Wireshark mailing list archives
Re: composite tvbuffs
From: Jeff Morriss <jeff.morriss.ws () gmail com>
Date: Tue, 12 Oct 2010 15:19:45 -0400
John Sullivan wrote:
On Tuesday, October 12, 2010, 3:27:09 PM, Jeff Morriss wrote:I think composite tvbuffs probably do work, but their use is hindered by the fact that each tvb that you get (each time a frame is handed to your dissector) is freed/reused after the packet is dissected. That is, Wireshark does _not_ keep the entire capture file in memory. Because of that, there's relatively little need/use for the composite functions and it's why you need to allocate copies of whatever data you want to reassemble.This. I was digging into the RTMPT dissector again over the weekend,
This? ;-)
hoping I could move from allocating memory for every reassembled pdu to just creating composite tvbs overlaying the original packets. This would have been a big win memory-wise, since the reassembled packets have to live for the entire capture session (multiple non-contiguous chunks based on multiple prior TCP segments), but turns out not to be possible for the above reason. So it looks like it's necessary to either tvb_memdup from the original data, then add a tvb_new_real_data to a composite and use that, or se_alloc the entire buffer up front, tvb_memcpy into it and use a tvb_new_real_data from that which has a lower overhead in terms of tvbuff structures. (I have a concern that neither of those will free up the used memory. tvb_memdup uses g_malloc and there is no notification of when this is safe to free so AFAICS it is leaked. The se_alloced blocks will of course be freed once the capture is closed, but what of the extra tvb structures on top of them? I presume they are permanent rather than seasonal so ought to be tvb_freed at some point, but again I can't see any notification of when that should happen, so it doesn't.)
The tvb structures should be freed whenever they're not used any more. If you want to g_malloc() the data, you can use tvb_set_free_cb(). From "tvbuff.h": * Although you may call tvb_free(), the tvbuff may still be in use * by other tvbuff's (TVBUFF_SUBSET or TVBUFF_COMPOSITE), so it is not * safe, unless you know otherwise, to free your guint8* data. If you * cannot be sure that your TVBUFF_REAL_DATA is not in use by another * tvbuff, register a callback with tvb_set_free_cb(); when your tvbuff * is _really_ freed, then your callback will be called, and at that time * you can free your original data. An example from packet-data.c: guint8 *real_data = tvb_memdup(tvb, 0, bytes); data_tvb = tvb_new_child_real_data(tvb,real_data,bytes,bytes); tvb_set_free_cb(data_tvb, g_free); ___________________________________________________________________________ Sent via: Wireshark-dev mailing list <wireshark-dev () wireshark org> Archives: http://www.wireshark.org/lists/wireshark-dev Unsubscribe: https://wireshark.org/mailman/options/wireshark-dev mailto:wireshark-dev-request () wireshark org?subject=unsubscribe
Current thread:
- Re: composite tvbuffs Jeff Morriss (Oct 12)
- Re: composite tvbuffs John Sullivan (Oct 12)
- Re: composite tvbuffs Jeff Morriss (Oct 12)
- Re: composite tvbuffs John Sullivan (Oct 12)