指针C语言-很好的教程

Summary: this tutorial introduces you to C pointer, which is an important concept in C programming language. Pointers give you a flexible and powerful way of manipulating data in your programs.

Introduction to C Pointer

When you define a variable in your program, the compiler allocates a memory location with a unique address to store that variable’s value. You then access the memory address through the variable name.

For example, when you define a variable:

int x = 10;

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intx =10;

You specified variable name ( x ), its data type int and its value, 10. The variable x resides in the memory with a unique memory address. To get the memory address of the variable x, you use unary operator  & as follows:

printf("The memory address of x is %p\n",&x);

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printf("The memory address of x is %p\n",&x);

In our system, the memory address of x is:

The memory address of x is 0028FF1C

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Thememory address of xis 0028FF1C

Because memory address of x is a number ( 28FF1C), you can use another variable to store it e.g., px as the following picture:

In C, we say that px points to x, or px is a pointer tox.

By definition, C pointer is a special variable that holds the memory address of another variable.

There is a special kind of pointer called function pointer that holds a memory address of a function. Check it out the C function pointer tutorial for more information.

Declaring a pointer

Like a variable, before using a pointer, you have to declare it.  The following illustrates the syntax of declaring a pointer:

<type> *<pointer>;

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<type>*<pointer>;

First, you specify the data type of the variable that the pointer point to. The type can be any validC data type such as int, char, float or evenvoid.

Second, you place the indirection operator ( *) in front of the pointer name to indicate that this is a pointer that points to a variable with a specific type e.g.int, not a variable of the type  int

Third, you provide the name for the pointer. The name of pointers must follow the naming rules ofvariables. By convention, in C,  pointer name begins with p to help distinguish between a pointer and a variable in your programs.

The following example defines 2 pointers that point to int variables, an a pointer that points to achar variable.

int *px, *py;char *ps;

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int*px,*py;

char *ps;

Initializing Pointers

If you declare a pointer without initializing it, you have an uninitialized pointer. With an uninitialized pointer, you cannot do anything with it.

To initialize a pointer, you assign the memory address of another variable to the pointer using the address-of operator (&) as follows:

pointer = &variable;

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pointer= &variable;

For example, to assign the address of the x variable to the px pointer you use the following syntax:

px = &x;

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px= &x;

Using pointers

After initializing a pointer, you can manipulate the variable which the pointer points to using the indirection operator (*).

For example, we can display the value of x through the px pointer as follows:

printf("%d",*px);

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printf("%d",*px);

We can change the value of x through the px pointer as well:

*px = 20;printf("*px = %d\n",*px); /* 20 */printf("x = %d\n",x); /* 20 */

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*px= 20;

printf("*px = %d\n",*px);/* 20 */

printf("x = %d\n",x);/* 20 */

In C, accessing the value of a variable through the variable name is called direct access, and accessing the value of a variable through a pointer that points to the variable is known asindirect access or indirection.

Putting it all together.

/* * File: main.c * Author: zentut.com * C Pointers Demo */#include <stdio.h>int main(){ int x = 10; int* px = &x; printf("Direct access, x = %d\n",x); printf("Indirect access, *px = %d\n",*px); *px = 20; printf("Direct access, x = %d\n",x); printf("Indirect access, *px = %d\n",*px); /* examining memory address of x and px */ printf("The memory address of x is %p\n",&x); printf("The value of the pointer px is %p\n",px); printf("The memory address of the pointer px is %p\n",&px); return 0;}

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/*

* File:   main.c

* Author: zentut.com

* C Pointers Demo

*/

#include <stdio.h>

 

int main()

{

    intx =10;

    int*px =&x;

 

    printf("Direct access, x = %d\n",x);

    printf("Indirect access, *px = %d\n",*px);

 

    *px= 20;

 

    printf("Direct access, x = %d\n",x);

    printf("Indirect access, *px = %d\n",*px);

 

    /* examining memory address of x and px */

    printf("The memory address of x is %p\n",&x);

    printf("The value of the pointer px is %p\n",px);

    printf("The memory address of the pointer px is %p\n",&px);

 

    return0;

}

The following is the output of the program:

Direct access, x = 10Indirect access, *px = 10Direct access, x = 20Indirect access, *px = 20The memory address of x is 0028FF1CThe value of the pointer px is 0028FF1CThe memory address of the pointer px is 0028FF18

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Directaccess,x =10

Indirect access,*px= 10

Directaccess,x =20

Indirect access,*px= 20

Thememory address of xis 0028FF1C

The value ofthe pointer pxis 0028FF1C

Thememory address of the pointerpx is0028FF18

Pointer & Array

Basically any operation on an array, which can be done using array subscript, can be achieved through a pointer to the array.

Let’s take a look at an example.

int a[10];

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inta[10];

This declares an array of 10 integers. Underneath, there is a block of memory with 10 consecutive objects defining as follows:

The a[i] refers the i^th element of the a array. The following defines a pointer that points to the first element of thea array:

int * pa;pa = &a[0];

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int* pa;

pa =&a[0];

Because the  pa pointer points to the first element of thea array, *pa is the value of the first element. And (pa+1) points to the next element of the array, and(pa+i) points to the i^th element.

Recall that the name of an array is the memory address of the first element, therefore the following assignment:

pa = &a[0];

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pa= &a[0];

can be rewritten as follows:

pa = a;

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pa= a;

After assigning the pointer to the first element of an array, you can perform any operations on that array. The following example illustrates how to manipulate an array via a pointer.

/* * File : main.c * Author : zentut.com * Purpose: pointer to an array demo */#include <stdio.h>#define MAX 10void display_array(int*,const int);int main(){ int i; int a[MAX]; int* pa; pa = a; /* fill array with random number between 0 and 100 */ for(i = 0; i < MAX; i++) { *(pa + i) = rand() % 100; } /* display array via pointer */ display_array(pa,MAX); /* display array via array name */ display_array(a,MAX); return 0;}/* Display an array of integers with predefined size*/void display_array(int* p,const int size){ int i; for(i = 0; i < size; i++) { printf("%d ",p[i]); } printf("\n");}

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/*

* File   : main.c

* Author : zentut.com

* Purpose: pointer to an array demo

*/

#include <stdio.h>

 

#define MAX 10

 

void display_array(int*,constint);

 

int main()

{

    inti;

    inta[MAX];

    int*pa;

    pa= a;

 

    /* fill array with random number between 0 and 100 */

    for(i= 0;i <MAX;i++)

    {

        *(pa+ i)= rand()% 100;

    }

    /* display array via pointer */

    display_array(pa,MAX);

 

    /* display array via array name */

    display_array(a,MAX);

 

    return0;

}

 

/*

    Display an array of integers with predefined size

*/

void display_array(int*p,constint size)

{

    inti;

    for(i= 0;i <size;i++)

    {

        printf("%d ",p[i]);

    }

    printf("\n");

}

In this tutorial, you have learned about C pointer, which is a special variable that store memory address of another variable. You also learned the relationship between a pointer and an array, and how to manipulate an array through a pointer.

转载自：http://www.zentut.com/c-tutorial/c-pointer/