> not nearly as elegant and generally involve ugly recasting of pointers.
ET>
ET>For those compilers that don't support zero-length arrays one can still
ET>use the same trick but with a one-element array at the end of the
ET>struct. One just has to remember to that element into account when
ET>allocating memory for the structure. Slightly uglier, but not much.
ET>
ET>It might be worth mentioning that this trick is not actually allowed
ET>according to the C standard and in principle invokes undefined
ET>behaviour. OTOH, AFAIK the trick does work on all existing compilers,
ET>so while it is not standard-conforming it is quite portable.
My ISO-C draft copy allows in section 6.7.2.1 paragraph 2 the last member
of a structure to be an incomplete array type and paragraph 16 shows an
example. Was this removed from the final standard?
harti
--
harti brandt, http://www.fokus.gmd.de/research/cc/cats/employees/hartmut.brandt/private
brandt@fokus.fhg.de
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