[C++] localtime 注意事项

1、localtime返回的指针是个静态成员变量

1    struct tm *pstTmp1, *psttTmp2;
2    time_t tTime1 = , tTime2;
3 
4    tTime1 = 1354852746;
5    tTime2 = 1350000000
6  
7   pstTmp1 = localtime(&tTime1);
8   pstTmp2 = localtime(&Time2);

   上述中的pstTmp1和pstTmp2指向的变量是同一个，所以它们的值是一样的。需要注意调用完localtime后，立马取出其值。

2、localtime传入的参数。

   VS 2010 中的localtime的参数是__int64，所以如果传进去的数是32位的地址，那么可能会导致localtime返回NULL(我就被这样的事情困扰了半个小时)。

    定义了如下一个结构体，其中m_uiTimeSaleBegin和m_uiTimeSaleEnd存放两个时间值

struct tmp
{
 int m_iDateSold;                    //当天已卖出的个数
 unsigned int m_uiTimeSaleBegin;    //限时销售开始时间
 unsigned int m_uiTimeSaleEnd;    //限时销售结束时间
 int m_iSaleTag;                    //标签（热卖，推荐等）
};       

然后调用一下代码来显示：

｛
       if (pstCommodity->m_uiTimeSaleBegin > 0)
    {
        TRACE("begintime = %u\n", pstCommodity->m_uiTimeSaleBegin);
        SYSTEMTIME stTimeTmp;
        struct tm *ptmTmp;
        ptmTmp = localtime((time_t *)&(pstCommodity->m_uiTimeSaleBegin));

        if(NULL == ptmTmp)
        {
            TRACE("ptmTmp is NULL.\n");
        }
        else
        {
            stTimeTmp.wYear = ptmTmp->tm_year + 1900;
            stTimeTmp.wMonth = ptmTmp->tm_mon + 1;
            stTimeTmp.wDay = ptmTmp->tm_mday;

            m_LimitBeginTime.SetTime(&stTimeTmp);
            m_LimitBeginTime.SetFormat(L"yyyy/MM/dd");
            SYSTEMTIME   SysTime; 
            m_LimitBeginTime.SendMessage(DTM_GETSYSTEMTIME,   NULL,   (LPARAM)&SysTime); 
            m_LimitBeginTime.SendMessage(DTM_SETSYSTEMTIME,   GDT_VALID,   (LPARAM)&SysTime); 
        }
    }
    else
    {
        m_LimitBeginTime.SetFormat(_T(" "));
        SYSTEMTIME   SysTime; 
        m_LimitBeginTime.SendMessage(DTM_GETSYSTEMTIME,   NULL,   (LPARAM)&SysTime); 
        m_LimitBeginTime.SendMessage(DTM_SETSYSTEMTIME,   GDT_NONE,   (LPARAM)&SysTime); 
    }


    if (pstCommodity->m_uiTimeSaleEnd > 0)
    {
        TRACE("endtime = %u\n", pstCommodity->m_uiTimeSaleEnd);
        time_t tt;
        tt = time(NULL);
        TRACE("tt = %u\n", tt);
        SYSTEMTIME stTimeTmp;
        struct tm *ptmTmp;
        
        ptmTmp = localtime((time_t *)&(pstCommodity->m_uiTimeSaleEnd));
        if(NULL == ptmTmp)
        {
            TRACE("ptmTmp is NULL.\n");
        }
        else
        {
            stTimeTmp.wYear = ptmTmp->tm_year+1900;
            stTimeTmp.wMonth = ptmTmp->tm_mon+1;
            stTimeTmp.wDay = ptmTmp->tm_mday;
            m_LimitEndTime.SetTime(&stTimeTmp);
            m_LimitEndTime.SetFormat(L"yyyy/MM/dd");
            SYSTEMTIME   SysTime; 
            m_LimitEndTime.SendMessage(DTM_GETSYSTEMTIME,   NULL,   (LPARAM)&SysTime); 
            m_LimitEndTime.SendMessage(DTM_SETSYSTEMTIME,   GDT_VALID,   (LPARAM)&SysTime); 
        }
｝

奇葩的事情出现了，传给pstCommodity->m_uiSaleBeginTime 和pstCommodity->m_uiSaleEndTime的值是一样的，结果在BeginTime的循环中判断ptmTmp为NULL，而EndTime的循环中却能够正常的将时间给显示出来，后来查了好久才知道，原来是32位的指针地址赋给了64位的指针地址。回头看下结构体的定义就明白了(m_sSaleTag为0)。