[PATCH] Cygwin: getdtablesize: always return OPEN_MAX_MAX

[Ken Brown]

On 1/28/2021 11:13 AM, Corinna Vinschen via Cygwin-patches wrote:
> On Jan 28 17:07, Corinna Vinschen via Cygwin-patches wrote:
>> 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?
> 
> ...ideally by adding a file include/cygwin/limits.h included by
> include/limits.h, which defines __OPEN_MAX et al, as required.

I'm not completely sure I follow.  Do you mean include/cygwin/limits.h should 
contain

   #define __OPEN_MAX 3200

and include/limits.h should contain

   #define OPEN_MAX __OPEN_MAX ?

For the sake of my education, could you explain the reason for this?

Thanks.

Ken