Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Fri, 27 Sep 2013 11:25:50 -0500

Hi Michael,

On 09/27/2013 02:57 AM, [email protected] wrote:

In EPICS 3.14.12 I am rather surprised to see the following warning
generated by asSetFilename:

asSetFilename: Warning - relative paths won't usually work

Forgive me, but EPICS has no business making such judgements, and
seeing this message flags up a very nasty future possibility that I'd
like to forestall: no library ever has any business changing the
working directory, and if anybody is thinking of adding such code,
please don't!

I know this is only a warning: but it's not true, and it has no
business being true.  So long as you avoid obvious mistakes, relative
paths will always work, so long as nobody calls chdir() without prior
agreement from the application.

The asSetFilename command is not called asLoadFile; it merely stores the

filename to be used for the access security rules file, but does /not/

cause the file to be loaded at the moment when the command is run. The

rules file will first be loaded during iocInit, and can subsequently be

reloaded at any time after that by the user running asInit from the

shell, or by processing a subroutine record that calls asSubProcess().

Thus if you give a relative path to asSetFilename it will be applied

relative to the CWD at the time the file gets loaded.

This warning therefore is not there because other libraries may change

the CWD, it's because a console user may run a cd command in the shell

at any time after iocInit. VxWorks and RTEMS only have a single CWD

which is shared by all threads — does anyone know if that is true of

Linux and/or Windows as well? Can you guarantee that they/you will

never leave the IOC's CWD pointing to somewhere other than where the

developer was expecting it to be (and is that the iocBoot/iocXxx startup

directory, or some other location?).

The consequences of a failure when reloading the access security rules

file is that the IOC turns off both read and write access to *all* PVs,

which is pretty drastic but when this system was designed it was felt

that this could be an indication that someone is trying to break into

the system.

Thus giving asSetFilename a relative path results in unintuitive

behaviour, and could easily result in your control system shutting down.

 Do you still think we should remove the warning message? If you have

suggestions for improvements in the text I will be happy to hear them,

but I don't think it's possible to condense the above description into

anything meaningful.

The Application Developers Guide does talk about this in 8.3.6

http://www.aps.anl.gov/epics/base/R3-14/12-docs/AppDevGuide/node9.html#SECTION00936000000000000000

- Andrew
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System