Why std::chrono::time_point is not large enough to store struct timespec?

问题

I'm trying the recent std::chrono api and I found that on 64 bit Linux architecture and gcc compiler the time_point and duration classes are not able to handle the maximum time range of the operating system at the maximum resolution (nanoseconds). In fact it seems the storage for these classes is a 64bit integral type, compared to timespec and timeval which are internally using two 64 bit integers, one for seconds and one for nanoseconds:

#include <iostream>
#include <chrono>
#include <typeinfo>
#include <time.h>

using namespace std;
using namespace std::chrono;

int main()
{
    cout << sizeof(time_point<nanoseconds>) << endl;                       // 8
    cout << sizeof(time_point<nanoseconds>::duration) << endl;             // 8
    cout << sizeof(time_point<nanoseconds>::duration::rep) << endl;        // 8
    cout << typeid(time_point<nanoseconds>::duration::rep).name() << endl; // l
    cout << sizeof(struct timespec) << endl;                               // 16
    cout << sizeof(struct timeval) << endl;                                // 16
    return 0;
}

On 64 bit Windows (MSVC2017) the situation is very similar: the storage type is also a 64 bit integer. This is not a problem when dealing with steady (aka monotonic) clocks, but storage limitations make the the different API implementations not suitable to store bigger dates and wider time spans, creating the ground for Y2K-like bugs. Is the problem acknowledged? Are there plans for better implementations or API improvements?

回答1:

This was done so that you get maximum flexibility along with compact size. If you need ultra-fine precision, you usually don't need a very large range. And if you need a very large range, you usually don't need very high precision.

For example, if you're trafficking in nanoseconds, do you regularly need to think about more than +/- 292 years? And if you need to think about a range greater than that, well microseconds gives you +/- 292 thousand years.

The macOS system_clock actually returns microseconds, not nanoseconds. So that clock can run for 292 thousand years from 1970 until it overflows.

The Windows system_clock has a precision of 100-ns units, and so has a range of +/- 29.2 thousand years.

If a couple hundred thousand years is still not enough, try out milliseconds. Now you're up to a range of +/- 292 million years.

Finally, if you just have to have nanosecond precision out for more than a couple hundred years, <chrono> allows you to customize the storage too:

using dnano = duration<double, nano>;

This gives you nanoseconds stored as a double. If your platform supports a 128 bit integral type, you can use that too:

using big_nano = duration<__int128_t, nano>;

Heck, if you write overloaded operators for timespec, you can even use that for the storage (I don't recommend it though).

You can also achieve precisions finer than nanoseconds, but you'll sacrifice range in doing so. For example:

using picoseconds = duration<int64_t, pico>;

This has a range of only +/- .292 years (a few months). So you do have to be careful with that. Great for timing things though if you have a source clock that gives you sub-nanosecond precision.

Check out this video for more information on <chrono>.

For creating, manipulating and storing dates with a range greater than the validity of the current Gregorian calendar, I've created this open-source date library which extends the <chrono> library with calendrical services. This library stores the year in a signed 16 bit integer, and so has a range of +/- 32K years. It can be used like this:

#include "date.h"

int
main()
{
    using namespace std::chrono;
    using namespace date;
    system_clock::time_point now = sys_days{may/30/2017} + 19h + 40min + 10s;
}

Update

In the comments below the question is asked how to "normalize" duration<int32_t, nano> into seconds and nanoseconds (and then add the seconds to a time_point).

First, I would be wary of stuffing nanoseconds into 32 bits. The range is just a little over +/- 2 seconds. But here's how I separate out units like this:

    using ns = duration<int32_t, nano>;
    auto n = ns::max();
    auto s = duration_cast<seconds>(n);
    n -= s;

Note that this only works if n is positive. To correctly handle negative n, the best thing to do is:

    auto n = ns::max();
    auto s = floor<seconds>(n);
    n -= s;

std::floor is introduced with C++17. If you want it earlier, you can grab it from here or here.

I'm partial to the subtraction operation above, as I just find it more readable. But this also works (if n is not negative):

    auto s = duration_cast<seconds>(n);
    n %= 1s;

The 1s is introduced in C++14. In C++11, you will have to use seconds{1} instead.

Once you have seconds (s), you can add that to your time_point.

回答2:

std::chrono::nanoseconds is a type alias for std::chrono::duration<some_t, std::nano> where some_t is a signed int with an storage of at least 64 bits. This still allows for at least 292 years of range with nanosecond precision.

Notably the only integral types with such characteristics mentioned by the standard are the int(|_fast|_least)64_t family.

You are free to choose a wider type to represent your times, if your implementation provides one. You are further free to provide a namespace with a bunch of typedef's that mirror the std::chrono ratios, with your wider type as the representation.

来源：https://stackoverflow.com/questions/44269533/why-stdchronotime-point-is-not-large-enough-to-store-struct-timespec