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<== Date ==> | <== Thread ==> |
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Subject: | Re: Strange warning from asSetFilename |
From: | Andrew Johnson <[email protected]> |
To: | <[email protected]> |
Cc: | [email protected] |
Date: | 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 -- Advertising may be described as the science of arresting the human intelligence long enough to get money from it. -- Stephen Leacock