What does Windows error code 151 (ERROR_INVALID_EVENT_COUNT) mean?

ERROR_INVALID_EVENT_COUNT

What ERROR_INVALID_EVENT_COUNT means

The number of semaphore events supplied to a multiple-wait operation is invalid. This legacy result is tied to multiplexed semaphore waits such as DosMuxSemWait. It means the caller supplied a count outside the supported range or inconsistent with the provided list.

Common causes

• The count is zero when at least one event is required
• The count exceeds the implementation limit
• The array length and declared count differ
• Integer truncation or structure-layout mismatch corrupts the count

How to investigate

1. Log the declared event count and the actual allocated array length
2. Validate every entry before crossing a module or ABI boundary
3. Check 32-bit versus 64-bit packing in compatibility structures
4. Reduce the wait set to the smallest failing example

Developer guidance

Represent the list and its length together rather than passing unrelated values. Validate the range before calling the legacy interface and split very large wait sets into a scalable dispatcher.

Administrator and support guidance

Configuration that controls worker or endpoint counts can trigger the limit, but confirm the exact count before changing capacity settings.

How this code differs from related results

ERROR_INVALID_LIST_FORMAT concerns the structure or contents of the list; this result specifically concerns its event count.

References

• Microsoft: System error codes 0–499