Experimental Physics and
Industrial Control System

The currently observed behaviour may be caused by an ARP lookup https://stackoverflow.com/questions/4165174/when-does-a-udp-sendto-block

Yes, this may be the problem. The blocking does not occur, when the non existent hosts are not in the local network which means that no ARP is involved.

We also have an old debian-6 system, which doesn't have the problem at all, so it may be a feature of newer linux kernels.

By the way, my test consists of a bunch of bash scripts, so it should be possible to test this on MacOS/Darwin. I have seen this problem
on Debian-9 and Fedora-33 linux.

 

From: Core-talk <core-talk-bounces at aps.anl.gov> On Behalf Of Freddie Akeroyd - STFC UKRI via Core-talk
Sent: 05 June 2021 13:40
To: EPICS core-talk (core-talk at aps.anl.gov) <core-talk at aps.anl.gov>
Subject: RE: EPICS Base 3.15.8: Name resolution requests directed to non-existing hosts block CA Server

 

Windows doesn’t look to provide MSG_DONTWAIT, you would probably need to set the socket into non-blocking mode with ioctlsocket() to get the equivalent behaviour, but that may cause undesirable consequences elsewhere (not looked);  however temporarily disabling/enabling blocking mode just around the call may be possible.

 

Freddie

 

From: Core-talk <core-talk-bounces at aps.anl.gov> On Behalf Of Johnson, Andrew N. via Core-talk
Sent: 04 June 2021 16:32
To: Michael Davidsaver <mdavidsaver at gmail.com>
Cc: EPICS core-talk <core-talk at aps.anl.gov>
Subject: Re: EPICS Base 3.15.8: Name resolution requests directed to non-existing hosts block CA Server

 

On Jun 4, 2021, at 8:24 AM, Michael Davidsaver via Core-talk <core-talk at aps.anl.gov> wrote:

 

On 6/4/21 1:50 AM, Goetz Pfeiffer via Core-talk wrote:

We provide the "MSG_DONTWAIT" flag to the "sendto" call. By this, sendto never blocks even if internal

UDP buffers are full.

I have added a patch file for this to this e-mail.

I don't see any issue with this change, though I would format it as:

#ifndef MSG_DONTWAIT
#  define MSG_DONTWAIT (0)
#endif

This seems more portable than my solution and should be compatible with all platforms.

 

Both VxWorks 5.5 and MacOS define MSG_DONTWAIT in sys/socket.h but in the MacOSX SDK it's inside this conditional:

 

#if !defined(_POSIX_C_SOURCE) || defined(_DARWIN_C_SOURCE)

#define MSG_DONTWAIT    0x80            /* this message should be nonblocking */

 

I doubt that _POSIX_C_SOURCE is defined so I think the flag should be available there, but Michael’s conditional is a good idea anyway.

 

- Andrew

 

-- 

Complexity comes for free, simplicity you have to work for.

 

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Greetings,

  Goetz

References:

EPICS Base 3.15.8: Name resolution requests directed to non-existing hosts block CA Server Goetz Pfeiffer via Core-talk

Re: EPICS Base 3.15.8: Name resolution requests directed to non-existing hosts block CA Server Michael Davidsaver via Core-talk

Re: EPICS Base 3.15.8: Name resolution requests directed to non-existing hosts block CA Server Johnson, Andrew N. via Core-talk

RE: EPICS Base 3.15.8: Name resolution requests directed to non-existing hosts block CA Server Freddie Akeroyd - STFC UKRI via Core-talk

RE: EPICS Base 3.15.8: Name resolution requests directed to non-existing hosts block CA Server Freddie Akeroyd - STFC UKRI via Core-talk

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