c++ - 一个 tm 结构中的错误破坏了其他 tm 结构

Question

我偶然发现了这种行为，想知道这是否符合预期(我觉得不对)。

我在一个特定的 tm 结构中强加了一个错误，所有其他的都被破坏了。

这是代码(精简到最低限度以重现问题)

int main()
{
    cout << "-----   Bug test - tm struc   -----" << endl;
    //--------------------------------------------

    //--- Setup struct tm ---
    time_t timet_Now = time(NULL);
    struct tm* tm1 = localtime(&timet_Now);
    struct tm* tm2 = localtime(&timet_Now);

    //--- Verify OK  -  cout shows "28/10/2016"---
    cout << tm1->tm_mday << " " << tm1->tm_mon << " " << tm1->tm_year << endl;
    cout << tm2->tm_mday << " " << tm2->tm_mon << " " << tm2->tm_year << endl;

    // ... so far, so good 

    // --- Force an error in a different tm struct (xxtm)
    time_t xtimet = 1464778020000;
    struct tm* xxtm = localtime(&xtimet);  //<<< xxtm = null - no runtime error

    //--- tm1 and tm2 are now corrupted - cout shows "-1/-1/-1"
    cout << tm1->tm_mday << " " << tm1->tm_mon << " " << tm1->tm_year << endl;
    cout << tm2->tm_mday << " " << tm2->tm_mon << " " << tm2->tm_year << endl;

    //--- This next line crashes the application, as tm1 is corrupted
    char* c = asctime(tm1);

    return 0;
}

崩溃错误是:MyTest.exe 中 0x0FA520B5 (ucrtbased.dll) 处未处理的异常:将无效参数传递给认为无效参数致命的函数。

Answer 1

引用 http://en.cppreference.com/w/cpp/chrono/c/localtime

Return value

pointer to a static internal std::tm object on success, or NULL otherwise. The structure may be shared between std::gmtime, std::localtime, and std::ctime, and may be overwritten on each invocation.

换句话说，您所有的 struct tm * 最终都指向完全相同的位置。你可能想要

struct tm tm1 = *localtime(&timet_Now);

如果您要保留它一段时间，请复制一份以供使用。

Kenny Ostrom 在评论中提出了一个很好的观点。我没有处理 NULL 返回情况和复制 NULL...这不是一个好主意。

struct tm * temp = localtime(&timet_Now);
if (temp == nullptr)
{
    // handle error. Throw exception, return, whatever, just don't run the next line 
}
struct tm tm1 = *temp;