Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cwjcc!abvax!ivan!aep
From: aep@ivan (Alex E. Pensky)
Newsgroups: comp.lang.c
Subject: Re: More NULL questions
Message-ID: <914@abvax.UUCP>
Date: 12 Oct 89 16:58:28 GMT
References: <5950001@hpldola.HP.COM> <443@shodha.dec.com> <903@abvax.UUCP> <448@shodha.dec.com>
Sender: news@abvax.UUCP
Reply-To: aep@ivan.UUCP (Alex E. Pensky)
Organization: Allen-Bradley Company, Inc.; Industrial Computer Division; Highland Hts. OH
Lines: 36

In article <903@abvax.UUCP>, I, aep@ivan (Alex E. Pensky) wrote:
>> Even if pointers and integers are the same size and have the same 
>> representation, you are still in trouble if your compiler passes int
>> parameters and pointer parameters via different mechanisms.  In such
>> a case, omitting both the prototype and the cast will mean that the *entire*
>> parameter list will be received incorrectly by my_func().

In article <448@shodha.dec.com> devine@shodha.dec.com (Bob Devine) responds:
>  What different mechanisms?  C only supports call-by-value for
>parameters (I'm ignoring the special casing of arrays here).  How
>can pointers be passed differently than ints?  It sounds like you
>used a broken compiler.

Imagine this:  Compiler supports passing in registers rather than
on the stack.  Motorola 68xxx compiler.  Scalars are passed in data
registers, pointers in address registers.

    foobar( (char *)NULL )	passes a 32-bit zero in register A2, but

    foobar( NULL )		passes a 32-bit zero in register D3

If foobar() is written to expect a char *, it will look for its argument
in register A2.  Who knows what's in A2, in the latter case above?

Welcome to the Tektronix CLANDS II cross-development system!!  No, the
compiler is not broken, it's just trying to make my program run faster
by reducing function call overhead.

Moral of the story:  If you ever assume that a cast is not really necessary
when mixing types, you are writing code that is not only machine-specific,
it is also COMPILER-SPECIFIC.

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