Re: I need wmem_alloc advice please

[Pascal Quantin <pascal.quantin () gmail com>]

Hi Paul,

Le 15 sept. 2016 19:39, "Paul Offord" <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] On Behalf Of Pascal Quantin
> Sent: 15 September 2016 17:49
> To: Developer support list for Wireshark <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>:
> > 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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