CODEWITHSUNDEEP
c++datedatetimelocaltimectime
ChiefHagno
ChiefHagno

Reputation: 178

C++ - 'localtime' this function or variable may be unsafe

I am writing a simple logging class in C++ for learning purposes. My code contains a function that returns a string of today's date. However, I get a compiler error whenever 'localtime' is called.

std::string get_date_string(time_t *time) {
    struct tm *now = localtime(time);
    std::string date = std::to_string(now->tm_mday) + std::to_string(now->tm_mon) + std::to_string(now->tm_year);
    return date;
}

I have tried using #define _CRT_SECURE_NO_WARNINGS. It didn't work and the same error appeared. I also tried putting _CRT_SECURE_NO_WARNINGS inside the preprocessor definitions in the project properties. This gave an unresolved external error.

Does anyone have any ideas on what to do?

Upvotes: 16

Views: 34248

Answers (3)

why stuff works
why stuff works

Reputation: 61

updated answer with C++20 chrono library

    const auto now            = std::chrono::system_clock::now();
    const auto time_zone      = std::chrono::current_zone();
    const auto local_time     = time_zone->to_local(now);
    const auto time_point     = std::chrono::time_point_cast<std::chrono::days>(local_time);
    const auto year_month_day = std::chrono::year_month_day{ time_point };

    int year  = static_cast<int>(year_month_day.year());
    int month = static_cast<unsigned>(year_month_day.month());
    int day   = static_cast<unsigned>(year_month_day.day());

Upvotes: 4

Galik
Galik

Reputation: 48605

The problem is that std::localtime is not thread-safe because it uses a static buffer (shared between threads). Both POSIX and Windows have safe alternatives: localtime_r and localtime_s.

Here is what I do:

inline std::tm localtime_xp(std::time_t timer)
{
    std::tm bt {};
#if defined(__unix__)
    localtime_r(&timer, &bt);
#elif defined(_MSC_VER)
    localtime_s(&bt, &timer);
#else
    static std::mutex mtx;
    std::lock_guard<std::mutex> lock(mtx);
    bt = *std::localtime(&timer);
#endif
    return bt;
}

// default = "YYYY-MM-DD HH:MM:SS"
inline std::string time_stamp(const std::string& fmt = "%F %T")
{
    auto bt = localtime_xp(std::time(0));
    char buf[64];
    return {buf, std::strftime(buf, sizeof(buf), fmt.c_str(), &bt)};
}

Upvotes: 67

Null
Null

Reputation: 703

Try to #define _CRT_SECURE_NO_WARNINGS before #include any other header files, like the following code

#define _CRT_SECURE_NO_WARNINGS
#include <ctime>
//your code

Upvotes: -4

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