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From: paul@u02.svl.cdc.com (Paul Kohlmiller)
Newsgroups: comp.std.c
Subject: Voids in a comma expression
Keywords: voids comma operator
Message-ID: <34405@shamash.cdc.com>
Date: 21 Jun 91 20:35:40 GMT
Sender: usenet@shamash.cdc.com
Distribution: usa
Lines: 35

This program gets a fatal diagnostic from at least one C compiler but works
okay on at least 3 others.

extern void foo(int x);
main() {
int j=3;
int i;
j=(i=4,foo(i),i++);
}

The diagnostic claims that ANSI disallows the void expression foo(i) in the
command expression. ANSI 3.3.17 claims that the left operand is evaluated as
a void. Does this mean the leftmost only? That seems wrong since only the
rightmost operand really needs to be non-void. However, the diagnostic says
that it is disallowed because of something in section 3.2.2.1 but I can't find
it.
The ANSI grammar under the comma operator says:                
expression:
  assignment-expression
  expression , assignment-expression
which might imply that (i=4,foo(i),i++) is the same as
(i=4,foo(i)),i++  /*Did I get that right? */
if this is right then foo(i) is the rightmost operand at some point in time?
Maybe?
Flipping the positions of i=4 and foo(i) makes it compile okay on all compilers.
Paul Kohlmiller
CDC
disclaimers, etc.


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