Re: [Discuss-gnuradio] Input items vs Output items

[Marcus Müller]

Hi Mike,

you might actually be doing something redundant here:
GNU Radio actually keeps tabs on how many items you produced; just call
nitems_written(0) to get that number.

I don't really understand where your string comes from or where it ends
up, but do make sure your string isn't going to be longer than your
noutput_items.

Generally, this is something that can most probably best be analyzed
using appropriate debugging methods -- I'd like to point you to [1].
As soon as you find out where your segfault happens, it will be easier
to understand under which conditions it happens. Also, GDB allows you to
`print` the value of local variables, so that might actually shine quite
a light on what goes wrong :)

On a more general note: what your block seems to do looks very much like
what tagged stream blocks (probably after gr-ax25) were designed to do.
Have a look at [2]; maybe I'm misjudging the situation, maybe you're
already using the TSB facilities.

Greetings,
Marcus

[1] http://gnuradio.org/redmine/projects/gnuradio/wiki/TutorialsGDB
[2] http://gnuradio.org/doc/doxygen/page_tagged_stream_blocks.html
On 04/18/2015 03:36 PM, Mike Willis wrote:
> I should have added, the packet decoding is based on Volkers rather nifty 
> gr-ax25, which I think is based on earlier implementations so I don't think 
> that is where the bug is. I have heavily modified that code as I am using 
> BPSK demodulator but the hdlc parts are the same. 
>
> Mike
>
> -----Original Message-----
> From: Mike Willis [mailto:address@hidden 
> Sent: 18 April 2015 13:58
> To: GNURadio Discussion List
> Subject: Input items vs Output items
>
> I am slightly confused with the way you implement a general work function - 
> here is my AX25 decoder which takes in bits and outputs decoded packets in 
> text format as a stream of 8 bit integers. This was based on the general work 
> example in the GR tutorial.
>
> int ax25_impl::general_work (int noutput_items,
>                   gr_vector_int &ninput_items,
>                   gr_vector_const_void_star &input_items,
>                   gr_vector_void_star &output_items) { int i; const char *bit 
> = (const char *) input_items[0]; out = (char *) output_items[0];
>
> d_numchars=0;
> for (i=0;i <noutput_items; i++)
>     {
>     hdlc_rxbit( bit[i] );
>     }
>
> // Tell runtime system how many output items we produced.
> consume_each (noutput_items);
> return d_numchars;
> }
>
> I keep a tally of how many characters I output in the class member variable 
> d_numchars. I do this by initially printing to the "out"
> buffer (see above code) with the vsprintf() function and then using the 
> strlen() function to find out how long the out string is.
>
> It works for a while, perhaps 200-300 packets and then crashes with a 
> segfault and error 7. I know the crash is related to the number of output 
> items because if I reduce the number of characters I output it will run for 
> longer. If I don't output anything its fine, but not very useful.
>
> It gets complicated because the block outputs nothing at all until it decodes 
> a packet, then it outputs that entire packet, which might be up to 1000 
> characters or so.
>
> I think I have made a fundamental error somewhere. Finding it is proving 
> problematic. I am also confused by ninput_items and noutput_items variables 
> as to me they appear reversed but that is how they are in the tutorial.
>
>
> Mike
>
>
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