linux内核中的_THIS_IP_宏

Question

The following macro appears in include/linux/kernel.h

#define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })

I don't understand what the second & applied to __here would do. The first takes the address of the local label, but what about the second?

Answer 1

The second & in && is necessary to make GCC lookup the name as a label, instead of as a variable. For example

foo: ;
int foo;

void *p1 = &&foo;
void *p2 = &foo;

The second initializer refers to the int variable.

Answer 2

I think && is to get the address of label.

this is gcc extention and I don't think C99 standard supports this behavior.

for more see this.. gcc Labels and values

And local label declaration

In your case ,

#define _THIS_IP_ ({ __label__ __here; __here: (unsigned long)&&__here; })

In actual code _THIS_IP_ will be replaced by the code below of block scope

{ __label__ __here;
 __here: 
(unsigned long) &&__here; 
}

You are declaring local label __here . Hence to get the address of label we use && while we get the address of variable with single & .