void pointer in C

A void pointer is a pointer that has no associated data type with it. A void pointer can hold address of any type and can be typcasted to any type.

int a = 10;
char b = 'x';

void *p = &a;  // void pointer holds address of int 'a'
p = &b; // void pointer holds address of char 'b'

Advantages of void pointers:
1) malloc() and calloc() return void * type and this allows these functions to be used to allocate memory of any data type (just because of void *)

int main(void)
{
    // Note that malloc() returns void * which can be 
    // typecasted to any type like int *, char *, ..
    int *x = malloc(sizeof(int) * n);
}

Note that the above program compiles in C, but doesn’t compile in C++. In C++, we must explicitly typecast return value of malloc to (int *).

2) void pointers in C are used to implement generic functions in C. For example compare function which is used in qsort().

Some Interesting Facts:
1) void pointers cannot be dereferenced. For example the following program doesn’t compile.

#include<stdio.h>
int main()
{
    int a = 10;
    void *ptr = &a;
    printf("%d", *ptr);
    return 0;
}

Output:

Compiler Error: 'void*' is not a pointer-to-object type 

The following program compiles and runs fine.

#include<stdio.h>
int main()
{
    int a = 10;
    void *ptr = &a;
    printf("%d", *(int *)ptr);
    return 0;
}

Output:

10

2) The C standard doesn’t allow pointer arithmetic with void pointers. However, in GNU C it is allowed by considering the size of void is 1. For example the following program compiles and runs fine in gcc.

#include<stdio.h>
int main()
{
    int a[2] = {1, 2};
    void *ptr = &a;
    ptr = ptr + sizeof(int);
    printf("%d", *(int *)ptr);
    return 0;
}

Output:

2

Note that the above program may not work in other compilers.

References:
http://stackoverflow.com/questions/20967868/should-the-compiler-warn-on-pointer-arithmetic-with-a-void-pointer
http://stackoverflow.com/questions/692564/concept-of-void-pointer-in-c-programming

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