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    SubjectRe: generic strncpy - off-by-one error

    Erik Andersen wrote:

    > char *strncpy(char * s1, const char * s2, size_t n)
    > {
    > register char *s = s1;
    > while (n) {
    > if ((*s = *s2) != 0) s2++;
    > ++s;
    > --n;
    > }
    > return s1;
    > }


    How about this:

    char *strncpy(char * s1, const char * s2, size_t n)
    register char *s = s1;

    while (n && *s2) {
    *s++ = *s2++;
    while (n--) {
    *s++ = 0;
    return s1;

    This reminds me a lot of the ORIGINAL, although I didn't pay much
    attention to it at the time, so I don't remember. It may be that the
    original had "n--" in the while () condition of the first loop, rather
    than inside the loop.

    I THINK the original complaint was that n would be off by 1 upon exiting
    the first loop. The fix is to only decrement n when n is nonzero.

    If s2 is short enough, then we'll exit the first loop on the nul byte
    and fill in the rest in the second loop. Since n is only decremented
    with we actually write to s, we will only ever write n bytes. No

    If s2 is too long, the first loop will exit on n being zero, and since
    it doesn't get decremented in that case, it'll be zero upon entering the
    second loop, thus bypassing it properly.

    Erik's code is actually quite elegant, and its efficiency is probably
    essentially the same as my first loop. But my second loop would
    probably be faster at doing the zero fill.

    Now, consider this for the second loop!

    while (n&3) {
    *s++ = 0;
    l = n>>2;
    while (l--) {
    *((int *)s)++ = 0;
    n &= 3;
    while (n--) {
    *s++ = 0;

    This is only a win for relatively long nul padding. How often is the
    padding long enough?

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      Last update: 2005-03-22 13:47    [W:0.042 / U:30.764 seconds]
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