[Xastir-dev] Switching string length discussion to here

[Tom Russo]

On Sun, Dec 19, 2021 at 03:12:50PM -0600, we recorded a bogon-computron collision of the <godfreja at gmail.com> flavor, containing:
> There is also strlcat.

Are these in the C standard, snprintf?

Looks like they may be.  If so, the fact that they take the full size of the
destination array instead of "whatever's left" may make it the right tool
for the job.

But the important thing is to pass the correct size in the first place (the
size of the destination, not of the source), and not misuse it to copy only
the first few characters of the source (both of these mistakes have been
present in Xastir in multiple places).

> On Sun, Dec 19, 2021 at 11:58 AM Tom Russo <russo at bogodyn.org> wrote:
> 
> > Oh, and certainly you *could* use "xastir_snprintf" to do a sort of strncat
> > operation:
> >
> >    xastir_snprintf(dest,dest_size,"%s%s",dest,src);
> >
> > instead of
> >    strncat(dest,src,dest_size-1-strnlen(dest));
> >
> > but that would be less efficient (because of all the format processing and
> > such in xastir_snprintf) and even more typing.  I advocate just undoing the
> > mistake of how strncat was used.
> >
> > On Sun, Dec 19, 2021 at 09:30:37AM -0800, we recorded a bogon-computron
> > collision of the <curt.we7u at gmail.com> flavor, containing:
> > > Tom,
> > >
> > > Thanks for your recent explanation in the issue discussion (Issue
> > > #188). This is good general "string" talk and applicable to a wide
> > > range of code. For that reason I'm moving the discussion here so it's
> > > more easily found later, instead of hidden in Github issue comments.
> > >
> > > Pardon me while I summarize my limited knowledge of "strings" at this
> > point:
> > >
> > > There aren't actually strings in C, only null-terminated character
> > > buffers. We can't run past the end or else we read/write from other
> > > stuff in memory.
> > >
> > > In Xastir we often use character buffers of size MAX_DEVICE_BUFFER,
> > > which is the case for the Opentrac code.
> > >
> > > At one point we got rid of almost all of the strnXX() type of calls,
> > > switching to xastir_snprintf() instead... Perhaps the Opentrac
> > > routines were coded up later or weren't converted at the time.
> > >
> > > >From Tom:
> > > -----
> > > "There was a lot if incorrect use of strncat in Xastir, mostly using
> > > incorrect values of "n" --- it's supposed to be the maximum number of
> > > characters that can fit into the destination string, and in many cases
> > > the code had been written to specify that number as the number of
> > > characters to copy out of the source string instead. We fixed a lot of
> > > that, but the opentrac code had a lot of it.
> > >
> > > The "right" fix is to fix the strncat calls so that one keeps track of
> > > how much room is left in the destination string, and use that as
> > > the"n" in strncat."
> > > -----
> > >
> > > I can certainly do that last but it seems like a lot of busy work in
> > > C. Other languages have string functions which handle all of this. Are
> > > there any C libraries that make it easier these days? I know I
> > > couldn't find a good one before and that's why xastir_snprintf() came
> > > about.
> > >
> > > --
> > > Curt, WE7U        http://xastir.org        http://www.sarguydigital.com
> > > _______________________________________________
> > > Xastir-dev mailing list
> > > Xastir-dev at lists.xastir.org
> > > http://xastir.org/mailman/listinfo/xastir-dev
> >
> > --
> > Tom Russo    KM5VY
> > Tijeras, NM
> >
> >  echo "prpv_a'rfg_cnf_har_cvcr" | sed -e 's/_/ /g' | tr [a-m][n-z]
> > [n-z][a-m]
> >
> > _______________________________________________
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> > Xastir-dev at lists.xastir.org
> > http://xastir.org/mailman/listinfo/xastir-dev
> >
> -- 
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> verify their authenticity." - *Abraham Lincoln*
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-- 
Tom Russo    KM5VY
Tijeras, NM  

 echo "prpv_a'rfg_cnf_har_cvcr" | sed -e 's/_/ /g' | tr [a-m][n-z] [n-z][a-m]