void pointer

Learn to use generic pointers with void*.

void pointer (void*)

A void pointer (void *) is a generic pointer that holds an address of unknown type. Any data pointer can be implicitly converted to void *.

Example: int x = 5; void *p = &x;. Here p points to an int but the type is lost. To access the value through p you must first cast it to the right type: *(int *)p gives you 5. void * is the return type of malloc and the foundation of generic data structures such as tree nodes.

Generic functions with void*

With void * a single function can work with different types. The standard library's qsort is the best-known example: it takes a void * array and sorts it regardless of element type.

A generic print example: void print_int(void *p) { printf("%d\n", *(int *)p); }. This function takes an int * through a void * and prints it. The advantage is that you can write similar functions for different types with the same signature; the disadvantage is that type safety is lost at compile time.

Exercise Print an int through void*

Write void print_int(void *p); inside it print *(int *)p with printf. In main declare int x = 99; and call print_int(&x).

Show example solution

#include <stdio.h>

void print_int(void *p) {
    printf("%d\n", *(int *)p);
}

int main(void) {
    int x = 99;
    print_int(&x);
    return 0;
}

Test yourself

How do you read an int through void *p?
- p->int
- *p
- p.int
- *(int *)p
Show answer

*(int *)p — Cast to (int *) first, then dereference with *.

Cast void* to int and print

int x = 7;
void *p = &x;
printf("%d\n", *(int *)____);

Show answer

int x = 7;
void *p = &x;
printf("%d\n", *(int *)p);

Match the void* usage

void *p

Generic pointer
(int *)p

Type cast
*(int *)p

Value access
void *malloc(...)

Allocation return

Next lesson: Summary - Advanced pointers

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