On Jun 25 01:17, Mark Geisert wrote:
> Corinna Vinschen wrote:
> > Hi Mark,
> >
> > On Jun 24 22:25, Mark Geisert wrote:
> > > Return what the documentation says, instead of a misreading of it.
> > > ---
> > > winsup/cygwin/sched.cc | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/winsup/cygwin/sched.cc b/winsup/cygwin/sched.cc
> > > index e7b44d319..8f24bf80d 100644
> > > --- a/winsup/cygwin/sched.cc
> > > +++ b/winsup/cygwin/sched.cc
> > > @@ -608,7 +608,7 @@ done:
> > > else
> > > {
> > > /* Emulate documented Linux kernel behavior on successful return
> > > */
> > > - status = wincap.cpu_count ();
> > > + status = sizeof (cpu_set_t);
> >
> > Wait... what docs are you referring to? The Linux man page in Fedora 29
> > says
> >
> > On success, sched_setaffinity() and sched_getaffinity() return 0. On
> > error, -1 is returned, and errno is set appropriately.
>
> I've been using http://man7.org/linux/man-pages/man2/sched_setaffinity.2.html
> which has the text you quoted under the RETURN VALUE heading, but has the
> following further down the page under the heading "C library/kernel
> differences":
> | This manual page describes the glibc interface for the CPU affinity
> | calls. The actual system call interface is slightly different, with
> | the mask being typed as unsigned long *, reflecting the fact that the
> | underlying implementation of CPU sets is a simple bit mask.
> |
> | On success, the raw sched_getaffinity() system call returns the
> | number of bytes placed copied into the mask buffer; this will be the
> | minimum of cpusetsize and the size (in bytes) of the cpumask_t data
> | type that is used internally by the kernel to represent the CPU set
> | bit mask.
>
> I see now that that 2nd paragraph has actually been updated since I printed
> it out in April so I'll need to update the patch yet again.
>
> The taskset(1) utility in util-linux actually depends on the kernel return
> value that glibc doesn't return. On Cygwin there is only one "syscall"
> interface so I have to have sched_getaffinity() return a nonzero value on
> success like the Linux kernel does.That's very unfortunate. Keep in mind that applications usually link against the C lib, so the expectation of developers is usually that the API follows the C lib definitions. The kernel API should only be considered if there's no equivalent/documented C lib API. And even then, the call is typically just a SYSCALL wrapper, something we don't support at all. The API of sched_getaffinity should definitely follow glibc. If you need the kernel API as well, create another function like, say, __sched_getaffinity_raw() (leading underscores) and export this as well. You can simply add a cpp macro in your application to support this. Make sure to document the differences in the source, lest we forget. Thanks, Corinna -- Corinna Vinschen Cygwin Maintainer
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