python doesn't allow descriptors > 63 in select

Corinna Vinschen
Wed Aug 3 07:50:00 GMT 2011

Hi Jason,

it looks like there's a build glitch in python:

  $ cat > <<EOF
  from socket import *
  from select import select

  ins = []

  for i in range(1024):
      s = socket(AF_INET, SOCK_STREAM)
      print "socket opened with fd", s.fileno()
      select(ins, [], [], 0)
  $ python
  socket opened with fd 3
  socket opened with fd 4
  socket opened with fd 5
  socket opened with fd 62
  socket opened with fd 63
  socket opened with fd 64
  socket opened with fd 64
  Traceback (most recent call last):
    File "", line 10, in <module>
      select(ins, [], [], 0)
  ValueError: filedescriptor out of range in select()

I debugged this and it turns out that python does not call Cygwin's
select function anymore, as soon as there's a file descriptor in the
set which is >= 64.

While the default value for FD_SETSIZE is 64 (see /usr/include/sys/types.h):

  #  ifndef      FD_SETSIZE
  #      define  FD_SETSIZE      64

AFAICS the python default for the fd set is 256, see the definition
in /usr/include/python2.6/pyport.h:

  #ifndef FD_SETSIZE
  #define FD_SETSIZE      256

But somehow this didn't make it into the python build.  On my Linux
box, the same script only stops when it hits the soft descriptor
limit, 1024 in my case.

Can you have a look and, perhaps, provide a new python with a bigger
internal FD_SETSIZE?  The 256 from /usr/include/python2.6/pyport.h
would already be ok, I guess, but 512 or so wouldn't hurt either...


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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