Hans,

on Fri, 26 May 2023 09:06:15 -0700 you (Hans Boehm <boehm_at_[hidden]>)
wrote:

> There are currently also a few more not-obviously-localized forms of
> undefined behavior, even in the C standard. In N3096, I found
>
> footnote 92 (header names)
>
> restrict seems tricky, buit can probably be handled like data races?
>
> "The execution of a non-directive preprocessing directive results in
> undefined behavior."

Yes there is a completely different source of undefined behavior that
is actually not related to faults in the execution but is merely there
to leave slackness to the implementors.

> Referring to signal(): "Use of this function in a multi-threaded
> program results in undefined behavior." Actually, I don't understand
> why that's there at all. I could see "implementation defined".

Implementation-defined behavior is different from undefined behavior
and unspecified behavior, in that it requires that the behavior is
documented. So here we do not even demand that they document what they
are doing.

> As it stands, if I call signal(), and then create threads, it
> encounters undefined behavior at the first thread creation?

This is a most pessimistic interpretation ;-)

More directly from the text follows that calling that `signal` in an
execution that has (had) multiple threads, is a fault. This was what
I was first thinking, too, but it may be more subtle than that,
actually. It is undefined what concerns the C standard, but it also
means that other standards such as POSIX may prescribe some specific
behavior here.

> Do we guarantee that the standard library doesn't create threads
> behind the scenes?

I think that the intent for an "as-if" property is there, but this is
kind of hidden. A C library implementation could under some
circumstances create multiple threads for `qsort`, for example, as
long as all the additional threads are well synchronized, they
guarantee that no signal is raised, and all additional threads are
gone when `qsort` returns.

Jₑₙₛ

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