From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <31f9a933eb03172e356de753a5092e5a@plan9.jp>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] cc lexer bug?
From: Joel Salomon <JoelCSalomon@Gmail.com>
Date: Sun, 18 Feb 2007 17:07:09 -0500
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> > cpu% cat t.c
> > void foo (void)
> > {
> > 	double d =3D 08.7;
> > 	USED(d);
> > }
> > cpu% 8c t.c
> > t.c:3 syntax error, last name: 8.7
> > cpu%=20
> >=20
> > This came up as I=E2=80=99m making my lexer for C able to scan number=
s.  I
> > tried to understand ken=E2=80=99s code, but it gets very hairy right =
around
> > /sys/src/cmd/cc/lex.c:751 =E2=80=94 and I think there=E2=80=99s a bug=
.
>=20
> you're right, that's wrong.  but the question is do you really want
> that to work?

In kencc, maybe not; in a class project designed to implement a C89 compi=
ler, definitely yes.

> in that case, one could always remove the goto ncu on line 757.

cpu% grep -n 'goto ncu;' /sys/src/cmd/cc/lex.c
778: 			goto ncu;
788: 		goto ncu;
801: 		goto ncu;
808: 		goto ncu;
cpu%=20

I think you mean the one on 788.  I'm far from comfortable enough with
the code here to suggest a patch, though.  Certainly not comfortable
enough to incorporate it into my lexer.

> however, for better or worse, octal constants start with a 0.  08.7
> feels like a syntax error to me.  i am a little suprised that gcc
> accepts 08.7.  are you sure that that's actually in the standard?

>>From the ISO standard (the freely available =E2=80=9Cdraft" C99+TG1+TG2 =
standard):

6.4.4.2 Floating constants
Syntax
floating-constant:
		decimal-floating-constant
		hexadecimal-floating-constant

decimal-floating-constant:
	fractional-constant exponent-part_{opt} floating-suffix_{opt}
	digit-sequence exponent-part floating-suffix_{opt}
=E2=80=A6

fractional-constant:
	digit-sequence_{opt} . digit-sequence
	digit-sequence .
=E2=80=A6

digit-sequence:
	digit
	digit-sequence digit


=E2=80=94 no restrictions on the initial digit.  You get the same table i=
n
Harbison & Steele, 5th Ed. =C2=A72.7.2.

--Joel