The Fa dispatch_time_t type is a semi-opaque integer, with only the special values Vt DISPATCH_TIME_NOW , Vt DISPATCH_WALLTIME_NOW and Vt 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 Fn dispatch_time function returns a milestone relative to an existing milestone after adding Fa offset nanoseconds. If the Fa base parameter maps internally to a wall clock or is Vt DISPATCH_WALLTIME_NOW , then the returned value is relative to the wall clock. Otherwise, if Fa base is Vt 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 Vt mach_absolute_time() .
The Fn 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 Fa base parameter. If Fa base is NULL, then the current time of the wall clock is used. Vt dispatch_walltime(NULL, offset) is equivalent to Vt dispatch_time(DISPATCH_WALLTIME_NOW, offset) .
Overflow causes Vt DISPATCH_TIME_FOREVER to be returned. When Fa base is Vt DISPATCH_TIME_FOREVER , then the Fa offset parameter is ignored.
Underflow causes the smallest representable value to be returned for a given 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:
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);
Use a negative delta to create a milestone an hour before the one above:
milestone = dispatch_walltime(&ts, -60 * 60 * NSEC_PER_SEC);