Using the GNU Compiler Collection (GCC): AVR Function Attributes

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#### 6.33.6 AVR Function Attributes

These function attributes are supported by the AVR back end:

`interrupt`

  Use this attribute to indicate that the specified function is an interrupt handler. The compiler generates function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

  On the AVR, the hardware globally disables interrupts when an interrupt is executed. The first instruction of an interrupt handler declared with this attribute is a `SEI` instruction to re-enable interrupts. See also the `signal` function attribute that does not insert a `SEI` instruction. If both `signal` and `interrupt` are specified for the same function, `signal` is silently ignored.

`naked`

  This attribute allows the compiler to construct the requisite function declaration, while allowing the body of the function to be assembly code. The specified function will not have prologue/epilogue sequences generated by the compiler. Only basic `asm` statements can safely be included in naked functions (see Basic Asm). While using extended `asm` or a mixture of basic `asm` and C code may appear to work, they cannot be depended upon to work reliably and are not supported.

`no_gccisr`

  Do not use `__gcc_isr` pseudo instructions in a function with the `interrupt` or `signal` attribute aka. interrupt service routine (ISR). Use this attribute if the preamble of the ISR prologue should always read

      push  __zero_reg__
      push  __tmp_reg__
      in    __tmp_reg__, __SREG__
      push  __tmp_reg__
      clr   __zero_reg__

  and accordingly for the postamble of the epilogue — no matter whether the mentioned registers are actually used in the ISR or not. Situations where you might want to use this attribute include:

  - Code that (effectively) clobbers bits of `SREG` other than the `I`-flag by writing to the memory location of `SREG`.
  - Code that uses inline assembler to jump to a different function which expects (parts of) the prologue code as outlined above to be present.

  To disable `__gcc_isr` generation for the whole compilation unit, there is option `-mno-gas-isr-prologues`, see AVR Options.

`OS_main`
`OS_task`

  On AVR, functions with the `OS_main` or `OS_task` attribute do not save/restore any call-saved register in their prologue/epilogue.

  The `OS_main` attribute can be used when there is guarantee that interrupts are disabled at the time when the function is entered. This saves resources when the stack pointer has to be changed to set up a frame for local variables.

  The `OS_task` attribute can be used when there is no guarantee that interrupts are disabled at that time when the function is entered like for, e.g. task functions in a multi-threading operating system. In that case, changing the stack pointer register is guarded by save/clear/restore of the global interrupt enable flag.

  The differences to the `naked` function attribute are:

  - `naked` functions do not have a return instruction whereas `OS_main` and `OS_task` functions have a `RET` or `RETI` return instruction.
  - `naked` functions do not set up a frame for local variables or a frame pointer whereas `OS_main` and `OS_task` do this as needed.
`signal`

  Use this attribute on the AVR to indicate that the specified function is an interrupt handler. The compiler generates function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

  See also the `interrupt` function attribute.

  The AVR hardware globally disables interrupts when an interrupt is executed. Interrupt handler functions defined with the `signal` attribute do not re-enable interrupts. It is save to enable interrupts in a `signal` handler. This “save” only applies to the code generated by the compiler and not to the IRQ layout of the application which is responsibility of the application.

  If both `signal` and `interrupt` are specified for the same function, `signal` is silently ignored.

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