What does Windows error code 209 (ERROR_INVALID_SIGNAL_NUMBER) mean?

ERROR_INVALID_SIGNAL_NUMBER

What ERROR_INVALID_SIGNAL_NUMBER means

The signal number supplied to the process-control operation is not valid. This code is a direct parameter validation failure in a legacy signaling API. Signal ranges and meanings can differ between compatibility subsystems, so a value valid in one environment may be invalid in another.

Common causes

• The caller uses a platform-specific signal constant in the wrong subsystem
• An uninitialized variable becomes the signal number
• A configuration value is outside the allowed range
• Enumeration values changed across module versions

How to investigate

1. Log the numeric signal and originating symbolic name
2. Check the allowed range for the exact API and subsystem
3. Validate configuration before conversion to the API type
4. Compare caller and callee header or SDK versions

Developer guidance

Use typed enumerations and reject unknown values at configuration boundaries. Avoid forwarding arbitrary integers directly to process-control APIs.

Administrator and support guidance

Review recently changed scripts or configuration that select signal actions. The operating system generally does not need repair.

How this code differs from related results

This result occurs before delivery. Codes such as ERROR_SIGNAL_REFUSED describe what happened after a valid signal was addressed to a target.

References

• Microsoft: System error codes 0–499