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VFW_E_PIN_ALREADY_BLOCKED
Meaning
Windows documents VFW_E_PIN_ALREADY_BLOCKED as “IPinFlowControl::Block() has been called on another thread. The current thread cannot make any assumptions about this pin's block state”. Here, IPinFlowControl::Block finds that another thread has already established the pin block, so the current thread cannot assume or replace that state.
Relevant contract
Start with the filter graph, device or persisted configuration that first returned the result.
Evidence worth preserving
- Evidence 1: pin identity, owning and calling thread IDs.
- Evidence 2: block event handle and operation correlation IDs.
- Evidence 3: graph lock order and outstanding reconfiguration.
Conditions that can produce it
- Cause 1: two reconfiguration controllers race for the same pin.
- Cause 2: stop and dynamic reconnect paths use separate locks.
- Cause 3: the owning thread is delayed or abandoned without cleanup.
Retry and recovery
Retry rule: Retry only after the owning thread releases the block and the graph generation is revalidated.
Correction strategy
- Action 1: serialize topology changes through one coordinator.
- Action 2: wait for the owner to complete rather than issuing a second block.
- Action 3: provide cancellation and cleanup for abandoned reconfiguration.
Example incident
A UI reconnect and a device-change handler both block the same output pin; a single graph-operation queue prevents it.
How it differs from nearby results
VFW_E_PIN_ALREADY_BLOCKED_ON_THIS_THREAD reports duplicate acquisition by one thread; VFW_E_PIN_ALREADY_BLOCKED reports conflicting ownership by another thread. Keep those cases separate because their next actions are different.
Official Microsoft references
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