errno 106 (EISCONN): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: code attempted to establish or supply a peer for a socket that is already connected.

Could be also:
Constant	Type	OS
ERROR_SEM_USER_LIMIT	Win32 error	Windows
EQFULL	errno	Mac
ELAST	errno	Mac
ECONNABORTED	errno	Windows
LPC_INITIALIZATION_FAILED	BugCheck Code	Windows

Previous	Next
ECANCELED	EQFULL

EISCONN

EISCONN is a state-machine error: the socket already has a peer, but the caller is trying to establish another connection or provide a destination where the API expects the existing peer to be used. It is most often caused by a duplicate connect(), reusing a connection object after the handshake, or passing a target address to a connection-mode send call.

For a stream socket, connection setup should happen exactly once for each socket lifetime. After successful completion of a nonblocking connect, transition the object out of the “connecting” state and send data without another destination. Linux documents that sendto() on a connected connection-mode socket ignores the destination arguments on some paths and may return EISCONN on others; portable code should not rely on either behavior.

What to check

Ensure only one component owns the connection state transition.
After a writable connect event, read SO_ERROR once and update the state atomically.
Use a separate socket for a different remote endpoint instead of re-calling connect() on the established stream.

POSIX connect() · Linux send(2) · UNIX-domain socket errors

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