Something useful about function pointers in C.

fptr.c

# include <stdio.h>
# include <stddef.h>

// x: A function taking short and returning char.
char x (short);						// Usage: char c = x ('H');
// x ();								// A function
// x (short);							// taking short
// char x (short);						// and returning char.

// y: A pointer to a function taking short and returning char.
char (* y) (short);					// Usage: char c = (* y) ('E');
// * y;									// A pointer
// (* y) ();							// to a function
// (* y) (short);						// taking short
// char (* y) (short);					// and returning char.
//									// Array: char (* arr [COUNT]) (short);
//									// Array usage: (* arr [index]) ('A');

// z: A function taking two ints and returning pointer
//     to a function taking short and returning char.
char (* z (int, int)) (short);		// Usage: char c = (* z (1, 2)) ('L');
// z ()									// A function
// z (int, int)							// taking two ints
// * z (int, int)						// and returning pointer
// (* z (int, int)) ()					// to a function
// (* z (int, int)) (short)				// taking short
// char (* z (int, int)) (short)		// and returning char.

// w: A pointer to a function taking two ints and returning
//     pointer to a function taking short and returning char.
char (* (* w) (int, int)) (short);	// Usage: char c = (* (* w) (1, 2)) ('L');
// * w;									// A pointer
// (* w) ();							// to a function
// (* w) (int, int);					// taking two ints
// * (* w) (int, int);					// and returning pointer
// (* (* w) (int, int)) ();				// to a function
// (* (* w) (int, int)) (short);		// taking short
// char (* (* w) (int, int)) (short)	// and returning char.
//									// Array: char (* (* arr [COUNT]) (int, int)) (short);
//									// Array usage: (* (* arr [index]) (1, 2)) ('B');

char * (* (* a (long, long, long)) (int, int)) (short);
char * (* (* (* b) (long, long, long)) (int, int)) (short);
char * (* (* (* c (double, double, double, double)) (long, long, long)) (int, int)) (short);
char * (* (* (* (* d) (double, double, double, double)) (long, long, long)) (int, int)) (short);
char * (* (* (* (* e (char, char, char, char, char)) (double, double, double, double)) (long, long, long)) (int, int)) (short);
char * (* (* (* (* (* f) (char, char, char, char, char)) (double, double, double, double)) (long, long, long)) (int, int)) (short);

// Typedefs
typedef char X (short);						// Usage: X _x;				// For function definitions only.
typedef char (* Y) (short);					// Usage: Y _y = & x;
//											// Array: Y _yy [COUNT];
typedef char (* Z (int, int)) (short);		// Usage: Z _z;				// For function definitions only.
typedef char (* (* W) (int, int)) (short);	// Usage: W _w = & z;
//											// Array: W _ww [COUNT];

char x (short a) { return (char) a; }
char (* z (int a, int b)) (short) { return & x; }

# define NL() putchar ('\n')

int main (void)
{
	y = & x;
	w = & z;

	putchar (x ('H'));
	NL ();
	putchar ((* y) ('E'));
	NL ();
	putchar ((* z (1, 2)) ('L'));
	NL ();
	putchar ((* (* w) (1, 2)) ('L'));
	NL ();
	puts (":D");

//	char (* yy [5]) (short) = {& x, & x, & x, & x, & x};
//	char (* (* ww [5]) (int, int)) (short) = {& z, & z, & z, & z, & z};
	Y yy [5] = {& x, & x, & x, & x, & x};
	W ww [5] = {& z, & z, & z, & z, & z};
	for (size_t i = 0; i < 5; i++)
		putchar ((* yy [i]) ('A'));
	NL ();
	for (size_t i = 0; i < 5; i ++)
		putchar ((* (* ww [i]) (1, 2)) ('B'));
	NL ();
}