On Tue, Dec 18, 2018 at 10:23:46AM -0500, Paul Koning wrote:
>
>
> > On Dec 17, 2018, at 2:23 PM, Szabolcs Nagy <[email protected]> wrote:
> >
> > On 17/12/2018 18:22, Uecker, Martin wrote:
> >>>
> >>> ...
> >>
> >> So a thread_local static variable for storing the static
> >> chain?
> >
> > something like that, but the more i think about it the
> > harder it seems: the call site of the nested function
> > may not be under control of the nested function writer,
> > in particular the nested function may be called on a
> > different thread, and extern library apis are unlikely
> > to provide guarantees about this, so in general if a
> > nested function escapes into an extern library then
> > this cannot be relied on, which limits my original
> > idea again to cases where there is no escape (which i
> > think is not that useful).
>
> I'm not sure I understand "escape" of a nested function pointer.
>
> Your description makes it sound like you're talking about a function being
> called by someone who has been given the pointer, from outside the scope of
> the function. That sounds like an illegal operation, exactly as it would be
> if you attempted to reference an automatic variable via a pointer from
> outside the scope of that variable.
>
> Did I misunderstand?
The most common case is when you pass a call to a nested function
to some function that has a function pointer argument, e.g. qsort.
This is well defined with GNU nested functions, but the function that calls
the callback (qsort in this case) doesn't know it is a call to a nested
function.
#include <stdlib.h>
#include <stdbool.h>
int
main ()
{
bool r = false;
auto int cmp (const void *a, const void *b)
{
const signed char *c = (const signed char *) a;
const signed char *d = (const signed char *) b;
return r ? *c - *d : *d - *c;
}
signed char l[8] = { 10, 2, 11, 21, 0, 7, 18, 12 };
qsort (l, 8, 1, cmp);
}
Jakub
