[PATCH] Cygwin: getdtablesize: always return OPEN_MAX_MAX

Ken Brown kbrown@cornell.edu
Fri Jan 29 19:23:21 GMT 2021

On 1/28/2021 5:28 PM, Ken Brown via Cygwin-patches wrote:
> 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?

Trying to answer my own question, I guess the idea is to hide implementation 
details from viewers of limits.h.  Is that right?  I took a stab at this and am 
about to send a patchset.  I'm not sure whether I made a reasonable choice of 
"et al" in "__OPEN_MAX et al".


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