Dive Into Systems

2.9.3. The `void *` Type and Type Recasting

The C type void * represents a generic pointer — a pointer to any type, or a pointer to an unspecified type. C allows for a generic pointer type because memory addresses on a system are always stored in the same number of bytes (e.g., addresses are four bytes on 32-bit systems and eight bytes on 64-bit systems). As a result, every pointer variable requires the same number of storage bytes, and because they’re all the same size, the compiler can allocate space for a void * variable without knowing the type it points to. Here’s an example:

void *gen_ptr;
int x;
char ch;

gen_ptr = &x;  // gen_ptr can be assigned the address of an int
gen_ptr = &ch; // or the address of a char (or the address of any type)

Typically, programmers do not declare variables of type void * as in the preceding example. Instead, it’s commonly used to specify generic return types from functions or generic parameters to functions. The void * type is often used as a return type by functions that return newly allocated memory that can be used to store any type (e.g., malloc). It’s also used as a function parameter for functions that can take any type of value. In this case, individual calls to the function pass in a pointer to some specific type, which can be passed to the function’s void * parameter because it can store the address of any type.

Because void * is a generic pointer type, it cannot be directly dereferenced — the compiler does not know the size of memory that the address points to. For example, the address could refer to an int storage location of four bytes or it could refer to a char storage location in memory of one byte. Therefore, the programmer must explicitly recast the void * pointer to a pointer of a specific type before dereferencing it. Recasting tells the compiler the specific type of pointer variable, allowing the compiler to generate the correct memory access code for pointer dereferences.

Here are two examples of void * use:

A call to malloc recasts its void * return type to the specific pointer type of the variable used to store its returned heap memory address:

int *array;
char *str;

array = (int *)malloc(sizeof(int) * 10); // recast void * return value
str = (char *)malloc(sizeof(char) * 20);

*array = 10;
str[0] = 'a';

Students often encounter the void * when creating threads. Using a void * parameter type in a thread function allows the thread to take any type of application-specific pointer. The pthread_create function has a parameter for the thread main function and a void * parameter for the argument value that it passes to the thread main function that the newly created thread will execute. The use of the void * parameter makes pthread_create a generic thread creation function; it can be used to point to any type of memory location. For a specific program that calls pthread_create, the programmer knows the type of the argument passed to the void * parameter, so the programmer must recast it to its known type before dereferencing it. In this example, suppose that the address passed to the args parameter contains the address of an integer variable:
```
/*
 * an application-specific pthread main function
 * must have this function prototype: int func_name(void *args)
 *
 * any given implementation knows what type is really passed in
 *  args: pointer to an int value
 */
int my_thr_main(void *args) {
    int num;

    // first recast args to an int *, then dereference to get int value
    num = *((int *)args);  // num gets 6
    ...
}

int main(void) {
    int ret, x;
    pthread_t tid;

    x = 6;
    // pass the address of int variable (x) to pthread_create's void * param
    // (we recast &x as a (void *) to match the type of pthread_create's param)
    ret = pthread_create(&tid, NULL,
                         my_thr_main,    // a thread main function
                         (void *)(&x));  // &x will be passed to my_thr_main
    // ...
```

2.9.3. The void * Type and Type Recasting

2.9.3. The `void *` Type and Type Recasting