Using the “naked” attribute for functions in GCC

Question

GCC documentation states in 6.30 Declaring Attributes of Functions:

naked

Use this attribute on the ARM, AVR, IP2K, RX and SPU ports

Answer 1

You can safely call functions from a naked function, provided that the called functions have a full prologue and epilogue.

Note that it is a bit of a nonsense to assert that you can 'safely' use assembly language in a naked function. You are entirely responsible for anything you do using assembly language, as you are for any calls you make to 'safe' functions.

To ensure that your generic called function is not static or inlined, it should be in a seperate compilation unit.

"naked" functions do not include any prologue or epilogue -- they are naked. In particular, they do not include operations on the stack for local variables, to save or restore registers, or to return to a calling function.

That does not mean that no stack exists -- the stack is initialized in the program initialisation, not in any function initialization. Since a stack exists, called function prologues and epilogues work correctly. A function call can safely push it's return address, any registers used, and space for any local variables. On return (using the return address), the registers are restored and the stack space is released.

Static or inlined-functions may not have a full prologue and epilogue. They can and may depend on the calling function to manage the stack and to restore corrupted registers.

This leads to the next point: you need the prologue and epilogue only to encapsulate the operations of the called function. If the called function is also safe (no explicit or implicit local variables, no changes to status registers), it can be safely static and/or inlined. As with asm, it would be your responsibility to make sure this is true.

Answer 2

The only statements that can be safely included in naked functions are asm statements that do not have operands. All other statements, including declarations of local variables, if statements, and so forth, should be avoided.

Based on the description you already gave, I would assume that even function calls are not suitable for the "naked" keyword.

Answer 3

If the only thing you do in the naked function is call another function you can just use a single JMP machine code instruction.

The function you jump to will have a valid prologue and it should return directly to the caller of the naked function since JMP doesn't push a return address on the stack.