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Re: [Discuss-gnuradio] usrp usb buffer minimum requirement

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] usrp usb buffer minimum requirement
Date: Thu, 27 Aug 2009 09:41:18 -0400
User-agent: Thunderbird (X11/20090105)

Mattias Kjellsson wrote:
> adib_sairi wrote:
>> Dear All,
>> As i being experimenting with USRP and GNU Radio, i notice that some
>> of the
>> last packet of my data that i receive is keep inside the USB buffer.
>> this
>> data will be flush out to my PC only if i transmit (receive) new data. i
>> believe that this is due to the minimum byte data needed by the USB
>> buffer
>> (fusb_block_size - 512). so even though there is some receive packet
>> inside
>> the USRP, it won't be send to the PC until it is a group of 512 byte of
>> data. am i correct?
>> so at the transmitter, i should send an extra 512byte of data for
>> each of my
>> last packet that is transmitted for one session..    
> I think there is more to it than just the usrp and usb. Today I have
> been working on a test- case
> where I have the following flow- graph (in c++ if that might matter)
> file_sink-> "some blocks" -> file_sink
> If the file- sink was not exactly 8k = 8192 bytes large the whole
> flow- graph gave unpredictable output in terms of size of the output.
> Note that I used "repeat = false" in the file- source.
> I'm not sure if it's the number of items in the source or the size of
> it, since it in my case is the same (using a bit- file- source).
> Hope this helps (and that someone else has more input)
> //Mattias
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> Discuss-gnuradio mailing list
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> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
File sinks use the stdio library, which by default buffers data, and it
only gets flushed out to your file at 8K intervals.  This can cause
  some strangeness sometimes, for sure.

There should be a way to ask the Gnu Radio library to do an fflush()
after every write.

Marcus Leech
Principal Investigator, Shirleys Bay Radio Astronomy Consortium

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