C / C++ / MFC

Re: FileMapping book exmple goes in loop

8-Aug-22 8:48

Hi,

I am following an example from a book and it doesn't work fine.

In a win32 c++ project I am trying to make the following codes to work:

At the window creation the mapped file is created

C++

case WM_CREATE:
		{
			// Create a file mapped object.
			//.............................
			hMapFile = CreateFileMapping( (HANDLE)0xFFFFFFFF, NULL,
				                          PAGE_READWRITE, 
										  0,
										  1024,
										  "MyFileMappingObject" );

			// Return if the mapping failed.
			//..............................
			if ( !hMapFile )
				return(-1);
		}
		break;

There are made modification in mapped file in the following thread function:

C++

DWORD AnotherProcess( LPDWORD lpdwParam )
{
	HANDLE hFileMap;
	LPTSTR lpMapAddress;

	// Open the file mapping.
	//.......................
	hFileMap = OpenFileMapping( FILE_MAP_ALL_ACCESS, FALSE,
		                        "MyFileMappingObject" );

	// Map a view to the mapping object.
	//..................................
	lpMapAddress =(LPTSTR)MapViewOfFileEx( hFileMap, FILE_MAP_ALL_ACCESS,
		                            0, 0, 0, lpdwParam );

	// Display the contents of the memory.
	//....................................
	MessageBox( NULL, lpMapAddress, "Mapped Memmory",
		        MB_OK | MB_ICONINFORMATION );

	// Set "Received" into the memory.
	//................................
	strcpy( lpMapAddress, "Received" );

	UnmapViewOfFile( lpMapAddress );

	return( 0 );
}

There are made modification on mapped file and the thread function is called on the following code:

C++

case IDM_TEST:
			{   
				DWORD dwID;
				// Map a view of the file mapped object.
				//......................................
				lpMapAddress = (LPTSTR)MapViewOfFile( hMapFile, 
					                          FILE_MAP_ALL_ACCESS,
											  0, 0, 0 );

				// Place data int the view.
				//.........................
				strcpy(lpMapAddress, "Data passed from main process.");

				// Make sure the data is flushed to the file.
				//...........................................
				FlushViewOfFile( lpMapAddress, 
					             lstrlen( lpMapAddress ) );

				// Create a thread to receive the data.
				//.....................................
				hThread = CreateThread( NULL, 0, 
					      (LPTHREAD_START_ROUTINE)AnotherProcess, 
						  lpMapAddress, 0, &dwID);

				// Set a timer to wait for the data to be processed.
				//..................................................
				SetTimer( hWnd, 1,	1000, NULL );
			}
			break;

If everything is ok all cycle is ended with the timer:

C++

case WM_TIMER:
		{
			// Check to see if the thread has processed the memory.
			//.....................................................
			if ( strcmp( lpMapAddress, "Received" ) == 0 )
			{
				// Kill the timer and thread.
				//...........................
				KillTimer( hWnd, 1 );
				TerminateThread( hThread, 0 );

				MessageBox( hWnd, lpMapAddress, "Done", MB_OK );

				UnmapViewOfFile( lpMapAddress );

			}

When I put breakpoint in the "AnotherProcess" thread function, after the message box, in the message box no message
is displayed, like lpMapAddress would be empty.
It could be from OpenFileMapping or MapViewOfFileEx to not doing their job? Because in watch table lpMapAddress
has value 0.

After that point, the compiler enters in a kind of assambler code in a inifinite main loop. Maybe here WM_TIMER goes crazy.

Can you help me to figure out what should be done? Where are the things that are nok?

Thank you very much.

Mircea Neacsu8-Aug-22 10:35

Re: FileMapping book exmple goes in loop

8-Aug-22 10:35

Took me some time to spot the problem (I think):

C++

lpMapAddress =(LPTSTR)MapViewOfFileEx( hFileMap, FILE_MAP_ALL_ACCESS,
		                            0, 0, 0, lpdwParam );

This doesn't make much sense: the last argument of the function is the address where you want it mapped. So you are kind of mapping the memory where it was already mapped. I's suggest to change it to:

C++

lpMapAddress =(LPTSTR)MapViewOfFile( hFileMap, FILE_MAP_ALL_ACCESS,
		                            0, 0, 0);

If you want a shared memory implementation that has been tested, you can find one here. The source file is mlib/src/shmem.cpp

Mircea

coco2438-Aug-22 11:45

Re: FileMapping book exmple goes in loop

8-Aug-22 11:45

Thank you for the rapid response.

Indeed, now, the code works as I think it should.

I supposed that this was the central idea, to modify the mapped file from two different places, but appears
that something went wrong. I supposed that the LPWORD type of lpdwParam was the problem but I haven't seen a solution.
It appears that it does't like the start address but the all file to be mapped.

Anyway it seems that it's working, and now I can move on in the learning process beacause I want to get the ideea not to
stay to long on each topic.

Thank you very much Mircea for your response and for the links.

Thank you.

Mircea Neacsu8-Aug-22 11:47

defining a function with #define

8-Aug-22 11:47

You are very welcome!

Mircea

coco24330-Jul-22 9:03

30-Jul-22 9:03

Hi,

Please help me understanding the following declaration:

C++

#define EVENBYTE( i )  ((i+7)/8*8)

It represents the definition of function?

If for example I call EVENBYTE( 1 ), the result will be ((1+7)/8*8)?

From where I have extracted de code, the EVENBYTE was called in that way:

C++

int    nIconBytes = (EVENBYTE(nWidth)/8)*nHeight;

I don't understand what this kind of declaring is and why to do it in that mode?

Thank you in advance.

Mircea Neacsu30-Jul-22 9:54

30-Jul-22 9:54

No, it is not a function, it is a macro definition. While, in your case a function defined as:

C++

int evenbyte (int i) {return (i+7)/8*8;}

would do the same thing, there is an important difference: a macro definition is expanded at compile time while a function is compiled and later on called at runtime.

When to use one over the other is a question of optimization speed vs memory: a macro definition like your would not incur the overhead of a call but might produce larger code. Meanwhile a function definition would produce smaller code but might take longer to execute.

To solve these type of issues, C++ has introduced inline functions, so in C++ you can write

C++

inline int evenbyte (int i) {return (i+7)/8*8;}

and you leave the compiler the choice of expanding it inline (like a macro definition) or make it a function and call it.

One more thing: the macro definition is insensitive to the type of argument. That is both a good thing and a bad thing like in the examples below:

C++

int i1 = 3;
int e1 = EVENBYTE (i1); //calling macro with an int argument yields an int result
long i2 = 129;
long e2 = EVENBYTE (e2); //calling macro with a long argument yields a long result

int e3 = EVENBYTE ("abc"); //generates some obscure error message

To accomplish the same thing in C++ you would have to use a function template:

C++

template <typename T>
T evenbyte (T i) { return (i+7)/8*8;}

Mircea

trønderen30-Jul-22 12:46

trønderen

30-Jul-22 12:46

Also worth noting: A macro does not define its own scope, the way a function does. It could refer to other symbols, say,

C++

#define PLUSX(i) (i + x)

Which x is added? That depends on where PLUSX is used. In one place it could be a different x than in another place. In a third place, there may be no x defined at all, and you get a compilation error, even if the two first uses of PLUSX went without any error.

A define is a pure textual replacement. If you in your code editor replace 'PLUSX(i)' with '(i + x)' (and change the 'i' in the replacement text to whatever was the name of your actual argument), you are doing by hand the work of the C preprocessor. Or in your case, if in your editor you replace

C++

int  nIconBytes = (EVENBYTE(nWidth)/8)*nHeight;

with

C++

int  nIconBytes = (((nWidth+7)/8*8)/8)*nHeight;

then you get the exact same result as with the #define.

Note: I am certainly not suggesting that referring to non-argument variables from macros is a good idea (usually is is not), but understanding how macros differ from functions includes understanding how such references would work.

Personal opinion: #define is among the most mis-used features of the C language. Except for giving symbolic names to simple constants, you should try to avoid it as far as possible.

Greg Utas30-Jul-22 13:58

Greg Utas

30-Jul-22 13:58

In C, #define is sometimes used to do the same thing as templates in C++, which I can understand.

In C++, #define shouldn't be used that way, but C hackers use their preprocessor crapola in C++. It shouldn't even be used to define constants, especially after constexpr showed up.

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Randor 30-Jul-22 9:55

Randor

30-Jul-22 9:55

coco243 wrote:
Please help me understanding the following declaration

Kinda looks like something used for calculating Image stride[^].

coco2433-Aug-22 9:01

How to read an xlsx file using CDatabase

3-Aug-22 9:01

Thank you all for your responses.

Sampath57927-Jul-22 19:54

27-Jul-22 19:54

Hi,

I have an xlsx file, which i need to read using CDatabase in c++.. Here is the sample application i tried but its throwing an exception while opening the file.

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPWSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    // TODO: Place code here.

    CDatabase* pExcelDatabase = NULL;//Pointer to database
    CString csExcelFile = _T("D:\\ExcelFile.xlsx");
    CString csDSN;
    //check for file existence
    if (TRUE == ::PathFileExists(csExcelFile))// checks whether the file exists or not
    {
        csDSN.Format(_T("DRIVER=Microsoft Excel Driver (*.xls,*.xlsx);DSN='';DBQ=%s"), (LPCTSTR)csExcelFile);
    }
    else
    {
        return NULL;
    }

    //Create the database
    pExcelDatabase = new CDatabase();
    HRESULT hr;
    try // to handle exceptions on opening database
    {
        hr = pExcelDatabase->OpenEx(csDSN, CDatabase::noOdbcDialog);//open the database
    }
    catch (...)
    {
        if (pExcelDatabase)
        {
            delete pExcelDatabase;
        }
        return NULL;
    }
    // Initialize global strings
    LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadStringW(hInstance, IDC_WINDOWSPROJECT1, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WINDOWSPROJECT1));

    MSG msg;

    // Main message loop:
    while (GetMessage(&msg, nullptr, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}

Note: Sometimes, i have to open xls file too, thats the reason in the query i added ".xls" and ".xlsx" file. If i give just ".xls" in the query and input is ".xls" file, the above code is working fine.

Any help is much appreciated.

Thanks

Victor Nijegorodov27-Jul-22 20:07

Victor Nijegorodov

27-Jul-22 20:07

Sampath579 wrote:
its throwing an exception while opening the file.

What exception exactly is throwing?

Sampath57931-Jul-22 23:27

31-Jul-22 23:27

Unhandled exception at 0x75B9CA42 in WindowsProject1.exe: Microsoft C++ exception: CDBException at memory location 0x00EFF6CC.

Richard MacCutchan31-Jul-22 23:38

Richard MacCutchan

31-Jul-22 23:38

Unfortunately that gives no clue as to where the exception occurred. So you will need to run the code in the debugger and step through it until you can find where it occurs, and why. Also note you have the following code:

C++

catch (...)
{
    if (pExcelDatabase)
    {
        delete pExcelDatabase;
    }
    return NULL;
}

So you are catching any exception but ignoring it, which is not a good idea. You need to put some proper code in there to get the type and reason information from the exception.

Message Closed

31-Jul-22 17:19

31-Jul-22 17:19

Message Closed

modified 1-Aug-22 10:39am.

Dave Kreskowiak31-Jul-22 17:30

Dave Kreskowiak

31-Jul-22 17:30

Try answering the question you were asked about the exception.

Asking questions is a skill
CodeProject Forum Guidelines
Google: C# How to debug code
Seriously, go read these articles.

Dave Kreskowiak

Sampath57931-Jul-22 23:40

31-Jul-22 23:40

Sorry for missing this question earlier.. i posted the exception details now.

Sampath5791-Aug-22 23:21

How to use UTF8 characters in C++

1-Aug-22 23:21

Figured it out on my own. Things to be asked or answered.

1) My application is 32 bit.
2) I have MDAC 64 bit on my windows 64bit OS.
3) When the CDatabase OpenEx executes with connection string i mentioned in my above code, it checks in ODBC32.exe in which the required driver (xlsx) is not available.
4) We must have to install the MDAC 32 bit and the required driver to read the xlsx file is will be available.

Hope this answer will be helpful to others in future and thought of writing my analysis.

Regards
Sampath.
Questioning is an Art and Answering will be done by only an Artist.

JohnCodding27-Jul-22 3:00

JohnCodding

27-Jul-22 3:00

What do I need to do to be able to use in C++ UTF8 characters for char * and std::string, or at least for std::string if char * doesn't support it? I'm using std:c++17. I tried the following code to see if it is working without changing anything but it didn't worked and returned ┬┴╩╦≡??Γε?:

#include <iostream>
#include <string>

int main() {
	std::string a = "ÂÁÊËðșțâîă";
	std::cout << a;
	std::cin.get();
}

Re: How to use UTF8 characters in C++

Mircea Neacsu27-Jul-22 3:54