冲突原因

写CString的时候，可能内存不够，于是释放、重新分配内存。可能刚释放，没来来得及分配，就被挂起了，然后另外的线程访问此变量，就崩了。　

测试说明

Button18测试了，没有防冲突，程序崩溃。Button19，测试了有防冲突处理，程序没崩溃。Button20，一个写锁若干个读锁，比全部写锁要快。SNMFC::CriticalReadWriteLock，自己整理的类库的类。

测试代码

class CTesNoLock
{
public:
static UINT Write(LPVOID)
{
Sleep(1);
s_str += _T("AB33333333333333333333");
return 0;
}
static UINT Read(LPVOID)
{
Sleep(1);
bool b = s_str[0] == _T('A');
ASSERT(b);
return 0;
}
protected:
static CString s_str;
};
CString CTesNoLock::s_str = "A";

class CTesReadWriteLock : public CTesNoLock
{
public:
static UINT LockWrite(LPVOID)
{
s_lock.Lock();
Write(NULL);
s_lock.UnLock();
return 0;
}
static UINT LockRead(LPVOID)
{
s_lock.LockRead();
Read(NULL);
s_lock.UnLockRead();
return 0;
}

static UINT LockWrite100ms(LPVOID)
{
s_lock.Lock();
Sleep(100);
s_lock.UnLock();
return 0;
}
static UINT LockRead100ms(LPVOID)
{
s_lock.LockRead();
Sleep(100);
s_lock.UnLockRead();
return 0;
}
protected:
static SNMFC::CriticalReadWriteLock s_lock;
};

SNMFC::CriticalReadWriteLock CTesReadWriteLock::s_lock;

void CSNMFCDlg::OnBnClickedButton18()
{
CTesNoLock::Write(NULL);
for (int i = 0; i < 1000; i++)
{
AfxBeginThread(CTesNoLock::Write, NULL);
AfxBeginThread(CTesNoLock::Read, NULL);
}
}

void CSNMFCDlg::OnBnClickedButton19()
{
for (int i = 0; i < 10; i++)
{
AfxBeginThread(CTesReadWriteLock::LockWrite, NULL);
for (int j = 0; j < 1000; j++)
{
AfxBeginThread(CTesReadWriteLock::LockRead, NULL);
}
}
}

void CSNMFCDlg::OnBnClickedButton20()
{
DWORD d1 = GetTickCount();
{//由于被锁，只能依次访问，用时约10秒
for (int i = 0; i < 99; i++)
{
AfxBeginThread(CTesReadWriteLock::LockWrite100ms, NULL);
}
}
CTesReadWriteLock::LockWrite100ms(NULL);
DWORD d2 = GetTickCount();
{//可以同时读，所以用时少的多
for (int i = 0; i < 99; i++)
{
AfxBeginThread(CTesReadWriteLock::LockRead100ms, NULL);
}
}
CTesReadWriteLock::LockWrite100ms(NULL);
DWORD d3 = GetTickCount();

ASSERT((d2 - d1) >(d3 - d2) * 2);
}