Why Is Escaping Data Considered So Magical?

[Carl Banks]

On Jun 27, 9:54 pm, Kushal Kumaran <kushal.kumaran+pyt... at gmail.com>
wrote:
> On Mon, Jun 28, 2010 at 2:00 AM, Jorgen Grahn <grahn+n... at snipabacken.se> wrote:
> > On Sun, 2010-06-27, Lawrence D'Oliveiro wrote:
> >> In message <roy-854954.20435125062... at news.panix.com>, Roy Smith wrote:
>
> >>> I recently fixed a bug in some production code.  The programmer was
> >>> careful to use snprintf() to avoid buffer overflows.  The only problem
> >>> is, he wrote something along the lines of:
>
> >>> snprintf(buf, strlen(foo), foo);
>
> >> A long while ago I came up with this macro:
>
> >>     #define Descr(v) &v, sizeof v
>
> >> making the correct version of the above become
>
> >>     snprintf(Descr(buf), foo);
>
> > This is off-topic, but I believe snprintf() in C can *never* safely be
> > the only thing you do to the buffer: you also have to NUL-terminate it
> > manually in some corner cases. See the documentation.
>
> snprintf goes to great lengths to be safe, in fact.  You might be
> thinking of strncpy.

Indeed, strncpy does not copy that final NUL if it's at or beyond the
nth element.  Probably the most mind-bogglingly stupid thing about the
standard C library, which has lots of mind-boggling stupidity.

Whenever I do an audit of someone's C code the first thing I do is
search for strncpy and see if they set the nth character to 0.  (They
usually didn't.)

Carl Banks