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RL-ARM: Possible Reasons for OS_ERR_FIFO_OVF Status

Information in this knowledgebase article applies to:

  • Keil RL-ARM RTX


On rare occasions, os_error() is being called with OS_ERR_FIFO_OVF. What can be the reason for this?


The OS_ERR_FIFO_OVF occurs when the FIFO queue for post-processing events sent via isr_xxx() functions becomes full. In this case, events will be lost. To inform the user about this critical situation, os_error() is called.

Interrupts can't access critical RTX resources. Because of that, such actions must be queued and later processed by the RTX. isr_xxx() functions will also trigger an RTX interrupt that runs when the current interrupt handler finishes.

This FIFO queue size is configurable in the RTX_Config. But selecting the maximum size will not always help if the RTX does not get the chance to process the FIFO queue. So possible reasons for the error are:

  • The interrupt rate for isr_xxx() function calls is too fast.
  • Too many isr_xxx() function calls from the same interrupt handler.
  • The task scheduler was locked too long (after tsk_lock() too long a delay before tsk_unlock()).
  • More interrupts are used and the IRQ rate is too high (that the system runs almost all of the time in the user IRQ handlers).


Check your application against the possible reasons mentioned above and avoid them. This should give RTX sufficient time to post-process the FIFO queue and the OS_ERR_FIFO_OVF error will not occur.



The following Discussion Forum threads may provide information related to this topic.

Last Reviewed: Thursday, November 12, 2020

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