How do nested functions get compiled?

Question

When I have 2 functions in D like this:

void func() {
    void innerFunc() {
        import std.stdio;
        writeln(x);
    }

    int x = 5;
    innerFunc();
}

When I call func this will print 5. How does it work? Where in memory does the 5 get stored? How does innerFunc know it has to print 5?

Answer 1

I attempt to answer this in broad terms. This type of issue arises in a number of languages that permit nested function definitions (including Ada and Pascal).

Normally, a variable like "x" is allocated on the processor stack. That's the normal process in any language that permits recursion.

When a nested function is called, a descriptor for the enclosing function's stack frame gets passed as hidden argument.

funct() then knows that x is located at some offset specified by the base pointer register.

innerFunct () knows the offset of x but has to derive the base from the hidden argument. It can't use its own base pointer value because it will be different from funct(). And, if innerFunct () called itself, the base pointer value would be different in each invocation.