Concatenating tokens that aren't parameters, in ANSI C

Thread View: comp.std.c

3 messages

[T] Back to Threads [V] Tree View [G] Group Articles

3 total messages Started by John Gilmore Tue, 26 Jul 1988 04:47

#5228

Author: John Gilmore
Date: Tue, 26 Jul 1988 04:47

43 lines
2039 bytes

I'm still trying to get Unix to compile and run under an ANSI C compiler
(gcc).  The latest wierd problem comes from the Fortran I/O library, where
they define the number of significant digits in a float and then want
to define a constant containing 1e(that number).  Under pcc, they used:

#define LFD 6
#define LHIGH 1.0e+LFD

which expanded to

1.0e+6

which worked just fine.  This no longer works, since the preprocessor
works on "pp-tokens" one of which is a "pp-number" which includes all
of "1.0e+LFD" (once you've seen a digit, it includes letters and digits
and "e"s followed by signs until kingdom come).  It never sees "LFD" as
a separate token like the old one did.  I have tried various kludges to
make this work; the obvious:

#define LHIGH 1.0e+ ## LFD

fails because ## does not fully expand its arguments before concatenating
them; it only expands them if they are parameters to the macro (boo!).
I tried various more complex alternatives and NEVER got any of them to
work.  Some of this may be due to gcc-1.24 bugs; I expect that:

#define cat_a(a,b) a ## b
#define cat_b(a,b) a ## b
#define LHIGH cat_a(1.0e+,cat_b(LFD,))

should work, but it produces:

1.0e+cat_b(6 ,)  

a wierd result, probably due to misimplementation of the overly complex
ANSI C anti-self-calling rules.

If C had exponentiation, I could define LHIGH (10.**(LFD-1)) and hope
for constant folding, but there's no **.  Does anyone have any suggestions?
-- 
John Gilmore    {sun,pacbell,uunet,pyramid,amdahl}!hoptoad!gnu    gnu@toad.com
      "And if there's danger don't you try to overlook it,
       Because you knew the job was dangerous when you took it"

Re: Concatenating tokens that aren't parameters, in ANSI C

#5230

Author: Paul Hilfinger
Date: Tue, 26 Jul 1988 08:55

50 lines
2209 bytes

In article <4963@hoptoad.uucp> gnu@hoptoad.uucp (John Gilmore) writes:
>I'm still trying to get Unix to compile and run under an ANSI C compiler
>(gcc).  The latest wierd problem comes from the Fortran I/O library, where
>they define the number of significant digits in a float and then want
>to define a constant containing 1e(that number).  Under pcc, they used:
>
>#define LFD 6
>#define LHIGH 1.0e+LFD
>
>which expanded to
>
>1.0e+6
>
>which worked just fine.  This no longer works,....

The following works (I have tried it under gcc):

    #define cat_a(a,b) a ## b
    #define cat_b(a,b) cat_a(a,b)
    #define LHIGH cat_b(1.0e+,LFD)

This is because, as specified in the ANSI Standard, the LFD argument is
expanded before substitution into the definition of cat_b, so that a 
macro invocation of LHIGH becomes
    cat_a(1.0e+,6)
 
>Some of this [weirdness] may be due to gcc-1.24 bugs; I expect that:
>
>#define cat_a(a,b) a ## b
>#define cat_b(a,b) a ## b
>#define LHIGH cat_a(1.0e+,cat_b(LFD,))
>
>should work, but it produces:
>
>1.0e+cat_b(6 ,)  
>
>a wierd result, probably due to misimplementation of the overly complex
>ANSI C anti-self-calling rules.

Not so weird.  Given that the original version didn't work in gcc because
LFD immediately after the 1.0e+ was not recognized as a macro name, it is
consistent that cat_b should not be recognized as a macro name 
right after 1.0e+.

I confess, however, that I am a relative newcomer to the ANSI C standard,
and am not sure whether gcc's behavior is correct.

Paul Hilfinger
University of California at Berkeley
Hilfinger@Berkeley.EDU, hilfingr@ginger.Berkeley.EDU

Re: Concatenating tokens that aren't parameters, in ANSI C

#5238

Author: ~XT4103000~Marc Mengel~C25~G25~6...
Date: Wed, 27 Jul 1988 14:54

33 lines
1186 bytes

In article <4963@hoptoad.uucp> gnu@hoptoad.uucp (John Gilmore) writes:
[old code breaks under ANSI cpp follows]
>#define LFD 6
>#define LHIGH 1.0e+LFD
>which expanded to
>1.0e+6
 [tried]
>#define LHIGH 1.0e+ ## LFD
 [and also]
>#define cat_a(a,b) a ## b
>#define cat_b(a,b) a ## b
>#define LHIGH cat_a(1.0e+,cat_b(LFD,))
>
>should work, but it produces:
>
>1.0e+cat_b(6 ,)  

Hmmm. seeems like maybe
#define LFD	6
#define BASE	1.0e
#define LHIGH	LFD+BASE

might just work, I don't have a gcc cpp handy to test it though. (It
works under SysV cc.

>John Gilmore    {sun,pacbell,uunet,pyramid,amdahl}!hoptoad!gnu    gnu@toad.com


-- 
 Marc Mengel			       
				
 attmail!mmengel	
 {lll-crg|mtune|ihnp4}!cuuxb!mmengel

Thread Navigation

This is a paginated view of messages in the thread with full content displayed inline.

Messages are displayed in chronological order, with the original post highlighted in green.

Use pagination controls to navigate through all messages in large threads.

Back to All Threads