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Groups > comp.lang.c > #163509
| From | Keith Thompson <Keith.S.Thompson+u@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' |
| Date | 2021-11-19 12:21 -0800 |
| Organization | None to speak of |
| Message-ID | <878rxjq4l9.fsf@nosuchdomain.example.com> (permalink) |
| References | <86pmqwkm7u.fsf@levado.to> |
Meredith Montgomery <mmontgomery@levado.to> writes:
> I did not get Brian Kernighan's concern with setting EOF to a char c.
> The context is
>
> char c = getchar();
>
> And Kernighan says:
>
> --8<---------------cut here---------------start------------->8---
> We must declare c to be a type big enough to hold any value that getchar
> returns. We can't use char since c must be big enough to hold EOF in
> addition to any possible char . Therefore we use int.
> --8<---------------cut here---------------end--------------->8---
>
>
> I'm trying to justify this using C99's document. Here's what I find.
>
> --8<---------------cut here---------------start------------->8---
> Section 6.25 Types
> [...]
>
> 3. An object declared as type char is large enough to store any member
> of the basic execution character set. If a member of the basic execution
> character set is stored in a char object, its value is guaranteed to be
> positive. If any other character is stored in a char object, the
> resulting value is implementation-defined but shall be within the range
> of values that can be represented in that type.
> --8<---------------cut here---------------end--------------->8---
>
> So I can't be sure a char would always be signed, for example. I can't
> be sure EOF would fit in a char.
>
> Am I looking at the right place, thinking the right thing? Thank you.
Yes, that's pretty much it -- except that the same issue applies (with
some differences) whether plain char is signed or unsigned.
For simplicity, assume CHAR_BIT==8, 2's-complement for signed types, and
EOF==-1. Then a char object can hold any of exactly 256 distinct
values, either 0..255 or -128..127.
A call to getchar() can return any of exactly 257 distinct values. If
it succeeds, the result is in the range 0..255. If it fails, it returns
EOF. (A cleaner design choice might have been to separate the data from
the status, but C does it this way and we're stuck with it.)
Suppose you (unwisely) store the result of getchar() in a char:
char c = getchar();
If char is signed and getchar() returns EOF, then c==-1 -- but -1 could
also be a valid character value. The test (c == EOF) will succeed if
you read a valid character with the value -1.
If char is unsigned and getchar() returns EOF, then c==255 -- and again,
255 could be a valid character value. But now the test (c == EOF) will
never succeed, because the converted value is no longer equal to EOF.
(In the 8-bit Latin-1 character set, character 255 is 'ΓΏ').
The byte value 0xff never appears in valid UTF-8, but of course it can
easily appear in binary data.
Note that if you have an exotic implementation with CHAR_BIT==16 and
sizeof(int)==1, storing the result of getchar() in an int still doesn't
let you distinguish between EOF and a valid character with the same
value. In practice, you probably wouldn't be doing normal character
input on such a system (which is likey to be a DSP). But if you had to,
you could call feof() and ferror() after the getchar() call to determine
whether the EOF value is an actual end-of-file indicator or a valid
16-bit character.
--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */
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K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Meredith Montgomery <mmontgomery@levado.to> - 2021-11-19 15:57 -0300
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-19 12:21 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Meredith Montgomery <mmontgomery@levado.to> - 2021-11-19 18:14 -0300
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-19 14:10 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Meredith Montgomery <mmontgomery@levado.to> - 2021-11-20 17:30 -0300
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' scott@slp53.sl.home (Scott Lurndal) - 2021-11-21 16:01 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Philipp Klaus Krause <pkk@spth.de> - 2021-11-22 11:23 +0100
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-22 12:55 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Philipp Klaus Krause <pkk@spth.de> - 2021-11-23 17:04 +0100
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-23 09:43 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Philipp Klaus Krause <pkk@spth.de> - 2021-11-24 11:26 +0100
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-10 01:41 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' David Brown <david.brown@hesbynett.no> - 2021-12-10 14:40 +0100
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Barry Schwarz <schwarzb@delq.com> - 2021-11-19 12:22 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Meredith Montgomery <mmontgomery@levado.to> - 2021-11-19 18:10 -0300
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' David Brown <david.brown@hesbynett.no> - 2021-11-20 13:24 +0100
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Meredith Montgomery <mmontgomery@levado.to> - 2021-11-20 17:32 -0300
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Mark Bluemel <mark.bluemel@gmail.com> - 2021-11-22 00:27 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Mark Bluemel <mark.bluemel@gmail.com> - 2021-11-22 00:34 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-22 02:04 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Mark Bluemel <mark.bluemel@gmail.com> - 2021-11-22 02:46 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2021-11-22 21:22 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-11-22 22:03 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' scott@slp53.sl.home (Scott Lurndal) - 2021-11-23 14:52 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Dave Dunfield <dave.dunfield@gmail.com> - 2021-11-26 07:31 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Dave Dunfield <dave.dunfield@gmail.com> - 2021-12-08 07:26 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-08 12:16 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Dave Dunfield <dave.dunfield@gmail.com> - 2021-12-08 18:59 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-08 19:23 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Dave Dunfield <dave.dunfield@gmail.com> - 2021-12-09 06:30 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Manfred <noname@add.invalid> - 2021-12-09 16:10 +0100
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-09 16:05 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Bart <bc@freeuk.com> - 2021-12-09 17:41 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Dave Dunfield <dave.dunfield@gmail.com> - 2021-12-09 12:24 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-09 22:59 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-09 21:26 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Bart <bc@freeuk.com> - 2021-12-09 22:37 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-09 23:09 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-10 01:58 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-10 10:46 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-12 08:21 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Paul <nospam@needed.invalid> - 2021-12-09 10:31 -0500
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-12-09 13:32 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' luser droog <luser.droog@gmail.com> - 2021-12-09 17:08 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Dave Dunfield <dave.dunfield@gmail.com> - 2021-12-10 12:45 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Manfred <noname@add.invalid> - 2021-12-11 19:17 +0100
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Bart <bc@freeuk.com> - 2021-12-11 23:19 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-12-15 00:31 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Mark Bluemel <mark.bluemel@gmail.com> - 2021-12-09 02:26 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Bart <bc@freeuk.com> - 2021-12-09 11:20 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Dave Dunfield <dave.dunfield@gmail.com> - 2021-12-09 06:50 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-12-08 20:00 -0800
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' Kaz Kylheku <480-992-1380@kylheku.com> - 2021-12-12 18:55 +0000
Re: K&R, 2nd edition, Brian's concerns with ``char c = EOF'' James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-12 14:38 -0500
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