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| From | Richard Heathfield <rjh@cpax.org.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Line read from csv file with fgets has two characters before the terminating one |
| Date | 2016-12-15 12:43 +0000 |
| Organization | Fix this later |
| Message-ID | <o2u334$6b0$1@dont-email.me> (permalink) |
| References | (5 earlier) <o2tudm$1qtv$1@gioia.aioe.org> <o2tuov$ml2$1@dont-email.me> <o2tvfe$ohk$1@news.albasani.net> <o2u0ig$t3a$1@dont-email.me> <20161215130922.0f41b9a7@maxa-pc> |
On 15/12/16 12:09, Melzzzzz wrote:
> On Thu, 15 Dec 2016 12:00:35 +0000
> Richard Heathfield <rjh@cpax.org.uk> wrote:
>
[non-standard fopen modes]
<snip>
>> It therefore makes perfect sense to have the behaviour undefined.
>> That way, if you want to take advantage of a particular
>> implementation's support for a particular mode, you can, but at the
>> cost of recognising that your code isn't going to work as you might
>> wish on implementations that don't support that mode.
>>
>
> Well, I have learned that undefined behavior means bad thing ;) If I
> use fopen in particular mode and it is undefined, in this case it
> actually does not means undefined as bad things can happen?
It means that the behaviour is not defined by the C Standard, and that's
all it means. So, if you want your program to work on many different
implementations, you should stick to the standard modes. But if you are
writing for a particular implementation, you are free to take up the
guarantees that /it/ provides, to the extent that you are prepared to
trust (or test) those guarantees.
Sometimes you have little choice, as with IBM's mainframe modes for
VSAMs and so on. But when you go down that road, you accept that you are
limiting the portability of your program.
> Or we can take advantage of undefined behavior elsewhere ;)?
Of course you can, /if/ you know the consequences and /if/ you are
prepared to accept those consequences in return for a benefit that
outweighs the cost.
Since the C Standard doesn't say what will happen in the case of
undefined behaviour (in fact, that's what "undefined behaviour" actually
means!), you had better have some other way of knowing what will happen,
and you had better be very, very sure, and there had better be a pay-off
that outweighs the fact that you are breaking the rules and thus
sacrificing portability.
Let me give you two examples, one good and one bad. The bad one first.
i = i++; /* BAD! */
The C Standard doesn't say what will happen here. Now, you might do some
experiments and learn that your implementation happens to interpret the
above in exactly the way you want --- but there's still no point in
writing the code that way. You know what you want to happen to i, and
it's easy to express in conforming code, so why not express it in
conforming code? That way, you know for sure that it'll work on /any/
implementation, not just the one you happen to be using today.
Now for the good example (well, not that good, since it doesn't do much
checking, but good enough for our purposes):
unsigned char *scrbase = (unsigned char *)0xb0000000L;
if(_getgraphmode() == _GMODE_COLOUR)
{
scrbase += 0x08000000L;
}
scrbase += row * 160;
scrbase += col * 2;
while(*s)
{
*scrbase++ = (unsigned char)*s++;
*scrbase++ = attr;
}
The C Standard doesn't say what will happen here, either. But if you
want to write a string to a particular place on the screen and you
happen to be using MS-DOS, this is a /really/ fast way to do it. You
give up portability, but you gain performance.
So, as with so many things in life, it's really important to know
exactly what the rules are, so that you know exactly what you're letting
yourself in for when you break them.
The i = i++ code breaks the rules for *no good reason*. There is a cost
(portability) without a commensurate gain (such as performance, for
example). The graphics code breaks the rules too, but it's for an
excellent reason, and we accept the cost in order to gain the benefit.
--
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Line read from csv file with fgets has two characters before the terminating one Alla _ <modelling.data@gmail.com> - 2016-12-14 00:04 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-14 11:10 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Alla _ <modelling.data@gmail.com> - 2016-12-14 23:50 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 08:52 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Alla _ <modelling.data@gmail.com> - 2016-12-15 00:58 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 09:09 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Alla _ <modelling.data@gmail.com> - 2016-12-15 04:23 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 12:50 +0000
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-15 14:14 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 14:42 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Keith Thompson <kst-u@mib.org> - 2016-12-15 09:49 -0800
Re: Line read from csv file with fgets has two characters before the terminating one gazelle@shell.xmission.com (Kenny McCormack) - 2016-12-15 17:58 +0000
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-15 18:22 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Keith Thompson <kst-u@mib.org> - 2016-12-15 11:51 -0800
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-15 20:35 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Keith Thompson <kst-u@mib.org> - 2016-12-15 12:58 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-12-15 13:11 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-16 02:14 +0000
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-16 11:25 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-16 16:41 +0000
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-16 22:08 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-17 02:48 +0000
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-17 11:54 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-12-17 15:33 +0000
Re: Line read from csv file with fgets has two characters before the terminating one supercat@casperkitty.com - 2016-12-17 08:27 -0800
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-17 17:25 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-12-17 09:53 -0800
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-17 18:21 +0000
Re: Line read from csv file with fgets has two characters before the terminating one supercat@casperkitty.com - 2016-12-17 10:30 -0800
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-17 22:39 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-12-17 14:51 -0800
Re: Line read from csv file with fgets has two characters before the terminating one BartC <bc@freeuk.com> - 2016-12-17 23:12 +0000
Re: Line read from csv file with fgets has two characters before the terminating one supercat@casperkitty.com - 2016-12-18 08:37 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Keith Thompson <kst-u@mib.org> - 2016-12-18 13:14 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Geoff <geoff@invalid.invalid> - 2016-12-15 15:03 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Keith Thompson <kst-u@mib.org> - 2016-12-15 15:29 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Geoff <geoff@invalid.invalid> - 2016-12-15 20:48 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Keith Thompson <kst-u@mib.org> - 2016-12-15 22:58 -0800
OSX line endings (Was: Line read from csv file with fgets has two characters before the terminating one) gazelle@shell.xmission.com (Kenny McCormack) - 2016-12-16 10:22 +0000
Re: OSX line endings (Was: Line read from csv file with fgets has two characters before the terminating one) Geoff <geoff@invalid.invalid> - 2016-12-16 07:29 -0800
Making OSX's boot drive case-sensitve (Was: OSX line endings (Was: Line read from csv file with fgets has two characters before the terminating one)) gazelle@shell.xmission.com (Kenny McCormack) - 2016-12-17 10:06 +0000
Re: Making OSX's boot drive case-sensitve (Was: OSX line endings (Was: Line read from csv file with fgets has two characters before the terminating one)) Geoff <geoff@invalid.invalid> - 2016-12-17 12:39 -0800
Re: Making OSX's boot drive case-sensitve (Was: OSX line endings (Was: Line read from csv file with fgets has two characters before the terminating one)) gazelle@shell.xmission.com (Kenny McCormack) - 2016-12-18 13:54 +0000
Re: Making OSX's boot drive case-sensitve (Was: OSX line endings (Was: Line read from csv file with fgets has two characters before the terminating one)) Geoff <geoff@invalid.invalid> - 2016-12-18 17:57 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Geoff <geoff@invalid.invalid> - 2016-12-16 07:12 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Keith Thompson <kst-u@mib.org> - 2016-12-16 12:02 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 23:29 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Alla _ <modelling.data@gmail.com> - 2016-12-16 02:37 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Geoff <geoff@invalid.invalid> - 2016-12-16 07:51 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Manfred <noname@invalid.add> - 2016-12-15 12:22 +0100
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 11:29 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Melzzzzz <Melzzzzz@zzzzz.com> - 2016-12-15 12:40 +0100
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 12:00 +0000
Re: Line read from csv file with fgets has two characters before the terminating one Melzzzzz <mel@zzzzz.com> - 2016-12-15 13:09 +0100
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 12:43 +0000
Re: Line read from csv file with fgets has two characters before the terminating one "James R. Kuyper" <jameskuyper@verizon.net> - 2016-12-15 09:18 -0500
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 14:49 +0000
Re: Line read from csv file with fgets has two characters before the terminating one "James R. Kuyper" <jameskuyper@verizon.net> - 2016-12-15 10:34 -0500
Re: Line read from csv file with fgets has two characters before the terminating one Manfred <noname@invalid.add> - 2016-12-15 16:34 +0100
Re: Line read from csv file with fgets has two characters before the terminating one Alla _ <modelling.data@gmail.com> - 2016-12-15 00:22 -0800
Re: Line read from csv file with fgets has two characters before the terminating one Richard Heathfield <rjh@cpax.org.uk> - 2016-12-15 09:04 +0000
Re: Line read from csv file with fgets has two characters before the terminating one mark.bluemel@gmail.com - 2016-12-14 03:38 -0800
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