Re: Questions about pcap_read() function

["Varuna De Silva" <varunax () gmail com>]

Hello

On 9/30/07, Guy Harris <guy () alum mit edu> wrote:

> Varuna De Silva wrote:
>
> > And when we use this FT_Read(), we wait and read in 3968 bytes at a time
>
> So, presumably, if the device has more than 3968 bytes available, only the
> first 3968 bytes are read?


Yes

What if it has fewer than 3968 bytes available?  Does the read return only
> what's available?


It waits checking until it receives 3968 bytes and then read it in

If no bytes are available, does FT_Read() just block waiting for data to
> be available?


Yes this case is similar to "fewer than 3968 bytes"

So the device un-does the bit stuffing, but, in order to preserve frame
> boundaries, the data it hands to the host is byte-stuffed?


Exactly

So what you'd do in the xxx_read() routine would be to call FT_Read(), and
> then scan through the buffer, do the filtering, and, for each SS7 frame
> you extract through the buffer, call the callback routine.


Yes, Exactly, that is what I hoped to do

> Now Do we need to extract all the frames, when we read in one chunk of
> > 3968
> > bytes? as I understand in septel_read(....,int cnt,....) function, it
> > processes only a number of packets specified by cnt.
>
> Yes.  If the cnt argument is non-zero, you should call the callback
> routine no more than cnt times; any frames that are still in your buffer
> should be left there, and processed by any subsequent xxx_read() call
> *before* reading another bufferful of frames from the device..


Thats right,

 If cnt is zero, keep processing frames and calling the callback routine
> until you
> run out of frames in the buffer.


Since I  hope to  do a FT_Read() in every call to xxx_read() I have to read
all the frames
in that buffer (3968 bytes), but I guess, I do not have control over cnt
variable. It
belongs to a higher layer sw ! is it? If so how will that piece of Higher
layer SW, know
to call my xxx_read() with a cnt value of zero or non-zero?

Will this reading a chunk of bytes together with reading one frame at a time
for
each frame in the chunk, work ? Or is it good for me to have a particular
library which
does all this filtering part and reading of chunks, (in the form of a dll
for the moment) and access it?

But I like to do it in the xxx_read() it self, because then the
implementation of our
device by another, would be quite easy I guess.

I guess that works, but it's not really necessary - adding it to the
> pcap_md structure will work, too, and means you don't have to modify any
> of the packet.dll code (and also means that if there's a driver for your
> device for some form of UN*X, you might be able to use the same code).


Alright Ill work it out.

Thanks for your reply

Varuna Xavier
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