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WSAEMFILE
WSAEMFILE means that Winsock could not provide another socket handle. The limit can be global, per process, or per thread depending on the provider and environment, so the failure is a resource-exhaustion signal rather than evidence that the network is down.
Typical root causes
The usual cause is a leak: accepted connections, failed connection attempts, event objects, or duplicated socket handles are created faster than they are released. A burst of legitimate connections can also expose an application design that has no admission control, no idle timeout, or no reserved capacity for the accept path.
What to measure
- Track socket creation, successful close, failed close, and the live-handle count by connection role.
- Separate listener exhaustion from per-client leakage; an accept failure may happen only after a connection is already queued.
- Apply back-pressure and bounded queues before increasing limits, otherwise the process may simply fail later under the same load.
Microsoft: accept · Microsoft: Winsock error codes · The Open Group: socket descriptors
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