From: jameskuyper@alumni.caltech.edu

On 2025-11-01 17:39, Tristan Wibberley wrote:
> Follow-up set comp.lang.c
>
> On 01/11/2025 02:45, Richard Damon wrote:
>
>> .. [stuff I think is about the invalidity of not completing execution]
isn't a valid definition ...
>
> Let's suppose C disallows abort due to external events... Then no
> conforming C implementation exists.
>
> I think we must suppose that susceptibility to power-cuts do not render
> C implementations non-conforming.

Correct. What a power cut does is make a conforming implementation of C
fail to continue qualifying as a implementation.

In 1.6 of C89, an implementation is defined as "a particular set of
software, running in a particular translation environment under
particular control options, that performs translation of programs for,
and supports execution of functions in, a particular execution environment."

Thus, when the power is cut, the software ceases to run, so it ceases to
be an implementation, and in particular, ceases to be a conforming
implementation.

There's important wording that wasn't added to the standard until C99
that clarified something that was already considered to be true: "A
program that is correct in all other aspects, operating on correct data,
containing unspecified behavior shall be a correct program and act in
accordance with 5.1.2.3." (4p3). The peculiar wording is intended to
make clear that unspecified behavior does not relieve an implementation
of this requirement.
Section 5.1.2.3 indirectly references most of the rest of the standard,
while defining that only the observable behavior of a program has to
match what the standard says - that behavior doesn't have to be
generated in the fashion that the standard describes. Section 5.1.2.3 in
C99 corresponds to 2.1.2.3 in C89.

The rest of the standard defines, among other things, how a program
exits. If the behavior of the program is not undefined, it can only exit
by reaching the end of the main() function, or by calling certain
library functions. If there is any observable behavior that allows you
determine that it ended by any other method, the implementation is
non-conforming.

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