What does macOS kernel return 51 (KERN_POLICY_STATIC) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: A requested Mach thread-policy change targets a thread whose policy parameters are static at that point.

Could be also:
Constant	Type	OS
ERROR_REM_NOT_LIST	Win32 error	Windows
ippStsFeaturesCombination	Intel Ipp Status	Any
EL2HLT	errno	Linux
ENETUNREACH	errno	Mac
EBADR	errno	Solaris
UNEXPECTED_INITIALIZATION_CALL	BugCheck Code	Windows

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KERN_CODESIGN_ERROR	KERN_INSUFFICIENT_BUFFER_SIZE

KERN_POLICY_STATIC

A policy change that the target thread does not accept

KERN_POLICY_STATIC means that the requested property cannot be changed at that point. For Mach thread-policy operations, this is different from a malformed request and different from a normal authorization failure: the target thread has static policy parameters, so the scheduler does not apply the requested change through that path.

Current XNU checks the thread’s static_param state before applying policy changes. Both the kernel implementation of thread_policy_set and the legacy absolute-priority path can return KERN_POLICY_STATIC for such a thread.

Why an application may not see code 51 directly

The public libsyscall wrappers for thread_policy_set, thread_policy, and thread_set_policy explicitly convert KERN_POLICY_STATIC to KERN_SUCCESS after the underlying kernel RPC. Therefore a raw Mach result, an internal trace, or a lower-level client can expose this code even when a caller using the public wrapper only observes success.

For diagnosis, treat a wrapper-level success as insufficient proof that a legacy policy change became effective. Inspect the policy actually in force on the target thread and the exact interface that produced the status.

What to check

Record whether the result came from a public wrapper, a direct Mach RPC, a kernel extension, or diagnostic instrumentation.
Identify the requested flavor and target thread. A retry with the same target and static state does not make the property mutable.
Do not classify this automatically as a privilege problem. KERN_PROTECTION_FAILURE and KERN_DENIED describe different failure classes.
When the goal is ordinary application responsiveness, use the documented higher-level scheduling or QoS mechanism appropriate to the application instead of assuming that a legacy Mach-policy request can override a static policy.

References

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