Reset a string?

[spike]

How do i reset a string?
I just want to empty it som that it does not contain any characters

Say it contains "hello world" at the time...
I want it to contain "". Nothing that is..

Thanx

[Leor Zolman]

Re: Reset a string?

On 5 Mar 2004 12:46:40 -0800, joan@ljungh.se (spike) wrote:
[color=blue]
>How do i reset a string?
>I just want to empty it som that it does not contain any characters
>
>Say it contains "hello world" at the time...
>I want it to contain "". Nothing that is..
>
>Thanx[/color]

Easy: put a '\0' into the first position. Whether "s" is defined as a
pointer-to-char or an array-of-char, you just say:
*s = '\0';
or
s[0] = '\0';

But beware that is isn't a pointer-to-const-char; doing this may invoke
undefined behavior.

Also note that this doesn't "empty" the string, it just marks the first
position as the terminating NUL. The memory it was using is still there and
needs to be contended with.
-leor




Leor Zolman
BD Software
leor@bdsoft.com
www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html

[Tor Husabø]

Re: Reset a string?

spike wrote:[color=blue]
> How do i reset a string?
> I just want to empty it som that it does not contain any characters
>
> Say it contains "hello world" at the time...
> I want it to contain "". Nothing that is..[/color]

char *str = "hello";
str[0] = '\0'; /* put a null terminator at the beginning */

Now the string is considered empty, and strlen(str) will return zero.

Tor

[Christopher Benson-Manica]

Re: Reset a string?

Tor Husabø <torhu@student. hf.uio.no> spoke thus:
[color=blue]
> char *str = "hello";
> str[0] = '\0'; /* put a null terminator at the beginning */[/color]

As Leor's post notes, this is incorrect code: str points to a string
literal, and thus may not be modified. You wanted something like

char str[]="hello";

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Roberto Nunnari]

Re: Reset a string?

Tor Husabø wrote:[color=blue]
> spike wrote:
>[color=green]
>> How do i reset a string?
>> I just want to empty it som that it does not contain any characters
>>
>> Say it contains "hello world" at the time...
>> I want it to contain "". Nothing that is..[/color]
>
>
> char *str = "hello";
> str[0] = '\0'; /* put a null terminator at the beginning */
>[/color]

be careful with char pointers like that.. it's not portable.
you have no garanties you'll be able to modify it like you did..
the compiler may put "hello" in readonly memory..

a correct version should be:

char str[] = "hello";
str[0] = '\0';

Best regards.
--
Roberto Nunnari -software engineer-
mailto:robi@nun nisoft.ch
Residenza Boschetto 12 tel/fax: +41-91-6046511
6935 Bosco Luganese """ mobile: +41-76-3208561
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[Clark Cox]

Re: Reset a string?

In article <0O52c.8568$rj4 .114557@news2.e .nsc.no>,
Tor Husabo <torhu@student. hf.uio.no> wrote:
[color=blue]
> spike wrote:[color=green]
> > How do i reset a string?
> > I just want to empty it som that it does not contain any characters
> >
> > Say it contains "hello world" at the time...
> > I want it to contain "". Nothing that is..[/color]
>
> char *str = "hello";
> str[0] = '\0'; /* put a null terminator at the beginning */[/color]

That's undefined behavior, I bealieve that you meant (Notice the change
in the first line):

char str[] = "hello";
str[0] = '\0'; /* put a null terminator at the beginning */

[Eric Sosman]

Re: Reset a string?

Roberto Nunnari wrote:[color=blue]
>
> Tor Husabø wrote:[color=green]
> > spike wrote:
> >[color=darkred]
> >> How do i reset a string?
> >> I just want to empty it som that it does not contain any characters
> >>
> >> Say it contains "hello world" at the time...
> >> I want it to contain "". Nothing that is..[/color]
> >
> >
> > char *str = "hello";
> > str[0] = '\0'; /* put a null terminator at the beginning */
> >[/color]
>
> be careful with char pointers like that.. it's not portable.
> you have no garanties you'll be able to modify it like you did..
> the compiler may put "hello" in readonly memory..[/color]

Even if no read-only memory is involved you can find
yourself in trouble. For example,

char *str = "hello";
str[0] = '\0';
puts ("Some rhymes: bellow, fellow, Jell-O, and hello");

may well produce the output

Some rhymes: bellow, fellow, Jell-O, and

More than one compiler performs the optimization that
produces this effect when abused by incorrect code.

--
Eric.Sosman@sun .com

[CBFalconer]

Re: Reset a string?

Tor Husabø wrote:[color=blue]
> spike wrote:
>[color=green]
>> How do i reset a string? I just want to empty it som that it
>> does not contain any characters
>>
>> Say it contains "hello world" at the time...
>> I want it to contain "". Nothing that is..[/color]
>
> char *str = "hello";
> str[0] = '\0'; /* put a null terminator at the beginning */
>
> Now the string is considered empty, and strlen(str) will return
> zero.[/color]

Nope. Now it has undefined (or possibly implementation defined)
behaviour, which may include going belly up. Think about it for a
while.

--
Chuck F (cbfalconer@yah oo.com) (cbfalconer@wor ldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net> USE worldnet address!

[Jack Klein]

Re: Reset a string?

On Fri, 05 Mar 2004 22:12:15 +0100, Roberto Nunnari
<robi@nunnisoft .ch> wrote in comp.lang.c:
[color=blue]
> Tor Husabø wrote:[color=green]
> > spike wrote:
> >[color=darkred]
> >> How do i reset a string?
> >> I just want to empty it som that it does not contain any characters
> >>
> >> Say it contains "hello world" at the time...
> >> I want it to contain "". Nothing that is..[/color]
> >
> >
> > char *str = "hello";
> > str[0] = '\0'; /* put a null terminator at the beginning */
> >[/color]
>
> be careful with char pointers like that.. it's not portable.
> you have no garanties you'll be able to modify it like you did..
> the compiler may put "hello" in readonly memory..
>
> a correct version should be:
>
> char str[] = "hello";
> str[0] = '\0';
>
> Best regards.[/color]

It's more than not portable, it is undefined behavior.

The C standard (all versions) does not state that string literals are
read-only, or whether or not they may share storage. Attempting to
modify string literals in C is undefined behavior because the C
standard (all versions) specifically says it is.

--
Jack Klein
Home: http://JK-Technology.Com
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http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

[Tor Husabø]

Re: Reset a string?

CBFalconer wrote:[color=blue]
> Tor Husabø wrote:
>[color=green]
>>spike wrote:
>>
>>[color=darkred]
>>>How do i reset a string? I just want to empty it som that it
>>>does not contain any characters
>>>
>>>Say it contains "hello world" at the time...
>>>I want it to contain "". Nothing that is..[/color]
>>
>>char *str = "hello";
>>str[0] = '\0'; /* put a null terminator at the beginning */
>>
>>Now the string is considered empty, and strlen(str) will return
>>zero.[/color]
>
>
> Nope. Now it has undefined (or possibly implementation defined)
> behaviour, which may include going belly up. Think about it for a
> while.
>[/color]

I know, just hasn't written C code for a while.

Maybe I should blame the fact that the two declarations below are equal?
Maybe it's more confusing than really helpful to differentiate in this way?
void func1(int a[]); /* argument is pointer to array */
void func2(int *a); /* argument is pointer to a single int */

I guess this could be what had me fooled for a moment.

[Adrian de los Santos]

Re: Reset a string?

On 2004-03-05 15:46:32 -0600, Eric Sosman <Eric.Sosman@su n.com> said:[color=blue]
>[color=green]
>> be careful with char pointers like that.. it's not portable.
>> you have no garanties you'll be able to modify it like you did..
>> the compiler may put "hello" in readonly memory..[/color]
>
> Even if no read-only memory is involved you can find
> yourself in trouble. For example,
>
> char *str = "hello";
> str[0] = '\0';
> puts ("Some rhymes: bellow, fellow, Jell-O, and hello");
>
> may well produce the output
>
> Some rhymes: bellow, fellow, Jell-O, and =
>
>
> More than one compiler performs the optimization that
> produces this effect when abused by incorrect code.[/color]


Intresting.. very intresting, but why the compiler do this ?

Thanks.

[Martin Dickopp]

Re: Reset a string?

Tor Husabø <torhu@student. hf.uio.no> writes:
[color=blue]
> Maybe I should blame the fact that the two declarations below are equal?
> Maybe it's more confusing than really helpful to differentiate in this way?
> void func1(int a[]); /* argument is pointer to array */[/color]

No, a pointer to an array cannot be used as an argument here, but a
pointer the first element of an array can.
[color=blue]
> void func2(int *a); /* argument is pointer to a single int */[/color]

Both declarations are equivalent. In both cases, `a' is pointer to an
object of type `int'. Whether this object is a single `int' or the first
element of an array makes no difference as far as the declaration is
concerned.

Martin


--
,--. Martin Dickopp, Dresden, Germany ,= ,-_-. =.
/ ,- ) http://www.zero-based.org/ ((_/)o o(\_))
\ `-' `-'(. .)`-'
`-. Debian, a variant of the GNU operating system. \_/

[J. J. Farrell]

Re: Reset a string?


"Adrian de los Santos" <demonREMOVE@de mon.com.mx> wrote in message news:c2c4pj$1q1 pnt$3@ID-86516.news.uni-berlin.de...[color=blue]
> On 2004-03-05 15:46:32 -0600, Eric Sosman <Eric.Sosman@su n.com> said:[color=green]
> >
> > Even if no read-only memory is involved you can find
> > yourself in trouble. For example,
> >
> > char *str = "hello";
> > str[0] = '\0';
> > puts ("Some rhymes: bellow, fellow, Jell-O, and hello");
> >
> > may well produce the output
> >
> > Some rhymes: bellow, fellow, Jell-O, and
> >
> >
> > More than one compiler performs the optimization that
> > produces this effect when abused by incorrect code.[/color]
>
> Intresting.. very intresting, but why the compiler do this ?[/color]

Because attempting to modify a string literal constant
results in undefined behaviour, and the compiler can do
what it wants. Since the writer of the code is not allowed
to invoke undefined behaviour if he wants predictable
results, the compiler can assume that he doesn't do so.
Hence the compiler can assume that the string literal
"hello" will never be modified, and hence it can optimise
memory usage by re-using the string "hello" that occurs
at the end of the bigger string. If the coder does risk
his life and sanity by modifying the string literal, in
this implementation he modifies others at the same time.

[CBFalconer]

Re: Reset a string?

Tor Husabø wrote:[color=blue][color=green]
> > Tor Husabø wrote:[color=darkred]
> >> spike wrote:
> >>[/color][/color][/color]
.... snip ...[color=blue][color=green][color=darkred]
> >>
> >> char *str = "hello";
> >> str[0] = '\0'; /* put a null terminator at the beginning */
> >>[/color][/color][/color]
.... snip ...[color=blue]
>
> Maybe I should blame the fact that the two declarations below
> are equal? Maybe it's more confusing than really helpful to
> differentiate in this way?
> void func1(int a[]); /* argument is pointer to array */
> void func2(int *a); /* argument is pointer to a single int */
>
> I guess this could be what had me fooled for a moment.[/color]

If the original declaration is of the (correct) form:

const char *str = "hello";

then the other misusages should get flagged at compile time.

--
Chuck F (cbfalconer@yah oo.com) (cbfalconer@wor ldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net> USE worldnet address!

[Adrian de los Santos]

Re: Reset a string?

On 2004-03-06 06:14:19 -0600, "J. J. Farrell" <jjf@bcs.org.uk > said:
[color=blue]
>
> "Adrian de los Santos" <demonREMOVE@de mon.com.mx> wrote in message
> news:c2c4pj$1q1 pnt$3@ID-86516.news.uni-berlin.de...[color=green]
>> On 2004-03-05 15:46:32 -0600, Eric Sosman <Eric.Sosman@su n.com> said:[color=darkred]
>>>
>>> Even if no read-only memory is involved you can find
>>> yourself in trouble. For example,
>>>
>>> char *str = "hello";
>>> str[0] = '\0';
>>> puts ("Some rhymes: bellow, fellow, Jell-O, and hello");
>>>
>>> may well produce the output
>>>
>>> Some rhymes: bellow, fellow, Jell-O, and
>>>
>>>
>>> More than one compiler performs the optimization that
>>> produces this effect when abused by incorrect code.[/color]
>>
>> Intresting.. very intresting, but why the compiler do this ?[/color]
>
> Because attempting to modify a string literal constant
> results in undefined behaviour[/color]

fist pardon me please for my poor knowledge of this topics.

As far as i understand

char *str="hello";

- will create a pointer to a char str
- will store "hello" on the memory
- will store the address of memory where "hello" starts in the str variable.

Am i right ?

So why *str becomes a constant ?
because it was assigned a rvalue at initialization ? (="hello")

Thanks for your kind explanation.