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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 of 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:
GNAT.Debug_Pools.Accessing_Not_Allocated_Storage
GNAT.Debug_Pools.Accessing_Deallocated_Storage
GNAT.Debug_Pools.Freeing_Not_Allocated_Storage
GNAT.Debug_Pools.Freeing_Deallocated_Storage
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. The deallocation strategy consists
in not releasing the memory to the underlying system but rather to fill
it with a memory pattern easily recognizable during debugging sessions:
The memory pattern is the old IBM hexadecimal convention: 16#DEADBEEF#.
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