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LukeW — Wed, 12 Nov 2008 05:53:00 GMT

C中的位运��?/strong>
能够�q�用��C�Q何整形的数据�c�d��?包括char, int), 无论有没有short, long, unsigned�q�样的限定词.

位运��的应用

// 交换指针变量x,y所指向的存储位�|�处存放的�?br /> // 优势是不需要第三个位置来��时存储另一个�?br /> // 但是�q�个�Ҏ��q�没有明昄��性能优势,只是一个智力上的消�?/span>
void inplace_swap(int *x, int *y)
{
*x = *x ^ *y;
*x = *x ^ *y;
*x = *x ^ *y;
}

位运��常见用�?
实现掩码�q�算

-----------------------------------
Java中的位运��?/strong>

LukeW 2008-11-12 13:53 发表评论

LukeW — Wed, 12 Nov 2008 03:58:00 GMT
因�ؓ现行的计��机都是以八位一个字节�ؓ存储单位,那么一�?6位的整数,也就是C语言中的short,在内存中可能有两�U�存储顺序big-

endian和litte-endian.考虑一个short整数0x3132(0x32是低�?0x31是高�?,把它赋值给一个short变量,那么它在内存中的存储�?br />
能有如下两种情况:
大端字节(Big-endian):

short变量地址
       0x1000                  0x1001
___________________________________
|                 |
|         0x31    |       0x32
|________________ | ________________
高位字节在低位字节的前面,也就是高位在内存地址低的一�?可以�q�样��C��(大端->高位->在前->正常的逻辑��序)

��端字节(little-endian):

short变量地址
       0x1000                  0x1001
_____________________________________
|                 |
|         0x32    |       0x31
|________________ | __________________
低位字节在高位字节的前面,也就是低位在内存地址低的一�?可以�q�样��C��(��端->低位->在前->与正帔R��辑��序相反)

可以做个实验
在windows上下如下�E�序

#include <stdio.h>
#include <assert.h>

void main( void )
{
        short test;
        FILE* fp;

        test = 0x3132;  //(31ASIIC码的’1’,32ASIIC码的’2’)
        if ((fp = fopen ("c:""test.txt", "wb")) == NULL)
              assert(0);
        fwrite(&test, sizeof(short), 1, fp);
        fclose(fp);
}

    然后在C盘下打开test.txt文�g,可以看见内容�?1,而test�{�于0x3132,可以明显的看出来x86的字节顺序是低位在前.如果我们
把这�D�同��L��代码攑ֈ�(big-endian)的机器上执行,那么打出来的文�g��是12.�q�在本机中��用是没有问题�?但当你把�q�个文�g从一
个big- endian机器复制��C��个little-endian机器上时��出现问题了.

    如上�q�C��?我们在big-endian的机器上创徏了这个test文�g,把其复制到little-endian的机器上再用fread��d��一�?short�?br /> �?我们得到的就不再�?x3132而是0x3231�?�q�样��d��的数据就是错误的,所以在两个字节��序不一��L��机器上传输数据时需要特�?br /> ��心字节��序,理解了字节顺序在可以帮助我们写出�U�L��行更高的代码.

正因为有字节��序的差�?所以在�|�络传输的时候定义了所有字节顺序相关的数据都��用big-endian,BSD的代码中定义了四个宏来处
�?

#define ntohs(n)     //�|�络字节��序��C��机字节顺�?nbsp;n代表net, h代表host, s代表short
#define htons(n)     //��L��字节��序到网�l�字节顺�?nbsp;n代表net, h代表host, s代表short
#define ntohl(n)      //�|�络字节��序��C��机字节顺�?nbsp;n代表net, h代表host, s代表 long
#define htonl(n)      //��L��字节��序到网�l�字节顺�?nbsp;n代表net, h代表host, s代表 long

举例说明下这其中一个宏的实�?

#define sw16(x) "
    ((short)( "
        (((short)(x) & (short)0x00ffU) << 8) | "
        (((short)(x) & (short)0xff00U) >> 8) ))

�q�里实现的是一个交换两个字节顺�?其他几个宏类�?

我们改写一下上面的�E�序

#include <stdio.h>
#include <assert.h>

#define sw16(x) "
    ((short)( "
        (((short)(x) & (short)0x00ffU) << 8) | "
        (((short)(x) & (short)0xff00U) >> 8) ))

// 因�ؓx86下面是低位在�?需要交换一下变成网�l�字节顺�?/span>
#define htons(x) sw16(x)

void main( void )
{
        short test;
        FILE* fp;

        test = htons(0x3132); //(31ASIIC码的’1’,32ASIIC码的’2’)
        if ((fp = fopen ("c:""test.txt", "wb")) == NULL)
              assert(0);
        fwrite(&test, sizeof(short), 1, fp);
        fclose(fp);
}

    如果在高字节在前的机器上,�׃��与网�l�字节顺序一�?所以我们什么都不干��可以了,只需要把#define htons(x) sw16(x)宏替

换�ؓ #define htons(x) (x).
    一开始我在理解这个问题时,��d��想�ؓ什么其他数据不用交换字节顺�?比如说我们write一块buffer到文�?最后终于想明白�?

因�ؓ都是unsigned char�c�d��一个字节一个字节的写进�?�q�个��序是固定的,不存在字节顺序的问题.

【用函数判断�pȝ��是Big Endian�q�是Little Endian�?/font>

bool IsBig_Endian()
//如果字节序�ؓbig-endian�Q�返回true;
//反之�?nbsp;  little-endian�Q�返回false
{
    unsigned short test = 0x1122;
    if(*( (unsigned char*) &test ) == 0x11)
       return TRUE;
else
    return FALSE;

}//IsBig_Endian()

【打印程序对象的字节表示�?/font>

// 可在不同�q�_��与硬件架构的机器中测试运行这�D�代�?理解大端表示和小端表�C�的不同.
// �q�段代码使用强制�c�d��转换规避�c�d��pȝ��
#incluede <stdio.h>

// 假设每个字节都是非负整数
typedef unsigned char *byte_pointer;

void show_bytes(byte_pointer start, int len)
{
for(int i = 0; i < len; i++)
  printf(" %.2x", start[i]);
printf("\n");
}

void show_int(int x)
{
show_bytes((byte_pointer) &x, sizeof(int));
}

void show_float(float x)
{
show_bytes((byte_pointer) &x, sizeof(float));
}

// 在��用相同编�?如ASCII�~�码)的系�l�中,字符串字节表�C�得到的�l�果一般是相同�?所以文本数据比二进制数据具有更强的�q�_��无关�?/span>
void show_string(char *x)
{
show_bytes((byte_pointer) x, strlen(x));
}

void show_pointer(void *x)
{
show_bytes((byte_pointer) &x, sizeof(void *));
}

void test_show_bytes(int val)
{
int ival = val;
float fval = (float)ival;
int *pval = &ival;

show_int(ival); // 各个机器因�ؓ大端表示和小端表�C�的不同,从而只是字节顺序不�?/span>
show_float(fval); // 各个机器因�ؓ大端表示和小端表�C�的不同,从而只是字节顺序不�?/span>
show_pointer(pval); // 指针值是与机器相关的(linux,sun使用4字节地址, 而alpha使用八字节地址)
}

---------------------------------------------
对于如数�?2345在int型和float型时的编码表�C?br />

LukeW 2008-11-12 11:58 发表评论

j2me 联网技术分析�ȝ��

LukeW — Tue, 04 Nov 2008 08:22:00 GMT
基本�?

Generic Connections

In the CLDC Generic Connection framework, all connections are created using the open static method from the Connector class. If successful, this method returns an object that implements one of the generic connection interfaces. Figure 1 shows how these interfaces form an is-a hierarchy. The Connection interface is the base interface such that StreamConnectionNotifier is a Connection and InputConnection is a Connection too.

Figure 1: Connection interface hierarchy

The Connection interface is the most basic connection type. It can only be opened and closed.

The InputConnection interface represents a device from which data can be read. Its openInputStream method returns an input stream for the connection.

The OuputConnection interface represents a device to which data can be written. Its openOutputStream method returns an output stream for the connection.

The StreamConnection interface combines the input and output connections.

The ContentConnection is a subinterface of StreamConnection. It provides access to some of the basic meta data information provided by HTTP connections.

The StreamConnectionNotified waits for a connection to be established. It returns a StreamConnection on which a communication link has ben established.

The DatagramConnection represents a datagram endpoint.

The open method of the Connector class has the following syntax, where the String parameter has the format "protocol:address;parameters".

Connector.open(String);

Here are a few examples:

HTTP Connection

Connector.open("http://java.sun.com/developer");

Datagram Connection

Connector.open("datagram://address:port#");

Communicate with a Port

Connector.open("comm:0;baudrate=9600');

Open Files

Connector.open("file:/myFile.txt");

The HttpConnection Interface:

The HTTP protocol is a request-response application protocol in which the parameters of the request must be set before the request is sent. The connection could be in one of the three following states:

Setup: No connection yet

Connected: Connection has been made, the request has been sent, and some response is expected

Closed: Connection is closed

In the setup state the following methods can be invoked:

setRequestMethod

setRequestProperty

For example, suppose you have this connection:

HttpConnection c = (HttpConnection)
Connector.open("http://java.sun.com/developer");

Then, you can set the request method to be of type POST as follows:

c.setRequestMethod(HttpConnection.POST);

And likewise, you can set some of the HTTP properties. For example, you can set the User-Agent as follows:

c.setRequestProperty("User-Agent","Profile/MIDP-1.0 Configuration/CLDC-1.0");

If there is a method that requires data to be sent or received from the server, there is a state transition from Setup to Connected. Examples of methods that cause the transition include:

openInputStream
openOutputStream
openDataInputStream
openDataOutputStream
getLength
getType
getDate
getExpiration

And while the connection is open, some of these methods that may be invoked:

getURL
getProtocol
getHost
getPort

------------------------------------------------------------
要注意的问题:
开发中遇到个很头疼的问�? 与服务器通信write()数据时报java.io.IOException: Couldn't write to socket.
但是服务器抓不到��M��? 一开始怀疑是�q�徏立连接出的问�? 实际上服务器抓不到包也有可能是流在没有close的时候就已经报错�?
�?
conn.open("url");
out = conn.openDataOutputStream();//此时��进行与服务器的三次握手;
                                  //但是如果在out.close()之前出现异常服务器是抓不��C�Q何包�?br /> out.write(byte[] bb);

关于�q�个的解释应该是��的�~�冲机制.
所以正��的写法应该是捕捉到异常之后在catch块中把流close�?strong>.
服务器端开发�h员一般会说收不到包所以连接有问题,会把责�Q推给客户�?抓住�q�个证据在跟服务器端的同事扯皮时��处于有利的位置,嘎嘎.
�q�有��是要多做小实验, 注意代码要规范严�?

发现的几个问�?
1. java.io.IOException: Couldn't write to socket

2. java.io.IOException: Couldn't read from socket

CMNET联网�Ҏ��:

CMWAP联网�Ҏ��:

�U�d��资费��늚�处理:

一个通用的HTTP�q�接��装:

LukeW 2008-11-04 16:22 发表评论

LukeW — Wed, 24 Sep 2008 09:57:00 GMT
一些讨�?
http://topic.csdn.net/t/20061214/22/5231907.html

LukeW 2008-09-24 17:57 发表评论

eclipse真机调试J2me�E�序

LukeW — Fri, 15 Aug 2008 07:22:00 GMT
��单记录一下遇到的几个问题:

1. 真机调试其实是厂商SDK�Ҏ��JAVA的调试API层实现的.

2. 要注意JAD文�g中字�D늚�长度不要�q�长, 否则会报�? 无法调试.

3. 相比�? netbeans�q�行真机调试要方便一�? 当然eclipse应该也没问题(懒得实验�?.).

LukeW 2008-08-15 15:22 发表评论