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Date: Mon, 3 Apr 2000 18:34:18 +0200
To: John Polstra <jdp@polstra.com>
Cc: bright@wintelcom.net, current@freebsd.org
Subject: Re: MLEN and crashes
References: <Pine.BSF.4.21.0004022118510.1442-100000@alphplex.bde.org> <20000403084330.6A6DC483D@hcswork.hcs.de> <20000403023858.F21029@fw.wintelcom.net> <200004031622.JAA19715@vashon.polstra.com>
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In-Reply-To: <200004031622.JAA19715@vashon.polstra.com>; from jdp@polstra.com on Mon, Apr 03, 2000 at 09:22:09AM -0700
From: Samuel Tardieu <sam@inf.enst.fr>
Organization: Ecole Nationale Superieure des Telecommunications
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On  3/04, John Polstra wrote:

| I doubt if it's possible to implement that at compile time.  Remember,
| the preprocessor doesn't understand "sizeof".  It doesn't recognize
| keywords in expressions at all.

Then don't use the preprocessor alone and use both the preprocessor and
the compiler. I suppose something like this will work:

   struct foobar {
      ...;
   }

   #ifdef CHECK_STRUCT_SIZE
   #define MAX_FOOBAR_SIZE 8
   static char dummy_foobar [MAX_FOOBAR_SIZE-sizeof(struct foobar)];
   #undef MAX_FOOBAR_SIZE
   #endif

If sizeof(struct foobar) is more than MAX_FOOBAR_SIZE, then the compiler
will try to create an array with a negative size, which will not compile.
Embed this in a macro and you're done:

   #ifdef CHECK_STRUCT_SIZE
   #define CSS(S,T,U) static char dummy_##T [U-sizeof(S)];
   #else
   #define CSS(S,T,U)
   #endif

Then use:

   CSS(struct foobar,foobar,8)

 Sam



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