What does errno 37 (EALREADY) mean?

EALREADY

The socket already has an unfinished request

EALREADY is a concurrency and state-management result. It commonly appears when code calls connect() again after a nonblocking connection attempt has already returned EINPROGRESS. The second call does not create a second independent connection attempt.

The useful question is not whether the destination is reachable in general, but which part of the program still owns this socket's pending operation. A retry timer, cancellation path, and event-loop callback must not all issue a new connect() against the same descriptor.

How to resolve the state safely

• Associate one explicit connection-state record with the file descriptor and let only that owner consume readiness and timeout events.
• After readiness, query SO_ERROR once and transition to connected, failed, or closed; do not infer success from an event notification alone.
• If the deadline expires, close or otherwise retire the pending socket before starting a fresh attempt. Reusing an ambiguous descriptor makes later errors hard to attribute.

Nearby errors have different meanings

EINPROGRESS describes the first pending result. EISCONN reports a completed connected state. ECONNREFUSED, ETIMEDOUT, and EHOSTUNREACH are final outcomes that may be discovered only after the pending attempt completes.

References

• XNU: Darwin errno definitions
• POSIX: connect()
• FreeBSD manual: connect()

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