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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
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.
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), or some
other implementation-defined time type, See section D.8 Monotonic Time.
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
implementation-defined time type, represents a time as reported by a
The following language-defined library package exists:
package Ada.Calendar is
type Time is private;
subtype Year_Number is Integer range 1901 .. 2099;
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)
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
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
See section 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 section 9.7.4 Asynchronous Transfer of Control, and See section 9.8 Abort of a Task - Abort of a Sequence of Statements, 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 section 13.7 The Package System, 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 timezone; 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
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
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
function Year or the procedure Split if the year number of the given
date is outside of the range of the subtype Year_Number.
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).
An implementation may define additional time types, See section D.8 Monotonic Time.
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.
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.
(31) A delay_relative_statement with a negative value of the
delay_expression is equivalent to one with a zero value.
(32) 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 section 10.2 Program Execution.
(33) A delay_statement is an abort completion point and a potentially
blocking operation, even if the task is not actually blocked.
(34) There is no necessary relationship between System.Tick (the
resolution of the clock of package Calendar) and Duration'Small (the
small of type Duration).
(35) Additional requirements associated with delay_statements are given
in See section D.9 Delay Accuracy.
Example of a relative delay statement:
delay 3.0; -- delay 3.0 seconds
Example of a periodic task:
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;
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