多线程中的volatile关键字

注：多线程中由于线程间需要同步，线程可能会不断检查某个同步值的是否改变，这必然牵涉到循环。由于编译器的优化，在循环中它不会每次都检查同步值。这就需要volatile关键字来说明这个同步值，告诉编译器不要对其进行优化。下面的程序源码摘自《Windows程序设计第五版》相关章节。

关键字：多线程同步循环监测 volatile 编译优化

/*----------------------------------------
BIGJOB1.C -- Multithreading Demo
(c) Charles Petzold, 1998
----------------------------------------*/
#include <windows.h>
#include <math.h>
#include <process.h>
#define REP 10000000 //Translator: the original value 1000000 is too small, increase 10 times to be 10000000
#define STATUS_READY 0
#define STATUS_WORKING 1
#define STATUS_DONE 2
#define WM_CALC_DONE (WM_USER + 0)
#define WM_CALC_ABORTED (WM_USER + 1)
typedef struct
{
HWND hwnd ;
BOOL bContinue ;
}
PARAMS, *PPARAMS ;
LRESULT APIENTRY WndProc (HWND, UINT, WPARAM, LPARAM) ;
int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
PSTR szCmdLine, int iCmdShow)
{
static TCHAR szAppName[] = TEXT ("BigJob1") ;
HWND hwnd ;
MSG msg ;
WNDCLASS wndclass ;
wndclass.style = CS_HREDRAW | CS_VREDRAW ;
wndclass.lpfnWndProc = WndProc ;
wndclass.cbClsExtra = 0 ;
wndclass.cbWndExtra = 0 ;
wndclass.hInstance = hInstance ;
wndclass.hIcon = LoadIcon (NULL, IDI_APPLICATION) ;
wndclass.hCursor = LoadCursor (NULL, IDC_ARROW) ;
wndclass.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH) ;
wndclass.lpszMenuName = NULL ;
wndclass.lpszClassName = szAppName ;
if (!RegisterClass (&wndclass))
{
MessageBox (NULL, TEXT ("This program requires Windows NT!"),
szAppName, MB_ICONERROR) ;
return 0 ;
}
hwnd = CreateWindow (szAppName, TEXT ("Multithreading Demo"),
WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, CW_USEDEFAULT,
CW_USEDEFAULT, CW_USEDEFAULT,
NULL, NULL, hInstance, NULL) ;
ShowWindow (hwnd, iCmdShow) ;
UpdateWindow (hwnd) ;
while (GetMessage (&msg, NULL, 0, 0))
{
TranslateMessage (&msg) ;
DispatchMessage (&msg) ;
}
return msg.wParam ;
}
void Thread (PVOID pvoid)
{
double A = 1.0 ;
INT i ;
LONG lTime ;
volatile PPARAMS pparams ;
/* 注意Thread中的pparams变量定义为volatile，这种型态限定字向编译器指出：该变量可能会在实际的程序叙述外被修改（例如被另一个线程）。 否则， 最佳化的编译器会假设pparams->bContinue不能被for循环内的程序代码修改，没有必要在每层循环中检查变量。volatile关键词防止这样的最佳化进行。 */
pparams = (PPARAMS) pvoid ;
lTime = GetCurrentTime () ;
for (i = 0 ; i < REP && pparams->bContinue ; i++)
A = tan (atan (exp (log (sqrt (A * A))))) + 1.0 ;
if (i == REP)
{
lTime = GetCurrentTime () - lTime ;
SendMessage (pparams->hwnd, WM_CALC_DONE, 0, lTime) ;
}
else
SendMessage (pparams->hwnd, WM_CALC_ABORTED, 0, 0) ;
_endthread () ;
}
LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
static INT iStatus ;
static LONG lTime ;
static PARAMS params ;
static TCHAR * szMessage[] = { TEXT ("Ready (left mouse button begins)"),
TEXT ("Working (right mouse button ends)"),
TEXT ("%d repetitions in %ld msec") } ;
HDC hdc ;
PAINTSTRUCT ps ;
RECT rect ;
TCHAR szBuffer[64] ;
switch (message)
{
case WM_LBUTTONDOWN:
if (iStatus == STATUS_WORKING)
{
MessageBeep (0) ;
return 0 ;
}
iStatus = STATUS_WORKING ;
params.hwnd = hwnd ;
params.bContinue = TRUE ;
_beginthread (Thread, 0, &params) ;
InvalidateRect (hwnd, NULL, TRUE) ;
return 0 ;
case WM_RBUTTONDOWN:
params.bContinue = FALSE ;
return 0 ;
case WM_CALC_DONE:
lTime = lParam ;
iStatus = STATUS_DONE ;
InvalidateRect (hwnd, NULL, TRUE) ;
return 0 ;
case WM_CALC_ABORTED:
iStatus = STATUS_READY ;
InvalidateRect (hwnd, NULL, TRUE) ;
return 0 ;
case WM_PAINT:
hdc = BeginPaint (hwnd, &ps) ;
GetClientRect (hwnd, &rect) ;
wsprintf (szBuffer, szMessage[iStatus], REP, lTime) ;
DrawText (hdc, szBuffer, -1, &rect,
DT_SINGLELINE | DT_CENTER | DT_VCENTER) ;
EndPaint (hwnd, &ps) ;
return 0 ;
case WM_DESTROY:
PostQuitMessage (0) ;
return 0 ;
}
return DefWindowProc (hwnd, message, wParam, lParam) ;
}