the minimum size when using strncpy(...)

[Simon]

Hi,

I am a bit confused with zero based items and strncpy(...)

assuming I have

char str1[] = "Hello world";
char str2[3];

I now have str2 that should look something like

str[0] == '?';
str[1] == '?';
str[2] == '?';

where '?' is any character that was there b4

and str2 does not have str[3] == '?';

am I right so far?
now can I also say that if I did

int nSize = strlen(str2);
I could get any value really 'cause there is no null char

now if I wanted to create a function to get 2 characters from a string I
could do something like..
the reason why I want a function is because the size(s) might change at a
later stage so I'd rather change one function and all the strings that call
it.
I also want to make sure that the data has a terminating null char and that
the rest of the data is set to null as well.

int MyCopy( char szDest[3], const char*szSource )
{
memset( szDest, 0, 3 );
strcpy( szDest, szSource, 2 );
szDest[2] = '\0';

strlen(szDest );
}

is my logic above correct? or would it be better to define my function as
(but I would I ensure that the size is correct?

int MyCopy( char *szDest, const char*szSource )
{
...
}

many thanks in advance.

regards.

"Simon" <sp********@sch oolsofafrica.co m> wrote in message
news:2q******** ****@uni-berlin.de...

Hi,

I am a bit confused with zero based items and strncpy(...)

assuming I have

char str1[] = "Hello world";
char str2[3];

I now have str2 that should look something like

str[0] == '?';
str[1] == '?';
str[2] == '?';

where '?' is any character that was there b4

and str2 does not have str[3] == '?';

am I right so far?
now can I also say that if I did

int nSize = strlen(str2);
I could get any value really 'cause there is no null char

I think the situation is 'undefined'.
now if I wanted to create a function to get 2 characters from a string I
could do something like..
the reason why I want a function is because the size(s) might change at a
later stage so I'd rather change one function and all the strings that
call
it.
I also want to make sure that the data has a terminating null char and
that
the rest of the data is set to null as well.

int MyCopy( char szDest[3], const char*szSource )
{
memset( szDest, 0, 3 );
strcpy( szDest, szSource, 2 );
szDest[2] = '\0';

strlen(szDest );
}

My problem with using a function like that are the design ramifications.
Hardcoding those kind of values is silly at best. :P
is my logic above correct? or would it be better to define my function as
(but I would I ensure that the size is correct?

int MyCopy( char *szDest, const char*szSource )
{
...
}

This would be fine assuming you zero terminated each passed string. Or why
not implement a 'limit' copy. Pass a number indicated the size. Or better
yet, use a supplied system function (most of the string and printf functions
have buffer 'safe' counterparts) for the desired results.
many thanks in advance.

regards.

[Ivan Vecerina]

"Simon" <sp********@sch oolsofafrica.co m> wrote in message
news:2q******** ****@uni-berlin.de...

int MyCopy( char szDest[3], const char*szSource )
{
memset( szDest, 0, 3 );
strcpy( szDest, szSource, 2 ); ~~~~~~ strncpy szDest[2] = '\0';

strlen(szDest );
}

is my logic above correct? or would it be better to define my function as
(but I would I ensure that the size is correct?

The function could be simplified to:
int MyCopy( char szDest[3], const char*szSource )
{
strncpy( szDest, szSource, 2 );
szDest[2] = '\0';

return strlen(szDest );
}

And if your library provides as an extension the safer strlcpy
function that was introducted by the OpenBSD team (see
http://www.gsp.com/cgi-bin/man.cgi?s...&topic=strlcpy ),
you can even write:
int MyCopy( char szDest[3], const char*szSource )
{
strlcpy( szDest, szSource, 3 );
return strlen(szDest);
}
.... and actually get rid of the function altogether.
This said, people in this NG prefer to use std::string ;)

Cheers,
Ivan
--
http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form

[Simon]

> >

is my logic above correct? or would it be better to define my function as (but I would I ensure that the size is correct?
The function could be simplified to:
int MyCopy( char szDest[3], const char*szSource )
{
strncpy( szDest, szSource, 2 );
szDest[2] = '\0';

return strlen(szDest );
}

I understand, but am I right in assuming there is nothing wrong with adding
memset( szDest, 0, 3 ); even if it is a waste of space.

And if your library provides as an extension the safer strlcpy
function that was introducted by the OpenBSD team (see
http://www.gsp.com/cgi-bin/man.cgi?s...&topic=strlcpy ),
you can even write:
no i do not seem to have it.


This said, people in this NG prefer to use std::string ;)
of course, but I am playing with chars for now :)

Cheers,
Ivan

Simon