Preprocessor errors?

[Sensei]

Hi!

I'm getting some weird results from a (probably) valid C expression
with some #define... I'm really puzzled.

A sample code is this (just the variables and defines, not actual code):

#define themacro(p, i) (p)[(i)]

int *ptr, *anotherptr, counter;

themacro(ptr, counter)=anothe rptr=MEMORY_ALL OCATION(...);

Where memory_allocati on is a function which mallocs and reallocs
memory. I get a weird null pointer somewhere, and just writing:

anotherptr=MEMO RY_ALLOCATION(. ..);
themacro(ptr, counter)=anothe rptr;

solves the problem. Am I doing something out of standard?

--
Sensei <senseiwa@mac.c om>

Part of the inhumanity of the computer is that, once it is competently
programmed and working smoothly, it is completely honest. (Isaac Asimov)

[Artie Gold]

Re: Preprocessor errors?

Sensei wrote:[color=blue]
> Hi!
>
> I'm getting some weird results from a (probably) valid C expression with
> some #define... I'm really puzzled.
>
> A sample code is this (just the variables and defines, not actual code):
>
> #define themacro(p, i) (p)[(i)]
>
> int *ptr, *anotherptr, counter;
>
> themacro(ptr, counter)=anothe rptr=MEMORY_ALL OCATION(...);
>
> Where memory_allocati on is a function which mallocs and reallocs memory.
> I get a weird null pointer somewhere, and just writing:
>
> anotherptr=MEMO RY_ALLOCATION(. ..);
> themacro(ptr, counter)=anothe rptr;
>
> solves the problem. Am I doing something out of standard?
>[/color]
Show a compilable snippet that exhibits the problem. *Real code*. Cut
and paste.

HTH,
--ag

--
Artie Gold -- Austin, Texas

http://goldsays.blogspot.com

http://www.cafepress.com/goldsays
"If you have nothing to hide, you're not trying!"

[Christopher Benson-Manica]

Re: Preprocessor errors?

Sensei <senseiwa@mac.c om> wrote:
[color=blue]
> themacro(ptr, counter)=anothe rptr=MEMORY_ALL OCATION(...);[/color]
[color=blue]
> anotherptr=MEMO RY_ALLOCATION(. ..);
> themacro(ptr, counter)=anothe rptr;[/color]
[color=blue]
> solves the problem. Am I doing something out of standard?[/color]

Possibly; I'm not much of a guru to tell you. If you haven't
already, however, I would investigate your platform's documentation
for how to invoke only the preprocessor on your code (gcc -E will do
it, for example) to make sure the preprocessed line looks the way you
expect it to.

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Mark McIntyre]

Re: Preprocessor errors?

On Wed, 14 Dec 2005 21:02:42 +0100, in comp.lang.c , Sensei
<senseiwa@mac.c om> wrote:
[color=blue]
>#define themacro(p, i) (p)[(i)][/color]

by convention, user-defined macros are UPPERCASE so they can be
identified easier.
[color=blue]
>int *ptr, *anotherptr, counter;
>
>themacro(ptr , counter)=anothe rptr=MEMORY_ALL OCATION(...);[/color]

themacro() will create an object (*ptr)[(counter)] which has type int.
This isn't compatible with anotherptr which is of type int*.
[color=blue]
>Where memory_allocati on is a function which mallocs and reallocs
>memory. I get a weird null pointer somewhere, and just writing:
>
>anotherptr=MEM ORY_ALLOCATION( ...);
>themacro(ptr , counter)=anothe rptr;[/color]
[color=blue]
>solves the problem.[/color]

this is just as bad - if you don't get a warning here, you should turn
up warninglevels in your compiler.
[color=blue]
>Am I doing something out of standard?[/color]

Remember how macro substitution works - its literal text replacement.
So you can write down what themacro() will do.


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