Web and Mobile Apps Dev

The statement below is from the last post:

void (*func_ptr)(emp *e);

The above variable can be assigned as follows:

func_ptr = read_emp; // Note that no & is required before read_emp

The following two statements are different from each other:

func_ptr = read_emp; // assign the function address of read_emp to a function to pointer variable func_ptr
func_ptr = read_emp();	// runs the function named read_emp() and saves function results to the variable func_ptr

Here is the sample code for the function pointers:

/*
 *
 * funcptrs.c
 *
 * Program to demonstrate the use of function pointers
 *
 * by Mark Virtue, 2001.
 *
 */

#include <stdio.h>
#include <stdlib.h>

static void goodbye() {
	while (getchar() != 'n') {}
	printf("nPress ENTER to exit: ");
	fflush(stdin);
	getchar();
}

static int add(int a, int b) {
	return a + b;
}

static int subtract(int a, int b) {
	return a - b;
}

main() {
	int x, y;
	int result;
	int (*func_ptr)(int, int);

	atexit(goodbye);

	printf("Please enter the first number: ");
	scanf("%d", &x);
	printf("Please enter the second number: ");
	scanf("%d", &y);

	while (getchar() != 'n') {}

	printf("Would you like to add or subtract (a/s)? ");
	fflush(stdin);

	if (getchar() == 'a') {
		func_ptr = add;
	}
	else {
		func_ptr = subtract;
	}

	result = func_ptr(x, y);

	printf("The result is %dn", result);

	return 0;
}

Note in the above code, the add and subtract functions need to have the same structure in order for use with the function pointer.

Here is another sample code which uses a function in the standard C library called qsort():

/*
 * qsort.c
 *
 * Program to demonstrate the use of function pointers
 * This program uses the library function qsort():
 *
 * void qsort(void *array, int count, int size, int (*comp_func)(const void *, const void *))
 *
 * by Mark Virtue, 2001.
 */

#include <stdio.h>

#include <stdlib.h>

#include <ctype.h>

struct employee {

  char name[31];

  char address[101];

  int age;

  float salary;

};

typedef struct employee emp;

int compare_ages(const void *v1, const void *v2) {

    // Create two temp pointers for convenience, to ensure that the void pointers are interpreted correctly

  const emp     *e1 = v1;

  const emp     *e2 = v2;

  return e1->age - e2->age;

    // Note: could have been written: return ((emp *)v1)->age - ((emp *)v2)->age;

}

int compare_names(const void *v1, const void *v2) {

    //Create two temp pointers for convenience, to ensure that the void pointers are interpreted correctly
  const emp *e1 = v1;

  const emp *e2 = v2;

  return strcmp(e1->name, e2->name);

}

static void goodbye() {

  while (getchar() != 'n') {}
  printf("nPress ENTER to exit: ");

  fflush(stdin);

  getchar();

}

main() {

  emp emps[] = {

    {"Bob",  "123 Hope St",   45, 45000.00},

    {"Tony", "124 Hope St",   99, 46000.00},

    {"Mary", "76 Banana Ave", 12, 20000.00},

    {"Sue",  "1000 Road Rd",  45, 90000.00},

    {"Fred", "32 Nowhere Rd", 51, 49000.00}

};

  int i;

  atexit(goodbye);

  qsort(emps, 5, sizeof emps[0], compare_ages);

  for (i = 0; i < 5; i++) {

    printf("%-10s is aged %dn", emps[i].name, emps[i].age);
  }

  qsort(emps, 5, sizeof emps[0], compare_names);

  for (i = 0; i < 5; i++) {

    printf("%-10sn", emps[i].name);

  }
}