[erlang-questions] prim_inet:ignorefd/2 affecting erts_poll_wait()

Michael Santos <>
Sun Aug 24 18:10:17 CEST 2014

On Fri, Aug 22, 2014 at 06:01:56PM -0700, Michael Truog wrote:
> To be a little clearer, my prim_inet:ignorefd/2 usage is to use a
> file descriptor created in the inet source code after passing it
> through gen_tcp:fdopen/2. So, the sequence is:
> ...
> {ok, FileDescriptorInternal} = prim_inet:getfd(Socket),
> ok = prim_inet:ignorefd(Socket, true),
> {ok, NewSocket} = gen_tcp:fdopen(FileDescriptorInternal, SocketOptions),
> ...
> While this might seem odd, this allows me to dup2 the file
> descriptor without causing obvious problems and handle UNIX domain
> sockets, which are currently unsupported within inet (by avoiding
> the internal checking that prevents their use).

Well, if you bypass the internal checks ... :)

This doesn't answer your question but I would suggest only using PF_INET
and PF_INET6 file descriptors with inet. The inet driver assumes it is
working with sockets described by a sockaddr_in structure. Using anything
else can lead to undefined behaviour.

For example, something trivial like calling getpeername(2) on a PF_UNIX
socket would cast the sockaddr_un as a sockaddr_in and presumably return
4 bytes of the Unix socket path as an IPv4 address.

The alternative is passing the file descriptor into a port:

    FD = 7,
    Port = erlang:open_port({fd, FD, FD}, [stream,binary]).

The disadvantage of using this method is the lack of flow control. If that
is a concern, a process could monitor the rate of a incoming messages
and close the port if it crossed some threshold, re-opening it when the
messages have been processed. Closing the port will not close the file
descriptor so the sender will eventually block.

Otherwise, it's simple to write a small driver that uses erts to poll
the fd on your behalf. I wrote one to use with a unix socket library so
I could retrieve the socket ancillary data:


This is very likely old news to you and you have good reasons for using
gen_tcp but maybe it will be helpful for other people going down this

> On 08/22/2014 05:46 PM, Michael Truog wrote:
> >I have been seeing the erts_poll_wait() thread consume 100% CPU
> >when my configuration makes prim_inet:ignorefd/2 ignore a fd
> >(inet_descriptor has is_ignored set to true) created external to
> >inet (10 file descriptors created this way).  I don't have this
> >problem when using the inet code to create tcp sockets, when
> >prim_inet:ignorefd/2 is not used with the same configuration. When
> >setting "#define INET_DRV_DEBUG 1" in
> >"./erts/emulator/drivers/common/inet_drv.c" and "#define
> >ERTS_POLL_DEBUG_PRINT" in "./erts/emulator/sys/common/erl_poll.c"
> >all the debugging output looks the same when exercising the file
> >descriptors in the same way.  The only difference seems to be the
> >"Entering erts_poll_wait(), timeout=NUMBER" output has non-zero
> >timeout values more often when prim_inet:ignorefd/2 is not used
> >when compare to the output when it is being used.  Also, the
> >NUMBER doesn't seem to go about 1000 for me when
> >prim_inet:ignorefd/2 is used, but it can go above 1000 when
> >prim_inet:ignorefd/2 is not used.
> >
> >Why would the erts_poll_wait() loop be refusing to timeout?  Is
> >this expected behaviour?  Is there an erts configuration flag
> >which is meant to address the problem?
> >
> >Thanks,
> >Michael
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