Clang currently uses avr-gcc's runtime libraries when linking AVR executables.

The ISR tables are provided by GCC's AVR CRT files. Projects that target AVR-GCC use the ISR(..) #define macro to define interrupt handlers. This macro hides the implementation details.

By default, the AVR-GCC runtime libraries are linked when generating ELF executables with AVR/clang.

You can handle interrupts using clang by either

1. write functions with __attribute__(interrupt). The function names must match those defined in the AVR-GCC CRT assembly and object files.

or

2. write your own ISR table in assembly and chose the ISR handler symbol names yourself.

This repository is an example of (1).

GCC defines the ISR table in assembly using weak symbol references. This allows linking to succeed when no ISR handlers are explicitly defined. If you name your AVR-Clang methods with the same names that GCC uses for its ISR tables, the linker will automatically point at your custom ISR handler.

GCC CRT ISR handler names:

• ADC interrupt handler: __vector_21
• Catch-all interrupt handler: __vector_default

This example repository was created in response to a question on llvm-dev: https://lists.llvm.org/pipermail/llvm-dev/2020-March/139732.html