What does errno 56 (EISCONN) mean?

EISCONN

The peer association already exists

EISCONN means the socket is already connected. A second connect() does not create another connection, and a send operation that supplies a destination can also be invalid or ignored when the socket already has a peer association.

This is not a reachability result. The local socket's state is the relevant fact. Inspecting DNS, routes, or a remote service cannot explain why the caller tried to attach a second peer to an object that already has one.

State checks that prevent the mistake

• Track socket ownership and connection state in one place; do not let two asynchronous completion paths independently start connection setup.
• For datagram sockets, decide whether the application uses a connected peer or per-message destinations, then keep that mode consistent.
• Log the prior successful or pending connect, current peer address, and the API that made the duplicate request.

Different from pending and absent states

EALREADY means a previous operation is still pending. ENOTCONN means no required peer exists. EISCONN means the connection state already exists and the caller must use it or replace it deliberately.

References

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

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