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

[Paul Offord <Paul.Offord@xxxxxxxxxxxx>]

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.

 

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

 

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.

 

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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