[USRP-users] UHD C++ API Questions

Perelman, Nathan nperelman at LGSInnovations.com
Tue Dec 2 14:59:51 EST 2014


Oops, meant to send this to the list as well instead of just to Peter.



My experience with USRP N210s has been that any call that involves talking to 
the USRP (so both the get and set calls) will throw an exception if the 
connection to the USRP has failed, so you should probably assume that all 
calls into UHD throw exceptions.

-Nathan Perelman



From: USRP-users [mailto:usrp-users-bounces at lists.ettus.com] On Behalf Of 
Peter Witkowski via USRP-users
Sent: Tuesday, December 2, 2014 1:29 PM
To: usrp-users at lists.ettus.com
Subject: Re: [USRP-users] UHD C++ API Questions



Where can I find a list or know whether or not a function throws an exception 
or not?  I've spent some time now looking through the Doxygen documentation 
that's available and am still, very much lost.  It would be nice to know how 
errors are handled so that I can deal with them accordingly.

In my original question, I asked to see how I can check for errors for 
functions to configure the device using the "set" functions in multi_usrp. 
The response I got indicated that it's best to check the value with the 
corresponding "get" function.  This led me to invalidly assume that these 
functions don't throw exceptions.

So my new question is what functions do throw exceptions, so that I can check 
for this case instead of just having my program exit due to the unhandled 
exception?

Also, once I call make() on a multi_usrp and issue a stream command to it to 
start streaming how do I clean up?  There is no reset() for multi_usrp that 
I've been able to find.  What does the shared pointer point to once this clean 
up is finished?  Finally, it appears that I am using a deprecated function for 
set_rx_freq since I am able to simply pass in a double for the frequency.



On Tue, Dec 2, 2014 at 1:12 PM, Marcus D. Leech via USRP-users 
<usrp-users at lists.ettus.com> wrote:






Finally, I just wanted to confirm that the UHD API does NOT throw exceptions. 
I guess I was used to lots of try-catch blocks in my code due to working with 
other libraries.

Not sure where you got the impression that UHD, doesn't in general, throw 
exceptions.  Here's a list I produced with "find" and "grep" on the source
  tree:

http://www.sbrac.org/files/uhd_exceptions.txt

I think that the general error-handling strategy in UHD is to "carry on if 
possible", so if, for example, you try for a sample-rate that cannot be 
satisfied,
  it will program the hardware for the closest matching sample-rate. 
Similarly for frequency and gain changes.

But for others, such as asking for hardware that doesn't exist, there's no 
"closest match" possible, so it throws.








Thanks again for the help.





On Mon, Dec 1, 2014 at 7:31 PM, Michael West <michael.west at ettus.com> wrote:

Answers inline:



On Mon, Dec 1, 2014 at 3:23 PM, Peter Witkowski via USRP-users 
<usrp-users at lists.ettus.com> wrote:

Hello,

I am in the process of writing a simple RF receiving program that follows many 
of the same steps as rx_samples_to file.  I am following the example code 
found here: 
https://github.com/GREO/uhd/blob/master/host/examples/rx_samples_to_file.cpp .

I had a couple questions come up as I was following the example:

1.  How are errors handled by the API?  For example, when I call 
uhd::usrp::multi_usrp::make() and it fails, how do I know that the pointer 
points to an invalid device?  Do I check the pointer for null or is there an 
exception thrown?

The return value of make() is a shared_ptr.  You can do a simple check of the 
validity of the shared pointer.  Assuming you use "uhd::usrp::multi_usrp::sptr 
usrp = uhd::usrp::multi_usrp::make(args);", you can simply add "if (not usrp) 
{}".

2.  Similar question to above, how do I know that device settings are actually 
set?  For example, for set_rx_rate() are any exceptions thrown if the rate is 
bad, or do I just need to call get_rx_rate() immediately afterwards?

I recommend doing the get_*() methods for sanity checks.

3.  In a newer version of rx_samples_to_file (see here: 
https://github.com/EttusResearch/uhd/blob/master/host/examples/rx_samples_to_file.cpp) I 
noticed that a tuning request is used to set the frequency of the device.  Is 
this the preferred method or does a call to set_rx_freq() suffice?

Either will work.  The tune request just gives you more control.

4.  In all the example code I've been able to dig up, the connection to the 
device is closed by simply exiting from the program.  Is there a way to close 
the device and perform the necessary clean-up (i.e. the opposite of make()) 
manually?

The necessary clean-up is done in the destructors, which are automatically 
called when the program exits cleanly.  You can force the destructors to be 
called by using "usrp.reset()".

5.  In the two versions of rx_samples_to_file that I found, I noted a 
difference in the way data is read from the device via recv().  Namely, I'm 
lost on the last two parameters that are passed in.  Do I have to specify an 
io_type and are there different options than RECV_MODE_FULL_BUFF?  Do I have 
to use a STL vector or can I pass it a different data structure (like a 
C-style buffer/array)?  I guess I just need to be pointed to a good few lines 
of code on how recv() works and is implemented.  Also, does recv() throw 
exceptions?

See the UHD API documentation for the information for the recv() call.  With 
UHD (and Boost), you have to learn to love templates. ;-)

Regards,

Michael E. West



Thanks in advance for the help.

-- 

Peter Witkowski
pwitkowski at gmail.com


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-- 

Peter Witkowski
pwitkowski at gmail.com


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-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org


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-- 

Peter Witkowski
pwitkowski at gmail.com

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