Re: Better alternative to strncpy in QEMU.

[Peter Maydell <peter.maydell@linaro.org>]

On Sun, 11 Apr 2021 at 14:52, Chetan <chetan4windows@gmail.com> wrote:
> char *qemu_strncpy(char destination[], char source[], size_t destination_size)
> {
>     /* Looping through the array and copying the characters from
>      * source to destination.
>      */
>     for (int i = 0; i < strlen(source); i++) {
>         destination[i] = source[i];
>
>         /* Check if value of i is equal to the second last index
>          * of destination array and if condition is true, mark last
>          * index as NULL and break from the loop.
>          */
>         if (i == (destination_size - 2)) {
>             destination[destination_size - 1] = '\0';
>             break;
>         }
>     }
>     return destination;
> }

This implementation is "accidentally quadratic", because it
calls strlen(source) every time through the loop, and thus
copying an N byte string will read N*N bytes of memory. (The
compiler can't pull the "strlen(source)" call up out of the loop
because it can't guarantee that source and destination don't
overlap.)

I think this is a good illustration of why we probably don't want
to roll our own string operation functions if we can avoid it
(ie without having a clear view of why we are improving on either
what libc or glib offer us).

thanks
-- PMM