What does errno 129 (ENOTSUP) mean?

ENOTSUP

ENOTSUP is a UCRT POSIX compatibility constant for a requested capability that the current implementation does not support. The relevant question is not merely whether an API call failed, but which feature, clock, option, object capability, or execution mode was requested.

For example, POSIX specifies ENOTSUP when clock_nanosleep() is asked to use a clock that the implementation does not support for that operation. That pattern is more useful than a generic retry: record the requested option and substitute a supported design only when the API contract allows it.

Diagnose the feature boundary

• Capture the function, option or flag, object type, and runtime or library version.
• Check whether the feature is optional, disabled by configuration, unavailable on the selected backend, or only unsupported for this argument combination.
• Keep ENOTSUP separate from ENOSYS: a known interface with an unsupported feature is a different compatibility problem from an unavailable interface.

Microsoft documents this number in the UCRT POSIX supplement. It is not enough to identify a Windows subsystem from the numeric value alone.

References

• POSIX: clock_nanosleep()
• Microsoft: errno constants

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