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Date: Sun, 16 Jul 2023 19:59:57 +0200
From: Robert Clausecker <fuz@....su>
To: musl@...ts.openwall.com
Subject: Re: strcmp() guarantees and assumptions
Hi NRK,
Thank you for your response.
Am Sun, Jul 16, 2023 at 11:49:45PM +0600 schrieb NRK:
> Hi Robert,
>
> > Or to phrase it differently, is the following a legal implementation of
> > strcmp()?
> >
> > int strcmp(char *a, char *b) {
> > size_t la = strlen(a), lb = strlen(b);
> >
> > if (la != lb)
> > return ((la > lb) - (lb > la));
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> I don't see how this can ever be a valid strcmp implementation. The
> return value of the comparison functions must be about the first
> mismatching byte, not about the string lengths.
>
> | The sign of a nonzero value returned by the comparison functions is
> | determined by the sign of the difference between the values of the
> | first pair of characters that differ in the objects being compared.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Yes, sorry. The code would have to be extended to call memcmp() on the
common prefix in case there is a mismatch in length. E.g.
if (la != lb)
return (memcmp(la, lb, la > lb ? lb + 1 : la + 1));
> ref: https://port70.net/~nsz/c/c11/n1570.html#7.24.4p1
>
> > Or is it generally agreed upon that libc implementations support
> > strcmp() calls on unterminated strings?
>
> memchr (since C11) has the following requirement:
>
> | The implementation shall behave as if it reads the characters
> | sequentially and stops as soon as a matching character is found.
>
> I don't believe any such requirement exists for strcmp, so unless
> someone proves otherwise, I'd say it's fair game for libc to assume that
> the strings are nul-terminated.
That's good to hear. Any idea on the “what do existing libc
implementations permit” bit?
Yours,
Robert Clausecker
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