Re: strncpy clarify result may not be null terminated

[Alejandro Colomar <alx@kernel.org>]

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Hi Paul,


On Fri, Nov 10, 2023 at 09:58:42AM -0800, Paul Eggert wrote:
> On 2023-11-10 03:05, Alejandro Colomar wrote:
> > Hopefully, it won't be so bad in terms of performance.
> 
> It's significantly slower than strncpy for typical use (smallish fixed-size
> destination buffers). So just use strncpy for that. It may be bad, but it's
> better than the alternatives you've mentioned. You can package strncpy
> inside a [[nodiscard]] inline wrapper if you like.
> 
> More importantly, the manual should not push strlcpy as being superior or
> being in any way a "fix" for strncpy's problems. strlcpy is worse than
> strncpy in important ways and besides - as mentioned in the glibc manual -
> neither function is a good choice for string processing.

Hmmmm, that sounds convincing.  How about this as a starting point?

diff --git a/man3/stpncpy.3 b/man3/stpncpy.3
index 3cf4eb371..3aff18106 100644
--- a/man3/stpncpy.3
+++ b/man3/stpncpy.3
@@ -67,6 +67,38 @@ .SH DESCRIPTION
 }
 .EE
 .in
+.\"
+.SS Copying a string with truncation
+Although this function wasn't designed to copy a string with truncation,
+it can be used with appropriate care for that purpose.
+Such use is prone to off-by-one bugs,
+so it is recommended that you write a wrapper function
+that encloses all the danger.
+.P
+.in +4n
+.EX
+[[nodiscard]]
+inline ssize_t
+strxcpy(char *restrict dst, const char *restrict src, char dsize)
+{
+    char  *p;
+
+    p = stpncpy(dst, src, dsize);
+    if (dst[dsize - 1] != '\0')
+        return -1;
+
+    return p - dst - 1;
+}
+.EE
+.in
+You could implement a similar function in terms of
+.BR strlen (3)
+and
+.BR memcpy (3),
+or in terms of
+.BR strlcpy (3),
+and it would be simpler,
+but this implementation is faster.
 .SH RETURN VALUE
 .TP
 .BR strncpy ()


I used stpncpy(3), assuming it will have the same performance of
strncpy(3), because it can be used to return the length.

Cheers,
Alex

-- 
<https://www.alejandro-colomar.es/>

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