What does errno 95 (EOPNOTSUPP) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: the selected socket type, protocol, or call flags do not support the requested operation.

Could be also:
Constant	Type	OS
EMULTIHOP	errno	Mac
ENOTSOCK	errno	Solaris
SECURITY_INITIALIZATION_FAILED	BugCheck Code	Windows

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EUSERS	EMULTIHOP

EOPNOTSUPP

EOPNOTSUPP means that the operation is incompatible with the socket that received it. The exact reason is API-specific. A listener operation such as accept() needs a connection-oriented socket; a send can reject flags that the socket type does not implement; and socketpair() can reject a protocol that does not permit creating a pair. This is therefore a contract mismatch, not evidence that the network is temporarily unavailable.

Do not merge it blindly with ENOPROTOOPT. The latter concerns an option name at a protocol level. EOPNOTSUPP can instead describe the operation itself, the socket's type, or an incompatible message flag. On Linux, ENOTSUP and EOPNOTSUPP share a numeric value, but portable code should not assume that relationship on every POSIX system.

What to compare

Identify the failed operation and record the socket domain, SOCK_* type, protocol, connection/listening state, and every nonzero message flag.
For servers, verify that listen() and accept() are used only with a connection-oriented transport.
For send and receive paths, validate protocol-specific support for flags such as out-of-band or peek behavior instead of retrying with the same flag set.
Surface the mismatch during startup or configuration validation where possible; a runtime retry cannot add unsupported semantics to a socket.

Linux send(2) · POSIX accept() · Linux socketpair(2)

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