The Kernel Abstraction - Operating Systems: Principles and Practice - page 87

intKernelStub_Open(){
char*localCopy[MaxFileNameSize+1];

//checkthatstackpointerisvalidandthatargumentsarestoredatvalidaddresses
if(!validUserAddressRange(userStackPointer,userStackPointer+sizeofargumentsonstack))
returnerror_code;

//fetchpointertofilenamefromuserstack,andconverttoakernelpointer
filename=VirtualToKernel(userStackPointer);

//makealocalcopyofthefilename,insidetheOS
//thispreventstheapplicationfromchangingthename
//surreptitiously,afterthecheck,butbeforetheread!

//thestringcopyneedstocheckeachaddressinthestringbeforeuse
//tomakesureeveryaddressinthestringisvalid

//thestringcopyterminatesafteritcopiesMaxFileNameSize
//toensurewedon'toverwriteourinternalbuffer

if(!VirtualToKernelStringCopy(filename,localCopy,MaxFileNameSize))
returnerror_code;

//let'smakesureourlocalcopyisnullterminated

localCopy[MaxFileNameSize]=0;

//wecannowcheckiftheuserispermittedtoaccessthisfile

if(!UserFileAccessPermitted(localCopy,current_process)
returnerror_code;

//finallywecancalltheactualroutinetoopenthefile
//thisreturnsafilehandleonsuccess,oranerror_codeonfailure

returnKernel_Open(localCopy);

}

Figure2.14: Stub routine for the open system call inside the kernel

start(arg1,arg2){
main(arg1,arg2); //calluser'smain
exit(); //ifmainreturns,callexit

}