Error: Label used but not defined when using && operator [closed]

Answer 1

What && operator? There is no unary && operator in C++. GCC has an extension that allows computed goto statements, and that extension uses && to get the address of a label.

Answer 2

Here && is the GNU C label address operator.

http://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html

int *q = &&i;

i must be a label. You have no label i in your program.

Example of a label:

int main(void)
{
    i:
    (void) 0;

     int i = 0;
     int *p = &i;
     int *q = &&i;
     return 0;
}

I added the (void) 0; statement as labels in C can only put before statements and not before declarations.

Answer 3

& is used two ways, first, you can get the address of a variable:

int *p = &i; // that's fine, p points to i's address

Second it's used to do a "bit-wise" and:

int i = 1;     // 01
int j = 3;     // 11
int k = i & j; // 01

&& is a logical operator, not an "address of address of" operator, it's use is in checking two conditions together and asserting that both are true.

if (something && something_else)

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EDIT: I just noticed you taged this as C and C++... if this is a C question, see above. If this is a C++ question there's another note on &&:

C++ Double Address Operator? (&&)

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EDIT 2: There's another use of the && as a label value operator to take the address of the label.

This is what the error message is about since the compiler is assuming that's what you wanted to do:

The label value operator && returns the address of its operand, which must be a label defined in the current function or a containing function. The value is a constant of type void* and should be used only in a computed goto statement. The language feature is an extension to C and C++, implemented to facilitate porting programs developed with GNU C.

int main()
{
   void * ptr1;
   label1: ...
   ptr1 = &&label1;

   ...

   if (...) {
      goto *ptr1;
   }
   ...
}