[TUHS] History of popularity of C

Larry McVoy lm at mcvoy.com
Thu May 28 06:13:47 AEST 2020

So I may have just gotten lucky in my 30+ years of writing C code but
I have yet to hit a single instance of this doom and gloom.

On Wed, May 27, 2020 at 12:49:25PM -0700, Greg A. Woods wrote:
> At Wed, 27 May 2020 18:11:33 +0200, "Thomas Paulsen" <thomas.paulsen at firemail.de> wrote:
> Subject: Re: [TUHS] History of popularity of C
> >
> > When I'm doing C I always have the CPU and its instructions in mind.
> And that's exactly what might trip you up unless you _exactly_
> understand how the language standard defines the operations of the
> abstract virtual machine (right down to the implications of every
> sequence point in the code); how compilers and optimizers do and (more
> importantly) do not work when mapping the abstract virtual machine
> operations into real-world machine instructions; and what how _all_
> instances of "undefined behaviour" can arise, and exactly what the
> optimizer is allowed to do when and if it spots UB conditions in the
> code.
> A big part of the problem is that the C Standard mandates compilation
> will and must succeed (and allows this success to be totally silent too)
> even if the code contains instances of undefined behaviour.  This means
> that the successful execution of the generated code may depend on what
> optimization level was chosen.  Code that does security tests on input
> values might be entirely and silently eliminated by the optimizer
> because of some innocuous-seeming UB instance, and this is exactly what
> has happened in the Linux kernel, for example (probably more than once).
> UB can be introduced quite innocently just by moving sequence points in
> variable references in ways that are not necessarily obvious even to
> seasoned programmers (and indeed "seasoned" programmers are often the
> ones who's old-fashioned coding habits might lead to introduction of
> serious problems in such a way).
> I've found dozens of instances of UB in mature and well tested code, and
> sometimes only by luck of having chosen the "right" compiler and enabled
> its feature of introducing illegal instructions in places where UB might
> occur, _and_ having had the luck to test in such a way as to encounter
> the specific code path where this UB occurred.
> I would claim it's truly safer now to write C without understanding the
> underlying mechanics of the CPU and memory, but rather by just paying
> very close attention to the detailed semantics of the language,
> understanding only the abstract virtual C machine, and hoping your
> compiler will at least warn if anything even remotely suspicious is done
> in your code; and lastly (but perhaps most importantly) avoiding like
> the plague any coding constructs which might make UB harder to spot
> (e.g. never ever initialize local variables with their definition when
> pointers are involved).
> Unfortunately the new "most advanced" C compilers also make it quite a
> bit more difficult for those of us writing C code that must have
> specific actions on the bare metal hardware, e.g. in embedded systems,
> kernels, hardware drivers, etc.; including especially where UB detection
> tools are far more difficult to use.
> --
> 					Greg A. Woods <gwoods at acm.org>
> Kelowna, BC     +1 250 762-7675           RoboHack <woods at robohack.ca>
> Planix, Inc. <woods at planix.com>     Avoncote Farms <woods at avoncote.ca>

Larry McVoy            	     lm at mcvoy.com             http://www.mcvoy.com/lm 

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