C / C++ / MFC

13-Feb-10 7:03

Hi everyone.
I've got a code snippet which looks like this

CInternetSession session(_T("MyAgent"));
CHttpConnection *pConn = NULL;
pConn = session.GetHttpConnection(_T("www.google.com"),
(INTERNET_PORT)INTERNET_DEFAULT_HTTP_PORT); // function in wininet.dll
......
delete pConn;

The last delete operator works fine. The problem is that it shouldn't, because the exe from which I call wininet.dll function is linked with CRT statically, so the delete operator should look for pConn in it's CRT's run time heap and shouldn't find it. Please share any ideas.

Thanks in advance.

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C^hris L^osinger13-Feb-10 17:52

C^hris L^osinger

13-Feb-10 17:52

here's the source for CInternetSession::GetHttpConnection:

CHttpConnection* CInternetSession::GetHttpConnection(LPCTSTR pstrServer,
	DWORD dwFlags, INTERNET_PORT nPort /* = INTERNET_INVALID_PORT_NUMBER */,
	LPCTSTR pstrUserName /* = NULL */, LPCTSTR pstrPassword /* = NULL */)
{
	ASSERT(AfxIsValidString(pstrServer));
	ASSERT((dwFlags & INTERNET_FLAG_ASYNC) == 0);

	CHttpConnection* pReturn = new CHttpConnection(this, pstrServer,
			dwFlags, nPort, pstrUserName, pstrPassword, m_dwContext);
	return pReturn;
}

it's allocated on the caller's heap, not in WinInet.DLL

that's in VC6's MFC\SRC\Inet.cpp

naro2415-Feb-10 11:32

15-Feb-10 11:32

Tnank you very much for your reply, but please take a look at this text whick I took from Jeffrey Richter's "Windows via C++"

VOID EXEFunc() {
PVOID pv = DLLFunc();
// Access the storage pointed to by pv...
// Assumes that pv is in EXE's C/C++ run-time heap
free(pv);
}

PVOID DLLFunc() {
// Allocate block from DLL's C/C++ run-time heap
return(malloc(100));
}

So, what do you think? Does the preceding code work correctly? Is the block allocated by the DLL's function freed by the EXE's function? The answer is: maybe. The code shown does not give you enough information. If both the EXE and the DLL link to the DLL C/C++ run-time library, the code works just fine. However, if one or both of the modules link to the static C/C++ run-time library, the call to free fails. I have seen developers write code similar to this too many times, and it has burned them all...

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C^hris L^osinger15-Feb-10 11:36

C^hris L^osinger

15-Feb-10 11:36

the code i posted above is the code that you are calling. the object you are deleting is allocated by your app's CRT.

the DLL/EXE CRT allocation issue is not relevant here.

modified on Monday, February 15, 2010 8:14 PM

naro2415-Feb-10 23:33

15-Feb-10 23:33

Thank you for your reply, please take a look at my reply to Richard in the next thread.

Richard MacCutchan13-Feb-10 21:45

13-Feb-10 21:45

naro24 wrote:
so the delete operator should look for pConn in it's CRT's run time heap

Library functions do not have their own heap. The heap is allocated at run time and shared by the exe file and all library functions.

txtspeak is the realm of 9 year old children, not developers. Christian Graus

naro2415-Feb-10 11:35

15-Feb-10 11:35

Richard MacCutchan15-Feb-10 11:58

15-Feb-10 11:58

naro24 wrote:
Is the block allocated by the DLL's function freed by the EXE's function? The answer is: maybe.

Actually the answer is "Yes". As I said earlier there is only one heap within a running application. Whether the actual code is part of the EXE or part of the DLL is irrelevant, the OS maps both parts into the same address space and they all share the same data area within that address space. If this was not so then 99% of commercial applications would not work correctly.

txtspeak is the realm of 9 year old children, not developers. Christian Graus

naro2415-Feb-10 23:30

15-Feb-10 23:30

Thanks for your reply, but let me to disagree with you.

Richard MacCutchan wrote:
As I said earlier there is only one heap within a running application.

An application can actually create and use as many heaps as needed using HeapCreate,HeapAlloc,HeapFree and other APIs.

Richard MacCutchan wrote:
Actually the answer is "Yes"

The answer is not always "Yes". I tested the following code in vs2005

int main()
{
int* a = fnHeapTestLib();
std::cout<<*a<<std::endl;
delete="" a;
="" std::cout<<*a<<std::endl;
="" return="" 0;=""
}

heaptestlib_api="" int*="" fnheaptestlib(void)
{
="" p="new" int;
="" *p="42;
" p;
}

i="" built="" in="" release="" version="" using="" 4="" different="" configurations,="" and="" here="" are="" the="" results
1)both="" exe="" dll="" linked="" to="" crt="" dynamically
="" works="" fine
2)exe="" is="" statically,="" fine(="" very="" strange="" fact="" )
3)exe="" dynamically,="" statically
="" fails(shows="" same="" value="" after="" before="" delete,="" debug="" raises="" assertion)
4)both="" assertion)

i="" can't="" explain="" this="" behavior,="" but="" makes="" obvious="" that="" allocation="" issue="" relevant="" here.

<blockquote="" class="FQ">

Richard MacCutchan wrote:

If this was not so then 99% of commercial applications would not work correctly.

May be 99% of commercial applications and their DLLs link to CRT dynamically.

Regards

Richard MacCutchan16-Feb-10 0:05

16-Feb-10 0:05

naro24 wrote:
delete fails(shows the same value after and before delete, in debug version raises assertion)

The delete operation does not clear the contents of the memory, so testing the contents before and after is not a valid test. When a block is deleted its header is merely marked to show it is available for reallocation within the running program. As to your comments about HeapCreate() etc, if they work as you believe then I am certain that many commercial programs would be failing on a regular basis. Take a look at the documentation for these functions.

txtspeak is the realm of 9 year old children, not developers. Christian Graus

naro2416-Feb-10 1:37

16-Feb-10 1:37

Could you please make this small test in vs2005 or vs2008
1)Make an exe and dll using the code in my previous post
2)Make both projects link to CRT statically
3)Build both in debug mode

As you claim everything must work properly, but you'll see an assertion on delete operator. Can you explain why?

Richard MacCutchan16-Feb-10 2:30