What does Windows error code 129 (ERROR_CHILD_NOT_COMPLETE) mean?

ERROR_CHILD_NOT_COMPLETE

What ERROR_CHILD_NOT_COMPLETE means

The requested child process has not completed, so its final termination state is not yet available. This status can be a normal nonblocking-wait result rather than a fatal failure. The caller queried completion before the child exited.

Common causes

• A zero-timeout or polling wait checks too early
• The child is blocked on I/O or synchronization
• The parent assumes launch implies immediate completion
• The child is hung during shutdown

How to investigate

1. Measure child runtime and record the wait timeout
2. Inspect the child process state and active threads
3. Distinguish expected long-running work from a hang
4. Check whether the parent should wait asynchronously instead of polling

Developer guidance

Treat the result as “not yet” when the API contract defines it that way. Use a bounded timeout, cancellation, and progress reporting rather than a tight polling loop.

Administrator and support guidance

Inspect the child before killing it; it may be performing legitimate work. Capture elapsed time and the resource on which it waits.

How this code differs from related results

Unlike ERROR_WAIT_NO_CHILDREN, a relevant child exists. The unresolved question is completion, not ownership.

References

• Microsoft: System error codes 0–499