macOS kernel return 11 (KERN_ALREADY_IN_SET): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The receive right is already a member of the specified Mach port set.

Could be also:
Constant	Type	OS
ERROR_BAD_FORMAT	Win32 error	Windows
ippStsResFloor	Intel Ipp Status	Any
EAGAIN	errno	Linux
EDEADLK	errno	Mac
EAGAIN	errno	Solaris
EAGAIN	errno	Windows
NO_EXCEPTION_HANDLING_SUPPORT	BugCheck Code	Windows

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KERN_MEMORY_ERROR	KERN_NOT_IN_SET

KERN_ALREADY_IN_SET

KERN_ALREADY_IN_SET is returned when code tries to add a receive right to a port set that already contains it. A Mach port set lets a receiver wait for activity from multiple member ports as one unit; it is not a duplicate list in which adding the same receive right again is meaningful.

This error is usually a lifecycle or state-tracking issue. Setup may run twice, two components may both claim ownership of the same receive right, or cleanup may have failed to reset the local state before initialization repeats. The port itself can still be valid; the conflict is the attempted membership transition.

How to handle it

Make port-set registration idempotent only when the existing membership is expected and verified.
Track the receive right and the port-set relationship together, not merely the numeric port name.
Review concurrent initialization and reconnect paths for duplicate registration.
Do not respond by creating another receive right for the same service unless the design explicitly requires one.

Mach port sets are receive-side constructs. The relevant capability is the receive right, which is why a caller with only send rights cannot solve this by re-adding the port.

References

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