Wireshark mailing list archives

Re: I need wmem_alloc advice please

From: Paul Offord <Paul.Offord () advance7 com>
Date: Sat, 17 Sep 2016 11:41:57 +0000

OK – another embarrassing mistake on my part.  Although I had changed the global definition for sub_packet from a 
static array to a pointer to a dynamically allocated space I hadn’t changed a corresponding extern statement in 
decoders.c.

Thanks to all for your help with this.

Best regards…Paul

From: wireshark-dev-bounces () wireshark org [mailto:wireshark-dev-bounces () wireshark org] On Behalf Of Paul Offord
Sent: 17 September 2016 10:23
To: Developer support list for Wireshark <wireshark-dev () wireshark org>
Subject: Re: [Wireshark-dev] I need wmem_alloc advice please

I moved the space allocation from inti_globals function to proto_register_transum and changed the scope to epan like 
this:

    sub_packet = (PKT_INFO *)wmem_alloc(wmem_epan_scope(), (MAX_SUBPKTS_PER_PACKET * sizeof(PKT_INFO)));

I still get the same problem where another structure is being overwritten.  I obviously have something wrong in the 
code but I can’t see it at the moment.

Best regards…Paul

From: wireshark-dev-bounces () wireshark org<mailto:wireshark-dev-bounces () wireshark org> 
[mailto:wireshark-dev-bounces () wireshark org] On Behalf Of Paul Offord
Sent: 16 September 2016 22:07
To: Developer support list for Wireshark <wireshark-dev () wireshark org<mailto:wireshark-dev () wireshark org>>
Subject: Re: [Wireshark-dev] I need wmem_alloc advice please

Hi Pascal,

I’ve submitted the code to Gerrit – see https://code.wireshark.org/review/#/c/17750/

Best regards…Paul

From: wireshark-dev-bounces () wireshark org<mailto:wireshark-dev-bounces () wireshark org> 
[mailto:wireshark-dev-bounces () wireshark org] On Behalf Of Pascal Quantin
Sent: 15 September 2016 18:45
To: Developer support list for Wireshark <wireshark-dev () wireshark org<mailto:wireshark-dev () wireshark org>>
Subject: Re: [Wireshark-dev] I need wmem_alloc advice please


Hi Paul,

Le 15 sept. 2016 19:39, "Paul Offord" <Paul.Offord () advance7 com<mailto:Paul.Offord () advance7 com>> a écrit :


Hi Pascal,



Thanks for the quick feedback.  My intention is to reallocate the memory each time I load a new file.



I have a structure called preferences that holds all of the parsed preference values.  Just after allocation the 
pointer to the allocated area is as per sub_packet in this screenshot:





Note that tsumenabled (a gboolean) is TRUE.  When I enter the dissector for the first time the pointer is still good:





tsumenabled is still TRUE.  But when code that updates a value in the sub_packet array is executed I get this:





Note how tsumenabled has been zapped to FALSE.



I reckon my memory allocation has been freed somewhere.  Doing a search across the entire solution in Visual Studio I 
can’t help noticing that there are no other uses of wmem_alloc(wmem_file_scope(), ….) which is a bit worrying.


That is surprising because there are plenty of dissectors that use file scope to store persistent data (like 
conversation info, hash map, trees,...).
We have no known issue with this memory scope so this must be related somehow to your code. Without seeing it, we can 
hardly tell more.




I’ll redesign the code to use the epan scope and try that.



Best regards…Paul



From: wireshark-dev-bounces () wireshark org<mailto:wireshark-dev-bounces () wireshark org> 
[mailto:wireshark-dev-bounces () wireshark org<mailto:wireshark-dev-bounces () wireshark org>] On Behalf Of Pascal 
Quantin
Sent: 15 September 2016 17:49
To: Developer support list for Wireshark <wireshark-dev () wireshark org<mailto:wireshark-dev () wireshark org>>
Subject: Re: [Wireshark-dev] I need wmem_alloc advice please



Hi Paul,



2016-09-15 18:44 GMT+02:00 Paul Offord <Paul.Offord () advance7 com<mailto:Paul.Offord () advance7 com>>:


My porting of TRANSUM from LUA to C continues.  I now have a working dissector but I then read README.developer and 
noted the guidance regarding use of static buffers.  I assume this applies to arrays too and so I’m now trying to 
convert the statically defined arrays to versions based on dynamic memory allocation.  I’ve converted some string 
buffers and a simple gboolean array without problems.  I then tried to convert an array of structures and this 
caused crashes all over the place.



Before the conversion I had this as a global variable:



    PKT_INFO sub_packet[MAX_SUBPKTS_PER_PACKET];



I replaced this with a global variable:



    PKT_INFO *sub_packet;



And this in my init function:



    sub_packet = (PKT_INFO *)wmem_alloc(wmem_file_scope(), (MAX_SUBPKTS_PER_PACKET * sizeof(PKT_INFO)));



I subsequently use it like this:



    sub_packet[i].frame_number = 0;



I seem to be getting memory corruption as a result of this change.  Weird things happen, like I lose preference 
variables.



What am I doing wrong here?

File scope memory is automatically freed each time a preference is changed, or a file is reloaded. If you need to
keep some things persistent during all Wireshark instance, consider using epan scope memory for those variables
instead.

Regards,

Pascal.

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