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
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;
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:
- 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.
- 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.
- 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.
The return value is optional, and can be used to return the size of the freed memory.
- 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 behaviour of the program.
- Is the same as getmem, only the allocated memory should be filled with zeros
before the call returns.
- 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
behaviour is equivalent to GetMem.
- 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.
- This routine is called when a new thread is started: it should initialize the heap
structures for the current thread (if any).
- This routine is called when a thread is ended: it should clean up any heap
structures for the current thread.
- Relocates the heap - this is only for thread-local heaps.
- should return a THeapStatus record with the status of the memory
manager. This record should be filled with Delphi-compliant values.
- 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
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
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
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’;
pptrint = ^ptrint;
Function CGetMem (Size : ptrint) : Pointer;
if (CGetMem <> nil) then
pptrint(CGetMem)^ := size;
Function CFreeMem (P : pointer) : ptrint;
if (p <> nil) then
if size<=0 then
if size<0 then
if (p <> nil) then
if (size <> pptrint(p-sizeof(ptrint))^) then
Function CAllocMem(Size : ptrint) : Pointer;
if (CAllocMem <> nil) then
pptrint(CAllocMem)^ := size;
Function CReAllocMem (var p:pointer;Size:ptrint):Pointer;
if size=0 then
if p<>nil then
if p=nil then
if (p <> nil) then
pptrint(p)^ := size-sizeof(ptrint);
Function CMemSize (p:pointer): ptrint;
var res: THeapStatus;
CMemoryManager : TMemoryManager =
NeedLock : false;
GetMem : @CGetmem;
FreeMem : @CFreeMem;
FreememSize : @CFreememSize;
AllocMem : @CAllocMem;
ReallocMem : @CReAllocMem;
MemSize : @CMemSize;
InitThread : Nil;
DoneThread : Nil;
RelocateHeap : Nil;
GetHeapStatus : @CGetHeapStatus;
OldMemoryManager : TMemoryManager;