scanf(), ungetc() behaviour.

[Argento]

I was curious at the start about how ungetc() returns the character to the
stream, so i did the following coding. Things work as expected except if I
change the scanf("%c",&j) to scanf("%d",&j). I don't understand how could
scanf() affect the content of i[0] and i[1]. Can someone tell me why?

#include <stdio.h>
#include <ctype.h>

void main()
{
char i[2], j;
printf("Please input a two digit number:");
i[0]= getchar();
i[1]= getchar();
ungetc(i[1],stdin);
ungetc(i[0],stdin);
scanf("%c", &j); //scanf("%d",&j);
fflush(stdin);
printf("\nYou entered %d in decimal and %c in character mode for
i[0]",i[0],i[0]);
printf("\nYou entered %d in decimal and %c in character mode for
i[1]",i[1],i[1]);
printf("\nYou entered %d in decimal and %c in character mode for j",j,j);

getchar();

}

[Ben Bacarisse]

On Sat, 04 Mar 2006 17:33:10 +0800, Argento wrote:

Publish the instructors email address here and we will collectively set
him/her right. It is not right to warp the minds of the innocent.

Are you serious about this? Before you answer that first question, I would
just want you to know, those course books are from OUHK (Open University
of Hong Kong, that's where I currently study about this) taken from the
course code MT258.

I would not push this too far if I were you. I have had a look at some
of the TMA solutions and there are worse errors in there the void main()
and fflush(stdin)! One solution has some very odd logic (I can't say
"wrong" because the spec. is not given in the solution paper so it is
possible the strange behaviour is allowed). In one place in the
supporting material by the same author gets and scanf("%[\n]", ...) are
given as examples. In another, the example can overwrite an array.
Others use sentinel values which I don't think of a good practise.

Even good book can have errors, but these just look like bad teaching
examples to me.

--
Ben.

[CBFalconer]

Argento wrote:

Publish the instructors email address here and we will collectively
set him/her right. It is not right to warp the minds of the
innocent.

Are you serious about this? Before you answer that first question, I
would just want you to know, those course books are from OUHK
(Open University of Hong Kong, that's where I currently study about
this) taken from the course code MT258.

Yes, I am serious. I don't know what we want to set him right
about, since you snipped that portion. However I vaguely remember
writing the above, although you snipped the attributions. Please
don't do that. I guess snipping is an acquired art.

We all have an interest in not having bad teachers go uncorrected.
They produce bad product, and make life harder for all of us
correcting those ex-students. Some of us will be more diplomatic
than others, and some will be outright rude.

I suspect if you, as a student, go to your instructor and insist
that he cease and desist from misinformation, it will adversely
affect your marks. However if peers tell him the same, usually
with bell, book, candle, and standards references in hand, s/he may
listen and no evil retribution will fall on you.

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell. org/google/>
Also see <http://www.safalra.com/special/googlegroupsrep ly/>

[Joe Wright]

CBFalconer wrote:

Argento wrote:

Publish the instructors email address here and we will collectively
set him/her right. It is not right to warp the minds of the
innocent.

Are you serious about this? Before you answer that first question, I
would just want you to know, those course books are from OUHK
(Open University of Hong Kong, that's where I currently study about
this) taken from the course code MT258.

Yes, I am serious. I don't know what we want to set him right
about, since you snipped that portion. However I vaguely remember
writing the above, although you snipped the attributions. Please
don't do that. I guess snipping is an acquired art.

We all have an interest in not having bad teachers go uncorrected.
They produce bad product, and make life harder for all of us
correcting those ex-students. Some of us will be more diplomatic
than others, and some will be outright rude.

I suspect if you, as a student, go to your instructor and insist
that he cease and desist from misinformation, it will adversely
affect your marks. However if peers tell him the same, usually
with bell, book, candle, and standards references in hand, s/he may
listen and no evil retribution will fall on you.

In my own experience, the young student who demands the instructor cease
and desist from anything at all, will lose big time. If student finds
instructor incompetent, student should change instructors. No student
has ever won the fight with the instructor, in my experience.

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[jaysome]

Richard Heathfield wrote:

Argento said:

The C standard requires that main() be defined with a return type of
int in a hosted environment, unless you have a C99 conforming compiler
that specifically documents that it accepts "void main()", and I will
most certainly bet that you do not. This means your program is
undefined.

Well, sorry. I don't know much about the standard but i am using DEV C++
and the compiler compiles it happily.

Nevertheless, it is incorrect. And now you *know* it's incorrect.

I know for certain that at least one regular contributor to this group, and
possibly more than one, has got into the habit of completely ignoring any
"plea-for-help" article in which the questioner has used void main, on the
grounds that they can't be all that serious about C if they can't even get
the entry point right.

Is "void main" really that bad?

Does anyone have any statistics on the compilers that "void main" causes
problems with?

I suspect that such statistics would show that the use of "void main"
has similar ramifications to tearing off the tag of a bed mattress. If
anyone can prove me wrong, please back it up with some numbers.

--
jaysome

[Vladimir S. Oka]

jaysome wrote:

Richard Heathfield wrote:

Argento said:

The C standard requires that main() be defined with a return type of
int in a hosted environment, unless you have a C99 conforming
compiler that specifically documents that it accepts "void main()",
and I will
most certainly bet that you do not. This means your program is
undefined .

Well, sorry. I don't know much about the standard but i am using DEV
C++ and the compiler compiles it happily.

Nevertheless, it is incorrect. And now you *know* it's incorrect.

I know for certain that at least one regular contributor to this
group, and possibly more than one, has got into the habit of
completely ignoring any "plea-for-help" article in which the
questioner has used void main, on the grounds that they can't be all
that serious about C if they can't even get the entry point right.

Is "void main" really that bad?

Does anyone have any statistics on the compilers that "void main"
causes problems with?

I suspect that such statistics would show that the use of "void main"
has similar ramifications to tearing off the tag of a bed mattress. If
anyone can prove me wrong, please back it up with some numbers.

I don't think `void main()` will really do any nasty things to you.
However, most examples come from posters who are obviously new and
inexperienced, and I believe it signifies either bad teaching they've
been exposed to or not really caring about learning proper C. Something
along the lines of not capitalising `i`s and sentences, and using `u`,
`ur` and similar contractions.

Ask any of them about whether their particular implementation actually
allows `void main()`, which it can if it documents it, they wouldn't
have a clue what you're talking about. And of course, even if it did,
it wouldn't be portable, Standard C, and so would be off-topic here.

--
BR, Vladimir

Cleanliness becomes more important when godliness is unlikely.
-- P.J. O'Rourke

[Keith Thompson]

jaysome <ja*****@spamco p.com> writes:

Richard Heathfield wrote:

Argento said:

The C standard requires that main() be defined with a return type of
int in a hosted environment, unless you have a C99 conforming compiler
that specifically documents that it accepts "void main()", and I will
most certainly bet that you do not. This means your program is
undefined .

Well, sorry. I don't know much about the standard but i am using DEV C++
and the compiler compiles it happily. Nevertheless, it is incorrect. And now you *know* it's incorrect.
I know for certain that at least one regular contributor to this
group, and possibly more than one, has got into the habit of
completely ignoring any "plea-for-help" article in which the
questioner has used void main, on the grounds that they can't be all
that serious about C if they can't even get the entry point right.

Is "void main" really that bad?

Yes.
Does anyone have any statistics on the compilers that "void main"
causes problems with?
I don't know, but that's not really the point.
I suspect that such statistics would show that the use of "void main"
has similar ramifications to tearing off the tag of a bed mattress. If
anyone can prove me wrong, please back it up with some numbers.

There is quite simply no reason to use "void main". Unless you happen
to be using an implementation that (unwisely, IMHO) documents the use
of "void main" as an extension, using it invokes undefined behavior.
Most likely it will work as expected; the worst *likely* problem is
that the program will return an unpredictable exit status to the
environment (which is a good enough reason in itself to avoid it).
But it also means that any time your program doesn't behave as you
expected, you can't be certain that your use of "void main" didn't
cause the problem.

Consistently using the *correct* form costs nothing, and it means you
always have one less thing to worry about.

"void main" *probably* won't cause any real problems, but unless
you're using random numbers, programming isn't about probabilities.
It's about writing correct code that does what you want it to do.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Richard Heathfield]

jaysome said:

Richard Heathfield wrote:

...they can't be all that serious about C if they can't
even get the entry point right.
Is "void main" really that bad?

Yes, for the reason I gave above.

It's called "cargo cult" programming. Setting aside freestanding
implementations for a moment, there is never a good reason to use void
main. So where does it come from? Not from thinking, that's for sure. No,
people use it because they've seen other people using it, without thinking
of the possibility that those they emulate may not be all that great at C
themselves. Computers copy stuff blindly about the place without
understanding it. People are supposed to be brighter than that.
Does anyone have any statistics on the compilers that "void main" causes
problems with?
Does anyone have statistics on the number of programs that have failed
because of undefined behaviour?
I suspect that such statistics would show that the use of "void main"
has similar ramifications to tearing off the tag of a bed mattress. If
anyone can prove me wrong, please back it up with some numbers.

I suspect that statistics would show that those who don't care enough about
correctness to get the entry point right are also likely not to care enough
about getting other things right. So why bother trying to teach them? Just
let them guess and hope.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)

[Charles Krug]

On 2006-03-05, Richard Heathfield <in*****@invali d.invalid> wrote:

jaysome said:

Richard Heathfield wrote:

...they can't be all that serious about C if they can't
even get the entry point right.

Is "void main" really that bad?

Yes, for the reason I gave above.

It's called "cargo cult" programming. Setting aside freestanding
implementations for a moment, there is never a good reason to use void
main.

My entire career is freestanding implementations or things like vxWorks
where you just don't use main() at all.

Why?

Requirments for those types of things often state "ANSI (or ISO) C, with
no extensions," "No warnings from the compiler set at its most stringent
level," and "No unreachable code"

Defining main() void or with the wrong parameters violates the first
requirement (non-ANSI).

Ignoring the warning from main() not returning violates the Second
requirement (causes warnings).

Sticking a return; after a never exits loop violates the third
(unreachable code).

While there exist freestanding, as well as hosted, implementations where
it works, there's rather an enormous gulf between:

"This works on this version of the compiler running under this
processor on this OS cross-compiling to that processor under that OS"

and:

"This is Standard and supported by any conforming implementation. "

[Ben Bacarisse]

On Sun, 05 Mar 2006 02:07:41 -0500, Joe Wright wrote:

No student has
ever won the fight with the instructor, in my experience.

That may be true, but there should be no fight in the first place. Good
instructors know they can make mistakes and learn from them (rather more
students think they know better that actually do, but that should be no
excuse). To swing back almost OT, I once presented something rather like
this:

int order2(int *a, int *b)
{
if (*a > *b) {
int tmp = *a; *a = *b; *b = tmp;
return 1;
}
return 0;
}

int order4(int *a, int *b, int *c, int *d)
{
int swaps = 0;
while (order2(a, b) || order2(b, c) || order2(c, d))
swaps++;
return swaps;
}

and asked for an informal argument about the termination of the loop in
order4. It was not actually C, but no matter. Most were doing what I
wanted (informally showing that the loop terminates) and getting good
grades for that part until someone pointed out what will happen with
aliased pointers. Obvious enough, maybe, but I was thinking about other
matters when writing the assignment. The next year, the question was
better and asked for two arguments -- including one for when the pointers
might be aliased!

I would be wary of any teacher who does not learn from their students.

--
Ben.

[Chris Torek]

In article <l8************ *************** ***@comcast.com >,
Charles Krug <cd****@aol.com > wrote:

My entire career is freestanding implementations or things like vxWorks
where you just don't use main() at all.

Of course, if you do not use a main() at all, there is no need to
concern yourself with the return-type and parameter(s) to main(). :-)

I actually prefer avoiding main() entirely in freestanding code,
for this reason.

(Also: although it is off topic, I will mention that as of 6.x, vxWorks
does use "main" functions in RTPs.)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.