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MACH_RCV_TIMED_OUT
An expected idle-state outcome
MACH_RCV_TIMED_OUT means that a receive operation used MACH_RCV_TIMEOUT and no eligible message arrived before the interval expired. No message was dequeued. In an event loop, this is often a normal opportunity to perform periodic work, check cancellation, or update liveness state; it is not by itself evidence that the port, server, or client is broken.
Timeout interpretation depends on the options. The historical Mach interface cautions that when receive interruption is not requested, an interrupted call may be retried with the original timeout, so frequent interrupts can make the observed wait longer than a simplistic wall-clock expectation. Use a monotonic deadline in higher-level logic if precise end-to-end timing matters.
How to use it well
- Distinguish an idle timeout from a protocol deadline or a failed peer in logs and metrics.
- Record the port or port-set name, timeout value, and queue state when available.
- Use a deadline-based loop if a total operation budget must include interruptions and retries.
- Avoid polling with tiny timeouts when an event-driven blocking receive is sufficient.
References
- Mach Kernel Interface Reference: mach_msg
- Apple Kernel Programming Guide: Mach overview
- GNU Mach Reference Manual: interprocess communication
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