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Date: Fri, 04 Oct 2002 15:16:00 -0000
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From: Neil Booth <neil@daikokuya.co.uk>
Subject: Re: preprocessor/8120: cpp concatenation doesn't work as described
Reply-To: Neil Booth <neil@daikokuya.co.uk>
X-SW-Source: 2002-10/txt/msg00168.txt.bz2
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The following reply was made to PR preprocessor/8120; it has been noted by GNATS.

From: Neil Booth <neil@daikokuya.co.uk>
To: Peter Breitenlohner <peb@mppmu.mpg.de>
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: preprocessor/8120: cpp concatenation doesn't work as described
Date: Fri, 4 Oct 2002 23:08:38 +0100

 Peter Breitenlohner wrote:-
 
 > I was using the following code, cut down to the bare essentials and
 > replacing `#include' by `include' in order to (a) get some output from the
 > preprocessor and (b) make visible what happens.
 > ##################  start of input file bug.c  ####################
 > #define XXX_H(name) XXX_I(prefix_ ## name ## .h)
 > #define XXX_I(file) #file
 > 
 > include XXX_H(all)
 > ###################  end of input file bug.c  #####################
 > The command `gcc -E bug.c -o bug.i' yields
 > ##################  start of output file bug.i  ###################
 > # 1 "bug.c"
 > # 1 "<built-in>"
 > # 1 "<command line>"
 > # 1 "bug.c"
 > 
 > 
 > 
 > include "prefix_all .h"
 > ###################  end of output file bug.i  ####################
 > together with the warning:
 > bug.c:4:18: warning: pasting "prefix_all" and "." does not give a valid
 > preprocessing token
 > 
 > The warning is in agreement with the above quote, but the output is not,
 > since the two tokens `prefix_all' and `.' are separated by a space (and
 > subsequently stringified).
 > 
 > This construct used to produce "prefix_all.h" with gcc-2.95.3 (without
 > warning). With gcc-3.1.1 and 3.2 I got the desired result (with a warning) when
 > I replaced `name ## .h' by `name ##.h' or by  `name##.h', but not with
 > `name## .h'. I would think something funny happens here with the handling of
 > whitespace!!
 
 I will correct the documentation to say "gives undefined behaviour"
 shortly.  3.3 gives a hard error.  To get the result you want, remove the
 offending ##.
 
 Neil.