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Path: utzoo!utgpu!water!watnot!watmath!clyde!cbatt!decuac!cvl!mimsy!chris
From: chris@mimsy.UUCP
Newsgroups: comp.lang.c,comp.bugs.4bsd
Subject: Re: Bad Pointer Declaration in 'scandir'
Message-ID: <6074@mimsy.UUCP>
Date: Wed, 1-Apr-87 17:12:16 EST
Article-I.D.: mimsy.6074
Posted: Wed Apr  1 17:12:16 1987
Date-Received: Sat, 4-Apr-87 06:30:46 EST
References: <2479@dalcs.UUCP> <5916@mimsy.UUCP> <2491@dalcs.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 43
Keywords: scandir, C, pointers, man
Xref: utgpu comp.lang.c:1416 comp.bugs.4bsd:256

>In article <5916@mimsy.UUCP> I wrote:
>>>struct direct *(*namelist)[];
>>No: for there is no such thing.  A declaration for a pointer to an
>>array must specify the size of the array.

In article <2491@dalcs.UUCP> dalegass@dalcs.UUCP (Dale Gass) writes:
>... I think a pointer to an array need not specify the array size.
>The compiler (all the one's I've dealt with) will allow this omission
>of array size.

Indeed, the declaration allows it---but just try to use the resulting
variable:

	char (*x)[];

Now, what is the address of (*x)[0]?  Well, that is easy: it is
`value of x plus zero time array-size plus 0', which is of course
just `value of x'.  What about (*(x+1))[0]?  Its address is `value
of x plus 1 times array-size plus 0'.  Uh oh.

(The 4.3BSD C compiler never notices that anything is wrong, and
merrily produces the same address as for (*x)[0].)

>In many cases, it is required to be able to allow pointers to
>different size arrays being passed to a function.

Not so.  Give an example, and I will show that you are not interpreting
C's array/pointer conversion rules correctly.

>struct direct ***namelist will work exactly the same as *(*namelist)[], but
>the latter is a bit more intuitive (if you can call *any* C declarations
>intuitive).  A pointer to an array of pointers to ..... is what the beast
>actually is.

Scandir creates a contiguous memory region---which is not quite
the same as an array---of pointers and modifies an argument to
point to that region.  Calling this `a pointer to an array of
pointers' is misleading, because there is no real `array':  An
array has a size that is known at compile time.  As long as you
do not need the size, you can pretend one is the other.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu