[C] Pointer

Pointer

 

Pointer is one of the most important ideas in C.

 

(You can scroll down if you already have background knowledge about what pointer is)

 

Pointer, not like the other types, has data of other variable's address.

 

It makes a big difference of C from the other languages.

 

A memory is a hardware which physically have memory spaces.

 

So each unit of memory space have it's own address.

 

The pointer is used like a mediator, and let us access memory indirectly to the variable.

 

 

How to use

 

 

int* pointer = NULL;  //(type) * (pointer name) = (address value) ;

 

 

If the pointer doesn't need to point something yet, just put 'NULL' like the example.

 

Warnings

 

If the pointer is not resetted as soon as you declare, it gets the garbage value.

 

Usually the pointer must be resetted by the address of a variable by using '&' or it might occur very serious problem.

(Exept when you manage the memory)

 

If the pointer have nothing to point yet, usually people reset the pointer into 'NULL' temporarily.

('NULL' is declared as '0' in <studio.h>)

 

Be sure that you've corrected the type of pointer same as the type of variable.

 

 

Array Pointer

void Func(int arr[], int num){
...
}​

 

'int arr[]' is not actually constructed, it is just a pointer of array which have address of array from the external.

 

If the array element changes in the function, it is same as changing the original array.

 

 

Function Pointer

 

This simple code is a simple example of function pointer.

#include <stdio.h>

int add(int a, int b)    //int type of return, two int type of parameter
{
      return a + b;
}

void executer(int (*fp)(int, int))
{
      printf("%d\n", fp(10, 20));    //30 : call function by a parameter
}

int main()
{
      executer(add);    // 1. pass the memory of add() while calling executer()
      /*******************
      // 2. instead of line15, pass pointer of function while calling executer()
      int (*fp)(int, int);
      fp = add;
      executer(fp);
      *******************/
      return 0;
}
/*******************************
//form of using pointer function
rt_ty func_n(rt_ty_of_func_p (*func_p_n)(pr_ty_of_func_p1, pr_ty_of_func_p2))
{
}
rt = return, ty = type
func = function, n = name
p = pointer, pr = parameter
/*******************************