What does Windows error code 164 (ERROR_MAX_THRDS_REACHED) mean?

ERROR_MAX_THRDS_REACHED

What ERROR_MAX_THRDS_REACHED means

No additional threads can be created because the applicable thread limit has been reached. This result can represent process address-space exhaustion, runtime limits, subsystem quotas, or an application thread leak. CPU utilization may still be low because the limiting resource is thread metadata or stack reservation.

Common causes

• Unbounded per-connection thread creation
• Large reserved stacks exhaust virtual address space
• A library keeps background threads after clients are closed
• The process runs inside a constrained job or compatibility subsystem

How to investigate

1. Record process thread count and virtual-memory layout
2. Group threads by start address and stack signature
3. Inspect configured stack size and linker settings
4. Test whether a bounded worker pool eliminates the failure

Developer guidance

Design backpressure before the limit is reached. Reject or queue excess work, and use asynchronous operations for high fan-out workloads.

Administrator and support guidance

Collect thread and memory diagnostics before restart. Increasing limits without correcting unbounded creation can move the failure to a more damaging resource.

How this code differs from related results

ERROR_TOO_MANY_TCBS is closely related but may be emitted specifically by older thread-control-block management.

References

• Microsoft: System error codes 0–499