Permissions: Setting date/time with system exec or script

[J.Harv]

Hello-

We recently tried to implement settable date and time on a SOM/9651 using the system config VI - set time.vi, during main application execution.  When we did this all loops stopped for up to 45 sec.  supposedly while the OS adjusted the time?  I dunno.

Working with NI support there is a CAR for set time where using this VI "affects determinism" ...understatement if I've ever heard one   This affects 2013 and 2014 as far as I could gather.  I was not given the CAR number but apparently it's been fixed in LabVIEW 2015 (specifically sys config 15).  Unfortunately, we are standardized on 2014 for now, so upgrading is not an option. 

So, I'm simply (or not simply...) looking for a way to update the date/time during regular execution.

I thought to try two options:

-System exec VI calling the OS command to change date

-Script (same command and also called from system exec VI)

Command:  date --set="YYYY-MM-DD HH:MM"

I have verified the command and syntax I'm using through the bash shell/ssh and the command works as expected.

When I use system exec vi in LabVIEW to call the date command, the standard error output says "Operation not permitted"

A little research and I've found when I use ssh, I login with admin, which is equivalent to root, I believe.  The LabVIEW app runs as a user with less permissions.

I tried calling system exec with "su" before the date command.  Standard error was "must be run from terminal".  Next was "sudo", and the error was "/bin/sh/: sudo: command not found"  I assume sudo isn't installed for security reasons, which makes sense.

I next tried to put my command into a script.  Same command into a text file with a *.sh extension.  Called from system exec vi.  Still get "Operation not permitted"  I can tell this would work with the right permissions because the standard output reflects the command and shows the date I requested.  I didn't really think this would help, but I wanted to try the "chmod" command on the script file.  Again "Operation not permitted"

So, how can I set the date during execution, without using system config VIs?

Also, I wasn't able to find much at ni.com regarding permissions and/or how-to.  What can we do and what can't we do within certain permissions?  And how can we change permissions?  (I would like to avoid having to install the external sudo package both for security reasons and field upgrade simplicity.)

Thanks for any/all ideas and help, in advance.  I'm not that Linux savy, so be gentle.  Thanks.

-John

[DAQjr · Accepted solution]

Hi John,

The reason you can't call the date command through System Exec is because the capabilities given to LabVIEW are not inheritable by a command called through the shell.

The LabVIEW process does have permission to change the date. You can see this here:

# getcap /usr/local/natinst/labview/lvrt

/usr/local/natinst/labview/lvrt = cap_net_bind_service,cap_ipc_lock,cap_sys_rawio,cap_sys_boot,cap_sys_nice,cap_sys_time+ep

cap_sys_time is the required capability and it is set.

You need to call the function settimeofday() directly with a Call Library Function Node.

Unfortunately the LabVIEW type for timestamp is not compatible with Linux. However, it can be converted fairly easily. I've attached a simple VI to convert the format and call the system library to set the time. Since the LabVIEW process has access to call that API, you should not see the -EPERM error you would see from an executable without that capability.

Cheers,

-Joe Hershberger

[J.Harv]

Joe--

Thank you so much!  I can confirm that VI works exactly as expected (sets date and time) and does not interfere with loop operation nearly as much as the system config set time.  While our loops all jumped into the seconds range, it was only for a 1 (maybe 2) loop iterations.  Completely managable and understandable.  This is what I'd expect (just not 45 sec. like the sys config set time).  I'd say this is solved! 

Also...wow!  Great help! Very quick reply with fully functioning VI.  I'm impressed. I wish all my issues turned out like this...

-John

[ScotSalmon]

Another option to consider is that NI supports a few different options for standard time sync protocols. Depending on why you need to set the clock, it could be that using a standard protocol like IEEE-1588 (PTP) or (S)NTP for setting the time will give you better behavior than setting it "manually" from LabVIEW or SysConfig. In particular, if you find yourself needing to periodically re-adjust the time as your target's clock drifts from your global reference, you'll affect your loops each time you have to do that. With a full time sync protocol, after initially setting the time, it will gradually adjust the observed clock frequency so that there are no further discontinuities in the clock.

[J.Harv]

Hi Scott-

Thanks for replying as well!  Those are certianly valid options for people to consider. Unfortunately, we really don't need the sync capabilities of 1588.  Only one RIO is the system has time that matters. Also, the system is on it's own private network and there isn't an sntp server.  So neither of those are options. 

Really, we just wanted a way for an end user to update the clock, if needed.  The end user won't have MAX.  So, basic time setting is all we needed.  We just got stuck with the sys config bug, but we're back on track now. 

Thanks again.

-John

[J.Harv]

So, a quick followup for anyone else that find this.  While the solution provided here does change the time, and there is some helpful info about permissions and scripts, this approach doesn't quite solve the original problem.<