Data Type Encapulation

void means “no type”. void * means “a pointer with no type”. void * can point to any type of address.

Usage 1: void is to encapulate the data type
- int initHardEnv(void ** handle). A typical example is memory opertation functions such as memcpy and memset
  - void * memcpy(void * dest, const void * src, size_t len);
  - void * memset(void * buffer, int c, size_t num);
Usage 2: void is to describe the return value and parameters. It only means “empty” or “nothing”.
- If a function has no return value, it shall declare the return value as void;
- if a function has no parameter, it shall declare the parameter to be void.
  
  Example: void function(void){ ... }
meaning of void pointer
- In C, only the same type of pointers can be mutally assigned to each other.
- void * pointer used as left operand will accept an a pointer assignment of any types
- void * pointer used as right operand doing the pointer assignment has to be exclusively casted to the specific type on intention
  
  Examples:
  1
  2
  int * p1 = null;
  char * p2 = (char*)malloc(sizeof(char)*20);
C does not support void type variable

Pointer

Pointer is also a data type. It has 8 principles.

1. Pointer is a Data Type

Pointer is a variable. It occupies memory space to store a memory address.

int main(int argc, const char * argv[]) {
    // insert code here...
    int * p1 = 100;  // to tell the compiler to allocate 8 bytes of memory space
    return 0;
}

*p can manupate the memory space.
- When declearing the pointer, we use * to claim the varialble it modifies is a pointer.
- When use the pointer, we use * to update or access the value that the pointer points to in the memory space.
- The left operand of *p means assignment, i.e. assign a value to a memory slot.
- The right operand of *p means to retrieve the value from the memory slot.
The pointer variable is different from the memory slot the it points to.
- Making p=0X1111 will only change the pointer variable’s value. It won’t change the value that p points to.
- Making *p='a'; will only change the value it points to, but not change the value of the pointer variable.
- If a pointer points to memory value stored in the global constant memory space, the value of that memory can’t not be changed.

// insert code here...
int x = 2;
int * p1 = &x;  // to tell the compiler to allocate 8 bytes of memory space

printf("the value of p1: %d\n", p1);

int * p2 = 3; // this assignment is misleading and not recommended, because you are assigning an int value to pointer variable.
printf("the value of p1: %d\n", p2);