Re: strncpy clarify result may not be null terminated

[Alejandro Colomar <alx@kernel.org>]

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Hi Matthew,

On Thu, Nov 09, 2023 at 01:23:14PM +0100, Alejandro Colomar wrote:
> > > > reasonable to highlight precisely why strncpy(3)'s output isn't a string
> > >
> > > How about this?:
> > >
> > > diff --git a/man3/stpncpy.3 b/man3/stpncpy.3
> > > index d4c2ce83d..c80c8b640 100644
> > > --- a/man3/stpncpy.3
> > > +++ b/man3/stpncpy.3
> > > @@ -108,7 +108,10 @@ .SH HISTORY
> > >  .SH CAVEATS
> > >  The name of these functions is confusing.
> > >  These functions produce a null-padded character sequence,
> > > -not a string (see
> > > +not a string.
> > > +While strings have a terminating NUL byte,
> > > +character sequences do not have any terminating byte
> > > +(see
> > >  .BR string_copying (7)).
> > >  .P
> > >  It's impossible to distinguish truncation by the result of the call,
> > 
> > Yes, I'd be perfectly happy with something like that. That way, the
> > scariness is far more immediate ("the output might not be terminated!?"),
> > and thus more accessible to the typical reader.
> 
> Ok; I'll add that.

I think DJ's suggestion of providing an example shows this without
needing a wordy explanation:

<https://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=f502d3290c9f6f13f870cc041f553073af434949>
Here's the page now:

CAVEATS
     The name of these functions is confusing.  These  functions  pro‐
     duce   a  null‐padded  character  sequence,  not  a  string  (see
     string_copying(7)).  For example:

         strncpy(buf, "1", 5);       // { '1',   0,   0,   0,   0 }
         strncpy(buf, "1234", 5);    // { '1', '2', '3', '4',   0 }
         strncpy(buf, "12345", 5);   // { '1', '2', '3', '4', '5' }
         strncpy(buf, "123456", 5);  // { '1', '2', '3', '4', '5' }

     It’s impossible to distinguish truncation by the  result  of  the
     call,  from  a  character sequence that just fits the destination
     buffer; truncation should be detected by comparing the length  of
     the input string with the size of the destination buffer.

I think this is quite clear regarding what this functions does and
doesn't.

I'll leave it like that, I think.

Cheers,
Alex

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



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