What does errno 41 (EWOULDBLOCK) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: a nonblocking operation would have to wait before it can complete.

Could be also:
Constant	Type	OS
KERN_LOCK_OWNED_SELF	Kern return	Mac
ippStsI18nMsgCatalogInvalid	Intel Ipp Status	Any
EPROTOTYPE	errno	Mac
ELNRNG	errno	Solaris
ENOTEMPTY	errno	Windows
SECURITY_SYSTEM	BugCheck Code	Windows

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ENOSYS	EPROTOTYPE

EWOULDBLOCK

EWOULDBLOCK belongs to the nonblocking I/O contract. It means the operation could not complete immediately without blocking the caller. This is a readiness and state condition, not data corruption and not proof that the peer has closed the connection.

Event-driven code should treat it like a normal backpressure signal: stop the current read or write attempt, preserve the pending buffer state, and wait for the next appropriate readiness notification. A readiness event is not a guarantee that a later syscall cannot still return EWOULDBLOCK, because another thread, another consumer, socket buffer changes, or edge-triggered draining can change the state before the call.

Diagnostics to keep

Whether the descriptor is nonblocking and which event mask was last observed.
The number of bytes already transferred before the error.
Whether the code uses level-triggered or edge-triggered polling.
Whether EAGAIN and EWOULDBLOCK are handled together in the portability layer.

Linux read(2) · Linux send(2) · Linux errno(3) · Linux UAPI errno definitions

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