dispatch_time - Man Page

Calculate temporal milestones


#include <dispatch/dispatch.h>

static const dispatch_time_t DISPATCH_TIME_NOW = 0ull;
static const dispatch_time_t DISPATCH_WALLTIME_NOW = ~1ull;
static const dispatch_time_t DISPATCH_TIME_FOREVER = ~0ull;

dispatch_time(dispatch_time_t base, int64_t offset);

dispatch_walltime(struct timespec *base, int64_t offset);


The dispatch_time() and dispatch_walltime() functions provide a simple mechanism for expressing temporal milestones for use with dispatch functions 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, DISPATCH_WALLTIME_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 or is DISPATCH_WALLTIME_NOW, then the returned value is relative to the wall clock. Otherwise, if base is DISPATCH_TIME_NOW, then the current time of the default host clock is used. On Apple platforms, the value of the default host clock is obtained from mach_absolute_time().

The dispatch_walltime() function is useful for creating a milestone relative 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. dispatch_walltime(NULL, offset) is equivalent to dispatch_time(DISPATCH_WALLTIME_NOW, offset).

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.


Create a milestone two seconds in the future, relative to the default clock:

milestone = dispatch_time(DISPATCH_TIME_NOW, 2 * NSEC_PER_SEC);

Create a milestone two seconds in the future, in wall clock time:

milestone = dispatch_time(DISPATCH_WALLTIME_NOW, 2 * NSEC_PER_SEC);

Create a milestone for use as an infinite timeout:


Create a milestone on Tuesday, January 19, 2038:

struct timespec ts;
ts.tv_sec = 0x7FFFFFFF;
ts.tv_nsec = 0;
milestone = dispatch_walltime(&ts, 0);

Use a negative delta to create a milestone an hour before the one above:

milestone = dispatch_walltime(&ts, -60 * 60 * NSEC_PER_SEC);

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)

Referenced By

dispatch(3), dispatch_after(3), dispatch_group_create(3), dispatch_semaphore_create(3), dispatch_source_create(3).

May 1, 2009