File Time Type

Motivation

The filesystem library provides interfaces for getting and setting the last write time of a file or directory. The interfaces use the file_time_type type, which is a specialization of chrono::time_point for the “filesystem clock”. According to [fs.filesystem.syn]

trivial-clock is an implementation-defined type that satisfies the Cpp17TrivialClock requirements ([time.clock.req]) and that is capable of representing and measuring file time values. Implementations should ensure that the resolution and range of file_time_type reflect the operating system dependent resolution and range of file time values.

On POSIX systems, file times are represented using the timespec struct, which is defined as follows:

struct timespec {
  time_t tv_sec;
  long   tv_nsec;
};

To represent the range and resolution of timespec, we need to (A) have nanosecond resolution, and (B) use more than 64 bits (assuming a 64 bit time_t).

As the standard requires us to use the chrono interface, we have to define our own filesystem clock which specifies the period and representation of the time points and duration it provides. It will look like this:

struct _FilesystemClock {
  using period = nano;
  using rep = TBD; // What is this?

  using duration = chrono::duration<rep, period>;
  using time_point = chrono::time_point<_FilesystemClock>;

  // ... //
};

using file_time_type = _FilesystemClock::time_point;

To get nanosecond resolution, we simply define period to be std::nano. But what type can we use as the arithmetic representation that is capable of representing the range of the timespec struct?

Problems To Consider

Before considering solutions, let’s consider the problems they should solve, and how important solving those problems are:

Having a Smaller Range than timespec

One solution to the range problem is to simply reduce the resolution of file_time_type to be less than that of nanoseconds. This is what libc++’s initial implementation of file_time_type did; it’s also what std::system_clock does. As a result, it can represent time points about 292 thousand years on either side of the epoch, as opposed to only 292 years at nanosecond resolution.

timespec can represent time points +/- 292 billion years from the epoch (just in case you needed a time point 200 billion years before the big bang, and with nanosecond resolution).

To get the same range, we would need to drop our resolution to that of seconds to come close to having the same range.

This begs the question, is the range problem “really a problem”? Sane usages of file time stamps shouldn’t exceed +/- 300 years, so should we care to support it?

I believe the answer is yes. We’re not designing the filesystem time API, we’re providing glorified C++ wrappers for it. If the underlying API supports a value, then we should too. Our wrappers should not place artificial restrictions on users that are not present in the underlying filesystem.

Having a smaller range that the underlying filesystem forces the implementation to report value_too_large errors when it encounters a time point that it can’t represent. This can cause the call to last_write_time to throw in cases where the user was confident the call should succeed. (See below)

#include <filesystem>
using namespace std::filesystem;

// Set the times using the system interface.
void set_file_times(const char* path, struct timespec ts) {
  timespec both_times[2];
  both_times[0] = ts;
  both_times[1] = ts;
  int result = ::utimensat(AT_FDCWD, path, both_times, 0);
  assert(result != -1);
}

// Called elsewhere to set the file time to something insane, and way
// out of the 300 year range we might expect.
void some_bad_persons_code() {
  struct timespec new_times;
  new_times.tv_sec = numeric_limits<time_t>::max();
  new_times.tv_nsec = 0;
  set_file_times("/tmp/foo", new_times); // OK, supported by most FSes
}

int main(int, char**) {
  path p = "/tmp/foo";
  file_status st = status(p);
  if (!exists(st) || !is_regular_file(st))
    return 1;
  if ((st.permissions() & perms::others_read) == perms::none)
    return 1;
  // It seems reasonable to assume this call should succeed.
  file_time_type tp = last_write_time(p); // BAD! Throws value_too_large.
  return 0;
}

Having a Smaller Resolution than timespec

As mentioned in the previous section, one way to solve the range problem is by reducing the resolution. But matching the range of timespec using a 64 bit representation requires limiting the resolution to seconds.

So we might ask: Do users “need” nanosecond precision? Is seconds not good enough? I limit my consideration of the point to this: Why was it not good enough for the underlying system interfaces? If it wasn’t good enough for them, then it isn’t good enough for us. Our job is to match the filesystems range and representation, not design it.

Having a Larger Range than timespec

We should also consider the opposite problem of having a file_time_type that is able to represent a larger range than timespec. At least in this case last_write_time can be used to get and set all possible values supported by the underlying filesystem; meaning last_write_time(p) will never throw an overflow error when retrieving a value.

However, this introduces a new problem, where users are allowed to attempt to create a time point beyond what the filesystem can represent. Two particular values which cause this are file_time_type::min() and file_time_type::max(). As a result, the following code would throw:

void test() {
  last_write_time("/tmp/foo", file_time_type::max()); // Throws
  last_write_time("/tmp/foo", file_time_type::min()); // Throws.
}

Apart from cases explicitly using min and max, I don’t see users taking a valid time point, adding a couple hundred billions of years in error, and then trying to update a file’s write time to that value very often.

Compared to having a smaller range, this problem seems preferable. At least now we can represent any time point the filesystem can, so users won’t be forced to revert back to system interfaces to avoid limitations in the C++ STL.

I posit that we should only consider this concern after we have something with at least the same range and resolution of the underlying filesystem. The latter two problems are much more important to solve.

Potential Solutions And Their Complications

Source Code Portability Across Implementations

As we’ve discussed, file_time_type needs a representation that uses more than 64 bits. The possible solutions include using __int128_t, emulating a 128 bit integer using a class, or potentially defining a timespec like arithmetic type. All three will allow us to, at minimum, match the range and resolution, and the last one might even allow us to match them exactly.

But when considering these potential solutions we need to consider more than just the values they can represent. We need to consider the effects they will have on users and their code. For example, each of them breaks the following code in some way:

// Bug caused by an unexpected 'rep' type returned by count.
void print_time(path p) {
  // __int128_t doesn't have streaming operators, and neither would our
  // custom arithmetic types.
  cout << last_write_time(p).time_since_epoch().count() << endl;
}

// Overflow during creation bug.
file_time_type timespec_to_file_time_type(struct timespec ts) {
  // woops! chrono::seconds and chrono::nanoseconds use a 64 bit representation
  // this may overflow before it's converted to a file_time_type.
  auto dur = seconds(ts.tv_sec) + nanoseconds(ts.tv_nsec);
  return file_time_type(dur);
}

file_time_type correct_timespec_to_file_time_type(struct timespec ts) {
  // This is the correct version of the above example, where we
  // avoid using the chrono typedefs as they're not sufficient.
  // Can we expect users to avoid this bug?
  using fs_seconds = chrono::duration<file_time_type::rep>;
  using fs_nanoseconds = chrono::duration<file_time_type::rep, nano>;
  auto dur = fs_seconds(ts.tv_sec) + fs_nanoseconds(tv.tv_nsec);
  return file_time_type(dur);
}

// Implicit truncation during conversion bug.
intmax_t get_time_in_seconds(path p) {
  using fs_seconds = duration<file_time_type::rep, ratio<1, 1> >;
  auto tp = last_write_time(p);

  // This works with truncation for __int128_t, but what does it do for
  // our custom arithmetic types.
  return duration_cast<fs_seconds>().count();
}

Each of the above examples would require a user to adjust their filesystem code to the particular eccentricities of the representation, hopefully only in such a way that the code is still portable across implementations.

At least some of the above issues are unavoidable, no matter what representation we choose. But some representations may be quirkier than others, and, as I’ll argue later, using an actual arithmetic type (__int128_t) provides the least aberrant behavior.

Chrono and timespec Emulation.

One of the options we’ve considered is using something akin to timespec to represent the file_time_type. It only seems natural seeing as that’s what the underlying system uses, and because it might allow us to match the range and resolution exactly. But would it work with chrono? And could it still act at all like a timespec struct?

For ease of consideration, let’s consider what the implementation might look like.

struct fs_timespec_rep {
  fs_timespec_rep(long long v)
    : tv_sec(v / nano::den), tv_nsec(v % nano::den)
  { }
private:
  time_t tv_sec;
  long tv_nsec;
};
bool operator==(fs_timespec_rep, fs_timespec_rep);
fs_int128_rep operator+(fs_timespec_rep, fs_timespec_rep);
// ... arithmetic operators ... //

The first thing to notice is that we can’t construct fs_timespec_rep like a timespec by passing {secs, nsecs}. Instead we’re limited to constructing it from a single 64 bit integer.

We also can’t allow the user to inspect the tv_sec or tv_nsec values directly. A chrono::duration represents its value as a tick period and a number of ticks stored using rep. The representation is unaware of the tick period it is being used to represent, but timespec is setup to assume a nanosecond tick period; which is the only case where the names tv_sec and tv_nsec match the values they store.

When we convert a nanosecond duration to seconds, fs_timespec_rep will use tv_sec to represent the number of giga seconds, and tv_nsec the remaining seconds. Let’s consider how this might cause a bug were users allowed to manipulate the fields directly.

template <class Period>
timespec convert_to_timespec(duration<fs_time_rep, Period> dur) {
  fs_timespec_rep rep = dur.count();
  return {rep.tv_sec, rep.tv_nsec}; // Oops! Period may not be nanoseconds.
}

template <class Duration>
Duration convert_to_duration(timespec ts) {
  Duration dur({ts.tv_sec, ts.tv_nsec}); // Oops! Period may not be nanoseconds.
  return file_time_type(dur);
  file_time_type tp = last_write_time(p);
  auto dur =
}

time_t extract_seconds(file_time_type tp) {
  // Converting to seconds is a silly bug, but I could see it happening.
  using SecsT = chrono::duration<file_time_type::rep, ratio<1, 1>>;
  auto secs = duration_cast<Secs>(tp.time_since_epoch());
  // tv_sec is now representing gigaseconds.
  return secs.count().tv_sec; // Oops!
}

Despite fs_timespec_rep not being usable in any manner resembling timespec, it still might buy us our goal of matching its range exactly, right?

Sort of. Chrono provides a specialization point which specifies the minimum and maximum values for a custom representation. It looks like this:

template <>
struct duration_values<fs_timespec_rep> {
  static fs_timespec_rep zero();
  static fs_timespec_rep min();
  static fs_timespec_rep max() { // assume friendship.
    fs_timespec_rep val;
    val.tv_sec = numeric_limits<time_t>::max();
    val.tv_nsec = nano::den - 1;
    return val;
  }
};

Notice that duration_values doesn’t tell the representation what tick period it’s actually representing. This would indeed correctly limit the range of duration<fs_timespec_rep, nano> to exactly that of timespec. But nanoseconds isn’t the only tick period it will be used to represent. For example:

void test() {
  using rep = file_time_type::rep;
  using fs_nsec = duration<rep, nano>;
  using fs_sec = duration<rep>;
  fs_nsec nsecs(fs_seconds::max()); // Truncates
}

Though the above example may appear silly, I think it follows from the incorrect notion that using a timespec rep in chrono actually makes it act as if it were an actual timespec.

Interactions with 32 bit time_t

Up until now we’ve only be considering cases where time_t is 64 bits, but what about 32 bit systems/builds where time_t is 32 bits? (this is the common case for 32 bit builds).

When time_t is 32 bits, we can implement file_time_type simply using 64-bit long long. There is no need to get either __int128_t or timespec emulation involved. And nor should we, as it would suffer from the numerous complications described by this paper.

Obviously our implementation for 32-bit builds should act as similarly to the 64-bit build as possible. Code which compiles in one, should compile in the other. This consideration is important when choosing between __int128_t and emulating timespec. The solution which provides the most uniformity with the least eccentricity is the preferable one.

Summary

The file_time_type time point is used to represent the write times for files. Its job is to act as part of a C++ wrapper for less ideal system interfaces. The underlying filesystem uses the timespec struct for the same purpose.

However, the initial implementation of file_time_type could not represent either the range or resolution of timespec, making it unsuitable. Fixing this requires an implementation which uses more than 64 bits to store the time point.

We primarily considered two solutions: Using __int128_t and using a arithmetic emulation of timespec. Each has its pros and cons, and both come with more than one complication.

The Potential Solutions

long long - The Status Quo

Pros:

  • As a type long long plays the nicest with others:

    • It works with streaming operators and other library entities which support builtin integer types, but don’t support __int128_t.

    • Its the representation used by chrono’s nanosecond and second typedefs.

Cons:

  • It cannot provide the same resolution as timespec unless we limit it to a range of +/- 300 years from the epoch.

  • It cannot provide the same range as timespec unless we limit its resolution to seconds.

  • last_write_time has to report an error when the time reported by the filesystem is unrepresentable.

__int128_t

Pros:

  • It is an integer type.

  • It makes the implementation simple and efficient.

  • Acts exactly like other arithmetic types.

  • Can be implicitly converted to a builtin integer type by the user.

    • This is important for doing things like:

      void c_interface_using_time_t(const char* p, time_t);

void foo(path p) {
  file_time_type tp = last_write_time(p);
  time_t secs = duration_cast<seconds>(tp.time_since_epoch()).count();
  c_interface_using_time_t(p.c_str(), secs);
}

Cons:

  • It isn’t always available (but on 64 bit machines, it normally is).

  • It causes file_time_type to have a larger range than timespec.

  • It doesn’t always act the same as other builtin integer types. For example with cout or to_string.

  • Allows implicit truncation to 64 bit integers.

  • It can be implicitly converted to a builtin integer type by the user, truncating its value.

Arithmetic timespec Emulation

Pros:

  • It has the exact same range and resolution of timespec when representing a nanosecond tick period.

  • It’s always available, unlike __int128_t.

Cons:

  • It has a larger range when representing any period longer than a nanosecond.

  • Doesn’t actually allow users to use it like a timespec.

  • The required representation of using tv_sec to store the giga tick count and tv_nsec to store the remainder adds nothing over a 128 bit integer, but complicates a lot.

  • It isn’t a builtin integer type, and can’t be used anything like one.

  • Chrono can be made to work with it, but not nicely.

  • Emulating arithmetic classes come with their own host of problems regarding overload resolution (Each operator needs three SFINAE constrained versions of it in order to act like builtin integer types).

  • It offers little over simply using __int128_t.

  • It acts the most differently than implementations using an actual integer type, which has a high chance of breaking source compatibility.

Selected Solution - Using __int128_t

The solution I selected for libc++ is using __int128_t when available, and otherwise falling back to using long long with nanosecond precision.

When __int128_t is available, or when time_t is 32-bits, the implementation provides same resolution and a greater range than timespec. Otherwise it still provides the same resolution, but is limited to a range of +/- 300 years. This final case should be rather rare, as __int128_t is normally available in 64-bit builds, and time_t is normally 32-bits during 32-bit builds.

Although falling back to long long and nanosecond precision is less than ideal, it also happens to be the implementation provided by both libstdc++ and MSVC. (So that makes it better, right?)

Although the timespec emulation solution is feasible and would largely do what we want, it comes with too many complications, potential problems and discrepancies when compared to “normal” chrono time points and durations.

An emulation of a builtin arithmetic type using a class is never going to act exactly the same, and the difference will be felt by users. It’s not reasonable to expect them to tolerate and work around these differences. And once we commit to an ABI it will be too late to change. Committing to this seems risky.

Therefore, __int128_t seems like the better solution.