C Pointer

#include <stdio.h>

int main ()
{
   int  var1;
   char var2[10];

   printf("var1 变量的地址： %x\n", &var1  );
   printf("var2 变量的地址： %x\n", &var2  );

   return 0;
}

var1 变量的地址： bff5a400
var2 变量的地址： bff5a3f6

What is a pointer?

A pointer is a variable whose value is the address of another variable, namely, the direct address of the memory location.Before Like other variables or constants, you must use a pointer memory address of other variables, it is declared. The general form of a pointer variable declaration is:

type *var-name;

Here, type is a pointer to the base type, it must be a valid C data type,var-name is the name of the pointer variable. Is used to declare a pointer asterisk * multiplication use the asterisk is the same. However, in this statement, the asterisk is used to specify a variable is a pointer. The following is a valid pointer declaration:

int    *ip;    /* 一个整型的指针 */
double *dp;    /* 一个 double 型的指针 */
float  *fp;    /* 一个浮点型的指针 */
char   *ch     /* 一个字符型的指针 */

All pointers actual data type of the value, whether it is an integer, float, string, or other data types are the same, is a hexadecimal number represents a long memory address. The only difference between the different types of data pointer, the pointer is a variable or constant data types.

How to use the pointer?

Will frequently use the pointer when the following actions: define a pointer variable, the variable address assigned to the pointer, access the value of the pointer variable available addresses. This is done by using the unary* operator to return the value of the variable is located in the operand address specified.The following examples relate to these actions:

#include <stdio.h>

int main ()
{
   int  var = 20;   /* 实际变量的声明 */
   int  *ip;        /* 指针变量的声明 */

   ip = &var;  /* 在指针变量中存储 var 的地址 */

   printf("Address of var variable: %x\n", &var  );

   /* 在指针变量中存储的地址 */
   printf("Address stored in ip variable: %x\n", ip );

   /* 使用指针访问值 */
   printf("Value of *ip variable: %d\n", *ip );

   return 0;
}

When the above code is compiled and executed, it produces the following results:

Address of var variable: bffd8b3c
Address stored in ip variable: bffd8b3c
Value of *ip variable: 20

The C NULL pointer

At the time of variable declarations, if not the exact address can be assigned, assign a NULL value for the pointer variable is a good programming practice. Fu NULL pointer value is called anull pointer.

A NULL pointer is defined in the standard library zero constants. Consider the following program:

#include <stdio.h>

int main ()
{
   int  *ptr = NULL;

   printf("ptr 的值是 %x\n", ptr  );
 
   return 0;
}

When the above code is compiled and executed, it produces the following results:

ptr 的值是 0

On most operating systems, the program does not allow access to memory address 0, because the memory is reserved for the operating system. However, the memory address 0 has a special significance, it indicates that the pointer does not point to an accessible memory location. But according to the convention, if the pointer contains a null value (zero value), it is assumed that it does not point to anything.

To check for a null pointer, you can use the if statement, as follows:

if(ptr)     /* 如果 p 非空，则完成 */
if(!ptr)    /* 如果 p 为空，则完成 */

C pointer Detailed

In C, there are many pointers related concepts that are very simple, but very important. Listed below are some important concepts associated with the pointer C programmers must be clear: