?????? little endian和big endian是表示計(jì)算機(jī)字節(jié)順序的兩種格式,所謂的字節(jié)順序指的是長(zhǎng)度跨越多個(gè)字節(jié)的數(shù)據(jù)的存放形式.
??????? 假設(shè)從地址0x00000000開始的一個(gè)字中保存有數(shù)據(jù)0x1234abcd,那么在兩種不同的內(nèi)存順序的機(jī)器上從字節(jié)的角度去看的話分別表示為:
?????? 1)little endian:在內(nèi)存中的順序是0x00000000-0xcd,0x00000001-0xab,0x00000002-0x34,0x00000003-0x12
?????? 2)big? endian:在內(nèi)存中的順序是0x00000000-0x12,0x00000001-0x34,0x00000002-0xab,0x00000003-0xcd
?????? 需要特別說明的是,以上假設(shè)機(jī)器是每個(gè)內(nèi)存單元以8位即一個(gè)字節(jié)為單位的.
?????? 簡(jiǎn)單的說,little endian把低位存放到高位,而big endian把低位存放到低位.
?????? 現(xiàn)在主流的CPU,intel系列的是采用的little endian的格式存放數(shù)據(jù),而motorola系列的CPU采用的是big endian.
??????
?????? 以下是判斷字節(jié)存儲(chǔ)順序的可移植的C語言代碼:
??????

/**/ /* *******************************************************************
????created:????2006-9-5
????filename:?????test.cpp
????author:????????李創(chuàng)
????
????purpose:????可移植的用于判斷存儲(chǔ)格式是
????????????????little?endian還是big?ednian的C代碼
????????????????取自<<C:?A?Reference?Manual>>
******************************************************************** */

#include? < stdio.h >

union
{
???? long ?Long;
???? char ?Char[ sizeof ( long )];
} u;

int ?main()
{????
????u.Long? = ? 1 ;
????
???? if ?(u.Char[ 0 ]? == ? 1 )????
???? {
????????printf( " Little?Endian!\n " );
????}
???? else ? if ?(u.Char[ sizeof ( long )? - ? 1 ]? == ? 1 )
???? {
????????printf( " Big?Endian!\n " );
????}
???? else
???? {
????????printf( " Unknown?Addressing!\n " );
????}

????printf( " Now,?Let's?look?at?every?byte?in?the?memory!\n " );
???? for ?( int ?i? = ? 0 ;?i? < ? sizeof ( long );? ++ i)
???? {
????????printf( " [%x]?=?%x\n " ,? & u.Char[i],?u.Char[i]);
????}

???? return ? 0 ;
}

?

?????? 很多人認(rèn)為掌握這個(gè)知識(shí)是不必要,其實(shí)不然.在網(wǎng)絡(luò)編程中,TCP/IP統(tǒng)一采用big endian方式傳送數(shù)據(jù),也就是說,假設(shè)現(xiàn)在是在一個(gè)字節(jié)順序是little endian的機(jī)器上傳送數(shù)據(jù),要求傳送的數(shù)據(jù)是0XCEFABOBO,那么你就要以0XBOBOFACE的順序在unsigned int中存放這個(gè)數(shù)據(jù),只有這樣才能保證存放的順序滿足TCP/IP的字節(jié)順序要求.很多時(shí)候,需要自己編寫應(yīng)用層的協(xié)議,字節(jié)順序的概念在這個(gè)時(shí)候就顯得及其的重要了.
?????? 下面給出的是在big endian和little endian中相互轉(zhuǎn)換的代碼,C語言強(qiáng)大的位操作的能力在這里顯示了出來:
????????

/**/ /* *******************************************************************
????created:????2006-9-5
????filename:?????get32put32.cpp
????author:????????李創(chuàng)
????
????purpose:????在little?endian和big?ednian之間相互轉(zhuǎn)化數(shù)據(jù)的演示代碼

******************************************************************** */


#include? < stdio.h >

const ?unsigned? char ?SIZE_OF_UNSIGNEDINT?? = ? sizeof (unsigned? int );
const ?unsigned? char ?SIZE_OF_UNSIGNEDCHAR? = ? sizeof (unsigned? char );

void ?put_32(unsigned? char ? * cmd,?unsigned? int ?data)
{
???? int ?i;
???? for ?(i? = ?SIZE_OF_UNSIGNEDINT? - ? 1 ;?i? >= ? 0 ;? -- i)
???? {
????????cmd[i]? = ?data? % ? 256 ;
???????? // ?或者可以:
???????? // cmd[i]?=?data?&?0xFF;
????????data? = ?data? >> ? 8 ;
????}
}

unsigned? int ?get_32(unsigned? char ? * cmd)
{
????unsigned? int ??ret;
???? int ?i;

???? for ?(ret? = ? 0 ,?i? = ?SIZE_OF_UNSIGNEDINT? - ? 1 ;?i? >= ? 0 ;? -- i)
???? {
????????ret?? = ?ret? << ? 8 ;
????????ret? |= ?cmd[i];????????
????}
????
???? return ?ret;
}

int ?main( void )
{
????unsigned? char ?cmd[SIZE_OF_UNSIGNEDINT];
????unsigned? int ?data,?ret;
????unsigned? char ? * p;
???? int ?i;

????data? = ? 0x12345678 ;
????printf( " data?=?%x\n " ,?data);
???? // ?以字節(jié)為單位打印出數(shù)據(jù)
????p? = ?(unsigned? char * )( & data);
???? for ?(i? = ? 0 ;?i? < ?SIZE_OF_UNSIGNEDINT;? ++ i)
???? {
????????printf( " %x " ,? * p ++ );
????}
????printf( " \n " );

???? // ?以相反的順序存放到cmd之中
????put_32(cmd,?data);
???? for ?(i? = ? 0 ;?i? < ?SIZE_OF_UNSIGNEDINT;? ++ i)
???? {
????????printf( " cmd[%d]?=?%x\n " ,?i,?cmd[i]);
????}

???? // ?再以相反的順序保存數(shù)據(jù)到ret中
???? // ?保存之后的ret數(shù)值應(yīng)該與data相同
????ret? = ?get_32(cmd);
????printf( " ret?=?%x\n " ,?ret);
????p? = ?(unsigned? char * )( & ret);
???? for ?(i? = ? 0 ;?i? < ?SIZE_OF_UNSIGNEDINT;? ++ i)
???? {
????????printf( " %x " ,? * p ++ );
????}
????printf( " \n " );

???? return ? 0 ;
}