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 thesignal
function attribute that does not insert aSEI
instruction. If bothsignal
andinterrupt
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 extendedasm
or a mixture of basicasm
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 theinterrupt
orsignal
attribute aka. interrupt service routine (ISR). Use this attribute if the preamble of the ISR prologue should always readpush __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 theI
-flag by writing to the memory location ofSREG
. - 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. - Code that (effectively) clobbers bits of
OS_main
OS_task
-
On AVR, functions with the
OS_main
orOS_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 whereasOS_main
andOS_task
functions have aRET
orRETI
return instruction. -
naked
functions do not set up a frame for local variables or a frame pointer whereasOS_main
andOS_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 asignal
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
andinterrupt
are specified for the same function,signal
is silently ignored.
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