New 'n' Improved comp.lang.c FAQ List

[Andreas Krey]

In article <1991Apr4.024300.19969 at garfield.cs.mun.ca>, matthew1 at garfield.cs.mun.ca (Matthew J. Newhook) writes:
|> bls at u02.svl.cdc.com (Brian Scearce) writes:
|> 
|> >grover at big-joe.cs.unlv.edu (Kevin Grover) writes:
|> >> Apparently non-obvious April Fool's Day bogus account posts:
|> >>> Q: Why doesn't this function work:
|> >>>
|> >>>         itoa(int i)
|> >>>         {
|> >>>         char retbuf[5];         /* biggest int: 32769 */
|> >>>         sprintf("%d", retbuf, i);
|> >>>         return retbuf;
|> >>>         }
|> >>>
|> 
|> >>    A correct version of this program is:
|> >>        char *itoa(int i)
|> >>          {
|> >>            static char retbuf[5];         /* biggest int: 32769 */
|> >>            sprintf(retbuf, "%d", i);
|> >>            return retbuf;
|> >>          }
|> 
|> >Almost, but not quite, Mr. Grover.  The *really* correct version of this:
|> 
|> >        char *itoa(int i)
|> >          {
|> >            static char retbuf[5];         /* biggest int: 32768 */
|> >            sprintf(retbuf, "%d", i);
|> >            return retbuf;
|> >          }
|> 
|> Actually here is a correct (portable) version (assuming that you want to 
|> keep a static character buffer, and not use a newly malloc'd one each time).
|> 
|> #include <math.h>
|> 
|> char *itoa(int i)
|> {
|>     static int first = 1;
|>     static char *s;
|> 
|>     if (first) { /* IF FIRST TIME ALLOCATE MEMORY FOR STRING */
|>         first = 0;
|>         s = (char *) 
|>         malloc((long) ceil(log10(pow(2, (double) sizeof(int) * 8))) +1);
|>         if (!s) {
|>             fprintf(stderr,"malloc failed\n");
|>             exit(1);
|>         }
|>     }
|> 
|>     sprintf(s, "%d", i);
|>     return s;
|> }
|> 
|> This is portable, 'cause it always assigns the correct value to the size of
|> the array, without depending on the sizeof(int) == 2.
|> 
|> Matthew Newhook
|>         
|> 
|> -- 
|> ----------------matthew1 at garfield.cs.mun.ca 
|> "Living in the limelight; the universal dream for those who wish to 
|> seem. Those who wish to be must put aside the alienation, get on with 
|> the facination, the real relation, the underlying theme" - Rush

Actually the correct simple solution is:
        char *itoa(int i)
          {
            static char retbuf[6];         /* biggest int: 32768 */
            sprintf(retbuf, "%d", i);
            return retbuf;
          }

since the resulting string contains five digits and a trailing zero.
(note the [6]). If you do not rely on sizeof(int)==2, I usually set
the array size to a sufficiently big value. 30 will probably be enough
for the next few computer generations, and will then generate really nice
(hard to find) bugs...

(I didn't bother to check whether the complicated expression takes the
trailing '\0' into account. Also I only read this group if there are
fewer than <limit> articles, so my point may have been pointed out.)
-- 
Andy

-------------------------------------------
Zeit ist Geld. Aber Geld ist keine Zeit.
[Intl: Time is money. But money isn't time.]