ERROR_TIMER_NOT_CANCELED - 541 (0x21D)
An attempt was made to cancel or set a timer that has an associated APC and the subject thread is not the thread that originally set the timer with an associated APC routine.
Updated: Feb 21, 2026
Technical Meaning
The ERROR_TIMER_NOT_CANCELED error code indicates that an attempt was made to cancel or set a timer, but the operation failed because the timer has an associated Asynchronous Procedure Call (APC) and the subject thread is not the original thread that set the timer with the APC routine.
Error Details
This error typically occurs in scenarios where timers are used for scheduling tasks. When a timer is created with an associated APC, it means that when the timer expires, a specific function will be called on the thread that originally set the timer. Attempting to cancel or modify this timer from another thread can result in this error.
Usage Context
This error code is relevant in environments where timers are used for scheduling tasks and where APCs are involved. It is commonly encountered in applications that rely heavily on timing mechanisms, such as event-driven systems or real-time processing.
Developer Interpretation
Developers should be aware of the context in which timers are being used. If a timer with an associated APC is set and later needs to be canceled or modified, it must be done from the original thread that created the timer. Attempting to do so from another thread will result in this error.
Related Errors
ERROR_INVALID_PARAMETER(0x57)ERROR_NOT_SUPPORTED(0x103)ERROR_ACCESS_DENIED(0x5)
FAQ
Q: What does the ERROR_TIMER_NOT_CANCELED error mean?
A: It indicates that an attempt was made to cancel or set a timer with an associated APC from a thread other than the original setter.
Q: How can I resolve this issue?
A: Ensure that any operations on timers with associated APCs are performed from the original thread that created them.
Summary
The ERROR_TIMER_NOT_CANCELED error code is specific to scenarios where timers with associated APCs are used. It highlights a situation where an operation was attempted from an incorrect thread context, leading to failure.