RE: gcc 3.0.2 preprocessor token stringization fails

[David Thompson <David dot Thompson at efi dot com>]

Title: RE: gcc 3.0.2 preprocessor token stringization fails

Hi Zack/Neil,

Thank you for your excellent analysis!  I will commence
code changes here ...

--
David

> -----Original Message-----
> From: Zack Weinberg [mailto:zack@codesourcery.com]
> Sent: Monday, October 29, 2001 1:21 PM
> To: David Thompson
> Cc: 'gcc-bugs@gcc.gnu.org'
> Subject: Re: gcc 3.0.2 preprocessor token stringization fails
>
>
> On Mon, Oct 29, 2001 at 12:47:36PM -0800, David Thompson wrote:
> >
> > Hi,
> > 1) This source code shows a preprocessor failure for gcc-3.0.1
> > and gcc-3.0.2 but works correctly in gcc-2.95.3.  Although I am
> > using a sun->linux cross compiler, I notice the same failure on
> > a native linux gcc-3.0.2 compiler as well.
> >   --begin-source: f.c--
> >   #define CONCAT2(x,y)        x##y
> >   #define CONCAT3(x,y,z)      x##y##z
> >   #define QUOTE(macro)        #macro
> >   #define QUOTE_MACRO(macro)  QUOTE(macro)
> >   #define DEFINE_DEBUGGER(defaultLevel, msgPrefix) \
> >      _DEFINE_DEBUGGER(DEBUG_MODULE_NAME, defaultLevel, msgPrefix)
> >   #define _DEFINE_DEBUGGER(name, defaultLevel, msgPrefix) \
> >     static char *CONCAT3(p,name,MsgPrefix) = \
> >     CONCAT3("<",QUOTE_MACRO(name),"> ");
> >   #define DEBUG_MODULE_NAME SFTP
> >   DEFINE_DEBUGGER(9,"SFTP")
>
> This is a classic case of unnecessary token pasting.  Your example
> winds up as (the equivalent of)
>
>    "<" ## "SFTP" ## "> "
>
> But you cannot paste two string literals together with ##.  You get
> the single token {"<""SFTP"} which is not a valid preprocessing token.
> (You do _not_ get what you probably expected, i.e. "<SFTP".)  The C
> standard describes this as undefined behavior.  GCC 2.95
> would silently
> ignore the ## when this happened; 3.0 complains.
>
> Also, the QUOTE_MACRO invocation in
>
>    ... CONCAT3 (..., QUOTE_MACRO(...), ...) ...
>
> is _not_ supposed to be expanded according to the standard.  This is
> a known, obscure, bug in the 2.95 preprocessor.
>
> All you have to do to fix your code is remove the CONCAT3 on the line
> defining the string:
>
> #define _DEFINE_DEBUGGER(name) \
>    static const char CONCAT3(p,name,MsgPrefix)[] = \
>      "<" QUOTE_MACRO(name) "> "
>
> which will expand to e.g.
>
> static const char pSFTPMsgPrefix[] = "<" "SFTP" "> ";
>
> which is the _same thing_ that you were getting with 2.95. 
> You may not
> be aware of the rule that adjacent string literals are combined into a
> single string constant.  They are, and that is why your code worked in
> the first place.
>    
> Note that the second two arguments to _DEFINE_DEBUGGER in
> your original
> code are unused, I suspect this is an accident...
>
> zw
>