Problem with strcat, strcpy,sprintf

[pete]

Re: Problem with strcat, strcpy,sprintf

Arctic Fidelity wrote:[color=blue]
>
> On Fri, 28 Oct 2005 18:34:47 -0400,
> Keith Thompson <kst-u@mib.org> wrote:[/color]
[color=blue][color=green][color=darkred]
> >> void main(void)[/color]
> >
> > No, no, no, no, no.
> >
> > main() returns int, not void.[/color]
>
> :-O I never knew...Wah?? Gah! Ouch.
> I'll keep that in definite mind next time.[/color]

The rules are that main returns int,
but implementations may accept alternate forms of main.

Since the alternate forms are not standard,
and this is a newsgroup about C and not about *your* compiler,
the alternate forms are off topic here.

--
pete

[Jordan Abel]

Re: Problem with strcat, strcpy,sprintf

On 2005-10-29, pete <pfiland@mindsp ring.com> wrote:[color=blue]
> Arctic Fidelity wrote:[color=green]
>>
>> On Fri, 28 Oct 2005 18:34:47 -0400,
>> Keith Thompson <kst-u@mib.org> wrote:[/color]
>[color=green][color=darkred]
>> >> void main(void)
>> >
>> > No, no, no, no, no.
>> >
>> > main() returns int, not void.[/color]
>>
>> :-O I never knew...Wah?? Gah! Ouch.
>> I'll keep that in definite mind next time.[/color]
>
> The rules are that main returns int, but implementations may
> accept alternate forms of main.
>
> Since the alternate forms are not standard, and this is a
> newsgroup about C and not about *your* compiler, the alternate
> forms are off topic here.[/color]

Furthermore, very few implementations _actually_ accept void main.
It's just that a few of the most common ones happen to not do
anything worse than having some arbitrary number as the exit status

[Mark McIntyre]

Re: Problem with strcat, strcpy,sprintf

On Fri, 28 Oct 2005 23:38:31 GMT, in comp.lang.c , Keith Thompson
<kst-u@mib.org> wrote:
[color=blue]
>Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=green]
>> On 28 Oct 2005 14:09:43 -0700, in comp.lang.c ,
>> diego.arias.vel @gmail.com wrote:[/color]
>[...][color=green][color=darkred]
>>>void copy(char *filename)
>>>{
>>> char *log_file;
>>> log_file=filena me;[/color]
>>
>> This points log_file to the same place as filename.
>>
>> Remember that in C, = is not the copy operator, its the assignment
>> operator. For pointer types, this sets the pointers to point to the
>> same place. It does /not/ copy the contents.[/color]
>[...]
>
>I don't think I'd phrase it that way.[/color]

Yeah, yours is perhaps more correct phrasing, but IMO that would have
totally confused anyone who thought of "string" as an actual type.
Since this is a very common newby view, I deliberately chose different
wording.

[color=blue]
>Another way to put it is that "=" does a shallow copy, not a deep
>copy; it copies only the value itself,[/color]

As a newby I'd expect this to copy the string...

--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

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[Mark McIntyre]

Re: Problem with strcat, strcpy,sprintf

On 28 Oct 2005 16:55:25 -0700, in comp.lang.c , "nelu"
<tandauioan@gma il.com> wrote:
[color=blue]
>I've seen this in a lot of places and I've been wandering if it's smart
>to write code like this:
>
>log_file=mallo c(STRGSIZE)[/color]

This is the right way.
[color=blue]
>instead of:
>
>log_file=(ch ar *)malloc(STRGSI ZE*sizeof(char) )[/color]

In this version,
a) the cast is not needed and can conceal a serious error
b) sizeof(char) is by definition 1, so its not needed.
[color=blue]
>because it is both a portability problem[/color]

only between C and C++
[color=blue]
>and a lot of students don't
>get the idea and do the exact same thing for types other than char?[/color]

in that case use the form
log_file = malloc (STRGSIZE * sizeof (*log_file));
and no matter what log_file is defined as, you're clear.
--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

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[Keith Thompson]

Re: Problem with strcat, strcpy,sprintf

Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=blue]
> On Fri, 28 Oct 2005 23:38:31 GMT, in comp.lang.c , Keith Thompson
> <kst-u@mib.org> wrote:[color=green]
>>Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=darkred]
>>> On 28 Oct 2005 14:09:43 -0700, in comp.lang.c ,
>>> diego.arias.vel @gmail.com wrote:[/color]
>>[...][color=darkred]
>>>>void copy(char *filename)
>>>>{
>>>> char *log_file;
>>>> log_file=filena me;
>>>
>>> This points log_file to the same place as filename.
>>>
>>> Remember that in C, = is not the copy operator, its the assignment
>>> operator. For pointer types, this sets the pointers to point to the
>>> same place. It does /not/ copy the contents.[/color]
>>[...]
>>
>>I don't think I'd phrase it that way.[/color]
>
> Yeah, yours is perhaps more correct phrasing, but IMO that would have
> totally confused anyone who thought of "string" as an actual type.
> Since this is a very common newby view, I deliberately chose different
> wording.[/color]

I think that just reinforces any confusion. Anyone who thinks that
"string" is an actual type should be told that it isn't.
[color=blue][color=green]
>>Another way to put it is that "=" does a shallow copy, not a deep
>>copy; it copies only the value itself,[/color]
>
> As a newby I'd expect this to copy the string...[/color]

Not if you understand what a "string" is. (You're not restricted to
explaining just one thing at a time.)

--
Keith Thompson (The_Other_Keit h) kst-u@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.

[Mark McIntyre]

Re: Problem with strcat, strcpy,sprintf

On Sat, 29 Oct 2005 22:14:45 GMT, in comp.lang.c , Keith Thompson
<kst-u@mib.org> wrote:
[color=blue]
>Not if you understand what a "string" is. (You're not restricted to
>explaining just one thing at a time.)[/color]

I've noticed when teaching undergrads (and new staff) that the
teach-several-things-at-once approach is fraught with peril... :-)

--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

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[Keith Thompson]

Re: Problem with strcat, strcpy,sprintf

Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=blue]
> On Sat, 29 Oct 2005 22:14:45 GMT, in comp.lang.c , Keith Thompson
> <kst-u@mib.org> wrote:
>[color=green]
>>Not if you understand what a "string" is. (You're not restricted to
>>explaining just one thing at a time.)[/color]
>
> I've noticed when teaching undergrads (and new staff) that the
> teach-several-things-at-once approach is fraught with peril... :-)[/color]

Perhaps. Is the students-don't-know-what-strings-are-so-pretend-
their-misconceptions-are-correct approach fraught with less peril?

--
Keith Thompson (The_Other_Keit h) kst-u@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.

[Mark McIntyre]

Re: Problem with strcat, strcpy,sprintf

On Sun, 30 Oct 2005 19:59:43 GMT, in comp.lang.c , Keith Thompson
<kst-u@mib.org> wrote:
[color=blue]
>Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=green]
>> On Sat, 29 Oct 2005 22:14:45 GMT, in comp.lang.c , Keith Thompson
>> <kst-u@mib.org> wrote:
>>[color=darkred]
>>>Not if you understand what a "string" is. (You're not restricted to
>>>explaining just one thing at a time.)[/color]
>>
>> I've noticed when teaching undergrads (and new staff) that the
>> teach-several-things-at-once approach is fraught with peril... :-)[/color]
>
>Perhaps. Is the students-don't-know-what-strings-are-so-pretend-
>their-misconceptions-are-correct approach fraught with less peril?[/color]

Pejorative, rigged question and therefore not worthy of an answer. :-)

Ignoring your question, I'd say its much less fraught with peril to
start with fundamentals so that by the time the students get to such
complexities as the = operator, they already don't think strings are
PODs.
--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

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[Keith Thompson]

Re: Problem with strcat, strcpy,sprintf

Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=blue]
> On Sun, 30 Oct 2005 19:59:43 GMT, in comp.lang.c , Keith Thompson
> <kst-u@mib.org> wrote:
>[color=green]
>>Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=darkred]
>>> On Sat, 29 Oct 2005 22:14:45 GMT, in comp.lang.c , Keith Thompson
>>> <kst-u@mib.org> wrote:
>>>
>>>>Not if you understand what a "string" is. (You're not restricted to
>>>>explainin g just one thing at a time.)
>>>
>>> I've noticed when teaching undergrads (and new staff) that the
>>> teach-several-things-at-once approach is fraught with peril... :-)[/color]
>>
>>Perhaps. Is the students-don't-know-what-strings-are-so-pretend-
>>their-misconceptions-are-correct approach fraught with less peril?[/color]
>
> Pejorative, rigged question and therefore not worthy of an answer. :-)[/color]

I see the smiley, but the question was based directly on what you
wrote upthread. It may have been a bit pejorative, but it was not
rigged, at least not deliberately.

Mark:
] >> Remember that in C, = is not the copy operator, its the assignment
] >> operator. For pointer types, this sets the pointers to point to the
] >> same place. It does /not/ copy the contents.
] >[...]
Keith:
] >I don't think I'd phrase it that way.
Mark:
] Yeah, yours is perhaps more correct phrasing, but IMO that would have
] totally confused anyone who thought of "string" as an actual type.
] Since this is a very common newby view, I deliberately chose different
] wording.

It certainly appeared to me that you were assuming certain
misconceptions about what strings are and deliberately not correcting
them for the sake of an explanation of what assignment does. For
example, I found your use of the word "contents" misleading; the
content of a pointer is an address, not the thing it points to.
Saying that "=" is not the copy operator" is not a simplification;
it's just incorrect. It might be a convenient fiction that can help
someone understand that in
char *a = "hello";
char *b;
b = a;
the assignment doesn't copy the string -- but it will inevitably lead
to confusion later on.

There are times when it's appropriate to give not-quite-correct
explanations early on, to be refined later. I don't think this is one
of those cases. The subtle relationship between arrays and pointers
is so central to the way C works that understanding it is an absolute
prerequesite to having a real understanding of the language.
[color=blue]
> Ignoring your question, I'd say its much less fraught with peril to
> start with fundamentals so that by the time the students get to such
> complexities as the = operator, they already don't think strings are
> PODs.[/color]

(POD being Plain Old Data, I presume; I think that's mostly a C++
term, so some here might not be familiar with it.)

I agree with you, but that's not the situation here. The question is
how to explain pointer assignment to someone who *already*
misunderstands C strings. You can give a quick and misleading
explanation that lets the newbie *think* he understands how a
particular program works, or you can give a longer and more correct
explanation, or at least a pointer to one (such as, "read section 6 of
the C FAQ").

--
Keith Thompson (The_Other_Keit h) kst-u@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.

[Greg Comeau]

Re: Problem with strcat, strcpy,sprintf

In article <ln1x22u66o.fsf @nuthaus.mib.or g>,
Keith Thompson <kst-u@mib.org> wrote:[color=blue]
>Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=green]
>> On Sat, 29 Oct 2005 22:14:45 GMT, in comp.lang.c , Keith Thompson
>> <kst-u@mib.org> wrote:
>>[color=darkred]
>>>Not if you understand what a "string" is. (You're not restricted to
>>>explaining just one thing at a time.)[/color]
>>
>> I've noticed when teaching undergrads (and new staff) that the
>> teach-several-things-at-once approach is fraught with peril... :-)[/color]
>
>Perhaps. Is the students-don't-know-what-strings-are-so-pretend-
>their-misconceptions-are-correct approach fraught with less peril?[/color]

I find teaching to misconceptions misconcepted and so always teach
the truth. The "small lies" (and big ones) just never make any sense
to me when I hear others saying that they teach that way.
--
Greg Comeau / Celebrating 20 years of Comeauity!
Comeau C/C++ ONLINE ==> http://www.comeaucomputing.com/tryitout
World Class Compilers: Breathtaking C++, Amazing C99, Fabulous C90.
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