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.

 

[Ivan Vecerina]

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?section=3&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

 

[Guest]

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.

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.

 

[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?section=3&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