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Re: c preprocessor and indent problem in c-mode?

From: Alan Mackenzie
Subject: Re: c preprocessor and indent problem in c-mode?
Date: Mon, 26 Jan 2009 09:00:47 +0000
User-agent: Mutt/1.5.9i

Hi, there,

On Sun, Jan 25, 2009 at 04:46:38PM -0800, YOUNG wrote:
> Hi,

> If there is a following code, emacs does not present indent correctly.

> #incude <stdio.h>

> int main(void) {
>   int a = 10;
> #ifdef AAA
>   if (a == 10) {       <====================== A
>     printf("a:%d\n", a);
> #else
>     if (b == 10) {       <====================== B
>       printf("b=10\n");
> #endif
>     }

> #endif
>   }

> If the curly brace is not closed before the preprocessor, emacs seems
> to confuse to make indent.


> Is there anyone who know this problem and how to resolve?

Yes.  CC Mode doesn't take account of the #ifdef and #else when parsing
braces.  So it looks like the brace on line B is nested within the brace
on line A.  This is a longstanding bug, which needs fundamental work in
CC Mode (and possibly the Emacs core) to fix.  It may get fixed in the
medium future.  Sorry!

A reasonable workaround, if the project you're working on will let you,
is to keep braces in #ifdefs (and #defines) balanced.  You could thus
write this instead:

#ifdef AAA
  if (a == 10) {
    printf("a:%d\n", a);
  if (b == 10) {

Alan Mackenzie (Nuremberg, Germany).

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