8.4.4 Writing your own memory manager

Free Pascal allows you to write and use your own memory manager. The standard functions GetMem, FreeMem, ReallocMem etc. use a special record in the system unit to do the actual memory management. The system unit initializes this record with the system unit’s own memory manager, but you can read and set this record using the GetMemoryManager and SetMemoryManager calls:

procedure GetMemoryManager(var MemMgr: TMemoryManager);
procedure SetMemoryManager(const MemMgr: TMemoryManager);

the TMemoryManager record is defined as follows:

  TMemoryManager = record
    NeedLock    : Boolean;
    Getmem      : Function(Size:PtrInt):Pointer;
    Freemem     : Function(var p:pointer):PtrInt;
    FreememSize : Function(var p:pointer;Size:PtrInt):PtrInt;
    AllocMem    : Function(Size:PtrInt):Pointer;
    ReAllocMem  : Function(var p:pointer;Size:PtrInt):Pointer;
    MemSize     : function(p:pointer):PtrInt;
    InitThread          : procedure;
    DoneThread          : procedure;
    RelocateHeap        : procedure;
    GetHeapStatus       : function :THeapStatus;
    GetFPCHeapStatus    : function :TFPCHeapStatus;
  end;

As you can see, the elements of this record are mostly procedural variables. The system unit does nothing but call these various variables when you allocate or deallocate memory.

Each of these fields corresponds to the corresponding call in the system unit. We’ll describe each one of them:

NeedLock

This boolean indicates whether the memory manager needs a lock: if the memory manager itself is not thread-safe, then this can be set to True and the Memory routines will use a lock for all memory routines. If this field is set to False, no lock will be used.

Getmem

This function allocates a new block on the heap. The block should be Size bytes long. The return value is a pointer to the newly allocated block.

Freemem

should release a previously allocated block. The pointer P points to a previously allocated block. The Memory manager should implement a mechanism to determine what the size of the memory block is. ² The return value is optional, and can be used to return the size of the freed memory.

FreememSize

This function should release the memory pointed to by P. The argument Size is the expected size of the memory block pointed to by P. This should be disregarded, but can be used to check the behavior of the program.

AllocMem

Is the same as getmem, only the allocated memory should be filled with zeros before the call returns.

ReAllocMem

Should allocate a memory block of the specified Size, and should fill it with the contents of the memory block pointed to by P, truncating this to the new size of needed. After that, the memory pointed to by P may be deallocated. The return value is a pointer to the new memory block. Note that P may be Nil, in which case the behavior is equivalent to GetMem.

MemSize

should return the size of the memory block P. This function may return zero if the memory manager does not allow to determine this information.

InitThread

This routine is called when a new thread is started: it should initialize the heap structures for the current thread (if any).

DoneThread

This routine is called when a thread is ended: it should clean up any heap structures for the current thread.

RelocateHeap

Relocates the heap - this is only for thread-local heaps.

GetHeapStatus

should return a THeapStatus record with the status of the memory manager. This record should be filled with Delphi-compliant values.

GetHeapStatus

should return a TFPCHeapStatus record with the status of the memory manager. This record should be filled with FPC-Compliant values.

To implement your own memory manager, it is sufficient to construct such a record and to issue a call to SetMemoryManager.

To avoid conflicts with the system memory manager, setting the memory manager should happen as soon as possible in the initialization of your program, i.e. before any call to getmem is processed.

This means in practice that the unit implementing the memory manager should be the first in the uses clause of your program or library, since it will then be initialized before all other units - except the system unit itself, of course.

This also means that it is not possible to use the heaptrc unit in combination with a custom memory manager, since the heaptrc unit uses the system memory manager to do all its allocation. Putting the heaptrc unit after the unit implementing the memory manager would overwrite the memory manager record installed by the custom memory manager, and vice versa.

The following unit shows a straightforward implementation of a custom memory manager using the memory manager of the C library. It is distributed as a package with Free Pascal.

unit cmem;

interface

Const
  LibName = 'libc';

Function Malloc (Size : ptrint) : Pointer;
  cdecl; external LibName name 'malloc';
Procedure Free (P : pointer);
  cdecl; external LibName name 'free';
function ReAlloc (P : Pointer; Size : ptrint) : pointer;
  cdecl; external LibName name 'realloc';
Function CAlloc (unitSize,UnitCount : ptrint) : pointer;
  cdecl; external LibName name 'calloc';

implementation

type
  pptrint = ^ptrint;

Function CGetMem  (Size : ptrint) : Pointer;

begin
  CGetMem:=Malloc(Size+sizeof(ptrint));
  if (CGetMem <> nil) then
    begin
      pptrint(CGetMem)^ := size;
      inc(CGetMem,sizeof(ptrint));
    end;
end;

Function CFreeMem (P : pointer) : ptrint;

begin
  if (p <> nil) then
    dec(p,sizeof(ptrint));
  Free(P);
  CFreeMem:=0;
end;

Function CFreeMemSize(p:pointer;Size:ptrint):ptrint;

begin
  if size<=0 then
    begin
      if size<0 then
        runerror(204);
      exit;
                                                                            

                                                                            
    end;
  if (p <> nil) then
    begin
      if (size <> pptrint(p-sizeof(ptrint))^) then
        runerror(204);
    end;
  CFreeMemSize:=CFreeMem(P);
end;

Function CAllocMem(Size : ptrint) : Pointer;

begin
  CAllocMem:=calloc(Size+sizeof(ptrint),1);
  if (CAllocMem <> nil) then
    begin
      pptrint(CAllocMem)^ := size;
      inc(CAllocMem,sizeof(ptrint));
    end;
end;

Function CReAllocMem (var p:pointer;Size:ptrint):Pointer;

begin
  if size=0 then
    begin
      if p<>nil then
        begin
          dec(p,sizeof(ptrint));
          free(p);
          p:=nil;
        end;
    end
  else
    begin
      inc(size,sizeof(ptrint));
      if p=nil then
        p:=malloc(Size)
      else
        begin
          dec(p,sizeof(ptrint));
          p:=realloc(p,size);
        end;
      if (p <> nil) then
        begin
          pptrint(p)^ := size-sizeof(ptrint);
          inc(p,sizeof(ptrint));
        end;
    end;
  CReAllocMem:=p;
end;

Function CMemSize (p:pointer): ptrint;

begin
                                                                            

                                                                            
  CMemSize:=pptrint(p-sizeof(ptrint))^;
end;

function CGetHeapStatus:THeapStatus;

var res: THeapStatus;

begin
  fillchar(res,sizeof(res),0);
  CGetHeapStatus:=res;
end;

function CGetFPCHeapStatus:TFPCHeapStatus;

begin
  fillchar(CGetFPCHeapStatus,sizeof(CGetFPCHeapStatus),0);
end;

Const
 CMemoryManager : TMemoryManager =
    (
      NeedLock : false;
      GetMem : @CGetmem;
      FreeMem : @CFreeMem;
      FreememSize : @CFreememSize;
      AllocMem : @CAllocMem;
      ReallocMem : @CReAllocMem;
      MemSize : @CMemSize;
      InitThread : Nil;
      DoneThread : Nil;
      RelocateHeap : Nil;
      GetHeapStatus : @CGetHeapStatus;
      GetFPCHeapStatus: @CGetFPCHeapStatus;
    );

Var
  OldMemoryManager : TMemoryManager;

Initialization
  GetMemoryManager (OldMemoryManager);
  SetMemoryManager (CmemoryManager);

Finalization
  SetMemoryManager (OldMemoryManager);
end.