CreateFileMapping写文件?(100分)

如何用 CreateFileMapping 对文件进行写操作呢？

那是内存文件，使用方法一样

举个简单的例子：

声明：
type TData = record
Value1: DWORD;
Value2: DWORD;
end;

var aData: aData:^TData;
HMapFile:THandle;

调用：

aData.Value1:=1;
aData.Value2:=2;
HMapFile:=CreateFileMapping($FFFFFFFF,nil,PAGE_READWRITE,0,SizeOf(TData),'myMapping');
Data:= MapViewOfFile(HMapFile, FILE_MAP_WRITE, 0, 0, 0);
FillChar(Data^, SizeOf(TData), 0);

写错了，是 var aData:^TData;

sherman兄，各位老兄：
比如在"D:/"生成一个只包含一个字节#1的文件"D:/test.dat"。
FFileHandle := CreateFile(......);
if FFileHandle = INVALID_HANDLE_VALUE then Exit;
FFileMapping := CreateFileMapping(......);
if FFileMapping = 0 then Exit;
FFileBase := MapViewOfFile(......);
这些参数如何设置，使文件可写，能否写几行代码？

帮帮我。

以前在网上找到的一个文件：
{******************************************************************************}
{ This unit contains: }
{ TMapStream - a descendant of TStream encapsulating a memory-mapped file, }
{ with automatic mutexes for read/write access
}
{ TTextMap - a descendant of TMapStream adding text input and output }
{ routines and optional encryption
}
{ TRecordMap - a descendant of TMapStream adding i/o routines for record }
{ files and customizable file headers. This is an abstract }
{ class
descendants need to set the record size property in }
{ their constructors and override the ReadRec and WriteRec }
{ abstract methods. }
{ }
{ Freeware by Deborah Pate }
{ Version 1.14 created on 18/09/99 }
{ Please send bug reports, suggestions etc to Debs@djpate.freeserve.co.uk }
{ }
{ **************************************************************************** }
unit MapFiles;

interface

uses Windows, Classes, SysUtils;

type
EMemoryMappedFile = class(Exception);
EMMEndOfFile = class(EMemoryMappedFile);
TFileOptions = (foCreateNew, foCreateAlways, foOpenExisting,
foOpenAlways, foTruncateExisting);
TFileType = (ftUnspecified, ftRandomAccess, ftSequentialAccess);

TMapStream = class(TStream)
private
fActive: boolean;
fFileName: string
// name of the file that's being mapped
fFileOptions: TFileOptions
// set to foOpenExisting by default
fFileType: TFileType
// set to ftUnspecified by default
fInMutex: boolean;
fMapHandle: THandle
// handle to the map
fMapName: string
// name of the map
fMemory: pointer
// pointer to the data itself
fMutexHandle: THandle
// mutex handle for sharing
fMutexName: string
// same as mapname by default
fPosition: cardinal;
fReadOnly: boolean
// false by default
fResizeOpenWarning: boolean
// true by default
fSize: cardinal
// size of file being mapped
fWasAlreadyOpen: boolean
// tells you whether the memory map had
// already been created by another process
procedure CloseMap;
procedure CreateFileHandle(var Handle: THandle
IgnoreOptions: boolean);
procedure CreateMap(NewSize: cardinal);
function CreateMutex: boolean;
function MinSize: integer
virtual;
procedure SetActive(Activate: boolean);
procedure SetFileName(const Name: string);
procedure SetMapName(const Name: string);
procedure SetPosition(Pos: cardinal);
procedure SetSize(NewSize: cardinal)
{$IFDEF VER120} reintroduce
{$ENDIF}
protected
{$IFOPT D+}
function AssertValid: boolean
virtual;
{$ENDIF}
function EnterCriticalSection: boolean;
function Flush: boolean;
procedure LeaveCriticalSection;
property FileType: TFileType read fFileType write fFileType;
property Memory: pointer read fMemory;
property WasAlreadyOpen: boolean read fWasAlreadyOpen;
public
constructor Create(const FileName: string);
destructor Destroy
override;
function Read(var Buffer
Count: Longint): Longint
override;
function Seek(Offset: Longint
Origin: Word): Longint
override;
function Write(const Buffer
Count: Longint): Longint
override;

property Active: boolean read fActive write SetActive;
property FileName: string read fFileName write SetFileName;
property FileOptions: TFileOptions read fFileOptions write fFileOptions;
property MapName: string read fMapName write SetMapName;
property MutexName: string read fMutexName write fMutexName;
property Position: cardinal read fPosition write SetPosition;
property ReadOnly: boolean read fReadOnly write fReadOnly;
property ResizeOpenWarning: boolean read fResizeOpenWarning
write fResizeOpenWarning;
property Size: cardinal read fSize write SetSize;
end;

TTextMap = class(TMapStream)
private
fAutoGrow: boolean;
fUseEncryption: boolean;
protected
procedure EnDecode(var InString: PChar
Encrypting: boolean)
virtual;
function ReadToken: string;
procedure SkipSpaces;
public
constructor Create(const FileName: string)
// sets flag for sequential access
function Read(var Buffer
Count: Longint): Longint
override;
function ReadChar: Char;
function ReadInteger: LongInt;
function ReadFloat: Extended;
function ReadLine: string;
function Write(const Buffer
Count: Longint): Longint
override;
procedure WriteStr(const Str: string);
procedure WriteStrFmt(const Fmt: string
Args: array of const);
procedure WriteLineFmt(const Fmt: string
Args: array of const);
procedure WriteLine(Str: string);
property AutoGrow: boolean read fAutoGrow write fAutoGrow;
property UseEncryption: boolean read fUseEncryption write fUseEncryption;
end;

TRecordMap = class(TMapStream)
private
fHeaderSize: cardinal;
fRecordSize: cardinal
// descendants must set this in their constructor
protected
{$IFOPT D+}
function AssertValid: boolean
override;
{$endif}
function GetCurrentRec: cardinal;
function GetRecCount: cardinal;
function MinSize: integer
override;
function ReadHeader: boolean
virtual;
procedure ReadRec(var Rec)
virtual
abstract;
procedure SetCurrentRec(RecNo: cardinal);
procedure SetRecCount(Count: cardinal);
procedure WriteHeader
virtual;
procedure WriteRec(const Rec)
virtual
abstract;
property CurrentRec: cardinal read GetCurrentRec write SetCurrentRec;
property HeaderSize: cardinal read fHeaderSize write fHeaderSize;
property RecCount: cardinal read GetRecCount write SetRecCount;
property RecordSize: cardinal read fRecordSize write fRecordSize;
public
constructor Create(const FileName: string)
// sets flag for random access
procedure AppendRec(const Rec);
{ Appends one record to the file. Changing the file size incurs }
{ a considerable overhead, so to add several records set the }
{ RecCount property to the required number first and then use WriteRec. }
procedure First;
procedure Last;
end;

implementation
{$IFDEF VER120}
resourcestring
{$ELSE}
const
{$ENDIF}
SCannotResize = 'Memory map %s is open in another process - cannot resize';
SCreateMapError = 'Couldn''t create map';
SCreateMapViewError = 'Could not create map view';
SDefaultMapName = 'Map';
SEOFError = 'Cannot read beyond end of file';
SFileOpenError = 'Could not open file %s';
SMapCloseError = 'Couldn''t close map handle';
SNameChangeWhileActiveError = 'Cannot change filename while map is active';
SReadOnly = 'The memory map %s is readonly';
SResizeAnywayQuery = 'You may lose any information in the existing map. Proceed?';
SResizeOpenWarning = 'Warning: attempt to resize open memory map';
SResizeWhileInactiveError = 'Can''t change size while map is not active';

constructor TMapStream.Create(const FileName: string);
begin
inherited Create;
fFileOptions := foOpenExisting;
fFileName := FileName;
fFileType := ftUnspecified;
if (FileName <> '') then
fMapName := ExtractFileName(FileName)
else fMapName := SDefaultMapName;
fMutexName := MapName + 'Mtx';
fReadOnly := False;
fResizeOpenWarning := True;
end;

destructor TMapStream.Destroy;
begin
CloseMap;
inherited Destroy;
end;

{$IFOPT D+}
function TMapStream.AssertValid: boolean;
{ Useful function for debugging. }
begin
Result := False;
if (Active) then begin
if ((fMapHandle = 0) or (fMemory = nil)) then Exit;
if (Size = 0) then Exit;
end
else begin
if ((fMapHandle <> 0) or (fMemory <> nil)) then Exit;
if (Size <> 0) then Exit;
end;
Result := True;
end;
{$ENDIF}

procedure TMapStream.CloseMap;
{ Closes the map and mutex handles, flushing the data first if necessary. }
begin
if not ReadOnly then Flush;
UnmapViewOfFile(fMemory);
if not CloseHandle(fMapHandle) then
EMemoryMappedFile.Create(SMapCloseError);
fMapHandle := 0;
CloseHandle(fMutexHandle);
fMemory := nil;
fPosition := 0;
fSize := 0;
end;

procedure TMapStream.CreateFileHandle(var Handle: THandle
IgnoreOptions: boolean);
{ Creates or opens the physical file and determines the access mode. }
{ Called by the CreateMap method. }
var
CreationDistribution: integer;
FileAttributes: integer;
begin
{ The IgnoreOptions flag ensures that we don't recreate the file every time
we resize it. }
if not IgnoreOptions then
case FileOptions of
foCreateNew:
CreationDistribution := CREATE_NEW;
foCreateAlways:
CreationDistribution := CREATE_ALWAYS;
foOpenAlways:
CreationDistribution := OPEN_ALWAYS;
foTruncateExisting:
CreationDistribution := TRUNCATE_EXISTING;
else CreationDistribution := OPEN_EXISTING;
end
else CreationDistribution := OPEN_EXISTING;

case FileType of
ftRandomAccess: FileAttributes := FILE_ATTRIBUTE_NORMAL or FILE_FLAG_RANDOM_ACCESS;
ftSequentialAccess: FileAttributes := FILE_ATTRIBUTE_NORMAL or FILE_FLAG_SEQUENTIAL_SCAN;
else FileAttributes := FILE_ATTRIBUTE_NORMAL;
end;

{ Call CreateFile, and check for success }
Handle := CreateFile(PChar(fFileName), GENERIC_READ or GENERIC_WRITE,
FILE_SHARE_READ or FILE_SHARE_WRITE, nil,
CreationDistribution, FileAttributes, 0);
if (Handle = INVALID_HANDLE_VALUE) then
raise EMemoryMappedFile.CreateFmt(SFileOpenError, [FileName]);
end;

procedure TMapStream.CreateMap(NewSize: cardinal);
{ Calls CreateFileHandle to open the physical file, opens or creates the file }
{ mapping, closes the file handle, then calls MapViewOfFile. Sets the map size }
{ based on the file size. If a filename is not specified, the paging file is }
{ mapped, and the size is determined by the system granularity - unless the }
{ map was already open, when its size can be determined only after the map }
{ view is created. }
{ Called by SetActive and SetSize. }
var
Access, Protection: integer;
DiscFileSize: cardinal;
FileHandle: THandle;
MemInfo: TMemoryBasicInformation;
SysInfo: TSystemInfo;
begin
if ReadOnly then begin
Access := FILE_MAP_READ;
Protection := PAGE_READONLY;
end
else begin
Access := FILE_MAP_ALL_ACCESS;
Protection := PAGE_READWRITE;
end;

{ Try to open the map. }
fMapHandle := OpenFileMapping(Access, BOOL(True), PChar(MapName));
fWasAlreadyOpen := (fMapHandle <> 0);

if (FileName <> '') then begin

{ Get the file handle and size. }
CreateFileHandle(FileHandle, NewSize <> 0);
DiscFileSize := GetFileSize(FileHandle, nil);

if (NewSize = 0) then begin // Creating the map the first time:
if (DiscFileSize = 0) then // make sure size > 0
fSize := MinSize
else fSize := DiscFileSize;
end

else begin // Trying to resize the map
if fWasAlreadyOpen then begin // Map was Open - can't resize
if fResizeOpenWarning then begin
MessageBox(0, PChar(Format(SCannotResize, [fMapName])),
PChar(SResizeOpenWarning), MB_OK or MB_ICONEXCLAMATION);

end;
fSize := 0
// fSize will be determined after MapViewOfFile
end
else begin // OK, can resize map
if (NewSize < DiscFileSize) then begin
FileSeek(FileHandle, NewSize, 0);
Win32Check(SetEndOfFile(FileHandle));
end;
fSize := NewSize;
end;
end;

end
else if not fWasAlreadyOpen then begin

{ No filename - so use the paging file. }
FileHandle := $FFFFFFFF;
GetSystemInfo(SysInfo);
if NewSize = 0 then
fSize := SysInfo.dwAllocationGranularity
else fSize := NewSize;

{ Note that at this point fSize can only be 0 if
the map was open *and* either no filename was specified, or
ResizeOnException is set to false. }
end;

try
{ If the map wasn't open, create it. }
if not fWasAlreadyOpen then begin
fMapHandle := CreateFileMapping(FileHandle, nil, Protection,
0, fSize, PChar(MapName));
if (fMapHandle = 0) then begin
fSize := 0;
raise EMemoryMappedFile.Create(SCreateMapError);
end;
end;
finally
CloseHandle(FileHandle);
end;

if not ReadOnly then
CreateMutex;

fMemory := MapViewOfFile(fMapHandle, Access, 0, 0, 0);

if (fMemory <> nil) then begin
if (Size = 0) then begin

{ Another process has determined the map size, so we need to find it here }
VirtualQuery(fMemory, MemInfo, sizeof(MemInfo));
fSize := MemInfo.RegionSize;
end;
end
else begin
CloseMap;
raise EMemoryMappedFile.Create(SCreateMapViewError);
end;

end;

function TMapStream.CreateMutex: boolean;
{ Create a mutex which will be used when writing to the mapfile. }
{ Called by CreateMap and MapExists, if write access to the map is required. }
begin
if (fMutexHandle = 0) then
fMutexHandle := Windows.CreateMutex(nil, False, PChar(fMutexName));
if (fMutexHandle = 0) then
Result := False
else Result := True;
end;

function TMapStream.EnterCriticalSection: boolean;
{ Grab the mutex to prevent other processes accessing the file while we write }
{ to it. }
{ Called by Write and by Flush. }
begin
Result := (fMutexHandle <> 0);
if ((not fInMutex) and Result) then
begin
fInMutex := (WaitForSingleObject(fMutexHandle, INFINITE) = WAIT_OBJECT_0);
Result := fInMutex;
end;
end;

function TMapStream.Flush: boolean;
{ Flushes the data to disc. }
{ Called by CloseMap. }
begin
if Active then begin
EnterCriticalSection;
try
Result := FlushViewOfFile(Memory, 0);
finally
LeaveCriticalSection;
end;
end
else Result := False;
end;

procedure TMapStream.LeaveCriticalSection;
{ Release the mutex now we've finished writing to the file. }
begin
if ((fInMutex) and (fMutexHandle > 0)) then
begin
ReleaseMutex(fMutexHandle);
fInMutex := False;
end;
end;

function TMapStream.MinSize: integer;
{ Returns the minimum size of the mapping. This is needed when you're mapping }
{ a new file (0 bytes) because CreateFileMapping doesn't return a valid handle }
{ if the size specified is 0. }
begin
Result := 64
// completely arbitrary value
end;

{$WARNINGS OFF}

function TMapStream.Read(var Buffer
Count: Longint): Longint;
{ Reads Count bytes from the map file into Buffer. }
{ *** NB: after using this function, the Position property is set to the end }
{ *** of the bytes read. }
begin
if (Count > 0) then begin
Result := Size - Position;
if (Result >= Count) then begin
Result := Count;
System.Move(Pointer(integer(Memory) + Position)^, Buffer, Count);
fPosition := Position + Count;
end
else raise EMMEndOfFile.Create(SEOFError);
end
else Result := 0;
end;

function TMapStream.Seek(Offset: Longint
Origin: Word): Longint;
{ Set the Position property to Origin + Offset. SeeTStream help. }
begin
case Origin of
soFromBeginning: Position := Offset;
soFromCurrent: Position := Position + Offset;
soFromEnd: Position := Size + Offset;
end;
Result := Position;
end;

{$WARNINGS ON}

procedure TMapStream.SetActive(Activate: boolean);
{ Opens an existing mapping or creates an new one. }
begin
if (Activate = fActive) then Exit;
if Activate then begin
CreateMap(0);
fActive := True;
end
else begin
CloseMap;
fActive := False;
end;
end;

procedure TMapStream.SetFileName(const Name: string);
{ Change the FileName. An exception is raised if the map is active. }
begin
if not Active then begin
fFileName := Name;
end
else raise EMemoryMappedFile.Create(SNameChangeWhileActiveError);
end;

procedure TMapStream.SetMapName(const Name: string);
{ Change the MapName. An exception is raised if the map is active. }
begin
if not Active then begin
fMapName := Name;
end
else raise EMemoryMappedFile.Create(SNameChangeWhileActiveError);
end;

procedure TMapStream.SetPosition(Pos: cardinal);
{ Set Position, making sure it is no greater than Size. }
begin
if (Pos <= Size) then
fPosition := Pos
else raise EMMEndOfFile.Create(SEOFError);
end;

procedure TMapStream.SetSize(NewSize: cardinal);
{ Change the size of the map file. There seems to be no way to do this without }
{ closing the map and opening it again - a major chore - so it would be better }
{ to avoid making lots of little size changes. }
var
Pos: cardinal;
begin
if Active then begin
if (NewSize <> fSize) then begin
if ((fFileName = '') and fResizeOpenWarning) then begin
if (MessageBox(0, PChar(SResizeAnywayQuery), PChar(SResizeOpenWarning),
MB_YESNO or MB_ICONEXCLAMATION))
<> IDYES then Exit;
end;
Pos := Position;
CloseMap;
CreateMap(NewSize);
Position := Pos;
end;
end
else raise EMemoryMappedFile.Create(SResizeWhileInactiveError);
end;

{$WARNINGS OFF}

function TMapStream.Write(const Buffer
Count: Longint): Longint;
{ Writes Count bytes from Buffer to map file, using a mutex. Raises an }
{ exception if you try to write beyond the end of the file. }
{ *** NB: after using this function, the Position property is set to the end }
{ *** of the bytes written. }
var
SetMutex: boolean;
Pos: integer;
begin
if ReadOnly then
raise EMemoryMappedFile.CreateFmt(SReadOnly,[Mapname]);
if (Count >= 0) then
begin
Pos := Position + Count;
if Pos > 0 then
begin
if Pos > Size then
raise EMMEndOfFile.Create(SEOFError);
SetMutex := not fInMutex;
EnterCriticalSection;
System.Move(Buffer, Pointer(integer(Memory) + Position)^, Count);
if SetMutex then
LeaveCriticalSection;
fPosition := Pos;
Result := Count;
Exit;
end;
end;
Result := 0;
end;

{$WARNINGS ON}

{******************************************************************************}
{ TTextMap: a memory mapped text file class. }
{ }
{******************************************************************************}

constructor TTextMap.Create(const FileName: string);
begin
inherited Create(FileName);
fFileType := ftSequentialAccess;
fUseEncryption := False;
end;

procedure TTextMap.EnDeCode(var InString: PChar
Encrypting: boolean);
{ Very simple encryption and decryption. The Encrypting flag is just to make }
{ it easier to override this algorithm with a more complicated scheme. }
{ Called by Read and Write. }
var
i: integer;
begin
for i := 0 to StrLen(InString) - 1 do begin
InString := Chr(not(Ord(InString)));
end;
end;

function TTextMap.Read(var Buffer
Count: Longint): Longint;
{ Overrides the write method to allow decryption, if required. }
var
Buf: PChar;
begin
Result := inherited Read(Buffer, Count);
if UseEncryption then begin
Buf := StrAlloc(Count + 1);
try
System.Move(Buffer, Buf^, Count);
Buf[Count] := #0;
EnDeCode(Buf, False);
System.Move(Buf^, Buffer, Count);
finally
StrDispose(Buf);
end;
end;
end;

function TTextMap.ReadChar: Char;
{ Read one character from the file. }
begin
ReadBuffer(Result, 1);
end;


function TTextMap.ReadFloat: Extended;
{ Read a float value from the file. }
begin
SkipSpaces;
Result := StrToFloat(ReadToken);
end;

function TTextMap.ReadInteger: LongInt;
{ Read an integer from the file. }
begin
SkipSpaces;
Result := StrToInt(ReadToken);
end;

function TTextMap.ReadLine: string;
{ Read a line of text, stripping the line ending characters. }
var
Ch: Char;
begin
Result := '';

{ Read characters until reaching an end-of-line character }
while (Read(Ch, 1) = 1) and not (Ch in [#10,#13]) do
AppendStr(Result, Ch);

{ If the end-of-line is CR, look for a subsequent LF }
if Ch = #13 then
if (Read(Ch, 1) = 1) and (Ch <> #10) then
Dec(fPosition);
end;

function TTextMap.ReadToken: string;
{ Read up to the next non-printing character. }
var
Ch: Char;
begin
Result := '';
while Read(Ch, 1) = 1 do begin
if (Ch <= ' ') then begin
Dec(fPosition);
Break;
end;
AppendStr(Result, Ch);
end;
end;

procedure TTextMap.SkipSpaces;
{ Skip over white space (<= ' ') characters. }
var
C: Char;
begin
while Read(C, 1) = 1 do
if C > ' ' then begin
{ Stop with the first nonspace character. }
Dec(fPosition);
Break;
end;
end;

{$WARNINGS OFF}

function TTextMap.Write(const Buffer
Count: Longint): Longint;
{ Overrides the write method to allow encryption and automatic resizing of }
{ the map file, if required. }
var
Buf: PChar;
begin
if (AutoGrow and ((Position + Count) > Size)) then
Size := Position + Count;
if not UseEncryption then
Result := inherited Write(Buffer, Count)
else begin
Buf := StrAlloc(Count + 1);
try
System.Move(Buffer, Buf^, Count);
Buf[Count] := #0;
EnDeCode(Buf, True);
Result := inherited Write(Buf^, Count);
finally
StrDispose(Buf);
end;
end;
end;

{$WARNINGS ON}

procedure TTextMap.WriteStr(const Str: string);
{ Write a string to the file. }
begin
WriteBuffer(Str[1], Length(Str));
end;

procedure TTextMap.WriteStrFmt(const Fmt: string
Args: array of const);
{ Format and write arbitrary arguments. }
begin
WriteStr(SysUtils.Format(Fmt, Args));
end;

procedure TTextMap.WriteLine(Str: string);
{ Print a line of text, appending a line ending. }
begin
Str := Str + #13 + #10;
WriteBuffer(Str[1], Length(Str));
end;

procedure TTextMap.WriteLineFmt(const Fmt: string
Args: array of const);
{ Format and write arbitrary arguments, appending a line ending. }
begin
WriteLine(SysUtils.Format(Fmt, Args));
end;


{******************************************************************************}
{ TRecordMap: an abstract base class for memory-mapped record files. }
{ Descendants should: }
{ a) set the RecordSize and FileHeader details in their constructors
}
{ b) override the ReadRec and WriteRec abstract methods. }
{******************************************************************************}

constructor TRecordMap.Create(const FileName: string);
begin
inherited Create(FileName);
fFileType := ftRandomAccess;
end;

procedure TRecordMap.AppendRec(const Rec);
{ Increases the size of the mapping if necessary, then adds Rec to end of the }
{ file. Changing the file size incurs a considerable overhead each time, so to }
{ add several records set the RecCount property to the required number first }
{ and then use WriteRec. }
begin
Size := Size + RecordSize;
Last;
Write(Rec, RecordSize);
end;

{$IFOPT D+}
function TRecordMap.AssertValid: boolean;
{ Useful debugging function. }
begin
Result := False;
if (RecordSize = 0) then Exit;
Result := inherited AssertValid;
end;
{$ENDIF}

procedure TRecordMap.First;
{ Move to the start of the first record in the file. }
begin
fPosition := HeaderSize;
end;

function TRecordMap.GetCurrentRec: cardinal;
{ Returns the number of the current record (with 0 being the first record in }
{ the file). }
var
RecPos: cardinal;
begin
RecPos := (Position - HeaderSize);
if (RecPos > 0) then
Result := RecPos div RecordSize
else Result := 0;
end;

function TRecordMap.GetRecCount: cardinal;
{ Returns the total number of records in the file. }
var
RecPos: cardinal;
begin
RecPos := (Size - HeaderSize);
if (RecPos > 0) then
Result := RecPos div RecordSize
else Result := 0;
end;

procedure TRecordMap.Last;
{ Sets the Position property to the beginning of the last record in the file. }
begin
if (Size > HeaderSize) then
fPosition := Size - RecordSize
else fPosition := Size;
end;

function TRecordMap.MinSize: integer;
{ Returns the minimum size of the mapping. This is needed when you're mapping }
{ a new file (0 bytes) because CreateFileMapping doesn't return a valid handle }
{ for an empty file (as far as I can see). }
begin
if (HeaderSize <> 0) then
Result := HeaderSize
else Result := RecordSize;
end;

function TRecordMap.ReadHeader: boolean;
{ Does nothing. Descendants can override to implement file headers
a boolean }
{ result enables easy file header checking, e.g.: }
{ if not Readheader then NotAValidFile
}
begin
Result := True
// just to stop compiler warnings about undefined result
end;

procedure TRecordMap.SetCurrentRec(RecNo: cardinal);
{ Moves the Position property to the start of record number RecNo. Note that }
{ the first record in the file is number 0. }
begin
Position := HeaderSize + (RecNo * RecordSize);
if Position >= Size then
raise EMemoryMappedFile.Create(SEOFError);
end;

procedure TRecordMap.SetRecCount(Count: cardinal);
{ Sets the size of the mapping. The file will grow to match it. Setting the }
{ map size has a large overhead, so avoid lots of small allocations. }
var
NewSize: cardinal;
begin
NewSize := HeaderSize + (Count * RecordSize);
Size := NewSize;
end;

procedure TRecordMap.WriteHeader;
{ Does nothing. Descendants can override to implement file headers - they must }
{ then set the appropriate HeaderSize in their constructors for the First }
{ procedure to work properly. }
begin
end;

end.

多人接受答案了。