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volatile 用法
时间：2023-02-04 06:33:42 下载该word文档
嵌入式编程中经常用到volatile这个关键字，在网上查了下他的用法找到了下面这3篇文章比较好的：一：告诉compiler不能做任何优化比如要往某一地址送两指令：int*ip=...;//设备地址*ip=1;//第一个指令*ip=2;//第二个指令以上程序compiler可能做优化而成：int*ip=...;*ip=2;结果第一个指令丢失。如果用volatile,compiler就不允许做任何的优化，从而保证程序的原意：volatileint*ip=...;*ip=1;*ip=2;即使你要compiler做优化，它也不会把两次付值语句间化为一。它只能做其它的优化。这对devicedriver程序员很有用。二：表示用volatile定义的变量会在程序外被改变,每次都必须从内存中读取，而不能把他放在cache或寄存器中重复使用。如volatilechara;a=0;while(!a{//dosomethings;}doother(;如果没有volatiledoother(不会被执行还找到一篇比较详细的比较CONST和Volatile的文章8.4.ConstandvolatileThesearenewinStandardC,althoughtheideaofconsthasbeenborrowedfromC++.Letusgetonethingstraight:theconceptsofconstandvolatilearecompletelyindependent.Acommonmisconceptionistoimaginethatsomehowconstistheoppositeofvolatileandviceversa.Theyareunrelatedandyoushouldrememberthefact.Sinceconstdeclarationsarethesimpler,we'lllookatthemfirst,butonlyafterwehaveseenwherebothofthesetypequalifiersmaybeused.Thecompletelistofrelevantkeywordsis
charlongfloatvolatileshortsigneddoublevoidintunsignedconstInthatlist,constandvolatilearetypequalifiers,therestaretypespecifiers.Variouscombinationsoftypespecifiersarepermitted:char,signedchar,unsignedcharint,signedint,unsignedintshortint,signedshortint,unsignedshortintlongint,signedlongint,unsignedlongintfloatdoublelongdoubleAfewpointsshouldbenoted.Alldeclarationstodowithanintwillbesignedanyway,sosignedisredundantinthatcontext.Ifanyothertypespecifierorqualifierispresent,thentheintpartmaybedropped,asthatisthedefault.Thekeywordsconstandvolatilecanbeappliedtoanydeclaration,includingthoseofstructures,unions,enumeratedtypesortypedefnames.Applyingthemtoadeclarationiscalledqualifyingthedeclaration—that'swhyconstandvolatilearecalledtypequalifiers,ratherthantypespecifiers.Hereareafewrepresentativeexamples:volatilei;volatileintj;constlongq;constvolatileunsignedlongintrt_clk;struct{