Xref: utzoo comp.lang.c:21134 comp.std.c:1562
Path: utzoo!attcan!uunet!wuarchive!brutus.cs.uiuc.edu!tut.cis.ohio-state.edu!att!cbnewsl!dfp
From: dfp@cbnewsl.ATT.COM (david.f.prosser)
Newsgroups: comp.lang.c,comp.std.c
Subject: Re: C puzzle
Keywords: arrays, compilers
Message-ID: <1614@cbnewsl.ATT.COM>
Date: 24 Aug 89 18:21:14 GMT
References: <404@opusys.UUCP>
Reply-To: dfp@cbnewsl.ATT.COM (david.f.prosser)
Followup-To: comp.std.c
Organization: AT&T Bell Laboratories
Lines: 56

In article <404@opusys.UUCP> rlkd@opusys.UUCP (R.L.K.Dattatri) writes:
>Here is a small puzzle.  The program below will compile and run on some
>compilers but will not compile on others.
>
>main()
>{
>	int *p;
>	p = (int *) malloc(1024);

Since there is no visible declaration for malloc(), it must be
assumed to return an int.  Since it actually returns a void *
(equivalent in this case to char *), it is quite possible that
a "garbage" int value is consequently converted to int *.  (For
example, most 68k implementations return pointer values in a
different register than integral values.)

>	test_it(p);
>}
>test_it (x)
>int (*x)[];

A pointer to an int was passed to test_it().  This parameter
declaration is for a pointer to an unspecified-sized array of
int.  These two are not equivalent, even though the number of
indirection necessary to get to an int may be the same.

>/* If a dimension is specified all compilers will accept it */
>{
>	int i;
>	for (i=0; i < 10; ++i)
>		(*x)[i] = i;
>	/* x++; */
>}
>
>The parameter 'x' in test_it without dimensions works on some compilers
>but others complain incomplete array'.
>I can understand that the compiler is trying to guess the size of the rows
>in the array. But that should come only when the commented x++ is used.
>Without the dimension all compilers generated an increment of 'int' size for
>'x++' but with the dimension it was the size of dimension * int size.
>
>Does ANSI specify anything in this regard?

(Follow-up directed to comp.std.c.)

You are correct that the length of the array is only necessary
in this example if the "x++;" statement is included.  Pointers
to unspecified-sized arrays are valid in the pANS.

Existing implementation were of varying minds with respect to
pointers to arrays.  For example, some took a construct such as
``&array_name'' and slapped your hand with a warning stating that
the & was ignored since it was "incorrect".  This is one of the
areas where the pANS has helped the language.

Dave Prosser	...not an official X3J11 answer...