c - 为什么 gcc 使用 && 一元运算符而不是给出标签的直接地址？

Question

我有这样的疑问，为什么 gcc 使用 && 来访问标签，为什么它不直接提供对与标签关联的位置值的访问。就像下面的代码:

void main()
{
    static void* loc = &&locA;
    printf("locA : %p\n", locA); //will give error
    printf("loc : %p\n",loc); //will not give error
    //statments X
    locA :
    //statements Y

}

Answer 1

这是文档的 gcc 扩展 Extensions to the C Language Family of the gcc documentation在 Labels as Values 部分其中说:

You can get the address of a label defined in the current function (or a containing function) with the unary operator ‘&&’. The value has type void *. This value is a constant and can be used wherever a constant of that type is valid. For example:

 void *ptr;
 /* ... */
 ptr = &&foo;

To use these values, you need to be able to jump to one. This is done with the computed goto statement1, goto *exp;. For example,

 goto *ptr;

One way of using these constants is in initializing a static array that serves as a jump table:

 static void *array[] = { &&foo, &&bar, &&hack };

Then you can select a label with indexing, like this:

 goto *array[i];