The basic purpose of developing a C programming tutorial for this website – CircuitsToday – is to make it useful for people who wish to work with embedded systems. Really good C programming skill is an essential to work with embedded systems and “Pointers” is the most important concept in C that should be mastered by an embedded systems programmer. “Pointers” are so important because it enables a programmer to work directly with memory of the system. Memory of a system is organized as a sequence of byte sized locations (1 byte = 8 bits). If the total memory of the system is 128 bytes then there will be 128 accessible locations of 1 byte each. Each of these 128 locations are numbered from 0 to 127 in a special manner like 0000, 0001, 0002 …etc. The number associated with a byte is known as the address of the memory location.

You may refer the figure below to get an idea – how memory is organized with in 8051

A pointer is an entity which holds the address of a memory location. So if the address of a location is 2050H, pointer is used to hold this particular address.

Note:- Address of a memory location is always a positive integer. The range of address is from zero to a positive integer constant (which is the address of the last memory location ).

Pointer variables

We can use variables to hold address of a memory location and such variables are known as pointer variables. We have seen before that normal variables are used to store data items of a particular data type (char, int, float etc). Before using a variable in a program, we declare it at the beginning. Similarly we need to declare a pointer variable too in a special way – to let the compiler know we have declared a variable as a pointer (not as a normal variable). To do this we have the *operator – known as indirection operator in C.

Pointer variable declaration

The syntax to declare a pointer variable is

(data type)   *(variable name);

Ex:-  int  *ptr ;

Here we have declared a pointer variable of name ‘ptr’ and it is of type integer (int).

Why we need data types in pointers ?

The first doubt that may come to many is, why we need data types to declare a pointer variable. Well, here is the explanation. The address of a memory location will contain a data – rite? And it can be of type char, int, float etc. The difference between all these data types is in the size allocated to each data type. Char – is 1 byte where as int – is 2 byte and float is 4 bytes. Memory is allocated to all these data types as sequential blocks.

Just consider a scenario like this:-

char a ;

int b;

float c; 

Lets start memory allocation from 2000H.

Now the character variable ‘a‘ will be allocated 2000H (1 byte), where as integer variable ‘b’ will be allocated 2 bytes using 2001H and 2002H. Finally the float variable ‘c’ will be allocated 4 bytes using 4 consecutive locations – 2003H, 2004H, 2005H, 2006H. Now you might get an idea of why we need data types to declare pointer variables. It is because memories are allocated in sequential blocks according to the type of data holded in those locations.

So when we declare a pointer variable as float *ptr and then assign address of the normal float variable c to ptr – what really happens is – ptr is assigned the sequential block from 2003H to 2006H as a whole. But the variable ptr will hold only the starting address of the sequential block i.e 2003H

So a pointer variable must be declared with a data type. And this data type should be the same data type as of the contents inside the memory location address – which is assigned to the pointer variable. Ex:- If 2000H is assigned to a pointer variable ptr and the contents inside 2000H is a character. In this case the pointer variable ptr should be declared as a character pointer as shown below:-

char  *ptr; 

Note:- In fact we can actually declare a pointer variable without any data type using the keyword void. It is known as a void pointer. The topic of void pointer has to be explained separately – so I will explain it in my next post.

Assigning address to a pointer variable

To make use of a pointer and it’s capabilities – the address of a particular memory location must be assigned to the pointer. It is possible to assign address of single variable or that of an array or the address of a structure etc to a pointer variable. This capability makes pointers the most powerful tool in C programming. We can literally play with the memory of a system using pointers.

To assign address of an entity to a pointer variable – we use the & operator (address of operator). The operator & fetches the memory location of a variable.

So taking our earlier case as example:-

#include

void main()

{

int *ptr; // Declaring the pointer variable 'ptr' of data type int

int a; // Declaring a normal variable 'a'

ptr=&a; // Assigning address of variable 'a' to pointer variable 'ptr'

}

Dereferencing a pointer (getting the real content from a memory location)

We have seen upto assigning an address to a pointer variable. Now how to get the content inside the memory location using the same pointer variable? For that we use the same indirection operator * used to declare a pointer variable.

Consider the scenario:-

#include

void main()

{

int *ptr; // Declaring the pointer variable 'ptr' of data type int

int a=; // Declaring a normal variable 'a' and assigning value 10 to it.

ptr=&a; // Assigning address of variable 'a' to pointer variable 'ptr'

printf("The value inside pointer is= %d",*ptr); // See the value is printed using *ptr;

}

Introduction to pointers in C的更多相关文章

  1. Void pointers in C

    In this article we are learning about “void pointers” in C language. Before going further it will be ...

  2. <转>年终盘点!2017年超有价值的Golang文章

    马上就要进入2018年了,作为年终的盘点,本文列出了一些2017年的关于Go编程的一些文章,并加上简短的介绍. 文章排名不分先后, 文章也不一定完全按照日期来排列.我按照文章的大致内容分了类,便于查找 ...

  3. Introduction of OpenCascade Foundation Classes

    Introduction of OpenCascade Foundation Classes Open CASCADE基础类简介 eryar@163.com 一.简介 1. 基础类概述 Foundat ...

  4. Introduction of Open CASCADE Foundation Classes

    Open CASCADE Foundation Classes Open CASCADE基础类 eryar@163.com 一.简介 1. 基础类概述 Foundation Classes Overv ...

  5. Brief introduction to Scala and Breeze for statistical computing

    Brief introduction to Scala and Breeze for statistical computing 时间 2013-12-31 03:17:19  Darren Wilk ...

  6. Introduction the naive“scull” 《linux设备驱动》 学习笔记

    Introduction the naive "scull" 首先.什么是scull? scull (Simple Character Utility for Loading Lo ...

  7. Reading task(Introduction to Algorithms. 2nd)

    Introduction to Algorithms 2nd ed. Cambridge, MA: MIT Press, 2001. ISBN: 9780262032933. Introduction ...

  8. Introduction to Parallel Computing

    Copied From:https://computing.llnl.gov/tutorials/parallel_comp/ Author: Blaise Barney, Lawrence Live ...

  9. [中英对照]Introduction to DPDK: Architecture and Principles | DPDK概论: 体系结构与实现原理

    [中英对照]Introduction to DPDK: Architecture and Principles | DPDK概论: 体系结构与实现原理   Introduction to DPDK: ...

随机推荐

  1. oracle学习1 基于oracle数据库的PLSQL编程以及存储过程的创建和使用视频

    https://www.bilibili.com/video/av46777605 plsql中选择testWindow中可以进行测试 1.编写函数在plsql的testwindow中 begin d ...

  2. Python中的操作符及优先级

    附注: 1.  Python中的按位运算符是把数字看作二进制来进行计算的.Python中的按位运算法则如下: 按位与   ( bitwise and of x and y ) &  举例: 5 ...

  3. libusb_transfer

    http://libusb.sourceforge.net/api-1.0/structlibusb__transfer.html int libusb_control_transfer(libusb ...

  4. 在ASP.NET MVC项目中使用极验验证(geetest)

    时间 2016-03-02 18:22:37 smallerpig 原文  http://www.smallerpig.com/979.html 主题 ASP.NET MVC   geetest开发体 ...

  5. pycharm中报错:ImportError: No module named 'skimage'

    pycharm中直接setting是没办法成功的,会提示is requied C++ bool.....先进入那个页面安装好需要的tools,这个安装比较慢:http://landinghub.vis ...

  6. Redux应用多人协作的思路和实现

    先上Demo动图,效果如下: 基本思路 由于redux更改数据是dispatch(action),所以很自然而然想到以action作为基本单位在服务端和客户端进行传送,在客户端和服务端用数组来存放ac ...

  7. 解决SpringBootApplication右键没有启动项的问题

    转载于 https://blog.csdn.net/weixin_41876599/article/details/82711302 感觉很有用

  8. C#静态变量 总结

    在C#程序中,没有全局变量的概念,这意味着所有的成员变量只有该类的实例才能操作这些数据,这起到了“信息隐藏”的作用.但有些时候,这样做却不是个明智的选择. 假设我们要定义一个图书类,要求该类能保存图书 ...

  9. easyui datagrid 选中行效果

    转:http://blog.csdn.net/hzw2312/article/details/27534065 jquery easyui datagrid 获取Checked选择行(勾选行)数据 g ...

  10. Kamil and Making a Stream

    E. Kamil and Making a Stream 参考:Codeforces Round #588 (Div. 2)-E. Kamil and Making a Stream-求树上同一直径上 ...