What does errno 42 (ENOPROTOOPT) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: a requested socket option is unavailable at the specified protocol level.

Could be also:
Constant	Type	OS
KERN_SEMAPHORE_DESTROYED	Kern return	Mac
ippStsI18nGetMessageFail	Intel Ipp Status	Any
ENOMSG	errno	Linux
EUNATCH	errno	Solaris
EILSEQ	errno	Windows
INCONSISTENT_IRP	BugCheck Code	Windows

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ENOMSG	EUNATCH

ENOPROTOOPT

The option lookup failed before any network exchange

ENOPROTOOPT usually identifies an unavailable protocol option. It is most useful when logged with the exact getsockopt() or setsockopt() arguments: the descriptor, option level, option name, buffer length, and socket family/type.

An option name is not globally meaningful. The same numeric value can have a different meaning at another protocol level, and an option supported for one socket family may be unavailable for another. A build-time platform check does not prove that a runtime socket has the same capabilities.

Separate option availability from operation support

ENOPROTOOPT concerns finding or applying a specific protocol option at a level.
EOPNOTSUPP is broader: an operation, flag, or socket type does not support what the caller requested.
EPROTONOSUPPORT arises earlier when the protocol itself cannot be used for the chosen family.

Actionable checks

Preserve symbolic and numeric level/option values in logs, not just a translated label. Then verify the option against the target macOS SDK and the actual socket family. Retrying the same unrecognized option cannot change its availability.

References

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