Nios® V Processor Software Developer Handbook

ID 743810
Date 7/20/2023
Public

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8.2.2.2. Running in a Restricted Environment

ISRs run in a restricted environment. Most of the HAL API calls are not available from ISRs. For example, accesses to the HAL file system are not permitted. As a general rule, when writing your own ISR, never include function calls that can block for any reason (such as waiting for a hardware interrupt).

For more information about identifying these API functions that are not available to ISRs, refer to the HAL API Reference chapter.

Be careful when calling ANSI C standard library functions inside of an ISR. Avoid using the C standard library I/O API, because calling these functions can result in deadlock within the system, that is, the system can become permanently blocked in the ISR.

In particular, do not call printf() from within an ISR unless you are certain that stdout is mapped to a non-interrupt-based device driver. Otherwise, printf() can deadlock the system, waiting for a hardware interrupt that never occurs because interrupts are disabled.