MFC 多线程访问同一个变量之互斥锁(CSingleLock )
先来一个例子：
一个进程开两个线程，这两个线程修改一个变量，并把这个变量的值打印出来
以下是代码（VS2010）：
新建一个解决方案：MutexTest
修改MutexTestDlg.h
// MutexTestDlg.h : header file
//
#pragma once
#include <Windows.h>
// CMutexTestDlg dialog
class CMutexTestDlg : public CDialogEx
{
// Construction
public:
CMutexTestDlg(CWnd* pParent = NULL); // standard constructor
// Dialog Data
enum { IDD = IDD_MUTEXTEST_DIALOG };
protected:
virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support
int m_nCount;
CString m_strCount;
BOOL m_bKillThread;
static UINT MyThread1(LPVOID* pParam);//thread1 function
CWinThread *Thread1;//thread1
static UINT MyThread2(LPVOID* pParam);//thread2 function
CWinThread *Thread2;//thread2
// Implementation
protected:
HICON m_hIcon;
// Generated message map functions
virtual BOOL OnInitDialog();
afx_msg void OnSysCommand(UINT nID, LPARAM lParam);
afx_msg void OnPaint();
afx_msg HCURSOR OnQueryDragIcon();
DECLARE_MESSAGE_MAP()
public:
afx_msg void OnBnClickedButton1();
afx_msg void OnBnClickedButton2();
afx_msg void OnBnClickedButton3();
afx_msg void OnBnClickedButton4();
};
修改MutexTestDlg.cpp
// MutexTestDlg.cpp : implementation file
//
#include "stdafx.h"
#include "MutexTest.h"
#include "MutexTestDlg.h"
#include "afxdialogex.h"
#ifdef _DEBUG
#define new DEBUG_NEW
#endif
// CAboutDlg dialog used for App About
class CAboutDlg : public CDialogEx
{
public:
CAboutDlg();
// Dialog Data
enum { IDD = IDD_ABOUTBOX };
protected:
virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support
// Implementation
protected:
DECLARE_MESSAGE_MAP()
};
CAboutDlg::CAboutDlg() : CDialogEx(CAboutDlg::IDD)
{
}
void CAboutDlg::DoDataExchange(CDataExchange* pDX)
{
CDialogEx::DoDataExchange(pDX);
}
BEGIN_MESSAGE_MAP(CAboutDlg, CDialogEx)
END_MESSAGE_MAP()
// CMutexTestDlg dialog
CMutexTestDlg::CMutexTestDlg(CWnd* pParent /*=NULL*/)
: CDialogEx(CMutexTestDlg::IDD, pParent)
{
m_hIcon = AfxGetApp()->LoadIcon(IDR_MAINFRAME);
m_nCount = 0;
m_strCount = "";
m_bKillThread = FALSE;
}
void CMutexTestDlg::DoDataExchange(CDataExchange* pDX)
{
CDialogEx::DoDataExchange(pDX);
}
BEGIN_MESSAGE_MAP(CMutexTestDlg, CDialogEx)
ON_WM_SYSCOMMAND()
ON_WM_PAINT()
ON_WM_QUERYDRAGICON()
ON_BN_CLICKED(IDC_BUTTON1, &CMutexTestDlg::OnBnClickedButton1)
ON_BN_CLICKED(IDC_BUTTON2, &CMutexTestDlg::OnBnClickedButton2)
ON_BN_CLICKED(IDC_BUTTON3, &CMutexTestDlg::OnBnClickedButton3)
ON_BN_CLICKED(IDC_BUTTON4, &CMutexTestDlg::OnBnClickedButton4)
END_MESSAGE_MAP()
// CMutexTestDlg message handlers
BOOL CMutexTestDlg::OnInitDialog()
{
CDialogEx::OnInitDialog();
// Add "About..." menu item to system menu.
// IDM_ABOUTBOX must be in the system command range.
ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
ASSERT(IDM_ABOUTBOX < 0xF000);
CMenu* pSysMenu = GetSystemMenu(FALSE);
if (pSysMenu != NULL)
{
BOOL bNameValid;
CString strAboutMenu;
bNameValid = strAboutMenu.LoadString(IDS_ABOUTBOX);
ASSERT(bNameValid);
if (!strAboutMenu.IsEmpty())
{
pSysMenu->AppendMenu(MF_SEPARATOR);
pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu);
}
}
// Set the icon for this dialog. The framework does this automatically
// when the application's main window is not a dialog
SetIcon(m_hIcon, TRUE); // Set big icon
SetIcon(m_hIcon, FALSE); // Set small icon
// TODO: Add extra initialization here
return TRUE; // return TRUE unless you set the focus to a control
}
void CMutexTestDlg::OnSysCommand(UINT nID, LPARAM lParam)
{
if ((nID & 0xFFF0) == IDM_ABOUTBOX)
{
CAboutDlg dlgAbout;
dlgAbout.DoModal();
}
else
{
CDialogEx::OnSysCommand(nID, lParam);
}
}
// If you add a minimize button to your dialog, you will need the code below
// to draw the icon. For MFC applications using the document/view model,
// this is automatically done for you by the framework.
void CMutexTestDlg::OnPaint()
{
if (IsIconic())
{
CPaintDC dc(this); // device context for painting
SendMessage(WM_ICONERASEBKGND, reinterpret_cast<WPARAM>(dc.GetSafeHdc()), 0);
// Center icon in client rectangle
int cxIcon = GetSystemMetrics(SM_CXICON);
int cyIcon = GetSystemMetrics(SM_CYICON);
CRect rect;
GetClientRect(&rect);
int x = (rect.Width() - cxIcon + 1) / 2;
int y = (rect.Height() - cyIcon + 1) / 2;
// Draw the icon
dc.DrawIcon(x, y, m_hIcon);
}
else
{
CDialogEx::OnPaint();
}
}
// The system calls this function to obtain the cursor to display while the user drags
// the minimized window.
HCURSOR CMutexTestDlg::OnQueryDragIcon()
{
return static_cast<HCURSOR>(m_hIcon);
}
UINT CMutexTestDlg::MyThread1(LPVOID* pParam)
{
CMutexTestDlg * pMutexTestDlg= (CMutexTestDlg *)pParam;
while(1)
{
if(pMutexTestDlg->m_bKillThread)
{
DWORD dwExitCode;
GetExitCodeThread(pMutexTestDlg->Thread1, &dwExitCode);
AfxEndThread(dwExitCode, TRUE);
}
else
{
pMutexTestDlg->m_nCount = 1;
pMutexTestDlg->m_strCount.Format(_T("%d"), pMutexTestDlg->m_nCount);
pMutexTestDlg->SetDlgItemText(IDC_EDIT1, pMutexTestDlg->m_strCount);
Sleep(300);
}
} return 0;
}
UINT CMutexTestDlg::MyThread2(LPVOID* pParam)
{
CMutexTestDlg * pMutexTestDlg= (CMutexTestDlg *)pParam;
while(1)
{
if(pMutexTestDlg->m_bKillThread)
{
DWORD dwExitCode;
GetExitCodeThread(pMutexTestDlg->Thread2, &dwExitCode);
AfxEndThread(dwExitCode, TRUE);
}
else
{
pMutexTestDlg->m_nCount = 2;
pMutexTestDlg->m_strCount.Format(_T("%d"), pMutexTestDlg->m_nCount);
pMutexTestDlg->SetDlgItemText(IDC_EDIT2, pMutexTestDlg->m_strCount);
Sleep(300);
}
} return 0;
}
void CMutexTestDlg::OnBnClickedButton1()
{
// TODO: Add your control notification handler code here
Thread1 = AfxBeginThread((AFX_THREADPROC)MyThread1, this); //Start thread
Thread2 = AfxBeginThread((AFX_THREADPROC)MyThread2, this); //Start thread
m_bKillThread = FALSE;
}
void CMutexTestDlg::OnBnClickedButton2()
{
// TODO: Add your control notification handler code here
Thread1->SuspendThread(); //Pause thread
Thread2->SuspendThread(); //Pause thread
}
void CMutexTestDlg::OnBnClickedButton3()
{
// TODO: Add your control notification handler code here
Thread1->ResumeThread(); //Continue
Thread2->ResumeThread(); //Continue
}
void CMutexTestDlg::OnBnClickedButton4()
{
// TODO: Add your control notification handler code here
m_bKillThread = TRUE;
Thread1->ResumeThread(); //Continue thread run (if thread is suspend, end thread will failed.)
Thread2->ResumeThread(); //Continue thread run (if thread is suspend, end thread will failed.)
}
编译-运行，点击【Start】按钮后，【Pause】和【Continue】互相切换点击， 会发现这两个编辑框会显示相同的值，并不是代码中写的第一个编辑框设置1，第二个编辑框设置2
以上的原因时因为两个线程修改设置同一个变量，导致一个线程准备显示值得时候，被另一个线程修改了，导致两个编辑框显示的值一致
解决办法就是加互斥锁Mutex
将代码做如下修改：
1.在MutexTestDlg.cpp中加入全局类变量 CMutex Mutex;
2.在两个线程中定义互斥锁类变量 CSingleLock singleLock(&Mutex);
3.把在线程中需要保护的代码保护起来
singleLock.Lock();
if (singleLock.IsLocked())
{
//需要保护的变量
}
singleLock.Unlock();
修改后的Cpp如下：
// MutexTestDlg.cpp : implementation file // #include "stdafx.h" #include "MutexTest.h" #include "MutexTestDlg.h" #include "afxdialogex.h" #ifdef _DEBUG #define new DEBUG_NEW #endif CMutex Mutex; //mutex // CAboutDlg dialog used for App About class CAboutDlg : public CDialogEx { public: CAboutDlg(); // Dialog Data enum { IDD = IDD_ABOUTBOX }; protected: virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support // Implementation protected: DECLARE_MESSAGE_MAP() }; CAboutDlg::CAboutDlg() : CDialogEx(CAboutDlg::IDD) { } void CAboutDlg::DoDataExchange(CDataExchange* pDX) { CDialogEx::DoDataExchange(pDX); } BEGIN_MESSAGE_MAP(CAboutDlg, CDialogEx) END_MESSAGE_MAP() // CMutexTestDlg dialog CMutexTestDlg::CMutexTestDlg(CWnd* pParent /*=NULL*/) : CDialogEx(CMutexTestDlg::IDD, pParent) { m_hIcon = AfxGetApp()->LoadIcon(IDR_MAINFRAME); m_nCount = 0; m_strCount = ""; m_bKillThread = FALSE; } void CMutexTestDlg::DoDataExchange(CDataExchange* pDX) { CDialogEx::DoDataExchange(pDX); } BEGIN_MESSAGE_MAP(CMutexTestDlg, CDialogEx) ON_WM_SYSCOMMAND() ON_WM_PAINT() ON_WM_QUERYDRAGICON() ON_BN_CLICKED(IDC_BUTTON1, &CMutexTestDlg::OnBnClickedButton1) ON_BN_CLICKED(IDC_BUTTON2, &CMutexTestDlg::OnBnClickedButton2) ON_BN_CLICKED(IDC_BUTTON3, &CMutexTestDlg::OnBnClickedButton3) ON_BN_CLICKED(IDC_BUTTON4, &CMutexTestDlg::OnBnClickedButton4) END_MESSAGE_MAP() // CMutexTestDlg message handlers BOOL CMutexTestDlg::OnInitDialog() { CDialogEx::OnInitDialog(); // Add "About..." menu item to system menu. // IDM_ABOUTBOX must be in the system command range. ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX); ASSERT(IDM_ABOUTBOX < 0xF000); CMenu* pSysMenu = GetSystemMenu(FALSE); if (pSysMenu != NULL) { BOOL bNameValid; CString strAboutMenu; bNameValid = strAboutMenu.LoadString(IDS_ABOUTBOX); ASSERT(bNameValid); if (!strAboutMenu.IsEmpty()) { pSysMenu->AppendMenu(MF_SEPARATOR); pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu); } } // Set the icon for this dialog. The framework does this automatically // when the application's main window is not a dialog SetIcon(m_hIcon, TRUE); // Set big icon SetIcon(m_hIcon, FALSE); // Set small icon // TODO: Add extra initialization here return TRUE; // return TRUE unless you set the focus to a control } void CMutexTestDlg::OnSysCommand(UINT nID, LPARAM lParam) { if ((nID & 0xFFF0) == IDM_ABOUTBOX) { CAboutDlg dlgAbout; dlgAbout.DoModal(); } else { CDialogEx::OnSysCommand(nID, lParam); } } // If you add a minimize button to your dialog, you will need the code below // to draw the icon. For MFC applications using the document/view model, // this is automatically done for you by the framework. void CMutexTestDlg::OnPaint() { if (IsIconic()) { CPaintDC dc(this); // device context for painting SendMessage(WM_ICONERASEBKGND, reinterpret_cast<WPARAM>(dc.GetSafeHdc()), 0); // Center icon in client rectangle int cxIcon = GetSystemMetrics(SM_CXICON); int cyIcon = GetSystemMetrics(SM_CYICON); CRect rect; GetClientRect(&rect); int x = (rect.Width() - cxIcon + 1) / 2; int y = (rect.Height() - cyIcon + 1) / 2; // Draw the icon dc.DrawIcon(x, y, m_hIcon); } else { CDialogEx::OnPaint(); } } // The system calls this function to obtain the cursor to display while the user drags // the minimized window. HCURSOR CMutexTestDlg::OnQueryDragIcon() { return static_cast<HCURSOR>(m_hIcon); } UINT CMutexTestDlg::MyThread1(LPVOID* pParam) { CSingleLock singleLock(&Mutex); //Mutex CMutexTestDlg * pMutexTestDlg= (CMutexTestDlg *)pParam; while(1) { if(pMutexTestDlg->m_bKillThread) { DWORD dwExitCode; GetExitCodeThread(pMutexTestDlg->Thread1, &dwExitCode); AfxEndThread(dwExitCode, TRUE); } else { singleLock.Lock(); //Mutex if (singleLock.IsLocked()) { pMutexTestDlg->m_nCount = 1; pMutexTestDlg->m_strCount.Format(_T("%d"), pMutexTestDlg->m_nCount); pMutexTestDlg->SetDlgItemText(IDC_EDIT1, pMutexTestDlg->m_strCount); Sleep(300); singleLock.Unlock(); //Mutex } } } return 0; } UINT CMutexTestDlg::MyThread2(LPVOID* pParam) { CSingleLock singleLock(&Mutex); //mutex CMutexTestDlg * pMutexTestDlg= (CMutexTestDlg *)pParam; while(1) { if(pMutexTestDlg->m_bKillThread) { DWORD dwExitCode; GetExitCodeThread(pMutexTestDlg->Thread2, &dwExitCode); AfxEndThread(dwExitCode, TRUE); } else { singleLock.Lock(); //mutex if (singleLock.IsLocked()) //mutex { pMutexTestDlg->m_nCount = 2; pMutexTestDlg->m_strCount.Format(_T("%d"), pMutexTestDlg->m_nCount); pMutexTestDlg->SetDlgItemText(IDC_EDIT2, pMutexTestDlg->m_strCount); Sleep(300); singleLock.Unlock(); //mutex } } } return 0; } void CMutexTestDlg::OnBnClickedButton1() { // TODO: Add your control notification handler code here Thread1 = AfxBeginThread((AFX_THREADPROC)MyThread1, this); //Start thread Thread2 = AfxBeginThread((AFX_THREADPROC)MyThread2, this); //Start thread m_bKillThread = FALSE; } void CMutexTestDlg::OnBnClickedButton2() { // TODO: Add your control notification handler code here Thread1->SuspendThread(); //Pause thread Thread2->SuspendThread(); //Pause thread } void CMutexTestDlg::OnBnClickedButton3() { // TODO: Add your control notification handler code here Thread1->ResumeThread(); //Continue Thread2->ResumeThread(); //Continue } void CMutexTestDlg::OnBnClickedButton4() { // TODO: Add your control notification handler code here m_bKillThread = TRUE; Thread1->ResumeThread(); //Continue thread run (if thread is suspend, end thread will failed.) Thread2->ResumeThread(); //Continue thread run (if thread is suspend, end thread will failed.) }
之后就可以正常运行了
【注意】：多线程涉及访问修改同一变量，一定要加🔒进行保护。
扩展知识：临界区也可以实现以上的功能，区别在于临界区速度快，只可用于进程中的多个线程，
互斥量可以防止多个进程对共享数据的同时修改操作。
另外，用 WaitForSingleObject 和 ReleaseMutex也可以
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