RHEL6: Why do the poll routines fail to set EPOLLOUT under some conditions for TCP connections?

Solution In Progress - Updated -


  • We have written some network code using epoll and have noticed that for the standard RHEL 6.x is does not work as expected. RHEL 7.* and MRG seems to work correctly
  • A program that tries to connect() to non existent port via the loopback device demonstrates this. It calls epoll_wait() that returns without the EPOLLOUT bit set on RHEL6.
  • For RHEL7
flags = IN OUT HUP ERR
socket is writable, hurrah!
  • For RHEL6:
flags = IN HUP ERR
socket is NOT writable, oh dear...


  • Red Hat Enterprise Linux.
  • All RHEL6 releases up RHEL 6.7.
  • All RHEL7 releases have the fix.

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