dispatch_time(3) BSD Library Functions Manual dispatch_time(3)
NAME
dispatch_time, dispatch_walltime -- Calculate temporal milestones
SYNOPSIS
#include <dispatch/dispatch.h>
static const dispatch_time_t DISPATCH_TIME_NOW = 0ull;
static const dispatch_time_t DISPATCH_TIME_FOREVER = ~0ull;
dispatch_time_t
dispatch_time(dispatch_time_t base, int64_t offset);
dispatch_time_t
dispatch_walltime(struct timespec *base, int64_t offset);
DESCRIPTION
The dispatch_time() and dispatch_walltime() functions provide a simple
mechanism for expressing temporal milestones for use with dispatch func-
tions that need timeouts or operate on a schedule.
The dispatch_time_t type is a semi-opaque integer, with only the special
values DISPATCH_TIME_NOW and DISPATCH_TIME_FOREVER being externally
defined. All other values are represented using an internal format that
is not safe for integer arithmetic or comparison. The internal format is
subject to change.
The dispatch_time() function returns a milestone relative to an existing
milestone after adding offset nanoseconds. If the base parameter maps
internally to a wall clock, then the returned value is relative to the
wall clock. Otherwise, if base is DISPATCH_TIME_NOW, then the the cur-
rent time of the default host clock is used.
The dispatch_walltime() function is useful for creating a milestone rela-
tive to a fixed point in time using the wall clock, as specified by the
optional base parameter. If base is NULL, then the current time of the
wall clock is used.
EDGE CONDITIONS
The dispatch_time() and dispatch_walltime() functions detect overflow and
underflow conditions when applying the offset parameter.
Overflow causes DISPATCH_TIME_FOREVER to be returned. When base is
DISPATCH_TIME_FOREVER, then the offset parameter is ignored.
Underflow causes the smallest representable value to be returned for a
given clock.
CAVEATS
Under the C language, untyped numbers default to the int type. This can
lead to truncation bugs when arithmetic operations with other numbers are
expected to generate a int64_t sized result, such as the offset argument
to dispatch_time() and dispatch_walltime(). When in doubt, use ull as a
suffix. For example:
3ull * NSEC_PER_SEC
EXAMPLES
Create a milestone two seconds in the future:
milestone = dispatch_time(DISPATCH_TIME_NOW, 2LL * NSEC_PER_SEC);
Create a milestone for use as an infinite timeout:
milestone = DISPATCH_TIME_FOREVER;
Create a milestone on Tuesday, January 19, 2038:
struct timespec ts;
ts.tv_sec = 0x7FFFFFFF;
ts.tv_nsec = 0;
milestone = dispatch_walltime(&ts, 0);
RETURN VALUE
These functions return an abstract value for use with dispatch_after(),
dispatch_group_wait(), dispatch_semaphore_wait(), or
dispatch_source_set_timer().
SEE ALSO
dispatch(3), dispatch_after(3), dispatch_group_create(3),
dispatch_semaphore_create(3)
Darwin May 1, 2009 Darwin
Mac OS X 10.9.1 - Generated Tue Jan 7 18:55:34 CST 2014