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dispatch_time(3) [osx man page]

dispatch_time(3)					   BSD Library Functions Manual 					  dispatch_time(3)

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 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 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 for- mat 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 current time of the default host clock is used. 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. 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

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dispatch_semaphore_create(3)				   BSD Library Functions Manual 			      dispatch_semaphore_create(3)

dispatch_semaphore_create, dispatch_semaphore_signal, dispatch_semaphore_wait -- synchronized counting semaphore SYNOPSIS
#include <dispatch/dispatch.h> dispatch_semaphore_t dispatch_semaphore_create(long count); long dispatch_semaphore_signal(dispatch_semaphore_t semaphore); long dispatch_semaphore_wait(dispatch_semaphore_t semaphore, dispatch_time_t timeout); DESCRIPTION
Dispatch semaphores are used to synchronize threads. The timeout parameter is creatable with the dispatch_time(3) or dispatch_walltime(3) functions. COMPLETION SYNCHRONIZATION
If the count parameter is equal to zero, then the semaphore is useful for synchronizing completion of work. For example: sema = dispatch_semaphore_create(0); dispatch_async(queue, ^{ foo(); dispatch_semaphore_signal(sema); }); bar(); dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); FINITE RESOURCE POOL
If the count parameter is greater than zero, then the semaphore is useful for managing a finite pool of resources. For example, a library that wants to limit Unix descriptor usage: sema = dispatch_semaphore_create(getdtablesize() / 4); At each Unix FD allocation: dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); fd = open("/etc/services", O_RDONLY); When each FD is closed: close(fd); dispatch_semaphore_signal(sema); RETURN VALUES
The dispatch_semaphore_create() function returns NULL if no memory is available or if the count parameter is less than zero. The dispatch_semaphore_signal() function returns non-zero when a thread is woken. Otherwise, zero is returned. The dispatch_semaphore_wait() function returns zero upon success and non-zero after the timeout expires. If the timeout is DISPATCH_TIME_FOR- EVER, then dispatch_semaphore_wait() waits forever and always returns zero. MEMORY MODEL
Dispatch semaphores are retained and released via calls to dispatch_retain() and dispatch_release(). CAVEATS
Unbalanced dispatch semaphores cannot be released. For a given semaphore, calls to dispatch_semaphore_signal() and dispatch_semaphore_wait() must be balanced before dispatch_release() is called on it. SEE ALSO
dispatch(3), dispatch_object(3) Darwin May 1, 2009 Darwin
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