Susam Pal

Here is a silly puzzle I solved exactly one month ago.

If the above problem statement is difficult to understand, here is the same problem expressed in terms of typedef.

typedef void (*func_t)(int *);
func_t (*x)(func_t [10]);

I'll describe how I solve such problems. I start from the right end of the problem and work my way to the left most end defining each part one by one.

Let me start.

I verified the declaration with a short code.

#include <stdio.h>

/* A function which has int * argument and return type void. */
void g(int *a)
{
    printf("g(): a = %d\n", *a);
}

/* A function which has an array of 10 pointers to functions that has
 * int * argument and return type void as argument, and returns a
 * pointer to a function which has int * argument and return type void.
 */
void (*f(void (*a[10])(int *)))(int *)
{
    int i;
    for (i = 0; i < 10; i++)
        a[i](&i);
    return g;
}

int main()
{
    /* An array of 10 pointers to functions that has int * argument and
     * return type void. */
    void (*a[10])(int *) = {g, g, g, g, g, g, g, g, g, g};

    /* A pointer to the function which has an array of 10 pointers to
     * functions that has int * argument and return type void as
     * argument, and returns a pointer to a function which has int *
     * argument and return type void. */
    void (*(*x)(void (*[10])(int *)))(int *) = f;

    /* A pointer to a function that has int * argument and return type
     * void. */
    void (*y)(int *a) = x(a);

    int i = 10;
    y(&i);
    return 0;
}

Section 5.12 (Complicated Declarations) of the second edition of The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie has some good examples of complicated declarations of pointers although they are less clumsy than the one in this blog post. Let me mention a couple of interesting ones from that section.

char (*(*x())[])()
x: function returning pointer to array[] of pointer to function returning char.

char (*(*x[3])())[5]
x: array[3] of pointer to function returning pointer to array[5] of char.