Why value changed after context switch?
in [Visual C]
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From: bucher on 9 May 2008 06:15 Hi, I have a function exported from dll. Here is the declaration: typedef struct tagPolicyBufEntry{ char* pPolicyBuf; int iSize; }POLICYBUFENTRY; extern "C" _declspec(dllexport) DWORD DBIPolicy_QueryPolicyListByHosts(const int iPolicyType, const CArray<int, int&>& iHostIDList, map<int, POLICYBUFENTRY>& policyList); I called it in another DLL named Policy_GetUSBDenyPolicy, here is the code: map<int, POLICYBUFENTRY> policyBufList; CArray<int, int&> iHostIDList; typedef DWORD (*lpDBIPolicy_QueryPolicyListByHosts)(const int iPolicyType, const CArray<int, int&>& iHostIDList, map<int, POLICYBUFENTRY>& policyList); lpDBIPolicy_QueryPolicyListByHosts pDBIPolicy_QueryPolicyListByHosts = (lpDBIPolicy_QueryPolicyListByHosts)(GetProcAddress(hDllPolicy, "DBIPolicy_QueryPolicyListByHosts")); dwError = pDBIPolicy_QueryPolicyListByHosts(DBOPT_USBDENYPOLICY, iHostIDList, policyBufList); I found the value of policyBufList._Tr._Nil is not NULL in "Policy_GetUSBDenyPolicy". But after I switched the context to "DBIPolicy_QueryPolicyListByHosts", the value of policyBufList._Tr._Nil was changed to NULL immediately. And the address of this member value was changed after context switch. This is very strange. Because the parameter was passed by reference. And I have checked the project settings, I set both dlls to "/Zp1 /MD". Why the value changed after context switch? Thanks in advanced. Bucher
From: bucher on 9 May 2008 06:43 Sorry, I misunderstanded the mean of context switch. What I want to ask is why value changed after call stack switch.
From: Igor Tandetnik on 9 May 2008 07:29 "bucher" <bucher(a)xxxx.com> wrote in message news:epRk22bsIHA.2068(a)TK2MSFTNGP05.phx.gbl > I found the value of policyBufList._Tr._Nil is not NULL in > "Policy_GetUSBDenyPolicy". But after I switched the context to > "DBIPolicy_QueryPolicyListByHosts", the value of > policyBufList._Tr._Nil was changed to NULL immediately. _Nil is a static data member. Each module (in your case, the EXE and the DLL) have its own separate copy of it. You are confusing yourself when you access it as if it were a regular data member, part of the map object you pass as a parameter: it's not. -- With best wishes, Igor Tandetnik With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925
From: Igor Tandetnik on 9 May 2008 07:36
"Igor Tandetnik" <itandetnik(a)mvps.org> wrote in message news:%233vS2fcsIHA.3716(a)TK2MSFTNGP04.phx.gbl > "bucher" <bucher(a)xxxx.com> wrote in message > news:epRk22bsIHA.2068(a)TK2MSFTNGP05.phx.gbl >> I found the value of policyBufList._Tr._Nil is not NULL in >> "Policy_GetUSBDenyPolicy". But after I switched the context to >> "DBIPolicy_QueryPolicyListByHosts", the value of >> policyBufList._Tr._Nil was changed to NULL immediately. > > _Nil is a static data member. Each module (in your case, the EXE and > the DLL) have its own separate copy of it. You are confusing yourself > when you access it as if it were a regular data member, part of the > map object you pass as a parameter: it's not. Do you use VC6, by any chance? If so, see http://www.dinkumware.com/vc_fixes.html "Fix to <xtree>" section. While you are at it, you may want to apply all the other fixes (or just switch to a modern version of VC compiler). -- With best wishes, Igor Tandetnik With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925 |