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6.7.2 The GNAT Debug Pool Facility

The use of unchecked deallocation and unchecked conversion can easily lead to incorrect memory references. The problems generated by such references are usually difficult to tackle because the symptoms can be very remote from the origin of the problem. In such cases, it is very helpful to detect the problem as early as possible. This is the purpose of the Storage Pool provided by GNAT.Debug_Pools.

In order to use the GNAT specific debugging pool, the user must associate a debug pool object with each of the access types that may be related to suspected memory problems. See Ada Reference Manual 13.11.

    type Ptr is access Some_Type;
    Pool : GNAT.Debug_Pools.Debug_Pool;
    for Ptr'Storage_Pool use Pool;

GNAT.Debug_Pools is derived from a GNAT-specific kind of pool: the Checked_Pool. Such pools, like standard Ada storage pools, allow the user to redefine allocation and deallocation strategies. They also provide a checkpoint for each dereference, through the use of the primitive operation Dereference which is implicitly called at each dereference of an access value.

Once an access type has been associated with a debug pool, operations on values of the type may raise four distinct exceptions, which correspond to four potential kinds of memory corruption:

For types associated with a Debug_Pool, dynamic allocation is performed using the standard GNAT allocation routine. References to all allocated chunks of memory are kept in an internal dictionary. Several deallocation strategies are provided, whereupon the user can choose to release the memory to the system, keep it allocated for further invalid access checks, or fill it with an easily recognizable pattern for debug sessions. The memory pattern is the old IBM hexadecimal convention: 16#DEADBEEF#.

See the documentation in the file g-debpoo.ads for more information on the various strategies.

Upon each dereference, a check is made that the access value denotes a properly allocated memory location. Here is a complete example of use of Debug_Pools, that includes typical instances of memory corruption:

    with Gnat.Io; use Gnat.Io;
    with Unchecked_Deallocation;
    with Unchecked_Conversion;
    with GNAT.Debug_Pools;
    with System.Storage_Elements;
    with Ada.Exceptions; use Ada.Exceptions;
    procedure Debug_Pool_Test is
    
       type T is access Integer;
       type U is access all T;
    
       P : GNAT.Debug_Pools.Debug_Pool;
       for T'Storage_Pool use P;
    
       procedure Free is new Unchecked_Deallocation (Integer, T);
       function UC is new Unchecked_Conversion (U, T);
       A, B : aliased T;
    
       procedure Info is new GNAT.Debug_Pools.Print_Info(Put_Line);
    
    begin
       Info (P);
       A := new Integer;
       B := new Integer;
       B := A;
       Info (P);
       Free (A);
       begin
          Put_Line (Integer'Image(B.all));
       exception
          when E : others => Put_Line ("raised: " & Exception_Name (E));
       end;
       begin
          Free (B);
       exception
          when E : others => Put_Line ("raised: " & Exception_Name (E));
       end;
       B := UC(A'Access);
       begin
          Put_Line (Integer'Image(B.all));
       exception
          when E : others => Put_Line ("raised: " & Exception_Name (E));
       end;
       begin
          Free (B);
       exception
          when E : others => Put_Line ("raised: " & Exception_Name (E));
       end;
       Info (P);
    end Debug_Pool_Test;

The debug pool mechanism provides the following precise diagnostics on the execution of this erroneous program:

    Debug Pool info:
      Total allocated bytes :  0
      Total deallocated bytes :  0
      Current Water Mark:  0
      High Water Mark:  0
    
    Debug Pool info:
      Total allocated bytes :  8
      Total deallocated bytes :  0
      Current Water Mark:  8
      High Water Mark:  8
    
    raised: GNAT.DEBUG_POOLS.ACCESSING_DEALLOCATED_STORAGE
    raised: GNAT.DEBUG_POOLS.FREEING_DEALLOCATED_STORAGE
    raised: GNAT.DEBUG_POOLS.ACCESSING_NOT_ALLOCATED_STORAGE
    raised: GNAT.DEBUG_POOLS.FREEING_NOT_ALLOCATED_STORAGE
    Debug Pool info:
      Total allocated bytes :  8
      Total deallocated bytes :  4
      Current Water Mark:  4
      High Water Mark:  8