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ENOTSUP
Unsupported operation versus unsupported endpoint
ENOTSUP means that the operation does not apply to the referenced object type. It is more specific than a generic invalid argument: the call may be syntactically valid, but the selected file, device, filesystem, or service object does not provide that capability.
On Darwin, keep this distinct from EOPNOTSUPP. The latter is the socket-specific status defined for an operation that the referenced socket type cannot perform. The fix is therefore to verify the object and capability contract, not to retry the same operation or substitute unrelated flags.
Useful checks
- Log the API, object type, open flags, filesystem type or device class, and the capability that the caller expected.
- Inspect feature negotiation and version checks before enabling an optional operation.
- Compare with
ENOSYS, which indicates that a function is not implemented, rather than an operation unsupported by this object.
References
- Apple XNU: Darwin errno definitions
- Apple system-call reference: ENOTSUP and EOPNOTSUPP
- POSIX: fcntl() interface and optional operations
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