What does a && operator do when there is no left side in C?

Question

I saw a program in C that had code like the following:

static void *arr[1]  = {&& varOne,&& varTwo,&& varThree};

varOne: printf(\"On         


        

Answer 1

This is a gcc extension, known as "Labels as Values". Link to gcc documentation.

In this extension, && is a unary operator that can be applied to a label. The result is a value of type void *. This value may later be dereferenced in a goto statement to cause execution to jump to that label. Also, pointer arithmetic is permitted on this value.

The label must be in the same function; or in an enclosing function in case the code is also using the gcc extension of "nested functions".

Here is a sample program where the feature is used to implement a state machine:

#include 
#include 
#include 

int main(void)
{
    void *tab[] = { &&foo, &&bar, &&qux };

    // Alternative method
    //ptrdiff_t otab[] = { &&foo - &&foo, &&bar - &&foo, &&qux - &&foo };

    int i, state = 0;

    srand(time(NULL));

    for (i = 0; i < 10; ++i)
    {
        goto *tab[state];

        //goto *(&&foo + otab[state]);

    foo:
        printf("Foo\n");
        state = 2;
        continue;
    bar:
        printf("Bar\n");
        state = 0;
        continue;
    qux:
        printf("Qux\n");
        state = rand() % 3;
        continue;
    }
}

Compiling and execution:

$ gcc -o x x.c && ./x
Foo
Qux
Foo
Qux
Bar
Foo
Qux
Qux
Bar
Foo