GaudiKernel: Gaudi::Time Class Reference

Calendar time in nanoseconds since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).

Time is represented internally as UTC time, but it can also be converted to the local time as necessary. Most methods take an argument flag local to indicate which time interpretation is desired by the client, and automatically perform the necessary adjustments. The client can also find out about the difference between UTC time and local time using the utcoffset() method, and the time zone name with timezone() method. Both allow the client to discover whether daylight savings is in effect.

The native representation of Time is not well suited for human handling of time. Time provides access in more convenient terms such as year(), month() and day(); more are available through conversion into a TimeSpan. Time can also be converted to and from ISO C standard tm structure. Note however that unlike C's mktime() which always assumes tm in local time, Time fully supports all conversions between local and universal time. Thus it is possible for example to build() a UTC time directly from a tm.

Time behaves as an integral type. Differences in time values are represented as a TimeSpan. Usual integral arithmetic works with both types. Output works in general as any other integral type, however since the ValueType can be a wide type, it may be poorly supported by the iostream; if so, including the LongLong.h header will help. Note that the output value will usually be very large as Time is represented in nanoseconds, not seconds! When constructing Time values in seconds, such as when reading in, do remember to use the two-argument constructor taking seconds and nanoseconds instead of the default single-argument one.

Time can be formatted into a string using the format() method, which uses the versatile strftime() function. Since the latter works on seconds at best (through a struct tm), the subsecond part cannot be formatted; the nanoformat() method is provided to overcome this limitation. To combine format() and nanoformat() output use a suitable StringFormat pattern.

Time is linked to the system's concept of calendar time and is therefore may not be linear nor monotonic. System time can jump arbitrarily in either direction as real time clock is corrected or the system is suspended. The local time may also jump due to daylight savings. The process' ability to sample system time can be limited for reasons such as getting swapped out. TimeInfo provides an alternative time measurement facility not linked to calendar and guaranteed to grow monotonically -- though not always linearly. Note that few systems actually provide wall-clock time in nanosecond resolution. Not all system provide an interface to get time at that resolution, let alone track it so precisely.

Because of the time warp issues, scheduling events using Time is not straightforward. Application code should understand whether it is dealing with concrete or abstract calendar calculations, and how the events it schedules are linked to wall clock time.

For calculations on concrete calendar as perceived by people use local() after plain Time and TimeSpan integer arithmetic. The method accounts for timezone and daylight savings definitions. To schedule events use build() to derive times from local() time to get values comparable to the system time returned by current(). The applications should know whether events are scheduled in UTC or local time---"meeting at 9:00 on Wednesday morning" when the device switches timezones may be known to be at 9:00 in the new timezone (= locked to local time), or in the timezone where the event was created (= locked to UTC). The build() and split() methods allow either format to be used, the application just needs to know which one to use. It is also easy to convert between the two using utcoffset().

For calculations using an abstract calendar, without timezone or daylight savings, use Time in its native UTC representation and integer arithmetic with Time and TimeSpan. Do note however that "T + 24 hours" may not be the same hour the next day in the local calendar time -- timezone changes and daylight savings make a difference. This may require the application to accept as user input exception rules to its usual calendar calculations.

To schedule events, one should choose between three choices: UTC time, local time, or delta time. For the first two cases system time should be polled regularly to see if any of the recorded events have expired. It is not a good idea to sleep until the next scheduled event, as the system time may jump during the nap; instead sleep small increments, recheck the current time after each nap and trigger the events that have expired. A policy must be applied when the system time warps; this can happen both forwards and backwards with both local and UTC time (daylight savings or timezone changes for mobile devices are common local time change reasons, but the system time can be updated for any reason, e.g. when the real time clock is wrong, or if the system is suspended for a long time). Some events should be executed only once in case of time warps backwards. If the time jumps forwards, several events may need to be dealt with in one go. In either case the application should guard against major time changes: long system suspends, moving mobile devices and major time updates may result in a large number of "missed" events. One possibility is to provide a user-configurable "excessive time drift limit" (e.g. N hours): if time changes by more than that, missed events are not triggered.

For the final case of using delta times, sort upcoming events by their deltas from the previous event---not by the time they are anticipated to occur. Capture current time before and after the sleep and pull events off the queue based on the difference (the sleep time may exceed the requested time). Either guard against long time warps like suspends or schedule timer events cautiously. Using TimeInfo as schedule base solves such issues simply. To cope with backward system time jumps when using Time as schedule base, assume that sleeps always last at least the requested time; if the time delta over the nap is less than the requested, assume time warp (this is not foolproof against interrupted system calls but works for many event scheduling situations).

Member Typedef Documentation

typedef longlong Gaudi::Time::ValueType

Definition at line 219 of file Time.h.

Member Enumeration Documentation

enum Gaudi::Time::Months

Symbolic names for months.

Enumerator:

January
February
March
April
May
June
July
August
September
October
November
December

Definition at line 222 of file Time.h.

00222                 {
00223       January  = 0,
00224       February = 1,
00225       March  = 2,
00226       April  = 3,
00227       May  = 4,
00228       June  = 5,
00229       July  = 6,
00230       August  = 7,
00231       September = 8,
00232       October  = 9,
00233       November = 10,
00234       December = 11
00235     };

Constructor & Destructor Documentation

Gaudi::Time::Time ( void )

Gaudi::Time::Time ( TimeSpan ts )

Gaudi::Time::Time ( ValueType nsecs )

Gaudi::Time::Time	(	ValueType	secs,
		int	nsecs
	)

Gaudi::Time::Time	(	int	year,
		int	month,
		int	day,
		int	hour,
		int	min,
		int	sec,
		ValueType	nsecs,
		bool	local = `true`
	)

Member Function Documentation

static Time Gaudi::Time::epoch ( void ) [static]

Returns the minimum time.

static Time Gaudi::Time::max ( void ) [static]

Returns the maximum time.

static Time Gaudi::Time::current ( void ) [static]

Returns the current time.

static Time Gaudi::Time::build	(	bool	local,
		const tm &	base,
		TimeSpan	diff = `0`
	)			`[static]`

tm Gaudi::Time::split	(	bool	local,
		int *	nsecpart = `0`
	)			const

tm Gaudi::Time::utc ( int * nsecpart = 0 ) const

tm Gaudi::Time::local ( int * nsecpart = 0 ) const

int Gaudi::Time::year ( bool local ) const

int Gaudi::Time::month ( bool local ) const

int Gaudi::Time::day ( bool local ) const

int Gaudi::Time::hour ( bool local ) const

int Gaudi::Time::minute ( bool local ) const

int Gaudi::Time::second ( bool local ) const

int Gaudi::Time::nsecond ( void ) const

int Gaudi::Time::weekday ( bool local ) const

bool Gaudi::Time::isdst ( bool local ) const

ValueType Gaudi::Time::utcoffset ( int * daylight = 0 ) const

const char* Gaudi::Time::timezone ( int * daylight = 0 ) const

Time& Gaudi::Time::operator+= ( const TimeSpan & x )

Time& Gaudi::Time::operator-= ( const TimeSpan & x )

ValueType Gaudi::Time::ns ( void ) const

std::string Gaudi::Time::format	(	bool	local,
		const std::string &	spec
	)			const

std::string Gaudi::Time::nanoformat	(	int	minwidth = `1`,
		int	maxwidth = `9`
	)			const

static bool Gaudi::Time::isLeap ( int year ) [static]

static unsigned Gaudi::Time::toDosDate ( Time time ) [static]

static Time Gaudi::Time::fromDosDate ( unsigned dosDate ) [static]

void Gaudi::Time::TimeAssert	(	bool	cond,
		const std::string &	msg = `"time assertion failed"`
	)			const `[inline, private]`

Definition at line 301 of file Time.h.

00301                                                                                             {
00302       if (!cond) throw TimeException(msg);
00303     }

Friends And Related Function Documentation

friend class TimeSpan [friend]

Definition at line 217 of file Time.h.

Member Data Documentation

const int Gaudi::Time::SECS_PER_DAY = 86400 [static]

Seconds in 24 hours.

Definition at line 238 of file Time.h.

const int Gaudi::Time::SECS_PER_HOUR = 3600 [static]

Seconds in one hour hour.

Definition at line 241 of file Time.h.

const ValueType Gaudi::Time::SEC_NSECS = 1000000000 [static]

Nanoseconds in one second.

Definition at line 244 of file Time.h.

ValueType Gaudi::Time::m_nsecs [private]

Definition at line 299 of file Time.h.

In This Package:

Gaudi::Time Class Reference

Public Types

Public Member Functions

Static Public Member Functions

Static Public Attributes

Private Member Functions

Private Attributes

Friends