Activation Record In GNU C (Nested Function)

Question

In GNU C, The Result is 13. Because the static link is used.
Otherwise, If the dynamic link is used, the result would be 16.

#include <stdio.h>

int h(){
    int x = 1;
    int g(int z){
        return z + x;     <------------------ P
    }
    int f(int y){
        int x = y + 1;
        return g(x * y);   
    }
    return f(3);
}

int main(){
    int a = h();
    printf("%d\n", a);
}

At the P point, Activation Record is

z = 12

x = 4

y = 3

f and pointer to code f

g and pointer to code g

x = 1

h and pointer to code h

a

main and pointer to code main

Is that right?
However if function g returns, how is it going?
The Activation for g and Activation for variable z is deleted.
Then In stack frame, The hole is looked.
The hole really appear?
And According to In-line block, In function h,
variable x is the most outer block. (It means that function g' block is nested in variable x's block) next outer block is function g, next function f... Then, Do function f's Static link point to function g's frame pointer? Or function h's frame pointer? What about function g's Static link?

+1 for the concept. WOW! I've been writing C code for ages now. Never knew nested functions were possible. I only thought Pascal and Modula had it. Ignorant me - smiles... — itsols, Jun 11 '11 at 15:43
@itsols: Nested functions are not supported by C. They are an extension of GCC, and notably one which works in C code but not C++. For an exploration of whether they're a good idea, see: http://stackoverflow.com/questions/2929281/are-nested-functions-a-bad-thing-in-gcc — John Zwinck, Jun 11 '11 at 15:52
And they're NOT CLOSURES. When a function that defines a nested function returns, any pointers to the functions it defines get invalidated. This drastically limits their usefulness. Also, they're not implemented on all architectures! — David Given, Jun 11 '11 at 18:18

score 2 · Accepted Answer · answered Jun 12 '11 at 17:13

At point p, there are 4 activation records on the stack:

activation record for g:

return address to f
static link to h activation record
z = 12

activation record for f:

x = 4
return address to h
static link to h activation record
y = 3

activation record for h:

x = 1
return address to main

activation record for main:

a = undefined
return address to OS

Each activation record for a nested function contains a link to the lexically enclosing activation record (h in both cases here) which is set up when the function is called and the activation record is created. At point p, the code will dereference that link to find the value of x, and looking through such links is the ONLY time a function will ever look at some other function's activation record.

score 0 · Answer 2 · answered Jun 12 '11 at 03:03

0

I think that at point P, x can only refer to the x defined in h(); it could only refer to the x in g() if it were itself nested inside g().

answered Jun 12 '11 at 03:03

Jonathan Leffler

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Activation Record In GNU C (Nested Function)

2 Answers2