6.33.6 AVR Function Attributes

These function attributes are supported by the AVR back end:


The function is an interrupt service routine (ISR). The compiler generates function entry and exit sequences suitable for use in an interrupt handler when one of the attributes is present.

The AVR hardware globally disables interrupts when an interrupt is executed.

  • ISRs 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.
  • ISRs with the interrupt attribute re-enable interrupts. The first instruction of the routine is a SEI instruction to globally enable interrupts.

The recommended way to use these attributes is by means of the ISR macro provided by avr/interrupt.h from AVR-LibC:

#include <avr/interrupt.h>

ISR (INT0_vect) // Uses the "signal" attribute.
    // Code

ISR (ADC_vect, ISR_NOBLOCK) // Uses the "interrupt" attribute.
    // Code

When both signal and interrupt are specified for the same function, then signal is silently ignored.


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 — Assembler Instructions Without Operands). 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.


Do not use the __gcc_isr pseudo instruction 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.


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.