What does errno 57 (ENOTCONN) mean?

ENOTCONN

The operation requires peer state that the socket does not have

ENOTCONN occurs when an API requires a connected peer but the socket is not connected. It can follow an attempt to send or receive on a connection-mode socket before connection setup completed, after teardown, or on a datagram-style socket that has no default peer.

The error is local state, not remote refusal. For a connectionless send, the correct API may be one that supplies a destination with the message; for a connection-mode socket, the program must complete and verify connection establishment first.

What to retain

• Log the socket type, current connection state, last connect result, and whether the call was send(), sendto(), recv(), or shutdown().
• Check for a race between connection completion and a worker that starts I/O, especially after a nonblocking EINPROGRESS result.
• After a failure or close, retire the old socket state rather than allowing queued application work to continue as though it still had a peer.

Contrast with address omission

EDESTADDRREQ is the connectionless case where a send needs an explicit destination. ENOTCONN is the connected-operation case where the API needs a peer association that has not been established.

References

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

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