c - void* type to char* type leadng -

the man has code

#include <conio.h> #include <stdio.h>  void dbleint(void *a) {     *((int*) a) *= 2; }  void deleteevenint(void* a) {     int tmp = *((int*) a);     if (tmp % 2 == 0) {         *((int*) a) = 0;     } }  void dbledouble(void *a) {     *((double*) a) *= 2.0; }  void deleteevendouble(void* a) {     int tmp = *((double*) a);     if (tmp % 2 == 0) {         *((double*) a) = 0;     } }  // function takes array, size, size of 1 element , function //pointer, // applied elements of array void map(void *arr, unsigned num, size_t size, void (*fun)(void *)) {     unsigned i;     char *ptr = (char*) arr;     (i = 0; < num; i++) {         fun((void*) (ptr + i*size));     } } 

why leads void* type char* type? can see thats error, when change code , dont lead it, why?

you're not allowed perform arithmetic on void pointers in c. following code illegal:

void *foo = whatever; foo + 1; 

by casting foo char *, can perform arithmetic on pointer.