SoftBank Robotics documentation What's new in NAOqi 2.8?

qi clocks

Libqi provides types to model clocks, time points and durations. The implementation is based on boost::chrono. You can thus refer to boost::chrono documentation or to your favourite C++11 book for a detailed description. Here is a short introduction to the concepts though.

Concepts

Duration
A span of time, defined as some number of ticks of some time unit. libqi defines durations types with various units.
Clocks
A clock consists of a starting point (or epoch) and a tick rate. libqi defines several clocks, see Clocks.
Time point
A reference to a specific point in time defined as the duration of time that has passed since the epoch of a specific clock.

Warning

While a default-constructed time point has a well-defined value (equal to its clock epoch), a default-constructed duration is not required to be zero-initialized.

Clock properties

libqi exposes several clocks, each of which having a distinctive set of properties.

Possible properties are:

monotonicity

A clock is monotonic if the time points of this clock cannot decrease as physical time moves forward.

../../../../../_images/clock_monotonic_or_not.png
steadiness

A clock is steady if the physical time between its ticks is constant. This property implies monotonicity.

../../../../../_images/clock_steady_or_not.png
system-wideness
A clock C may be system-wide. In such a case, all programs on the system calling C::now() at the same time should get the same value.
relation to “real time wall clock”
A clock may represent “real wall clock” time. In such a case its time points are convertible to calendar time. Such a clock is usually not steady nor monotonic in order to enable the adjustment of the clock.

Clocks

Libqi provides three clocks:

qi::SteadyClock
monotonic and steady clock, but not system-wide. Its epoch is unspecified, but typically related only to the program invocation (eg. the program starting time). Similar to std::chrono::steady_clock. This clock is best suitable for measuring intervals.
qi::Clock
system-wide clock, best suitable for timestamps. Typically monotonic and unaffected by the system clock adjustment, altough this is not guaranteed.
qi::SystemClock
system-wide real time wall clock. Similar to std::chrono::system_clock. This clock is best used when calendar time is needed. It may also be useful in specific cases for inter-system communication, assuming the systems involved do synchronize their system clocks.

Example

Let’s use libqi to time some long computation and display it duration in milliseconds.

#include <qi/clock.hpp>
#include <iostream>

void long_computation() {
// ...
}

int main() {
  qi::SteadyClock::time_point start = qi::SteadyClock::now();
  long_computation();
  qi::SteadyClock::time_point end = qi::SteadyClock::now();
  qi::MilliSeconds ms = boost::chrono::duration_cast<qi::MilliSeconds>(end - start);
  std::cout << "spent " << qi::to_string(ms) << std::endl;

  if (ms > qi::Seconds(10)) {
    std::cout << "This is more than 10 seconds." << std::endl;
  }

  return 0;
}

Or a slightly shorter version

#include <qi/clock.hpp>
#include <iostream>

void long_computation() {
// ...
}

int main() {
  qi::SteadyClock::time_point start = qi::SteadyClock::now();
  long_computation();
  std::cout << "spent " << qi::to_string(qi::durationSince<qi::MilliSeconds>(start)) << std::endl;
  return 0;
}

Rationale

libqi heavily uses boost::chrono but does not directly expose the boost::chrono clocks. There are several reasons for this choice:

  • in order to be usable through the middleware, the concrete duration and time point types need to be defined in a platform-independent way. This is not the case of boost::chrono clocks types.
  • we could add some time control API for simulation purposes.
  • boost::chrono::steady_clock is defined to be system-wide, but C++11’s std::chrono::steady_clock does not make this promise. By default, we prefer to stick to the standard over boost.
  • we need a monotonic system-wide clock. Since std::chrono::steady_clock is not system-wide, then we need to add a third clock (qi::Clock) anyway.

Summary

Global Namespaces

Global Members

Detailed Description

Reference

typedef qi::NanoSeconds Duration
typedef DurationType< int64_t, boost::nano > NanoSeconds
typedef DurationType< int64_t, boost::micro > MicroSeconds
typedef DurationType< int64_t, boost::milli > MilliSeconds
typedef DurationType< int64_t, boost::ratio< 1 >> Seconds
typedef DurationType< int64_t, boost::ratio< 60 >> Minutes
typedef DurationType< int64_t, boost::ratio< 3600 >> Hours

qi::SteadyClock Class Reference

Introduction

The SteadyClock class represents a monotonic clock. . More...

#include <qi/clock.hpp>

Enumerations

enum Expect

Brief: Enum expected argument.

Name Brief
Expect_SoonerOrLater Pick the nearest result to user-provided reference.
Expect_Later Result is expected to be later than user-provided reference.
Expect_Sooner Result is expected to be sooner than user-provided reference.

Public Members

typedef time_point
bool is_steady

Public Static Functions

time_point now()

Types

typedef Duration::rep rep
The representation type of the duration and time_point.
typedef Duration::period period
The tick period of the clock in nanoseconds.
typedef qi::Duration duration
The duration type of the clock.
typedef TimePoint< SteadyClock > time_point
The time_point type of the clock. Different clocks are permitted to share a time_point definition if it is valid to compare their time_points by comparing their respective durations.

Detailed Description

The time points of this clock cannot decrease as physical time moves forward. This clock is not related to wall clock time, and is best suitable for measuring intervals.

Members Documentation

typedef time_point
bool qi::SteadyClock::is_steady

true if t1 <= t2 is always true, else false. A clock that can be adjusted backwards is not steady.

Function Documentation

static time_point qi::SteadyClock::now()

Returns a time_point representing the current value of the clock.

qi::Clock Class Reference

Introduction

The Clock class represents a system-wide clock, best suitable for timestamping events. Typically monotonic and unaffected by the system clock adjustment, altough this is not guaranteed. More...

#include <qi/clock.hpp>

Enumerations

enum Expect

Brief: Enum expected argument.

Name Brief
Expect_SoonerOrLater Pick the nearest result to user-provided reference.
Expect_Later Result is expected to be later than user-provided reference.
Expect_Sooner Result is expected to be sooner than user-provided reference.

Public Members

bool is_steady

Public Static Functions

time_point now()
uint32_t toUint32ms(const time_point& t)
int32_t toInt32ms(const time_point& t)
time_point fromUint32ms(uint32_t t_ms, time_point guess, Expect expect)
time_point fromInt32ms(int32_t t_ms, time_point guess, Expect expect)

Types

typedef Duration::rep rep
The representation type of the duration and time_point.
typedef Duration::period period
The tick period of the clock in nanoseconds.
typedef qi::Duration duration
The duration type of the clock.
typedef boost::chrono::time_point< Clock > time_point
The time_point type of the clock. Different clocks are permitted to share a time_point definition if it is valid to compare their time_points by comparing their respective durations.

Detailed Description

Members Documentation

bool qi::Clock::is_steady

true if t1 <= t2 is always true, else false. A clock that can be adjusted backwards is not steady.

Function Documentation

static time_point qi::Clock::now()

Returns a time_point representing the current value of the clock.

static uint32_t qi::Clock::toUint32ms(const time_point& t)

Brief: Convert the time point to a number of milliseconds on 32 bits.

Parameters:
  • t – The time point to convert.
Returns:

Unsigned int representing the time.

Since the 32 bits number overflows every 2^32 ms ~ 50 days, this is a lossy operation.

static int32_t qi::Clock::toInt32ms(const time_point& t)

Brief: Convert the time point to a number of milliseconds on 32 bits.

Parameters:
  • t – The time point to convert.
Returns:

Integer (int) representing the time.

Since the 32 bits number overflows every 2^32 ms ~ 50 days, this is a lossy operation.

static time_point qi::Clock::fromUint32ms(uint32_t t_ms, time_point guess, Expect expect = Expect_SoonerOrLater)

Since the 32 bits number overflows every ~50 days, an infinity of time points match a given 32 bits number (all modulo ~50 days). This function picks the result near the guess timepoint depending on the expect argument:

if expect == LATER, result is expected to be later than guess: guess <= result < guess + periodif expect == SOONER, result is expected to be sooner than guess: guess - period < result <= guessif expect == SOONER_OR_LATER, pick the nearest result: guess - period/2 < result <= guess + period/2

where period == 2^32 ms ~ 50 days

static time_point qi::Clock::fromInt32ms(int32_t t_ms, time_point guess, Expect expect = Expect_SoonerOrLater)

Since the 32 bits number overflows every ~50 days, an infinity of time points match a given 32 bits number (all modulo ~50 days). This function picks the result near the guess timepoint depending on the expect argument:

if expect == LATER, result is expected to be later than guess: guess <= result < guess + periodif expect == SOONER, result is expected to be sooner than guess: guess - period < result <= guessif expect == SOONER_OR_LATER, pick the nearest result: guess - period/2 < result <= guess + period/2

where period == 2^32 ms ~ 50 days

qi::SystemClock Class Reference

Introduction

The SystemClock class represents the system-wide real time wall clock. It may not be monotonic: on most systems, the system time can be adjusted at any moment. More...

#include <qi/clock.hpp>

Public Members

typedef time_point
bool is_steady

Public Static Functions

time_point now()
std::time_t to_time_t(const time_point& t)
time_point from_time_t(const std::time_t& t)

Types

typedef Duration::rep rep
The representation type of the duration and time_point.
typedef Duration::period period
The tick period of the clock in nanoseconds.
typedef qi::Duration duration
The duration type of the clock.
typedef boost::chrono::time_point< SystemClock > time_point
The time_point type of the clock. Different clocks are permitted to share a time_point definition if it is valid to compare their time_points by comparing their respective durations.

Detailed Description

Members Documentation

typedef time_point
bool qi::SystemClock::is_steady

true if t1 <= t2 is always true, else false. A SystemClock is never steady.

Function Documentation

static time_point qi::SystemClock::now()

Returns a time_point representing the current value of the clock.

static std::time_t qi::SystemClock::to_time_t(const time_point& t)

Brief: Converts a system clock time point to std::time_t.

Parameters:
  • t – Time point to convert.
Returns:

A std::time_t representing t.

static time_point qi::SystemClock::from_time_t(const std::time_t& t)

Brief: Converts std::time_t to a system clock time point.

Parameters:
  • t – std::time to convert.
Returns:

A time point representing t.

typedef qi::SteadyClock::time_point SteadyClockTimePoint
Steady clock time point.
typedef qi::Clock::time_point ClockTimePoint
qi::Clock time point.
typedef qi::SystemClock::time_point SystemClockTimePoint
System clock time point.

Functions

void qi::sleepFor(const qi::Duration& d)

Blocks the execution of the current thread for at least d.

void qi::sleepUntil(const SteadyClockTimePoint& t)

This is equivalent to sleepFor(t - SteadyClockTimePoint::now())

void qi::sleepUntil(const ClockTimePoint& t)

Blocks the execution of the current thread until t has been reached.

void qi::sleepUntil(const SystemClockTimePoint& t)

Adjustments of the clock are taken into account. Thus the duration of the block might, but might not, be less or more than t - SystemClock::now()

std::string qi::toISO8601String(const SystemClockTimePoint& t)

For instance the string for a quarter past nine PM on April 3rd, 2001 is “2001-04-03T211500.000Z”

template<class DurationTo, class TimePointFrom>
DurationTo qi::durationSince(const TimePointFrom& t)

Returns the duration elapsed since t.