[PATCH] Cygwin: getdtablesize: always return OPEN_MAX_MAX

[Corinna Vinschen]

On Jan 28 08:42, Ken Brown via Cygwin-patches wrote:
> On 1/28/2021 5:20 AM, Corinna Vinschen via Cygwin-patches wrote:
> > On Jan 27 21:51, Ken Brown via Cygwin-patches wrote:
> > > According to the Linux man page for getdtablesize(3), the latter is
> > > supposed to return "the maximum number of files a process can have
> > > open, one more than the largest possible value for a file descriptor."
> > > The constant OPEN_MAX_MAX is the only limit enforced by Cygwin, so we
> > > now return that.
> > > 
> > > Previously getdtablesize returned the current size of cygheap->fdtab,
> > > Cygwin's internal file descriptor table.  But this is a dynamically
> > > growing table, and its current size does not reflect an actual limit
> > > on the number of open files.
> > > 
> > > With this change, gnulib now reports that getdtablesize and
> > > fcntl(F_DUPFD) work on Cygwin.  Packages like GNU tar that use the
> > > corresponding gnulib modules will no longer use gnulib replacements on
> > > Cygwin.
> > > ---
> > >   winsup/cygwin/syscalls.cc | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
> > > index 5da05b18a..1f16d54b9 100644
> > > --- a/winsup/cygwin/syscalls.cc
> > > +++ b/winsup/cygwin/syscalls.cc
> > > @@ -2887,7 +2887,7 @@ setdtablesize (int size)
> > >   extern "C" int
> > >   getdtablesize ()
> > >   {
> > > -  return cygheap->fdtab.size;
> > > +  return OPEN_MAX_MAX;
> > >   }
> > 
> > getdtablesize is used internally, too.  After this change, the values
> > returned by sysconf and getrlimit should be revisited as well.
> 
> They will now return OPEN_MAX_MAX, as I think they should.  The only
> question in my mind is whether to simplify the code by removing the calls to
> getdtablesize, something like this (untested):

But then again, what happens with OPEN_MAX in limits.h?  Linux removed
it entirely.  Given we have such a limit and it's not flexible as on
Linux, should we go ahead, drop OPEN_MAX_MAX entirely and define
OPEN_MAX as 3200?

One problem is that there are some applications in the wild which run
loops up to either sysconf(_SC_OPEN_MAX) or OPEN_MAX to handle open
descriptors.  tcsh is one of them.  It may slow done tcsh quite a bit
if the loop runs to 3200 now every time.


Corinna