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-boun...@wireshark.org 
[mailto:wireshark-dev-boun...@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-boun...@wireshark.org<mailto:wireshark-dev-boun...@wireshark.org> 
[mailto:wireshark-dev-boun...@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-boun...@wireshark.org<mailto:wireshark-dev-boun...@wireshark.org> 
[mailto:wireshark-dev-boun...@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.off...@advance7.com<mailto:paul.off...@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-boun...@wireshark.org<mailto:wireshark-dev-boun...@wireshark.org>
>  
> [mailto:wireshark-dev-boun...@wireshark.org<mailto:wireshark-dev-boun...@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.off...@advance7.com<mailto:paul.off...@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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