What does errno 100 (EADDRINUSE) mean?

EADDRINUSE

EADDRINUSE is the POSIX-style name for a local-address collision. In a socket context, the conflicting identity is not just a port number: it includes address family, local IP address or wildcard binding, port, protocol, and reuse policy. A listener on one specific address may coexist with another listener on the same port under conditions where a wildcard listener cannot.

This entry is a UCRT POSIX-supplement macro. Native Winsock code normally receives WSAEADDRINUSE from WSAGetLastError(), not EADDRINUSE. Preserve that native status when diagnosing a real socket failure; a wrapper may map it later for portability.

What distinguishes the common cases

• Capture the intended sockaddr, address family, socket type, protocol, and every reuse option before bind or connect.
• Check wildcard versus specific-address conflicts, dual-stack IPv6 behavior, and whether another process or service owns the endpoint.
• For a quick restart, inspect the prior process's shutdown path and active connections before adding reuse options; those options alter ownership rules and are not interchangeable.
• Do not infer the owner from a port scan alone. The local IP address and transport protocol are part of the collision.

References

• Microsoft: bind()
• Microsoft: WSAConnect()
• Microsoft: errno and Winsock error channels

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