little endian和big endian的概念解释,判定与用途
2006-09-11 08:54
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little endian和big endian的概念解释,判定与用途
little endian和big endian是表示计算机字节顺序的两种格式,所谓的字节顺序指的是长度跨越多个字节的数据的存放形式.
假设从地址0x00000000开始的一个字中保存有数据0x1234abcd,那么在两种不同的内存顺序的机器上从字节的角度去看的话分别表示为:
1)little endian:在内存中的顺序是0x00000000-0xcd,0x00000001-0xab,0x00000002-0x34,0x00000003-0x12
2)big endian:在内存中的顺序是0x00000000-0x12,0x00000001-0x34,0x00000002-0xab,0x00000003-0xcd
需要特别说明的是,以上假设机器是每个内存单元以8位即一个字节为单位的.
简单的说,little endian把低位存放到高位,而big endian把低位存放到低位.
现在主流的CPU,intel系列的是采用的little endian的格式存放数据,而motorola系列的CPU采用的是big endian.
以下是判断字节存储顺序的可移植的C语言代码:
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
/**/ /* *******************************************************************
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
created: 2006-9-5
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
filename: test.cpp
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
author: 李创
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
purpose: 可移植的用于判断存储格式是
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
little endian还是big ednian的C代码
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
取自<<C: A Reference Manual>>
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
******************************************************************** */
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
#include < stdio.h >
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
union
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
long Long;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
char Char[ sizeof ( long )];
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
} u;
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
int main()
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
u.Long = 1 ;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
if (u.Char[ 0 ] == 1 )
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " Little Endian!/n " );
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
else if (u.Char[ sizeof ( long ) - 1 ] == 1 )
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " Big Endian!/n " );
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
else
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " Unknown Addressing!/n " );
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " Now, Let's look at every byte in the memory!/n " );
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
for ( int i = 0 ; i < sizeof ( long ); ++ i)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " [%x] = %x/n " , & u.Char[i], u.Char[i]);
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
return 0 ;
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
很多人认为掌握这个知识是不必要,其实不然.在网络编程中,TCP/IP统一采用big endian方式传送数据,也就是说,假设现在是在一个字节顺序是little endian的机器上传送数据,要求传送的数据是0XCEFABOBO,那么你就要以0XBOBOFACE的顺序在unsigned int中存放这个数据,只有这样才能保证存放的顺序满足TCP/IP的字节顺序要求.很多时候,需要自己编写应用层的协议,字节顺序的概念在这个时候就显 得及其的重要了.
下面给出的是在big endian和little endian中相互转换的代码,C语言强大的位操作的能力在这里显示了出来:
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
/**/ /* *******************************************************************
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
created: 2006-9-5
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
filename: get32put32.cpp
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
author: 李创
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
purpose: 在little endian和big ednian之间相互转化数据的演示代码
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
******************************************************************** */
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
#include < stdio.h >
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
const unsigned char SIZE_OF_UNSIGNEDINT = sizeof (unsigned int );
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
const unsigned char SIZE_OF_UNSIGNEDCHAR = sizeof (unsigned char );
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
void put_32(unsigned char * cmd, unsigned int data)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
int i;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
for (i = SIZE_OF_UNSIGNEDINT - 1 ; i >= 0 ; -- i)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
cmd[i] = data % 256 ;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
// 或者可以:
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
// cmd[i] = data & 0xFF;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
data = data >> 8 ;
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
unsigned int get_32(unsigned char * cmd)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
unsigned int ret;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
int i;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
for (ret = 0 , i = SIZE_OF_UNSIGNEDINT - 1 ; i >= 0 ; -- i)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
ret = ret << 8 ;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
ret |= cmd[i];
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
return ret;
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
int main( void )
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
unsigned char cmd[SIZE_OF_UNSIGNEDINT];
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
unsigned int data, ret;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
unsigned char * p;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
int i;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
data = 0x12345678 ;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " data = %x/n " , data);
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
// 以字节为单位打印出数据
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
p = (unsigned char * )( & data);
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
for (i = 0 ; i < SIZE_OF_UNSIGNEDINT; ++ i)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " %x " , * p ++ );
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " /n " );
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
// 以相反的顺序存放到cmd之中
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
put_32(cmd, data);
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
for (i = 0 ; i < SIZE_OF_UNSIGNEDINT; ++ i)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " cmd[%d] = %x/n " , i, cmd[i]);
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
// 再以相反的顺序保存数据到ret中
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
// 保存之后的ret数值应该与data相同
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
ret = get_32(cmd);
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " ret = %x/n " , ret);
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
p = (unsigned char * )( & ret);
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
for (i = 0 ; i < SIZE_OF_UNSIGNEDINT; ++ i)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " %x " , * p ++ );
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " /n " );
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
return 0 ;
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
}
字节顺序:大端字节(big-endian)和小端字节(little-endian)
2007-06-05 17:50
little endian和big endian是表示计算机字节顺序的两种格式,所谓的字节顺序指的是长度跨越多个字节的数据的存放形式.
假设从地址0x00000000开始的一个字中保存有数据0x1234abcd,那么在两种不同的内存顺序的机器上从字节的角度去看的话分别表示为:
1)little endian:在内存中的顺序是0x00000000-0xcd,0x00000001-0xab,0x00000002-0x34,0x00000003-0x12
2)big endian:在内存中的顺序是0x00000000-0x12,0x00000001-0x34,0x00000002-0xab,0x00000003-0xcd
需要特别说明的是,以上假设机器是每个内存单元以8位即一个字节为单位的.
简单的说,little endian把低位存放到高位,而big endian把低位存放到低位.
现在主流的CPU,intel系列的是采用的little endian的格式存放数据,而motorola系列的CPU采用的是big endian.
以下是判断字节存储顺序的可移植的C语言代码:
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
/**/ /* *******************************************************************
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
created: 2006-9-5
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
filename: test.cpp
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
author: 李创
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
purpose: 可移植的用于判断存储格式是
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
little endian还是big ednian的C代码
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
取自<<C: A Reference Manual>>
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
******************************************************************** */
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
#include < stdio.h >
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
union
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
long Long;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
char Char[ sizeof ( long )];
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
} u;
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
int main()
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
u.Long = 1 ;
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
if (u.Char[ 0 ] == 1 )
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " Little Endian!/n " );
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
else if (u.Char[ sizeof ( long ) - 1 ] == 1 )
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " Big Endian!/n " );
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
else
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " Unknown Addressing!/n " );
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " Now, Let's look at every byte in the memory!/n " );
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
for ( int i = 0 ; i < sizeof ( long ); ++ i)
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedSubBlock.gif)
![](http://www.cppblog.com/Images/dot.gif)
{
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
printf( " [%x] = %x/n " , & u.Char[i], u.Char[i]);
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
return 0 ;
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://www.cppblog.com/Images/OutliningIndicators/None.gif)
很多人认为掌握这个知识是不必要,其实不然.在网络编程中,TCP/IP统一采用big endian方式传送数据,也就是说,假设现在是在一个字节顺序是little endian的机器上传送数据,要求传送的数据是0XCEFABOBO,那么你就要以0XBOBOFACE的顺序在unsigned int中存放这个数据,只有这样才能保证存放的顺序满足TCP/IP的字节顺序要求.很多时候,需要自己编写应用层的协议,字节顺序的概念在这个时候就显 得及其的重要了.
下面给出的是在big endian和little endian中相互转换的代码,C语言强大的位操作的能力在这里显示了出来:
![](http://www.cppblog.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/ContractedBlock.gif)
/**/ /* *******************************************************************
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
created: 2006-9-5
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
filename: get32put32.cpp
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
author: 李创
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
![](http://www.cppblog.com/Images/OutliningIndicators/InBlock.gif)
purpose: 在little endian和big ednian之间相互转化数据的演示代码
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******************************************************************** */
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#include < stdio.h >
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const unsigned char SIZE_OF_UNSIGNEDINT = sizeof (unsigned int );
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const unsigned char SIZE_OF_UNSIGNEDCHAR = sizeof (unsigned char );
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void put_32(unsigned char * cmd, unsigned int data)
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{
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int i;
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for (i = SIZE_OF_UNSIGNEDINT - 1 ; i >= 0 ; -- i)
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{
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cmd[i] = data % 256 ;
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// 或者可以:
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// cmd[i] = data & 0xFF;
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data = data >> 8 ;
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}
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}
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unsigned int get_32(unsigned char * cmd)
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{
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unsigned int ret;
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int i;
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for (ret = 0 , i = SIZE_OF_UNSIGNEDINT - 1 ; i >= 0 ; -- i)
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{
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ret = ret << 8 ;
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ret |= cmd[i];
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}
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return ret;
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}
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int main( void )
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{
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unsigned char cmd[SIZE_OF_UNSIGNEDINT];
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unsigned int data, ret;
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unsigned char * p;
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int i;
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data = 0x12345678 ;
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printf( " data = %x/n " , data);
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// 以字节为单位打印出数据
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p = (unsigned char * )( & data);
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for (i = 0 ; i < SIZE_OF_UNSIGNEDINT; ++ i)
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{
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printf( " %x " , * p ++ );
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}
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printf( " /n " );
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// 以相反的顺序存放到cmd之中
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put_32(cmd, data);
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for (i = 0 ; i < SIZE_OF_UNSIGNEDINT; ++ i)
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{
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printf( " cmd[%d] = %x/n " , i, cmd[i]);
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}
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// 再以相反的顺序保存数据到ret中
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// 保存之后的ret数值应该与data相同
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ret = get_32(cmd);
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printf( " ret = %x/n " , ret);
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p = (unsigned char * )( & ret);
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for (i = 0 ; i < SIZE_OF_UNSIGNEDINT; ++ i)
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{
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printf( " %x " , * p ++ );
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}
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printf( " /n " );
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return 0 ;
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}
字节顺序:大端字节(big-endian)和小端字节(little-endian)
2007-06-05 17:50
因为现行的计算机都是以八位一个字节为存储单位,那么一个16位的整数,也就是C语言中的short,在内存中可能有两种存储顺序big-endian和 litte-endian.考虑一个short整数0x3132(0x32是低位,0x31是高位),把它赋值给一个short变量,那么它在内存中的存储可能有如下两种情况 因为现行的计算机都是以八位一个字节为存储单位,那么一个16位的整数,也就是C语言中的short,在内存中可能有两种存储顺序big-endian和litte-endian.考虑一个short整数0x3132(0x32是低位,0x31是高位),把它赋值给一个short变量,那么它在内存中的存储可能有如下两种情况: 大端字节(Big-endian): ----------------->>>>>>>>内存地址增大方向 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端.可以这样记住(大端->高位->在前->正常的逻辑顺序) 小端字节(little-endian): ----------------->>>>>>>>内存地址增大方向 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端.可以这样记住(小端->低位->在前->与正常逻辑顺序相反) 可以做个实验 在windows上下如下程序 #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文件,可以看见内容是21,而test等于0x3132,可以明显的看出来x86的字节顺序是低位在前.如果我们把这段同样的代码放到(big-endian)的机器上执行,那么打出来的文件就是12.这在本机中使用是没有问题的.但当你把这个文件从一个big-endian机器复制到一个little-endian机器上时就出现问题了. 如上述例子,我们在big-endian的机器上创建了这个test文件,把其复制到little-endian的机器上再用fread读到一个short里面,我们得到的就不再是0x3132而是0x3231了,这样读到的数据就是错误的,所以在两个字节顺序不一样的机器上传输数据时需要特别小心字节顺序,理解了字节顺序在可以帮助我们写出移植行更高的代码. 正因为有字节顺序的差别,所以在网络传输的时候定义了所有字节顺序相关的数据都使用big-endian,BSD的代码中定义了四个宏来处理: #define ntohs(n) //网络字节顺序到主机字节顺序 n代表net, h代表host, s代表short #define htons(n) //主机字节顺序到网络字节顺序 n代表net, h代表host, s代表short #define ntohl(n) //网络字节顺序到主机字节顺序 n代表net, h代表host, s代表 long #define htonl(n) //主机字节顺序到网络字节顺序 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现: #define sw16(x) / ((short)( / (((short)(x) & (short)0x00ffU) << 8) | / (((short)(x) & (short)0xff00U) >> 8) )) 这里实现的是一个交换两个字节顺序.其他几个宏类似. 我们改写一下上面的程序 #include <stdio.h> #include <assert.h> #define sw16(x) / ((short)( / (((short)(x) & (short)0x00ffU) << 8) | / (((short)(x) & (short)0xff00U) >> 8) )) // 因为x86下面是低位在前,需要交换一下变成网络字节顺序 #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,由于与网络字节顺序一致,所以我们什么都不干就可以了,只需要把#define htons(x) sw16(x)宏替换为 #define htons(x) (x). |
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