What does errno 42 (ENOPROTOOPT) mean?

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

• XNU: Darwin errno definitions
• POSIX: socket()
• POSIX: connect()

Looking for a different code? Search another status or error code.