c++ - 如何导致 std::mktime 失败？

Question

我在程序中使用 std::mktime 设置/获取时间。手册页说明:

The values in time are permitted to be outside their normal ranges.

如果它不能将结果表示为 std::time_t，则返回 -1

我在单元测试中尝试了一切方法来使这个函数失败，但没有成功。 These 只是一些尝试:

#include <iostream>
#include <iomanip>
#include <ctime>
#include <stdlib.h>

int main()
{
    setenv("TZ", "/usr/share/zoneinfo/America/New_York", 1); // POSIX-specific

    std::time_t t = std::time(nullptr);
    std::tm tm = *std::localtime(&t);
    std::cout << "Today is           " << std::put_time(&tm, "%c %Z")
              << " and DST is " << (tm.tm_isdst ? "in effect" : "not in effect") << '\n';
    tm.tm_year = 550;
    tm.tm_year = 123456789;
    tm.tm_mon = 88;
    tm.tm_mday = 200;
    //tm.tm_mon -= 100;  // tm_mon is now outside its normal range
    std::mktime(&tm);  // tm_dst is not set to -1; today's DST status is used
    std::cout << "100 months ago was " << std::put_time(&tm, "%c %Z")
              << " and DST was " << (tm.tm_isdst ? "in effect" : "not in effect") << '\n';
}

那么，如何设置参数才能让这个函数失败呢？

---

正如评论中所建议的， I tried INT_MAX 仍然没有失败。所以，这个:

tm.tm_year = std::numeric_limits< decltype( tm.tm_year ) >::max();

    if ( -1 == std::mktime(&tm) )
        std::cout << "std::mktime() failed)" << '\n'; 

仍然不会让它失败。

---

$CXX --version
aarch64-pdm3-linux-g++ (GCC) 6.4.0

Answer 1

您的示例对于建议的输入 std::numeric_limits< decltype( tm.tm_year ) >::max() 确实失败。您的示例不会检查 -1 返回值。如果添加

auto err = std::mktime(&tm);  // tm_dst is not set to -1, err is.

err对于此输入设置为 -1。

您可以在https://ideone.com/GUcM3N处看到失败

您还可以在http://coliru.stacked-crooked.com/a/8288579ec8924d7e处看到失败

两个服务的输出相同:

Today is           Thu Apr 19 09:07:40 2018 EDT and DST is in effect
100 months ago was Thu ? 200 09:07:40 -2147481749 EDT and DST was in effect
-1