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From: ned@pebbles.cad.mcc.com (CME Ned Nowotny)
Newsgroups: comp.lang.c
Subject: Re: macros w/o side effects (was Re: declarations in include files)
Keywords: declarations include
Message-ID: <1310@cadillac.CAD.MCC.COM>
Date: 19 Jun 89 20:02:15 GMT
References: <179@larry.sal.wisc.edu> <10251@smoke.BRL.MIL> <181@larry.sal.wisc.edu> <2337@mit-caf.MIT.EDU> <8420@phoenix.Princeton.EDU>
Sender: news@cadillac.CAD.MCC.COM
Reply-To: ned%cad@MCC.COM (CME Ned Nowotny)
Distribution: usa
Organization: MCC CAD Program, Austin, TX
Lines: 48

In article <8420@phoenix.Princeton.EDU> bernsten@phoenix.Princeton.EDU (Dan Bernstein) writes:
=>In article <2337@mit-caf.MIT.EDU> vlcek@mit-caf.UUCP (Jim Vlcek) writes:
=>> In article <181@larry.sal.wisc.edu> jwp@larry.sal.wisc.edu.UUCP
=>> (Jeffrey W Percival) writes: 
=>  [if you rely upon .h files for function declarations,
=>   it is easy to later make a function into a macro]
=>> >Is that interpretation unflawed?
=>> If what was once a function is turned into a macro, programs which
=>> passed arguments with side effects to the function may not work
=>> correctly with the macro.  You know the tune:
=>> #define square(x) ((x)*(x))  /* Used to be a function ... needed speed */
=>> y = square(++i);
=>> Any macro->function sleight of hand had best not take place behind the
=>> scenes, and in fact should probably not take place at all.  Going the
=>> other direction is probably considered safe, although some programs
=>> may have depended upon the side effects of the macro which is now a
=>> function.  Probably anyone who does so deserves to suffer, though...
=>
=>Oh dear. I should be burned at the stake for some of the macros I write.
=>I even take functions like
=>
=>  sqaure(in,out) int in; int *out; { *out = in * in; }
=>
=>and convert them into macros like
=>
=>  #define square(in,out) { int Tsquare1 = in; int *Tsquare2 = out;
=>			   *Tsquare2 = Tsquare1 * Tsquare1; }
=>
=>Gasp! Where's the error when I do square(i++,&result)? Where have the
=>side effects gone? How come this macro does exactly the same thing as
=>the function except that it's faster? And I would have thought from the
=>above article that this was impossible!
=>

However, it is not much fun to debug code with something like the following
call to square():

	square( Tsquare1, &Tsquare2 );

Of course, the author would never use Tsquare[12] as variable names in
code that calls square.  However, other users of his header file might.
There are variable name constructions which would be extremely unlikely
to cause problems, but none guaranteed safe.

Ned Nowotny, MCC CAD Program, Box 200195, Austin, TX  78720  Ph: (512) 338-3715
ARPA: ned@mcc.com                   UUCP: ...!cs.utexas.edu!milano!cadillac!ned
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