Walter,
Thank you for the in-depth explanation!
John Zoltak
North American Mfg Co
> -----Original Message-----
> From: Walter Murray [SMTP:[log in to unmask]]
> Sent: Friday, October 30, 1998 4:43 PM
> To: [log in to unmask]
> Subject: Re: [HP3000-L] HP C/iX Warning 604
>
> John Zoltak ([log in to unmask]) wrote:
> : Here is a compilable code snippet in HP C/iX. I always get the
> Warning
> : 604. I cannot think of a cast to eliminate it. According to the
> book;
> : "604 Pointers are not assignment-compatible.
> : In an assignment expression, if both the left hand and right hand
> side
> : expressions are pointers, then the right hand side expression must
> : either be the same type as the left side expression, or a pointer to
> : void.
> : Correct the expression (possibly by inserting a cast).
> : ANSI 3.3.16.1"
> : So what am I missing. The right hand side IS a void so why do I get
> the
> : warning?
>
> : :ccxl ctest01;info="-Aa"
> : WED OCT 28 1998 9:29 AM Copyright Hewlett-Packard Co. 1984.
> : PAGE 1
> : HP C/iX HP31506 A.05.19
> : "CTEST01"
>
> : 1 0 # 1 "CTEST01.PUB.LIB"
> : 1 0 #pragma intrinsic HPGETPROCPLABEL
> : 2 0
> : 3 0 short (*pASCII)(short word, short base, char *string);
> : 4 0
> : 5 0 void *load()
> : 6 0 {
> : 7 1 int pLabel;
> : 8 1 HPGETPROCPLABEL("%ASCII%", &pLabel);
> : 9 1 return (void*) pLabel;
> : 10 1 }
> : 11 0
> : 12 0 void main()
> : 13 0 {
> : 14 1 pASCII = load();
> : 15 1 }
> : cc: "CTEST01.PUB.LIB", line 14: warning 604: Pointers are not
> : assignment-compatible.
>
> As I confessed earlier, the wording in the manual, an attempted
> gross simplification of the wording in the C standard, is wrong.
>
> The compiler is producing this ANSI-required diagnostic because,
> strictly speaking, C does not approve of mixing function pointers
> with other pointers, even pointers-to-void. It's common to use
> void* as a generic pointer, because any data pointer can be converted
> to a void* and back again, without losing anything. That's not
> true of function pointers, however, because some implementations
> may store function pointers in a format that won't fit in the
> storage allocated for a void*. (It works OK in HP C/iX; it
> just isn't guaranteed to work with other compilers.) To suppress
> this warning, you can use a cast to promise the compiler you know
> what you're doing. Assuming you want to retain the prototype in
> the declaration of pASCII, and assuming you really want to have
> load() return a void*, simply rewrite your assignment as follows:
>
> pASCII = (short(*)(short, short, char*))load();
>
> This says: "Take the pointer-to-void returned by load(), pretend
> that it's really a pointer to a function that takes two shorts and
> a char* and returns a short, and store that pointer in pASCII."
> (Isn't C beautiful! :-)
>
> (Stop here if you are a pragmatist and just want to get rid of
> the warning.)
>
> To be more of a C purist, you would probably want to change the
> return type of your load() function. Assuming it will ultimately
> be used to return function pointers for a variety of functions with
> different signatures, you would probably have it return some generic
> function pointer type, analogous to void*, but valid for functions.
> Then the declaration might look like this:
>
> void (*load(void))()
>
> This says that load is a function that returns a pointer to a
> function that has no parameters and returns nothing, i.e., load()
> returns a generic function pointer.
>
> And you'd put a corresponding cast in the return statement:
>
> return (void(*)(void)) pLabel;
>
> This will cause the compiler to recognize you as a C guru, and
> it will be pleased. However, you will still need a cast in the
> assignment to pASCII.
>
> Walter Murray
> Hewlett-Packard
> Support Technology Lab
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