Wireshark · Wireshark-dev: Re: [Wireshark-dev] I need wmem_alloc advice please

[Paul Offord <Paul.Offord@xxxxxxxxxxxx>]

Thanks Evan and Pascal.

I'll tackle my other issue first then -submitting the code.  I've had problems submitting the code via Gerrit.  I'll try to get that working.

Thanks and regards...Paul

Sent from Samsung Mobile on O2

-------- Original message --------

From: Pascal Quantin

Date:15/09/2016 18:45 (GMT+00:00)

To: Developer support list for Wireshark

Subject: Re: [Wireshark-dev] I need wmem_alloc advice please

Hi Paul,

Le 15 sept. 2016 19:39, "Paul Offord" <Paul.Offord@xxxxxxxxxxxx> 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:
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>  
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>  
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> Note that tsumenabled (a gboolean) is TRUE.  When I enter the dissector for the first time the pointer is still good:
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>  
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>  
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> tsumenabled is still TRUE.  But when code that updates a value in the sub_packet array is executed I get this:
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>  
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> Note how tsumenabled has been zapped to FALSE.
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>  
>
> 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.

>
>  
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> I’ll redesign the code to use the epan scope and try that.
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>  
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> Best regards…Paul
>
>  
>
> From: wireshark-dev-bounces@xxxxxxxxxxxxx [mailto:wireshark-dev-bounces@xxxxxxxxxxxxx] On Behalf Of Pascal Quantin
> Sent: 15 September 2016 17:49
> To: Developer support list for Wireshark <wireshark-dev@xxxxxxxxxxxxx>
> Subject: Re: [Wireshark-dev] I need wmem_alloc advice please
>
>  
>
> Hi Paul,
>
>  
>
> 2016-09-15 18:44 GMT+02:00 Paul Offord <Paul.Offord@xxxxxxxxxxxx>:
>>
>> 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.
>>
>>  
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>> Before the conversion I had this as a global variable:
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>>  
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>>     PKT_INFO sub_packet[MAX_SUBPKTS_PER_PACKET];
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>>  
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>> I replaced this with a global variable:
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>>  
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>>     PKT_INFO *sub_packet;
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>>  
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>> And this in my init function:
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>>  
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>>     sub_packet = (PKT_INFO *)wmem_alloc(wmem_file_scope(), (MAX_SUBPKTS_PER_PACKET * sizeof(PKT_INFO)));
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>>  
>>
>> I subsequently use it like this:
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>>  
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>>     sub_packet[i].frame_number = 0;
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>>  
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>> I seem to be getting memory corruption as a result of this change.  Weird things happen, like I lose preference variables.
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>> What am I doing wrong here?
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>  
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> 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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