why

[Darklight]

I am using suse 9.3 pro
Why is it when i run this program in the konsole

i get the out put:
1st string: To be or not to be?
2nd string: To bTo be or not to be?

which it should not be

but when i run the same program in the ddd debugger i get
1st string: to be or not to be:
2nd string: to be

which it should be according to the book: c programming in easy steps
the program is below:

#include<stdio. h>
#include<string .h> /* make strncpy available */

int main(void)
{
char string1[] = "To be or not to be?";
char string2[6];

/* copy first 5 characters in string1 to string2 */
strncpy(string2 , string1, 5);

printf("1st string: %s\n", string1);
printf("2nd string: %s\n", string2);
return 0;
}

[Robert Gamble]

Re: why

Darklight wrote:[color=blue]
> I am using suse 9.3 pro
> Why is it when i run this program in the konsole
>
> i get the out put:
> 1st string: To be or not to be?
> 2nd string: To bTo be or not to be?
>
> which it should not be
>
> but when i run the same program in the ddd debugger i get
> 1st string: to be or not to be:
> 2nd string: to be
>
> which it should be according to the book: c programming in easy steps
> the program is below:
>
> #include<stdio. h>
> #include<string .h> /* make strncpy available */
>
> int main(void)
> {
> char string1[] = "To be or not to be?";
> char string2[6];
>
> /* copy first 5 characters in string1 to string2 */
> strncpy(string2 , string1, 5);
>
> printf("1st string: %s\n", string1);
> printf("2nd string: %s\n", string2);
> return 0;
> }[/color]

Despite its name, strncpy does not guarantee that the destination will
have a null terminator (and thus be a string). In your case, since
there is no null character in the first 5 characters of the source
string, 5 non-null characters will be stored in string2, the last
character is indeterminate. When you printf string2, undefined
behavior occurs because it is not a string. If you initialized string2
like this:
char string2[6] = {0}
you would get the behavior you expect.

Robert Gamble

[Skarmander]

Re: why

Darklight wrote:[color=blue]
> I am using suse 9.3 pro
> Why is it when i run this program in the konsole
>
> i get the out put:
> 1st string: To be or not to be?
> 2nd string: To bTo be or not to be?
>
> which it should not be
>
> but when i run the same program in the ddd debugger i get
> 1st string: to be or not to be:
> 2nd string: to be
>
> which it should be according to the book: c programming in easy steps
> the program is below:
>
> #include<stdio. h>
> #include<string .h> /* make strncpy available */
>
> int main(void)
> {
> char string1[] = "To be or not to be?";
> char string2[6];
>
> /* copy first 5 characters in string1 to string2 */
> strncpy(string2 , string1, 5);
>[/color]
<snip>

Strings in C are null-terminated. strncpy copies only the first five
bytes of the string, which does not include a terminating null.
Therefore the string is not well-defined. You need either

string2[5] = '\0';

after the strncpy or

char string2[6] = {'\0'};

to initialize string2 with all NULs.

The reason you do not see this problem when debugging is because the
contents of uninitialized memory apparently include a NUL character, and
it happens to work. In general, anything could happen, including a
program crash.

S.

[Skarmander]

Re: why

Skarmander wrote:[color=blue]
> Darklight wrote:
>[color=green]
>> I am using suse 9.3 pro
>> Why is it when i run this program in the konsole
>>
>> i get the out put:
>> 1st string: To be or not to be?
>> 2nd string: To bTo be or not to be?
>>
>> which it should not be
>>
>> but when i run the same program in the ddd debugger i get
>> 1st string: to be or not to be:
>> 2nd string: to be
>>
>> which it should be according to the book: c programming in easy steps
>> the program is below:
>>
>> #include<stdio. h>
>> #include<string .h> /* make strncpy available */
>>
>> int main(void)
>> {
>> char string1[] = "To be or not to be?";
>> char string2[6];
>>
>> /* copy first 5 characters in string1 to string2 */
>> strncpy(string2 , string1, 5);
>>[/color]
> <snip>
>
> Strings in C are null-terminated. strncpy copies only the first five
> bytes of the string, which does not include a terminating null.[/color]

Uh oh, overloading alert. Make this "NUL-terminated" and "terminatin g
NUL", to avoid confusion with null pointers and NULL.

S.

[pete]

Re: why

Darklight wrote:[color=blue]
>
> I am using suse 9.3 pro
> Why is it when i run this program in the konsole
>
> i get the out put:
> 1st string: To be or not to be?
> 2nd string: To bTo be or not to be?
>
> which it should not be
>
> but when i run the same program in the ddd debugger i get
> 1st string: to be or not to be:
> 2nd string: to be
>
> which it should be according to the book: c programming in easy steps
> the program is below:[/color]

Really? That's some bad code.
It doesn't put a null character in string2,
and there's no such thing a string without a null character, in C.

[color=blue]
>
> #include<stdio. h>
> #include<string .h> /* make strncpy available */
>
> int main(void)
> {
> char string1[] = "To be or not to be?";
> char string2[6];[/color]

If it were

char string2[6] = "";

then that would be OK
[color=blue]
>
> /* copy first 5 characters in string1 to string2 */
> strncpy(string2 , string1, 5);[/color]


string2[5] = '\0';

/*
** If string2 is not initialized with null characters
** then you have to put one in there at the end.
*/
[color=blue]
> printf("1st string: %s\n", string1);
> printf("2nd string: %s\n", string2);
> return 0;
> }[/color]


--
pete

[Darklight]

Re: why

Thanks for your help, but this is what it says in the book

The example below copies a string of five characters into an array
of six elements-so string2[5] receives a \0 null character.

[Skarmander]

Re: why

Darklight wrote:[color=blue]
> Thanks for your help, but this is what it says in the book
>
> The example below copies a string of five characters into an array
> of six elements-so string2[5] receives a \0 null character.
>[/color]
This goes to show you that you can't believe everything you read.

This is a pretty explicit mistake; I'd look around for another book.

According to the ACCU, this book is "Not Recommended".

http://www.accu.org/bookreviews/public/reviews/c/c003566.htm

S.

[pete]

Re: why

Skarmander wrote:[color=blue]
>
> Darklight wrote:[color=green]
> > Thanks for your help, but this is what it says in the book
> >
> > The example below copies a string of five characters into an array
> > of six elements-so string2[5] receives a \0 null character..[/color][/color]

I'm not a proponent of book burning per se,
but if you have fireplace and it gets cold this Winter ...
[color=blue]
> This goes to show you that you can't believe everything you read.
>
> This is a pretty explicit mistake; I'd look around for another book.
>
> According to the ACCU, this book is "Not Recommended".
> http://www.accu.org/bookreviews/publ.../c/c003566.htm
>
> S.[/color]

--
pete