從主線程更改線程的變量值是安全的嗎？

Question

我寫了一個簡單的組件來監視文件夾，並在檢測到更改時觸發事件。它運作良好......顯然。但我不確定一件事。有時主線程可能需要更新受監控的路徑，我不知道我是否做得對。這是關於SetNewPath程序。這是從主線程執行的，它將從另一個線程中更改UpdatePath變量。當主線程寫入UpdatePath並且組件線程嘗試讀取其Execute週期中的值時，可能會產生衝突？

FolderMonitor.pas

unit FolderMonitor; 

interface 

uses 
    SysUtils, Windows, Classes, ExtCtrls; 

type 
    TOnFolderChange = procedure(Sender: TObject) of object; 

    TFolderMonitor = class(TThread) 
    private 
    MainWait: THandle; 
    UpdatePath: Boolean; 
    TimeOut: Cardinal; 
    FPath: String; 
    FOnFolderChange: TOnFolderChange; 
    procedure DoOnFolderChange; 
    procedure SetNewPath(Path:String); 
    protected 
    procedure Execute; override; 
    public 
    constructor Create(const FolderPath: String; OnFolderChangeHandler: TOnFolderChange); 
    destructor Destroy; override; 
    procedure Unblock; 
    property Path: String read FPath write SetNewPath; 
    property OnFolderChange: TOnFolderChange read FOnFolderChange write FOnFolderChange; 
    end; 

implementation 

constructor TFolderMonitor.Create(const FolderPath: String; OnFolderChangeHandler: TOnFolderChange); 
begin 
    inherited Create(True); 
    FOnFolderChange:=OnFolderChangeHandler; 
    FPath:=FolderPath; 
    UpdatePath:=false; 
    FreeOnTerminate:=false; 
    MainWait:=CreateEvent(nil,true,false,nil); 
    Resume; 
end; 

destructor TFolderMonitor.Destroy; 
begin 
    CloseHandle(MainWait); 
    inherited; 
end; 

procedure TFolderMonitor.DoOnFolderChange; 
begin 
    if Assigned(FOnFolderChange) then 
    Synchronize(procedure 
    begin 
    FOnFolderChange(Self); 
    end); 
end; 

procedure TFolderMonitor.Unblock; 
begin 
    PulseEvent(MainWait); 
end; 

procedure TFolderMonitor.SetNewPath(Path:String); 
begin 
    FPath:=Path; 
    UpdatePath:=true; 
    PulseEvent(MainWait); 
end; 

procedure TFolderMonitor.Execute; 
var Filter,WaitResult: Cardinal; 
    WaitHandles: array[0..1] of THandle; 
begin 
    Filter:=FILE_NOTIFY_CHANGE_DIR_NAME + FILE_NOTIFY_CHANGE_FILE_NAME + FILE_NOTIFY_CHANGE_SIZE; 
    WaitHandles[0]:=MainWait; 
    WaitHandles[1]:=FindFirstChangeNotification(PWideChar(FPath),false,Filter); 
    TimeOut:=INFINITE; 

    while not Terminated do begin 
    if UpdatePath then begin 
    if WaitHandles[1]<>INVALID_HANDLE_VALUE then FindCloseChangeNotification(WaitHandles[1]); 
    WaitHandles[1]:=FindFirstChangeNotification(PWideChar(FPath),false,Filter); 
    TimeOut:=INFINITE; 
    UpdatePath:=false; 
    end; 

    if WaitHandles[1] = INVALID_HANDLE_VALUE 
    then WaitResult:=WaitForSingleObject(WaitHandles[0],INFINITE) 
    else WaitResult:=WaitForMultipleObjects(2,@WaitHandles,false,TimeOut); 

    case WaitResult of 
    WAIT_OBJECT_0: Continue; 
    WAIT_OBJECT_0+1: TimeOut:=200; 
    WAIT_TIMEOUT: begin DoOnFolderChange; TimeOut:=INFINITE; end; 
    end; 

    if WaitHandles[1] <> INVALID_HANDLE_VALUE then 
    FindNextChangeNotification(WaitHandles[1]); 
    end; 

    if WaitHandles[1] <> INVALID_HANDLE_VALUE then 
    FindCloseChangeNotification(WaitHandles[1]); 
end; 

end. 

UnitMain.pas

unit UnitMain; 

interface 

uses 
    Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, 
    Dialogs, StdCtrls, FolderMonitor; 

type 
    TForm1 = class(TForm) 
    Memo1: TMemo; 
    Edit1: TEdit; 
    Button1: TButton; 
    procedure FormCreate(Sender: TObject); 
    procedure OnFolderChange(Sender: TObject); 
    procedure FormDestroy(Sender: TObject); 
    procedure Button1Click(Sender: TObject); 
    end; 

var 
    Form1: TForm1; 
    Mon: TFolderMonitor; 
    X: integer; 

implementation 

{$R *.dfm} 

procedure TForm1.FormCreate(Sender: TObject); 
begin 
X:=0; 
Mon:=TFolderMonitor.Create('D:\Test',OnFolderChange); 
end; 

procedure TForm1.FormDestroy(Sender: TObject); 
begin 
Mon.Terminate; 
Mon.Unblock; 
Mon.WaitFor; 
Mon.Free; 
end; 

procedure TForm1.OnFolderChange(Sender: TObject); 
begin 
inc(x); 
Memo1.Lines.Add('changed! '+IntToStr(x)); 
end; 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
Mon.Path:=Edit1.Text; 
end; 

end. 

Answer 1

您有多個線程之間共享的變量，與一個線程修改變量。這種情況被稱爲數據競賽。

一些比賽可能是良性的。這一個不是。如果一個線程修改該變量而另一個線程讀取該變量，則可能會發生錯誤。由於數據類型非常複雜（指向堆分配的字符數組），讀線程很可能試圖從釋放的內存中讀取數據。

對於像這樣的複雜類型，只要您訪問該值，就需要使用互斥鎖。所有的讀寫操作必須由鎖序列化。使用關鍵部分或顯示器。

爲確保您永遠不會執行不受保護的訪問，明智的做法是在代碼中執行此規則。例如，我在這裏描述的TThreadSafe<T>：Generic Threadsafe Property