dynamically(dynamically是什么意思)

知识点9:动态内存分配(dynamic memory allocation)

Dynamicmemoryallocationsortofallowsustheway

togetaroundthisparticularproblem:

Wecanusepointerstogetaccesstodynamicallyallocatedmemory,memorythatisallocatedasyourprogramisrunning.It'snotallocatedatcompiletime.

Whenyoudynamicallyallocatememoryitcomesfromapoolofmemoryknownastheheap.

Previouslyallthememorywe'vebeenworkingwithinthecoursehasbeencomingfromapoolofmemoryknownasthestack.

Agoodwaytogenerallykeepinmindisthatanytimeyougiveavariableaname,itprobablylivesonthestack.

Andanytimeyoudon'tgiveavariableaname,whichyoucandowithdynamicmemoryallocation,itlivesontheheap.

NowI'mkindofpresentingthisasifthere'sthesetwopoolsofmemory.

thisdiagramisisgenerallyarepresentationofwhatmemorylookslike,andwe'renotgoingtocareaboutallthestuffatthetopandthebottom.

Whatwecareaboutisthispartinthemiddlehere,heapandstack.

Asyoucanseebylookingatthisdiagram,theseactuallyaren'ttwoseparatepoolsofmemory.

It'sonesharedpoolofmemorywhereyoustart,inthisvisualyoustartatthebottomandstartfillingupfromthebottomwiththestack,andyoustartatthetopandstartfillingupfromthetopdownwiththeheap.

Butitreallyisthesamepool,it'sjustdifferentspots,differentlocationsinmemorythatarebeingallocated.

Andyoucanrunoutofmemorybyeitherhavingtheheapgoallthewaytothebottom,orhavethestackgoallthewaytothetop,orhavingtheheapandthestackmeetupagainsteachother.

Allofthosecanbeconditionsthatcauseyourprogramtorunoutofmemory.

Sohowdowegetdynamicallyallocatedmemoryinthefirstplace?Howdoesourprogramgetmemoryasit'srunning?

WellCprovidesafunctioncalledmalloc,memoryallocator,whichyoumakeacallto,andyoupassinhowmanybytesofmemorythatyouwant.

Soifyourprogramisrunningandyouwantanintegerruntime,youmightmallockfourbytesofmemory,mallocparenthesesfour.

mallockwillgothroughlookingthroughtheheap,becausewe'redynamicallyallocatingmemory,anditwillreturntoyouapointertothatmemory.

Itdoesn'tgiveyouthatmemory--itdoesn'tgiveitaname,itgivesyouapointertoit.

Ifmallockcan'tgiveyouanymemorybecauseyou'verunout,it'llgiveyoubackanullpointer.Wesufferasegfault.

Soeverytimeyoumakeacalltomallocyoualways,alwaysneedtocheckwhetherornotthepointeritgaveyoubackisnull.

Ifitis,youneedtoendyourprogrambecauseifyoutryanddereferencethenullpointeryou'regoingtosufferasegmentationfaultandyourprogramisgoingtocrashanyway.

Sohowdowestaticallyobtainaninteger?

intx.

We'veprobablydonethatabunchoftimes,right?

Thiscreatesavariablecalledxthatlivesonthestack.

Howdowedynamicallyobtainaninteger?

Intstarpxequalsmalloc4.

Ormoreappropriatelywe'dsayintstarpxequalsmallocsizeofint,justtothrowsomefewermagicnumbersaroundourprogram.

Thisisgoingtoobtainforusfourbytesofmemoryfromtheheap,andthepointerwegetbacktoitiscalledpx.Andthenjustaswe'vedonepreviouslywecandereferencepxtoaccessthatmemory.

Whatifwewanttocreateanarrayofxfloatsthatliveonthestack?

floatstack_array--that'sthenameofourarray--squarebracketsx.Thatwillcreateforusanarrayofxfloatsthatliveonthestack.

Wecancreateanarrayoffloatsthatlivesontheheap,too.

Thesyntaxmightlookalittlemorecumbersome,butwecansayfloatstarheap_arrayequalsmallocxtimesthesizeofthefloat.Ineedenoughroomtoholdxfloatingpointvalues.

SosayIneed100floats,or1,000floats.Sointhatcaseitwouldbe400bytesfor100floats,or4,000bytesfor1,000floats,becauseeachfloattakesupfourbytesofspace.

AfterdoingthisIcanusethesquarebracketsyntaxonheap_array.

JustasIwouldonstack_array,Icanaccessitselementsindividuallyusingheap_arrayzero,heap_arrayone.ButrecallthereasonwecandothatisbecausethenameofanarrayinCisreallyapointertothatarray'sfirstelement.

Sothefactthatwe'redeclaringanarrayoffloatsonthestackhereisactuallyabitmisleading.Wereallyareinthesecondlineofcodetherealsocreatingapointertoachunkofmemorythatwethendosomeworkwith.

Here'sthebigproblemwithdynamicallyallocatedmemorythough,andthisiswhyit'sreallyimportanttodevelopsomegoodhabitswhenyou'reworkingwithit.

Unlikestaticallydeclaredmemory,yourmemoryisnotautomaticallyreturnedtothesystemwhenyourfunctionisdone.

Soifwehavemain,andmaincallsafunctionf,whenffinisheswhateverit'sdoingandreturnscontroloftheprogrambacktomain,allofthememorythatfusedisgivenback.Itcanbeusedagainbysomeotherprogram,orsomeotherfunctionthatgetscalledlateroninmain.Itcanusethatsamememoryoveragain.

Ifyoudynamicallyallocatememorythoughyouhavetoexplicitlytellthesystemthatyou'redonewithit.It'llholdontoitforyou,whichcouldleadtoaproblemofyourunningoutofmemory.

Andinfactwesometimesrefertothisasamemoryleak.

Andsometimesthesememoryleakscanactuallybereallydevastatingforsystemperformance.

Howdowegivememorybackwhenwe'redonewithit?Wellfortunatelyit'saveryeasywaytodoit.Wejustfreeit.

There'safunctioncalledfree,itacceptsapointertomemory,

Solet'ssaywe'reinthemiddleofourprogram,wewanttomalloc50characters.

Wewanttomallocanarraythatcancapableofholding50characters.Andwhenwegetapointerbacktothat,thatpointer'snameisword.Wedowhateverwe'regoingtodowithword,andthenwhenwe'redonewejustfreeit.Andnowwehavereturnedthose50bytesofmemorybacktothesystem.

Sotherearethreegoldenrulesthatshouldbekeptinmindwheneveryou'redynamicallyallocatingmemorywithmalloc.

Solet'sgothroughanexamplehereofwhatsomedynamicallyallocatedmemorymightlooklikemixedinwithsomestaticmemory.

Sowesayintm.

WhatifIthensayintstara?

SoI'mcoloringitgreen-ishaswell.Iknowithassomethingtodowithaninteger,butit'snotitselfaninteger.

Butit'sprettymuchthesameidea.I'vecreatedabox.Bothoftheserightnowliveonthestack.I'vegiventhembothnames.

Then,intstarbequalsmallocsizeofint.

Wellthisdoesn'tjustcreateonebox.Thisactuallycreatestwoboxes.Anditties(连接在一起),italsoestablishesapointinarelationship.

We'veallocatedoneblockofmemoryontheheap.Noticethatthetoprightboxtheredoesnothaveaname.

Wemallocdit.Itexistsontheheap.Butbhasaname.It'sapointervariablecalledb.Thatlivesonthestack.

Soit'sapieceofmemorythatpointstoanotherone.bcontainstheaddressofthatblockofmemory.Itdoesn'thaveanameotherwise.Butitpointstoit.

Nowwe'llgetlittlemorestraightforwardagain.aequalsampersandm.

Doyourecallwhataequalsampersandmis?Wellthat'sagetsm'saddress.Orputmorediagrammatically,apointstom.

OKsohere'sanotherone.

Aequalsb.

What'sgoingtohappentothediagramthistime?

Wellrecallthattheassignmentoperatorworksbyassigningthevalueontherighttothevalueontheleft.

Soinsteadofapointingtom,anowpointstothesameplacethatbpoints.adoesn'tpointtob,apointswherebpoints.

Ifapointedtobthatwouldhavebeenaequalsampersand(&)b.

Butinsteadaequalsbjustmeansthataandbarenowpointingtothesameaddress,becauseinsideofbisjustanaddress.Andnowinsideofaisthesameaddress.

mequals10,probablythemoststraightforwardthingwe'vedoneinalittlebit.Putthe10inthebox.

Starbequalsmplus2,recallfromourpointersvideowhatstarbmeans.

We'regoingtodereferencebandputsomevalueinthatmemorylocation.Inthiscase12.

Sowhenwedereferenceapointofrecallwejusttraveldownthearrow（遵循着箭头的轨迹）.

Orputanotherway,wegotothatmemoryaddressandwemanipulateitinsomeway.Weputsomevalueinthere.

Inthiscasestarbequalsmplus2isjustgotothevariablepointedtobyb,gotothememorypointedtobyb,andputmplus2inthere,12.

NowIfreeb.

WhathappenswhenIfreeb?

I'mdoneworkingwithit,right?Iessentiallygiveupthememory.Igiveitbacktothesystem.Idon'tneedthisanymoreiswhatI'mtellingthem,OK?

NowifIsaystaraequals11youcanprobablyalreadytellthatsomethingbadisgoingtohappenhere,right?AndindeedifItriedthatIprobablywouldsufferasegmentationfault.

Becausenow,althoughpreviouslythatchunkofmemorywassomethingthatIhadaccessto,atthispointnowI'maccessingmemorythatisnotlegalformetoaccess.

Andaswewillprobablyrecall,whenweaccessmemorythatwe'renotsupposedtotouch,that'sthemostcommoncauseofasegmentationfault.AndsomyprogramwouldcrashifItriedtodothis.

结语：

Soagainit'sagoodideatogetgoodpracticeandgoodhabitsingrainedwhenworkingwithmallocandfree,sothatyoudon'tsuffersegmentationfaults,andthatyouuseyourdynamicallyallocatedmemoryresponsibly.

dynamically是什么意思

dynamically

英[daɪˈnæmɪkli]美[daɪˈnæmɪkli]

adv.动态地；充满活力地；不断变化地

短语

dynamicallyequivalentmodel等效动力学模型;等效动力学模子;等效动力学

dynamicallybalanced动平衡的;做过动态平衡的

DynamicallyTypedLanguage动态类型语言;语言

DYNAMICALLYASSIGNEDPORTS端口

DynamicallyAllocated动态分配;使用静态指派空间;动态分配的;动态地分配

dynamicallycast动态地转换

dynamicallybound动态绑定

AdaptDynamically随需而动;动态适应;需而动;详细翻译

DynamicallySwitchable动态切换

双语例句

Theunhamperedmarketisaverydynamicallyefficientprocess.

无阻碍的市场是一个动态高效的过程。

Icanjustwhipoutavariable,andthistimeit'sgonnabedynamicallystoredthere.

我可以拿出一个变量，并且这次它会被动态地存储在那里。

He'soneofthemostdynamicallyimaginativejazzpianistsofourtime.

他是我们这个时代最具活跃想像力的爵士乐钢琴演奏家之一。

linux怎么查看oracle版本

1、命令获取法

以root用户登录系统后，执行‘sqlplus/assysdba’回车，这个时候可以看到oracle数据库的版本。

2、工具获取法，以plsqldeveloper为例。

登录成功后，输入‘select*fromv$version’,点击执行。

在查询结果里即可看到版本信息：

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