vppbuffer接口

　　头条创作挑战赛vppbuffer接口bufferVLIBbufferrepresentation。typedefunion｛使用union方便在结构体和byte（内存）之间转换struct｛CLIBCACHELINEALIGNMARK（cacheline0）；用来做cacheline对齐signedoffsetindata〔〕，predata〔〕thatwearecurrentlyprocessing。Ifnegativecurrentheaderpointsintopredataarea。i16currentdata；Nbytesbetweencurrentdataandtheendofthisbuffer。u16currentlength；bufferflags：
　　VLIBBUFFERFREELISTINDEXMASK：bitsusedtostorefreelistindex，
　　VLIBBUFFERISTRACED：tracethisbuffer。
　　VLIBBUFFERNEXTPRESENT：thisisamultichunkbuffer。
　　VLIBBUFFERTOTALLENGTHVALID：asitsays
　　VLIBBUFFEREXTHDRVALID：buffercontainsvalidexternalbuffermanagerheader，settoavoidaddingittoaflowreport
　　VLIBBUFFERFLAGUSER（n）：userdefinedbitNu32flags；Genericflowidentifieru32flowid；Referencecountforthisbuffer。volatileu8refcount；indexofbufferpoolthisbufferbelongs。u8bufferpoolindex；Errorcodeforbufferstobeenqueuedtoerrorhandler。vliberrorterror；Nextbufferforthislinkedlistofbuffers。OnlyvalidifVLIBBUFFERNEXTPRESENTflagisset。u32nextbuffer；Thefollowingfieldscanbeinaunionbecauseonceapacketentersthepuntpath，itisnolongeronafeaturearcunion｛Usedbyfeaturesubgrapharcstovisitenabledfeaturenodesu32currentconfigindex；thereasonthepacketoncepuntedu32puntreason；｝；Opaquedatausedbysubgraphsfortheirownpurposes。u32opaque〔10〕；partofbuffermetadatawhichisinitializedonallocendshere。STRUCTMARK（templateend）；startof2ndhalf（2ndcachelineonsystemswherecachelinesizeis64）CLIBALIGNMARK（secondhalf，64）；SpecifiestracebufferhandleifVLIBPACKETISTRACEDflagisset。u32tracehandle；Onlyvalidforfirstbufferinchain。Currentlengthplustotallengthgivenheregivetotalnumberofbytesinbufferchain。u32totallengthnotincludingfirstbuffer；Moreopaquedata，see。。vnetvnetbuffer。hu32opaque2〔14〕；ifVLIBBUFFERTRACETRAJECTORY0tracetrajectorydataweuseaspecificcachelineforthatinthebufferwhenitiscompiledindefineVLIBBUFFERTRACETRAJECTORYMAX31defineVLIBBUFFERTRACETRAJECTORYSZ64defineVLIBBUFFERTRACETRAJECTORYINIT（b）（b）trajectorynb0CLIBALIGNMARK（trajectory，64）；u16trajectorynb；u16trajectorytrace〔VLIBBUFFERTRACETRAJECTORYMAX〕；elseVLIBBUFFERTRACETRAJECTORYdefineVLIBBUFFERTRACETRAJECTORYSZ0defineVLIBBUFFERTRACETRAJECTORYINIT（b）endifVLIBBUFFERTRACETRAJECTORYstartofbufferheadroomCLIBALIGNMARK（headroom，64）；Spaceforinsertingdatabeforebufferstart。Packetrewritestringwillberewrittenbackwardsandmayextendbackbeforebufferdata〔0〕。Mustcomedirectlybeforepacketdata。u8predata〔VLIBBUFFERPREDATASIZE〕；Packetdatau8data〔〕；变长数组，用来保存数据部分｝；ifdefCLIBHAVEVEC128u8x16asu8x16〔4〕；endififdefCLIBHAVEVEC256u8x32asu8x32〔2〕；endififdefCLIBHAVEVEC512u8x64asu8x64〔1〕；endif｝vlibbuffert；常用接口增将数据data添加到buffer里，没有空余的buffer会申请新的内存，同时会将buffer的索引赋值给bufferindex，方便后续用于访问
　　intvlibbufferadddata（vlibmaintvm，vlibbufferfreelistindextfreelistindex，u32bufferindex，voiddata，u32ndatabytes）；删alwaysinlinevoid
　　vlibbufferfree（vlibmaintvm，
　　pointertofirstbuffer
　　u32buffers，
　　numberofbufferstofree
　　u32nbuffers）；改通过移动buffer的currentdata指针来取数据或者添加数据，并同时修改buffer的currentlength值
　　一般l为正数表示往前读取数据，当然也可以进行修改
　　一般l为负数表示往后添加数据
　　alwaysinlinevoidvlibbufferadvance（vlibbuffertb，wordl）；查vm是vpp运行时主结构体，bufferindex是buffer的索引，一般在添加buffer时可以获取到，或者遍历buffer时获取
　　alwaysinlinevlibbuffertvlibgetbuffer（vlibmaintvm，u32bufferindex）；
　　获取当前数据所在的指针位置，一般用于对buffer进行指针偏移后使用
　　alwaysinlinevoidvlibbuffergetcurrent（vlibbuffertb）；