C preprocessor

[selvesteen@hotmail.com]

Hello All,

I came across the following macro in a program, could someone explain
what it actually means.

#define BUFFER_MAX_HPN_ LEN (2>>21)-1

Is there any other way to achieve the same

Thanks for the help.
Mike

[Chris Dollin]

Re: C preprocessor

selvesteen@hotm ail.com wrote:
[color=blue]
> Hello All,
>
> I came across the following macro in a program, could someone explain
> what it actually means.
>
> #define BUFFER_MAX_HPN_ LEN (2>>21)-1[/color]

It means that wherever BUFFER_MAX_HPN_ LEN appears in the
following text, it is replaced by `(2>>21)-1`, which looks
like a typo on someone's part to me, since that evaluates
to -1. Are you sure it wasn't `(2<<21)-1`?

The latter would be a way of writing a mask of 1's from
position 21ish rightwards.

<fx:thinks/>
<fx:assume32Bit Ints>

2 << 21 = 1 << 22 = 1 followed by 22 0's; -1 then gives
us a mask of 21 ones, which is probably why the macro
would be written using 2<<, as the right operand of <<
would be the number of 1's in the mask.

I'd write `((2 << 21) - 1)` myself.

</>

--
Chris "electric hedgehog" Dollin
It's called *extreme* programming, not *stupid* programming.

[pete]

Re: C preprocessor

selvesteen@hotm ail.com wrote:[color=blue]
>
> Hello All,
>
> I came across the following macro in a program, could someone explain
> what it actually means.
>
> #define BUFFER_MAX_HPN_ LEN (2>>21)-1
>
> Is there any other way to achieve the same[/color]

It looks more like a typographical error, than anything else.

BUFFER_MAX_HPN_ LEN * 5 is -5

5 * BUFFER_MAX_HPN_ LEN is -1



/* BEGIN new.c */

#include <stdio.h>

#define BUFFER_MAX_HPN_ LEN (2>>21)-1

int main(void)
{
printf("\nBUFFE R_MAX_HPN_LEN * 5 is %d\n\n",
BUFFER_MAX_HPN_ LEN * 5);
printf("5 * BUFFER_MAX_HPN_ LEN is %d\n\n",
5 * BUFFER_MAX_HPN_ LEN);
return 0;
}

/* END new.c */

--
pete

[CBFalconer]

Re: C preprocessor

Chris Dollin wrote:[color=blue]
>[/color]
.... snip ...[color=blue]
>
> 2 << 21 = 1 << 22 = 1 followed by 22 0's; -1 then gives
> us a mask of 21 ones, which is probably why the macro
> would be written using 2<<, as the right operand of <<
> would be the number of 1's in the mask.
>
> I'd write `((2 << 21) - 1)` myself.[/color]

<nit> or '((2uL << 21) -1)' so it works on 16 bitters. </nit>

--
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[snnn]

Re: C preprocessor

selvesteen@hotm ail.com wrote:[color=blue]
> Hello All,
>
> I came across the following macro in a program, could someone explain
> what it actually means.
>
> #define BUFFER_MAX_HPN_ LEN (2>>21)-1
>
> Is there any other way to achieve the same
>
> Thanks for the help.
> Mike
>[/color]

How should the following codes work?

int x;
x=5*BUFFER_MAX_ HPN_LEN
x=BUFFER_MAX_HP N_LEN *5


please define "BUFFER_MAX_HPN _LEN" as "((2>>21)-1)" instead.

[pete]

Re: C preprocessor

snnn wrote:
[color=blue]
> please define "BUFFER_MAX_HPN _LEN" as "((2>>21)-1)" instead.[/color]

That's a strange way to write negative one.

--
pete