What does errno 100 (EADDRINUSE) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: A UCRT POSIX-supplement status for a local address that cannot be bound because the endpoint is already in use.

Could be also:
Constant	Type	OS
ERROR_TOO_MANY_SEMAPHORES	Win32 error	Windows
HTTP_STATUS_CONTINUE	HTTP Code	Any
ENETDOWN	errno	Linux
EPROTO	errno	Mac
SYMBOLIC_INITIALIZATION_FAILED	BugCheck Code	Windows

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EPROTO	ENETUNREACH

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

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