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

Question

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?

Answer 1

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)};
}