【MFC】0xC0000005: 读取位置 0x00000020 时发生访问冲突

原因：使用GetDlgItem()函数时需要先判断指针然后才可以使用。

错误代码：

//重新建一个线程，查询帧同步
DWORD WINAPI SCsync_Thread(LPVOID Lparam)
{
    CTABPAGE1 * ppPage = (CTABPAGE1 *)Lparam;;// 创建指针指向当前类
    ULONG SYNC;
    char SSync[4];
    while(1)
    {
        ppPage->ReadReg(hDev, 0x08, &SYNC, 1);//该地址读取出来低八位是同步信息
        memcpy(SSync,&SYNC,4*sizeof(char));
        if (SSync[3] == 1)//说明已经同步
        {
            ppPage->GetDlgItem(IDC_STATIC_SCsync_green)->ShowWindow(SW_SHOW);//显示绿灯
            ppPage->GetDlgItem(IDC_STATIC_SCsync_Red)->ShowWindow(SW_HIDE);
        }
        else
        {
            ppPage->GetDlgItem(IDC_STATIC_SCsync_Red)->ShowWindow(SW_SHOW);//显示红灯
            ppPage->GetDlgItem(IDC_STATIC_SCsync_green)->ShowWindow(SW_HIDE);
        }
    }

}

加上判断指针之后的正确的代码：

//重新建一个线程，查询帧同步
DWORD WINAPI SCsync_Thread(LPVOID Lparam)
{
    CTABPAGE1 * ppPage = (CTABPAGE1 *)Lparam;;// 创建指针指向当前类
    ULONG SYNC;
    char SSync[4];
    while(1)
    {
        ppPage->ReadReg(hDev, 0x08, &SYNC, 1);//该地址读取出来低八位是同步信息
        memcpy(SSync,&SYNC,4*sizeof(char));
        if (SSync[3] == 1)//说明已经同步
        {
            if (ppPage->GetDlgItem(IDC_STATIC_SCsync_green))
            ppPage->GetDlgItem(IDC_STATIC_SCsync_green)->ShowWindow(SW_SHOW);//显示绿灯
            if (ppPage->GetDlgItem(IDC_STATIC_SCsync_Red))
            ppPage->GetDlgItem(IDC_STATIC_SCsync_Red)->ShowWindow(SW_HIDE);
        }
        else
        {
            if (ppPage->GetDlgItem(IDC_STATIC_SCsync_Red))//判断之后才能使用该指针
            ppPage->GetDlgItem(IDC_STATIC_SCsync_Red)->ShowWindow(SW_SHOW);//显示红灯
            if (ppPage->GetDlgItem(IDC_STATIC_SCsync_green))
            ppPage->GetDlgItem(IDC_STATIC_SCsync_green)->ShowWindow(SW_HIDE);
        }
    }

}