errno 35 (EAGAIN): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The requested operation cannot make progress now, but its preconditions can become true without changing the request.

Could be also:
Constant	Type	OS
KERN_INVALID_SECURITY	Kern return	Mac
ippStsDoubleSize	Intel Ipp Status	Any
EDEADLK	errno	Linux
ENOMSG	errno	Solaris
FAT_FILE_SYSTEM	BugCheck Code	Windows

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EDEADLK	ENOMSG

EAGAIN

A readiness result, not proof that the peer failed

EAGAIN commonly appears when an operation is intentionally non-blocking and no immediate progress is possible. On a writable socket or pipe, this can mean flow control has stopped the current write; on other APIs it can indicate a transient resource condition. The call has not completed, so the application must preserve the exact unsent or unread state.

A busy retry loop is usually the wrong response. Wait for the readiness or completion mechanism associated with the descriptor, then resume from the byte or message boundary actually reported by the preceding call. Treat this result differently from ETIMEDOUT: the former says “not now,” while the latter says the caller’s deadline elapsed.

What makes the diagnosis actionable

Log whether the descriptor is non-blocking, the requested and completed byte counts, and the event source used to resume work.
For writes, retain the unwritten suffix rather than resending the whole buffer.
Measure queue depth and time spent waiting; repeated this result can expose a missing consumer or a backpressure policy problem.

References

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