|
| From: | George Edwards |
| Subject: | Re: Having trouble with C++ OOT block in restricting output to those input values I wish to pass |
| Date: | Tue, 15 Sep 2020 05:46:28 -0600 |
Dear George,
please keep the list in CC so other people can read this discussion in
the future.
general_work can be called with any number of input parameters. This
could be just 5, or 500 because it can buffer the repeated result from
your repeating source. You can somewhat steer this with the forecast
method by telling GNURadio that you need 100 inputs. In that case
GNURadio will not call general_work with less than 100 inputs, but it
might still call it with more than 100. Also for unittests that do not
repeat keep in mind that GNURadio will silently drop any excess inputs
that are less than the requested size.
I suggest you put a print statement both in forecast and general_work to
and then run your code multiple times (also under different loads) to
get a better idea of what GNURadio is doing under the hood.
Yours
Martin
On 15.09.20 06:28, George Edwards wrote:
> Hi Martin,
>
> I was out of town for the weekend, so I am just getting at my email.
>
> Thanks for your detailed reply as usual. I really appreciate your help
> and I know it is a lot of effort and time on your part so I really
> appreciate it.
> :
> The param cnt works well and is used to synch up the indices for the
> in and out arrays. For example, suppose the input data was
> {1,2,3,0,1,3,2,5,6} and the preamble pattern is {3,0,1,3,2}.
> Then after initialization, cnt = 7 and i = 7 and of course the number of
> sync bytes = buf_size = 5. Before leaving the initialization loop I
> memcpy the 5 preamble bytes to the out array, which means the next index in
> "out" to write on the next input data sample is out[5], however, this
> sample comes from in[i] = in[7]. Immediately after initialization, hence
> the relationship:
> out[i-cnt+buf_size] = in[i] is correct because out[5] = in[7] and as i
> increases, we get out[6] = in[8], etc.
>
> However, I see one big problem that will cause my logic to fail and it
> is based on your explanation that each time the general_work method is
> called it brings in a new set of data and i starts over in the "for loop".
> I will need to change my logic, but please allow me to ask another
> question. Let's say, I have a source which generates 100 bytes and it
> Repeats. In Gnuradio, does this mean that each time the general_work
> method is called it receives a block of100 bytes of data? Thus, in the C
> logic test "for (int i = 0; i < ninput_items[0]; i++)", can I assume the
> Gnuradio parameter ninput_items[0] is equal to 100 and each function call?
>
> Thanks for the help.
>
> Best Regards,
> George
>
>
> On Sun, Sep 13, 2020 at 2:51 AM Martin Luelf <mail@mluelf.de
> <mailto:mail@mluelf.de>> wrote:
>
> Hi George,
>
> noutput_items is a number given to you by GNURadio. I would not
> overwrite this variable, otherwise you no longer know how many items
> you
> can write into the output buffer. Create a new variable nitems_written
> or similar to track how many items you have written.
>
> Also have a look at what each variable is tracking exactly. cnt
> seems to
> be the number of input bytes you have checked for the preamble. But you
> also use it to compute the position in the out array, which does not
> make sense to me. cnt would increase with more spurious bytes in front
> of the preamble, but the output position should not depend on the
> number
> of spurious bytes. From the code you showed me cnt also seems to
> persist
> between multiple calls to general_work, but the out array is always
> clean when general_work is called again (meaning that on every
> general_work call you should always start writing to element 0).
>
> You loop over your entire input array and if the preamble was found you
> copy the entire preamble and then you copy every following byte one by
> one. That means you copy the entire preamble again (without the first
> byte) and if your input buffer is longer than your message size you
> also
> copy more bytes to the output than your message is long which could
> result in missing the next preamble. Also you are potentially writing
> more items into the out array than GNURadio has space for, which is why
> I recommend you not to overwrite noutput_items and while you are still
> learning check that your computed indices for out and in are within the
> allowed bounds before every single write and read (i.e. by using
> assertions).
>
> What messages are you using in your unittest and what kind of spurious
> bytes? Is is all zeros, or all ones? I would recommend you use
> different
> bytes, e.g. an increasing counter for your message and a decreasing
> counter starting from 255 for your spurious bytes so you can quickly
> spot if your messages are cropped, or off in some other way. Because
> the
> way you describe your results it seems like one message is copied
> twice.
> I could not find any obvious bug in your code that would output a
> message exactly twice, so I suppose it copies some data that look
> like a
> message on the first glance.
>
> Yours
> Martin
>
>
> On 11.09.20 03:06, George Edwards wrote:
> > Hi Martin,
> >
> > Thanks for your detailed answer. I really appreciate the great
> effort
> > you put into explaining how things work. I am still on the
> learning curve.
> >
> > I used your suggestions to the best of my understanding and it
> worked
> > beautifully for the one sync pattern test vector in the QA test.
> >
> > Then, I took your suggestion for repeated sync patterns using an
> init
> > flag which I reset to 0 to restart the process. For the QA test, I
> > repeated the original input data twice (now I have 2
> sync patterns), so
> > the expected QA output should be the original output repeated
> twice. I
> > modified the C code by adding an if statement at the end to check if
> > noutput_items == Buf_size+message_size (buf_size is the length of
> the
> > pattern, which I call preamble) and if it is, I reset the init
> flag to
> > zero as well as other params used in the initialization section
> of the
> > code. The QA test failed by producing an output with 3 repeated
> copies
> > of the original output rather than the expected 2 copies. I do not
> > expect you to send too much time looking at my code below,
> however, I
> > would appreciate it very much, if you would glance at it to see
> if you
> > can spot what I am doing wrong. The test to un-initialize
> (setting init
> > to 0) was done towards the end of the code block after the
> consume method.
> >
> > int kk = 0;____
> >
> > for (int i = 0; i < ninput_items[0]; i++)____
> >
> > {____
> >
> > if(!init){__
> >
> > cnt += 1; // cnt number of passing
> bytes____
> >
> > kk = initialize(in[i]);____
> >
> > if (kk == 0){____
> >
> > noutput_items = cnt;____
> >
> > }else{____
> >
> > memcpy((void*)out, (const void*)preamble,
> buf_size);____
> >
> > noutput_items = cnt;____
> >
> > }
> >
> > } else {
> >
> > out[i-cnt+buf_size] = in[i];____
> >
> > noutput_items = buf_size + message_size;____
> >
> > }____
> >
> > } ____
> >
> > consume_each (noutput_items);____
> >
> > if (noutput_items == buf_size + message_size){____
> >
> > init = 0; // re-initialize all____
> >
> > cnt = 0;____
> >
> > kk = 0;____
> >
> > }____
> >
> > return noutput_items;____
> >
> > }
> >
> >
> > Thanks very much for the help.
> >
> > Regards,
> > George
> >
> > On Thu, Sep 10, 2020 at 12:06 AM Martin Luelf <mail@mluelf.de
> <mailto:mail@mluelf.de>
> > <mailto:mail@mluelf.de <mailto:mail@mluelf.de>>> wrote:
> >
> > Dear George,
> >
> > this also caused me a lot of headache when I started with
> GNURadio, so
> > here is what I learned about it.
> >
> > Let's start with the forecast method. This tells GNURadio how
> many
> > input
> > samples you need on each input stream to generate the
> requested number
> > of output items. Usually GNURadio will run your forecast
> function a
> > couple of times with different output numbers to find a good
> data chunk
> > size to give to the general work function. Keep in mind that
> the number
> > of required input items you give here is a minimum number and
> GNURadio
> > might decide to give you more input data than you requested.
> It is also
> > important to know that the number of samples you request here is
> > just an
> > estimate. You are not forced to actually process that much data.
> >
> > Now to the general_work function. noutput_items tells you how
> many
> > samples GNURadio would like you to produce (this is a maximum
> number
> > you
> > may produce less). It also tells you how much memory is
> allocated in
> > every array in the output_items vector. If you have only one
> output and
> > you used the default <out type> *out = (<out type> *)
> output_items[0];
> > definition this tells you how many items you can place (at
> most) into
> > the out array.
> >
> > The ninput_items[] array tells you how many input items are
> > available in
> > the input arrays. Again if you just have one input and you
> use const
> > <in
> > type> *in = (const <in type> *) input_items[0];
> ninput_items[0] is the
> > number of inputs available in the in array. You may not read
> more items
> > than that from the array.
> >
> > Within the given input symbols you can start looking for your
> sync
> > pattern. If you generate output you have to write (in your case
> > copy) it
> > to the out array. At the end of general_work you call
> consume(0, K)
> > with
> > the number of input items K that you have consumed on input
> 0. That is
> > how many of the input items you have used and do not need to
> see again.
> > If you consume 0 symbols the next call to general_work will
> show you
> > the
> > exact same input samples again. If you consume
> ninput_items[0] you will
> > see completely new input samples on the first input the next time
> > general_work is called. And then you return the number of
> samples you
> > generated (i.e. how much samples you put into the out array).
> This
> > number must be smaller or equal noutput_items, because
> otherwise you
> > would have written out of the allocated memory which might
> result in a
> > segfault/core dump. Note that you don't have to call consume
> at the
> > very
> > end of work and there is also another way of telling GNURadio
> how many
> > samples you have produced, but let's leave that for another day.
> >
> > So a very easy (but not the most efficient) setup for your
> problem
> > could be:
> > Add a boolean flag to your _impl class that both forecast and
> > general_work can read/write to. This flag will indicate
> whether or not
> > you have found the sync pattern or not. You initialize this flag
> > with false.
> > Assume you have a sync pattern of length L and a message with
> M data
> > symbols afterwards.
> >
> > In forecast if the flag is set (meaning you have found the sync
> > pattern)
> > you need L+M symbols of input. If the flag is not set you
> need L input
> > samples, regardless of how many output samples GNURadio wants
> you to
> > generate.
> >
> > In general work if the flag is false you search the input for
> the sync
> > pattern. If you found it at position i (counting from 0) you
> set the
> > flag to true, consume i samples (i.e. everything before the sync
> > marker). If the sync marker is not found you keep the flag to
> false and
> > consume the inputs that you have searched so far. In both
> cases you
> > return 0 since you have not generated any output yet.
> >
> > If the flag is true you copy the first L+M samples from the
> input to
> > the
> > output, you set the flag to false (because after the data you
> have to
> > start searching for the sync marker again) you consume L+M
> samples and
> > return L+M samples.
> >
> > Note: This is a very easy to understand scheme, but
> unfortunately not
> > very efficient. You only process a single block of either
> unwanted
> > spurious symbols, or one sync marker and data at a time. So
> once you
> > have a good understanding of how this works you should tweak
> that block
> > to be able to process multiple blocks of spurious symbols and
> sync
> > patterns/data within once call to general_work. It uses the same
> > kind of
> > logic, but requires more housekeeping of counters and indices.
> >
> > If your input is symbols rather than bits/bytes you should
> also look at
> > the paper from J. Massey "Optimum Frame Synchronization" from
> 1972 on
> > how to perform optimum sync marker detection, which performs
> better
> > than
> > the intuitive correlation search.
> >
> > Hope that gets you started.
> >
> > Yours
> > Martin
> >
> >
> > On 10.09.20 04:34, George Edwards wrote:
> > > Hello,
> > >
> > > I am writing an OOT block in C++ that receives a sequence of
> > numbers and
> > > searches through for a sync pattern and passes to its
> output the
> > sync
> > > pattern and the bytes of data which follows it. The QA test
> > shows the
> > > block recognizes the pattern and will pass the pattern
> along with
> > the
> > > data that follow it, but there is a problem. The block
> does not
> > know a
> > > priori the number of spurious bytes preceding the sync
> pattern of
> > bytes,
> > > so I cannot set up the true relationship between the
> ninput_items
> > and
> > > noutput_items. However, the block can count the number of
> bytes that
> > > came in before the pattern. This is my problem:
> > >
> > > 1) In the OOT general_work method: If I set noutput_items =
> > > ninput_items[0], then in addition to passing the correct
> data to the
> > > output, it passes trailing zeros equal to the number of
> spurious
> > bytes
> > > that entered before the pattern.
> > >
> > > 2) If I set the return noutput_items = ninput_items - cnt
> (where
> > cnt is
> > > the number of spurious bytes before the pattern) depending on
> > where I
> > > put noutput_items in the code, it either throws a core dump or
> > cuts off
> > > the number of true data.
> > >
> > > Also, within the forecast method, I simply use a _for_
> loop and set
> > > ninput_items_required[i]=noutput_items;
> > >
> > > I will appreciate any help to point me in the right
> direction in
> > dealing
> > > with this non-regular output/ inputs relationship.
> > >
> > > Thank you!
> > >
> > > George
> >
>
| [Prev in Thread] | Current Thread | [Next in Thread] |