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9.6 Delay Statements, Duration, and Time

A delay_statement is used to block further execution until a specified expiration time is reached. The expiration time can be specified either as a particular point in time (in a delay_until_statement), or in seconds from the current time (in a delay_relative_statement). The language-defined package Calendar provides definitions for a type Time and associated operations, including a function Clock that returns the current time.


delay_statement ::= delay_until_statement | delay_relative_statement
delay_until_statement ::= delay until delay_expression;
delay_relative_statement ::= delay delay_expression;

Name Resolution Rules

The expected type for the delay_expression in a delay_relative_statement is the predefined type Duration. The delay_expression in a delay_until_statement is expected to be of any nonlimited type.

Legality Rules

There can be multiple time bases, each with a corresponding clock, and a corresponding time type. The type of the delay_expression in a delay_until_statement shall be a time type — either the type Time defined in the language-defined package Calendar (see below), the type Time in the package Real_Time (see D.8), or some other implementation-defined time type. 

Static Semantics

There is a predefined fixed point type named Duration, declared in the visible part of package Standard; a value of type Duration is used to represent the length of an interval of time, expressed in seconds. The type Duration is not specific to a particular time base, but can be used with any time base.
A value of the type Time in package Calendar, or of some other time type, represents a time as reported by a corresponding clock.
The following language-defined library package exists: 

package Ada.Calendar is
  type Time is private;
  subtype Year_Number  is Integer range 1901 .. 2399;
  subtype Month_Number is Integer range 1 .. 12;
  subtype Day_Number   is Integer range 1 .. 31;
  subtype Day_Duration is Duration range 0.0 .. 86_400.0;
  function Clock return Time;
  function Year   (Date : Time) return Year_Number;
  function Month  (Date : Time) return Month_Number;
  function Day    (Date : Time) return Day_Number;
  function Seconds(Date : Time) return Day_Duration;
  procedure Split (Date  : in Time;
                   Year    : out Year_Number;
                   Month   : out Month_Number;
                   Day     : out Day_Number;
                   Seconds : out Day_Duration);
  function Time_Of(Year  : Year_Number;
                   Month   : Month_Number;
                   Day     : Day_Number;
                   Seconds : Day_Duration := 0.0)
   return Time;
  function "+" (Left : Time;   Right : Duration) return Time;
  function "+" (Left : Duration; Right : Time) return Time;
  function "-" (Left : Time;   Right : Duration) return Time;
  function "-" (Left : Time;   Right : Time) return Duration;
  function "<" (Left, Right : Time) return Boolean;
  function "<="(Left, Right : Time) return Boolean;
  function ">" (Left, Right : Time) return Boolean;
  function ">="(Left, Right : Time) return Boolean;
  Time_Error : exception;
   ... -- not specified by the language
end Ada.Calendar;

Dynamic Semantics

For the execution of a delay_statement, the delay_expression is first evaluated. For a delay_until_statement, the expiration time for the delay is the value of the delay_expression, in the time base associated with the type of the expression. For a delay_relative_statement, the expiration time is defined as the current time, in the time base associated with relative delays, plus the value of the delay_expression converted to the type Duration, and then rounded up to the next clock tick. The time base associated with relative delays is as defined in D.9, “Delay Accuracy” or is implementation defined. 
The task executing a delay_statement is blocked until the expiration time is reached, at which point it becomes ready again. If the expiration time has already passed, the task is not blocked. 
 If an attempt is made to cancel the delay_statement (as part of an asynchronous_select or abort — see 9.7.4 and 9.8), the statement is cancelled if the expiration time has not yet passed, thereby completing the delay_statement.
The time base associated with the type Time of package Calendar is implementation defined. The function Clock of package Calendar returns a value representing the current time for this time base. The implementation-defined value of the named number System.Tick (see 13.7) is an approximation of the length of the real-time interval during which the value of Calendar.Clock remains constant. 
 The functions Year, Month, Day, and Seconds return the corresponding values for a given value of the type Time, as appropriate to an implementation-defined time zone; the procedure Split returns all four corresponding values. Conversely, the function Time_Of combines a year number, a month number, a day number, and a duration, into a value of type Time. The operators "+" and "–" for addition and subtraction of times and durations, and the relational operators for times, have the conventional meaning. 
If Time_Of is called with a seconds value of 86_400.0, the value returned is equal to the value of Time_Of for the next day with a seconds value of 0.0. The value returned by the function Seconds or through the Seconds parameter of the procedure Split is always less than 86_400.0.
 The exception Time_Error is raised by the function Time_Of if the actual parameters do not form a proper date. This exception is also raised by the operators "+" and "–" if the result is not representable in the type Time or Duration, as appropriate. This exception is also raised by the functions Year, Month, Day, and Seconds and the procedure Split if the year number of the given date is outside of the range of the subtype Year_Number. 

Implementation Requirements

The implementation of the type Duration shall allow representation of time intervals (both positive and negative) up to at least 86400 seconds (one day); Duration'Small shall not be greater than twenty milliseconds. The implementation of the type Time shall allow representation of all dates with year numbers in the range of Year_Number; it may allow representation of other dates as well (both earlier and later). 

Implementation Permissions

 An implementation may define additional time types.
An implementation may raise Time_Error if the value of a delay_expression in a delay_until_statement of a select_statement represents a time more than 90 days past the current time. The actual limit, if any, is implementation-defined. 

Implementation Advice

Whenever possible in an implementation, the value of Duration'Small should be no greater than 100 microseconds. 
The time base for delay_relative_statements should be monotonic; it need not be the same time base as used for Calendar.Clock.
33  A delay_relative_statement with a negative value of the delay_expression is equivalent to one with a zero value.
34  A delay_statement may be executed by the environment task; consequently delay_statements may be executed as part of the elaboration of a library_item or the execution of the main subprogram. Such statements delay the environment task (see 10.2).
35  A delay_statement is an abort completion point and a potentially blocking operation, even if the task is not actually blocked.
36  There is no necessary relationship between System.Tick (the resolution of the clock of package Calendar) and Duration'Small (the small of type Duration). 
37  Additional requirements associated with delay_statements are given in D.9, “Delay Accuracy”.


Example of a relative delay statement: 
delay 3.0;  -- delay 3.0 seconds
Example of a periodic task: 
   use Ada.Calendar;
   Next_Time : Time := Clock + Period;
                      -- Period is a global constant of type Duration
   loop               -- repeated every Period seconds
      delay until Next_Time;
      ... -- perform some actions
      Next_Time := Next_Time + Period;
   end loop;

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