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From: mosteo@gmail.com (Alex R. Mosteo)
Newsgroups: comp.lang.ada
Subject: Memory leak - What the ...?
Date: 10 Oct 2004 14:33:49 -0700
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Date: 2004-10-10T14:33:49-07:00
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Hi, 

as the topic says, this post is about some code which leaks. I'm now
sure of having trapped the leak, but I don't understand where is my
error.

The culprit is a controlled type, one of whose members is an access to
a Stream_Element_Array. The idea is to have an array of the exact
length required and not a worst case one, without using an
unconstrained type.

Platform is gnat 3.15p, compiling with assertions enabled (-gnata),
either win32 or linux.

Running a lot of tests with gnatmem, it always shows the allocation
not deallocated being at:

s-assert.adb:46                        
system.assertions.raise_assert_failure
adagio-g2-transceiver_types.adb:134    
adagio.g2.transceiver_types.adjust
<other stack calls not meaningful>

I don't know what the assertion means there, since I never get any
exception anyway.

The type declaration is:

   type Udp_message is new Finalization.Controlled with record
      Data : Stream_Element_Array_Access;
      Dest : Gnat.Sockets.Sock_addr_type;
      Date : Calendar.Time;
   end record;

The adjust procedure is:

   procedure Adjust (This : in out Udp_Message) is
   begin
      if This.Data /= null then
         This.Data := new Stream_Element_Array'(This.Data.all);
      end if;
   end Adjust;

And the finalize one is:

   procedure Finalize (This : in out Udp_Message) is
   begin
      Free (This.Data);
   end Finalize;

Now, where it gets interesting is that I've double and triple checked
that no objects of Udp_Message type are being leaked. Using counters
and traces I've certified that every object of that type which is
created is eventually finalized. No code mangles with the Data
pointer, so there's no outside manipulation which could cause the
leak.

Is there something flawled in this implementation? I just can't figure
why there is a leak. Changing the allocated array to a static array
removes the leak.

These objects are stored in an Ada.Containers.Doubly_Linked_List, if
that could add something.

Thanks in advance,

A. Mosteo.