errno 101 (EADDRNOTAVAIL): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: A UCRT POSIX-supplement status for an address unavailable in the current family, interface, or routing context.

Could be also:
Constant	Type	OS
ERROR_EXCL_SEM_ALREADY_OWNED	Win32 error	Windows
HTTP_STATUS_SWITCH_PROTOCOLS	HTTP Code	Any
ENETUNREACH	errno	Linux
ETIME	errno	Mac
MEMORY1_INITIALIZATION_FAILED	BugCheck Code	Windows

Previous	Next
ETIME	ENETRESET

EADDRNOTAVAIL

EADDRNOTAVAIL describes an address that cannot be used in the current socket operation. For a local bind, that often means the requested local IP address is not assigned to an active interface. For an outbound operation, it can also reflect an incompatible address family, a stale interface choice, or a source-address selection constraint.

Do not confuse it with a remote host failure. The address may be syntactically valid, yet unavailable on this machine at this moment. In native Winsock code the corresponding diagnostic is normally a WSAEADDRNOTAVAIL status returned through WSAGetLastError(), rather than this UCRT macro.

Diagnostic data that matters

Record the full local and remote sockaddr values, their family, scope ID for IPv6 where applicable, and whether the address was selected explicitly or automatically.
Capture the interface and route state at the same time as the failure; a VPN, address renewal, or network namespace change can invalidate cached choices.
Check for binding an IPv4 address to an IPv6 socket or the reverse before assuming a routing issue.
For clients, prefer automatic ephemeral binding unless the application has a real source-address requirement.

References

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