Why can't I use `&&a` in C?

Question

int main(int argc, char * argv[]) {
  int       a = 10;
  int *    sp = &a;
  int * * dp1 = &sp;
  int * * dp2 = &&a;           // NG
  int * * dp3 = &(&a);         // NG
  int * * dp4 = &((int *) &a); // NG
}

$ cc test.c
test.c: In function ‘main’:
test.c:6:17: error: lvalue required as unary ‘&’ operand
   int * * dp3 = &(&a);         // NG
                 ^
test.c:7:17: error: lvalue required as unary ‘&’ operand
   int * * dp4 = &((int *) &a); // NG
                 ^
test.c:5:3: error: label ‘a’ used but not defined
   int * * dp2 = &&a;           // NG
   ^

Answer 1

Why can't I use &&a

Because & gives you the address of a varibale and &a is not a variable.

The C 11 Draft specifies the following:

6.5.3.2 Address and indirection operators

Constraints

1 The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier.

[...]

Semantics

3 The unary & operator yields the address of its operand. [...]

To get around this "limitation" one could introduce temporary storage using a compound literal like this (assuming C99 at least):

int a = 42; 
int ** ppa = &((int *){&a});

---

As a side note referring to the error message below:

test.c:5:3: error: label ‘a’ used but not defined
   int * * dp2 = &&a;           // NG
   ^

gcc (and probably others) define an extension to the C-Standard allowing the use of the && operator on a single operant if the operant identifies a label. (More on this here: https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html)

Example:

void * pv = &&lbl;

goto *pv;

/* some dead code  */

lbl:;

/* living code again here */

Answer 2

In every statement, this part: &a is treated as number value (because it gets the address of the variable).

So, for the first two errors:

error: lvalue required as unary ‘&’ operand

The first & is treated as the bitwise AND operator and so it expects to have a value before it. (i.e. 2 & 3).

For the last error:

error: label ‘a’ used but not defined

Is due to the fact that because of the two && it is treating the a as a label (see alk's answer).

The thing you would like to achieve (get an address of an address) is not possible, because cannot get a address of a "constant value", since &a is already a value, but only the address of variables.

Answer 3

&var gives you the address of your variable, so i would not know what you expect &&var to be, the address of the address? Correct me if i am wrong.

Answer 4

Because you can get only the address of a variable not the address of a value (given by &a) and moreover && is boolean AND as @UnWind says.