On Thu, Sep 22, 2016 at 11:04 AM, <u...@netbeisser.de> wrote:
>
> Hi,
>
> how does noreturn attribute work? Trying to get this [0] to work, I
> error on noreturn.
>
> /* Note how buffer overruns are undefined behavior and the compilers tend to
> optimize these checks away if you wrote them yourself, this only works
> robustly because the compiler did it itself. */
> extern uintptr_t __stack_chk_guard;
> noreturn void __stack_chk_fail(void);
> void foo(const char* str)
> {
> uintptr_t canary = __stack_chk_guard;
> char buffer[16];
> strcpy(buffer, str);
> if ( (canary = canary ^ __stack_chk_guard) != 0 )
> __stack_chk_fail();
> }
>
>
>
> main.c:7:10: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before
> ‘void’
> noreturn void __stack_chk_fail(void);
>
> tried
> void __stack_chk_fail(void) __attribute ((noreturn));
> instead.
>
>
> [0] http://wiki.osdev.org/Stack_Smashing_Protector
>
`noreturn` is usually used in exception code, kernel code, libc [0],
or other systems code. It signifies to the compiler, whether there is
a return type or void, that it is possible that the callee function
will not return to the caller function. Often you will see a `while(
true );` or `for(;;);` at the end of the function to make it more
obvious for the next developer. Adding a `noreturn` will keep the
compiler from complaining/ warning/ erroring when it encounters this
function.
For your case, you may need to add that infinite loop at the end of
the function.
[0]
https://git.musl-libc.org/cgit/musl/tree/src/exit/_Exit.c?id=e738b8cbe64b6dd3ed9f47b6d4cd7eb2c422b38d
