Re: ANSI C and POSIX (was Re: C/C++ knocks the crap out of Ada)

[karish@pangea.Stanford.EDU (Chuck Karish)]

Dave Emery roped me into this discussion, and now that I see how
nastily some people choose to express their frustration with a
ill-defined problem, I'm not sure whether to thank him.

Anyway, the problem as he posed it to me is whether code like

    /* sample 1 */
    char buf[100];
    ...
    i = read (fd, buf, 200);

is legal under the C and/or POSIX.1 standards.

My understanding is that it conforms to the C and the POSIX.1
standards but that it is not guaranteed to work.

Neither ISO/IEC 9945-1 (POSIX.1) nor ISO/IEC 9989 (Standard C)
specifies the details of storage in the calling environment.
This means that a conforming implementation could, but need not,
automatically adjust storage to allow 200 bytes of data to be
stored though only 100 bytes were allocated.

POSIX.1 specifically allows the implementation to report an error
for a condition that is not spelled out in the Standard:

     Implementations may support additional errors not included
     in this clause, may generate errors included in this clause
     under circumstances other than those described in this
     clause, or may contain extensions or limitations that
     prevent some errors from occurring.

My understanding is that this thread started around a discussion
of bounds checking under Linux that seems to go beyond the
requirements of POSIX.1.  According to the wording quoted
above, such extra stringency is allowed by POSIX.1.

There are a lot of things that are intentionally not spelled
out by standards.  Sometimes this is because the standard
writers want to limit the scope of the document to keep it
legible and usable, and sometimes it's because they don't want
to preclude implementors from offering usable products
based on current technology or from adding capabilities and
value to future products.

Anyone who craves a standard that describes every possible
aspect of system behavior with complete logical precision
is invited to study the later writings of Rene Descartes
and then get back to us.

--

    Chuck Karish          karish@mindcraft.com
    (415) 323-9000 x117   karish@pangea.stanford.edu