inner-fn-gnu.c

// lambda lifting (http://en.wikipedia.org/wiki/Lambda_lifting)
// in GNU C; use inner functions which are not closures 
// of course, but have access to free variables
//
// by Dmitry A. Soshnikov

#include <stdio.h>

// a pointer to a function type
typedef int (*funcPtr)(int);

int foo(int x)
{
    int firstInner(int y)
    {
        return x + y;
    }
    return firstInner(3);
}

funcPtr bar(int x)
{
    int secondInner(int y)
    {
        return x + y;
    }
    // return the function
    return secondInner;
}


int main()
{
    printf("%d\n", foo(2)); // OK, 5

    // returned function
    funcPtr f1 = bar(3);
    printf("%d\n", f1(2)); // also OK, 5

    // however, it's not a closure of course
    // but just lambda lifting;
    // an attemt to call it again with
    // different arg will cause that the
    // first one will return a new result also
    funcPtr f2 = bar(5);
    printf("%d\n", f2(2)); // OK, 7

    // see that we have the same function
    printf("first: %p, second: %p\n", f1, f2);

    // call the fist one again
    printf("%d\n", f1(2)); // segmentation fault

    return 0;
}

out:

> 5
> Segmentation fault (core dumped)