WSAEISCONN - 10056 (0x2748)
A connect request was made on an already connected socket.
Updated: Feb 21, 2026
Technical Background
The WSAEISCONN error, with the numeric code 10056 (0x2748), is a specific error that occurs in Windows API socket operations. This error indicates that a connection attempt was made to a socket that is already in a connected state.
Error Details
The WSAEISCONN error typically arises when an application attempts to establish a connection on a socket that has already been successfully connected. In the context of network programming, this can happen if the same function call to connect() or similar functions is made multiple times without first closing the socket.
Common Causes
- Invalid Parameter Values: Repeatedly calling connect() on an already connected socket.
- Incorrect Object Type: Attempting to perform a connection operation on a non-socket object.
- Unsupported Operations: Trying to establish a connection when the socket is in a state that does not support it, such as being closed or listening.
Real-World Context
In real-world scenarios, this error can occur due to programming mistakes where developers might inadvertently call connect() multiple times on the same socket. It is important to ensure that each connection attempt is properly managed and that sockets are correctly closed before reusing them.
Is This Error Critical?
The criticality of this error depends on the application's requirements. While it does not necessarily indicate a severe system failure, it can lead to unexpected behavior or performance issues if not handled appropriately.
How to Diagnose
To diagnose WSAEISCONN, developers should:
- Review Operation Context: Ensure that each socket operation is performed in the correct context and state.
- Validate Parameters: Check that parameters passed to socket functions are valid and appropriate for the current state of the socket.
- Confirm Object Types: Verify that operations are being performed on sockets, not other types of objects.
How to Resolve
To resolve WSAEISCONN, developers should:
- Correct Parameter Usage: Ensure that each call to a socket function is appropriate for the current state of the socket.
- Adjust Operation Context: Manage the lifecycle of sockets properly by closing and reopening them as needed.
- Restore Data: If data corruption or invalid states are suspected, restore the socket to a valid state before attempting further operations.
Developer Notes
Developers should be aware that WSAEISCONN is specific to connection-oriented protocols like TCP. For connectionless protocols like UDP, similar issues may arise but would manifest differently.
Related Errors
- WSAEALREADY: A blocking operation was interrupted by a call to WSACancelBlockingCall().
- WSAENOTSOCK: The descriptor is not a socket.
- WSAEINVAL: An invalid argument was supplied, such as an invalid parameter or incorrect object type.
FAQ
Q: What does WSAEISCONN mean?
A: WSAEISCONN indicates that a connection attempt was made on an already connected socket. This error typically occurs when the same connect operation is called multiple times without closing the socket first.
Q: How can I prevent this error?
A: Ensure proper management of socket states and avoid calling connect() repeatedly on the same socket. Close sockets before reusing them to maintain correct connection states.
Q: Can WSAEISCONN occur in UDP applications?
A: No, WSAEISCONN is specific to TCP connections. For UDP, similar issues may arise but would be handled differently due to the nature of the protocol.
Summary
The WSAEISCONN error signifies that a connection attempt was made on an already connected socket. This can lead to unexpected behavior and should be managed carefully in application code. Proper state management and validation are crucial to avoid this error.