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From: zvs@bby.oz.au (Zev Sero)
Newsgroups: comp.lang.c
Subject: Re: How SHOULD these macros work?
Message-ID: <1991Feb19.024059.21585@melba.bby.oz.au>
Date: 19 Feb 91 02:40:59 GMT
References: <9102151608.AA03533@ucbvax.Berkeley.EDU>
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In-Reply-To: schweitz@lexvmc.iinus1.ibm.com's message of 15 Feb 91 16:05:32 GMT

Eric = schweitz@lexvmc.iinus1.ibm.com ("Eric Schweitz")

Eric> I have a question concerning the preprocessor. Namely, what behavior
Eric> SHOULD be expected from the following:

Eric> #define one val1,val2
Eric> #define two(arg1,arg2)  arg1+arg2
Eric> ....
Eric>    two (one)  // ?

Eric> Should this evaluate to:

Eric> a) val1+val2
Eric>   -or-
Eric> b) val1,val2+

The correct answer is b.  I quote from H&S 2, page 33:

  Once a macro call has been expanded, the scan for macro calls
  resumes at the beginning of the expansion; this is so that names of
  macros may be recognized within the expansion for the purpose of
  further macro replacement...Macro replacement is also not performed
  within the actual argument token strings of a complex macro call at
  the time the macro call is being scanned. Macro names are recognised
  within actual argument token strings only during the rescanning of
  the expansion.

Therefore your macro is expanded in the following steps:
  two (one) -->  one+
At this point one is not recognised as a macro. Then the expression is
rescanned:
  one+ --> val1,val2+
--
	                                Zev Sero  -  zvs@bby.oz.au
This I say unto you, be not sexist pigs.
		-  The prophetess, Morgori Oestrydingh  (S.Tepper)