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WSAEINPROGRESS
WSAEINPROGRESS reports that a blocking Winsock 1.1 call is already in progress on the thread, or that a service provider is still running a callback. It is not the normal completion status for a nonblocking connection.
Why it matters
Winsock can perform an alertable wait during a blocking call. Starting another blocking Winsock call from an APC that interrupted the original call can cause undefined behavior.
What to check
- Do not re-enter blocking socket calls from callbacks or APC handlers on the same thread.
- For scalable code, use nonblocking or overlapped I/O and a single owner for each pending operation.
- Inspect the call stack to find the earlier blocking call rather than treating this as a transient network error.
Microsoft: connect · Microsoft: Windows Sockets Error Codes
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