nodejs(三)Buffer module & Byte Order

一。目录

➤UnderstandingwhyyouneedbuffersinNode

➤Creatingabufferfromastring

➤Convertingabuffertoastring

➤Manipulatingthebytesinabuffer

➤Slicingandcopyingabuffer

二。UnderstandingwhyyouneedbuffersinNode

　JavaScriptisgoodathandlingstrings,butbecauseitwasinitiallydesignedtomanipulateHTMLdocuments,itisnotverygoodat

handlingbinarydata.JavaScriptdoesn’thaveabytetype—itjusthasnumbers—orstructuredtypes,orevenbytearrays:Itjust

hasstrings.　　

　　BecauseNodeisbasedonJavaScript,NodecanhandletextprotocolslikeHTTP,butyoucanalsouseittotalktodatabases,

manipulateimages,andhandlefileuploads.Asyoucanimagine,doingthisusingonlystringsisverydifficult.Intheearlydays,

Nodehandledbinarydatabyencodingeachbyteinsideatextcharacter,butthatprovedtobewasteful,slow,unreliable,andhard

tomanipulate.

　　Tomakethesetypesofbinary-handlingtaskseasier,Nodeincludesabinarybufferimplementation,whichisexposedasaJavaScript

APIundertheBufferpseudo-class.Abufferlengthisspecifiedinbytes,andyoucanrandomlysetandgetbytesfromabuffer.

　　AnotherthingthatisspecialaboutthisbufferclassisthatthememorywherethedatasitsisallocatedoutsideoftheJavaScriptVM

memoryheap.Thismeansthattheseobjectswillnotbemovedaroundbythegarbage-collectionalgorithm:Itwillsitinthispermanent

memoryaddresswithoutchanging,whichsavesCPUcyclesthatwouldbewastedmakingmemorycopiesofthebuffercontents.

三。GETTINGANDSETTINGBYTESINABUFFER

　　Afteryoucreateorreceiveabuffer,youmightwanttoinspectandchangeitscontents.Youcanaccessthebytevalueonany

positionofabufferbyusingthe[]operatorlikethis:

varbuf=newBuffer('mybuffercontent');
//accessingthe10thpositionofbuf
console.log(buf[10]);//->99

　　//Youcanalsomanipulatethecontentofanyposition:
　　buf[20]=125;//setthe21thbyteto125

四。注意

NOTEWhenyoucreateaninitializedbuffer,keepinmindthatitwillcontainrandombytes,notzeros.

varbuf=newBuffer(1024);
console.log(buf[100]);//->5(somerandomnumber)

　　

NOTEIncertaincases,somebufferoperationswillnotyieldanerror.Forinstance:

➤Ifyousetanypositionofthebuffertoanumbergreaterthan255,thatpositionwillbeassignedthe256modulovalue.

➤Ifyouassignthenumber256toabufferposition,youwillactuallybeassigningthevalue0.

➤Ifyouassignafractionalnumberlike100.7toabufferposition,thebufferpositionwillstoretheintegerpart—100inthiscase.

➤Ifyoutrytoassignapositionthatisoutofbounds,theassignmentwillfailandthebufferwillremainunaltered.

五。字节序

　　又称端序，尾序（英语：ByteOrder,orEndianness）。现代的计算机系统一般采用字节(8bitByte)作为逻辑寻址单位。当物理单位的长度大于

1个字节时，就要区分字节顺序。典型的情况是整数在内存中的存放方式和网络传输的传输顺序。目前在各种体系的计算机中通常采用的字节存储机制主要

有两种：big-edian和little-endian。





六。字节序示例

varb=newBuffer('123456789');
console.log(b.length);//9
console.log(b);　　　　　　　　　　//<Buffer313233343536373839>

varb2=newBuffer(4);
b2.writeInt32LE(123456789,0);
console.log(b2.length);　　　　　　//4
console.log(b2);　　　　　　　　　　//<Bufferb5e53907>