[ILUG] C string concat question revisited

[Dave Neary]

Conor Daly wrote:
> 
> >   if (my_buf)
> >   {
> >     char *tmp = realloc(my_buf, strlen(my_buf) +
> >                         strlen(next_string) + 1);
> >     if(tmp == NULL)
> >     {
> >       fprintf(stderr, "realloc failed - not modifying my_buf\n");
> >     }
> >     else
> >     {
> >       tmp = my_buf;
>         ^^^^^^^^^^^^^
> >       strncat(my_buf, next_string, strlen(next_string)+1);
> >     }   
> >   }
> > else /* The rest looked OK */
>
> What's this line doing?  AFAICT, "realloc(my_buf,<new-size>);" has already
> increased the amount of memory allocated to my_buf.  tmp is a pointer to
> the same chunk of memory but is not required for operations on my_buf.
> from 'man realloc'

Yup - big typo - should have been 
my_buf = tmp;

Also (as I've done) the strncat should be after my_buf = tmp; in the
same block, rather than outside it (as the code is currently).

> if( (realloc(my_buf, <size>)) == NULL ) {
>    fprintf(stderr, "realloc failed - not modifying my_buf\n");
> } else {
>    strncat(my_buf, next_string, strlen(next_string)+1);
> }

This is wrong... realloc doesn't necessarily extend the end of the
existing space in my_buf, it may well move it elsewhere. realloc returns
the address of the new memory block.

That'll teach me to test code I post to mailing lists :)

Cheers,
Dave.

-- 
David Neary,               E-Mail dave.neary at palamon.ie
Palamon Technologies Ltd.  Phone +353-1-634-5059