What does errno 57 (ENOTCONN) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: a socket operation requiring a connected peer was attempted without one.

Could be also:
Constant	Type	OS
ERROR_ADAP_HDW_ERR	Win32 error	Windows
EBADSLT	errno	Linux
EBFONT	errno	Solaris
SYSTEM_EXIT_OWNED_MUTEX	BugCheck Code	Windows

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EBADSLT	EBFONT

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

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