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| mach_absolute_time | |
|---|---|
| Name | mach_absolute_time |
| Developer | Apple Inc.; NeXT |
| Introduced | NeXTSTEP |
| Operating system | macOS, iOS, iPadOS, tvOS, watchOS, Darwin |
| License | Apple Public Source License |
mach_absolute_time mach_absolute_time is a low-level kernel timing function used on macOS, iOS, iPadOS, tvOS, and watchOS platforms derived from Darwin. It returns a monotonically increasing, high-resolution tick count sourced from the Mach kernel abstraction, enabling precise interval measurement for performance profiling in Xcode, low-latency audio in Core Audio, and real-time scheduling in Grand Central Dispatch. Developers often pair it with conversion data from mach_timebase_info to translate ticks into nanoseconds for use in APIs such as POSIX timers, CFAbsoluteTime, and NSDate interop.
mach_absolute_time originates from the Mach microkernel developed at Carnegie Mellon University and later integrated into NeXTSTEP by NeXT before Apple acquired NeXT, forming part of the foundation of macOS and iOS architectures. The call provides a 64-bit unsigned integer representing processor or platform ticks from a monotonic clock source similar in purpose to clock_gettime's CLOCK_MONOTONIC on Linux. It is used across projects such as OpenAL, Metal, Core Animation, and custom high-resolution profilers employed by teams at Apple Inc. and third-party vendors like Adobe Systems, Autodesk, and Unity Technologies.
The API for mach_absolute_time is exposed via the Mach interfaces available through headers included with Xcode, typically called from C, Objective-C, or Swift wrappers. Typical usage patterns involve calling mach_absolute_time to capture two timestamps and subtracting them to compute elapsed ticks, then applying a timebase conversion obtained through mach_timebase_info. Examples appear in sample code bundled with Xcode and in technical notes circulated by Apple Inc. engineers, and are referenced in performance debugging tools like Instruments and third-party profilers from JetBrains and Google. Interop cases convert results to CFTimeInterval or integrate with NSTimer and DispatchTime for scheduling and benchmarking.
The resolution and precision depend on underlying hardware such as Intel CPUs with Time Stamp Counter support, ARM cores with Generic Timer implementations, and SoCs designed by Apple Inc. (e.g., Apple silicon). On Intel-based Macintosh systems the reading may derive from the Time Stamp Counter which can be affected by frequency scaling technologies from Intel SpeedStep and Turbo Boost, while on ARM-based iPhone and iPad devices the hardware generic timer provides stable, serializable ticks. Real-world measurement comparisons appear in analyses from organizations like Phoronix and research groups at Stanford University and MIT highlighting nanosecond-level resolution under typical conditions.
Because mach_absolute_time returns platform-specific ticks, developers must convert ticks to standard units using mach_timebase_info, which supplies a numerator and denominator to scale ticks into nanoseconds. This conversion is critical when correlating mach_absolute_time with system APIs such as CFAbsoluteTime or interoperating with POSIX time structs used in OpenCL or Metal tasks. Sample code in Apple Developer documentation demonstrates obtaining a mach_timebase_info_data_t structure, computing a 64-bit scaled result, and avoiding overflow via 128-bit arithmetic or careful ordering—techniques discussed in community posts from Stack Overflow and articles published by Ars Technica and IEEE Spectrum.
Implementations vary across CPU microarchitectures and SoC designs; on Intel platforms handlers reconcile invariant TSC behaviors with power-management events, while on ARM the generic timer provides a consistent virtual counter. Kernel-level interactions tie into subsystems like I/O Kit and scheduler timing in XNU, with contributions from teams associated with Apple Inc. and earlier work at Carnegie Mellon University. The function is exposed to user space through provided system libraries and headers bundled with macOS SDK and iOS SDK, used by large software projects including LLVM, Clang, Chromium, Firefox, and proprietary engines like Unreal Engine and Unity.
Common pitfalls include assuming ticks map directly to nanoseconds without conversion, mishandling 64-bit overflow on long-running processes, and comparing values across system suspends or CPU frequency transitions without understanding platform invariants. Best practices: call mach_timebase_info once and cache the ratio, use 128-bit temporaries or careful arithmetic to avoid overflow when scaling large intervals, prefer higher-level timing abstractions like DispatchTime or CFAbsoluteTime when interoperability or portability across POSIX platforms (e.g., Linux, FreeBSD) is required, and validate behavior on target hardware such as MacBook Pro or iPhone models during profiling sessions conducted with Instruments or DTrace.
mach_absolute_time’s lineage traces to the Mach research at Carnegie Mellon University and its adoption in NeXTSTEP and later macOS and iOS following Apple’s 1996 acquisition of NeXT. Over time, Apple introduced higher-level APIs and abstractions in Grand Central Dispatch, DispatchTime, and CFAbsoluteTime to simplify usage and portability; engineering guidance in Apple Developer documentation encourages these higher-level APIs for many common tasks. While not formally deprecated across supported SDKs as of current documentation, platform shifts such as the transition to Apple silicon and evolving best practices have led many projects to favor newer timing primitives, and developers should monitor Apple Developer release notes and SDK headers for changes.
Category:Apple APIs