plzzzz help me with this prg any one ....urgent

[vikrant3mahajan]

hey guys any one plz help me i would be highly thankful...avtually i a m trying to read and write raw disk sectors using vc++.i am able to read properly but as i write i get an exception which i am not able to sort out..the problem is the bytes are written on destination but after writing the application gives exception plz help me out..exception is in the writefile function line

here goes the program

// cod.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "cod.h"
//#include <iostream.h>
#include <winnt.h>
#include <Winbase.h>
#include <conio.h>
#ifdef _DEBUG
#define new DEBUG_NEW
#endif
#ifdef UNICODE
#undef UNICODE
#endif

// The one and only application object

CWinApp theApp;

//using namespace std;


        

        int _tmain(int argc, TCHAR* argv[], TCHAR* envp[])
{		
	
	DWORD bytesread;
	char hexcode[10];
	DWORD sector=0;
DWORD sectorN=1;

		char* buffer = (char*)malloc (512);
		
	int drive=0;
	HANDLE hDevice= CreateFileA("\\\\.\\N:",GENERIC_READ|GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE,NULL, OPEN_EXISTING, FILE_FLAG_WRITE_THROUGH, NULL); 
	if (hDevice == INVALID_HANDLE_VALUE) 
        printf("Error");
	else
		printf("Success\n");

	SetFilePointer (hDevice,(sector*512), NULL, FILE_BEGIN); 
if (!ReadFile (hDevice, buffer, 512*sectorN, &bytesread, NULL) )
		 printf("unable to read");

for (int i=0; i<512; i++)
	{
		if(i%16==0)
			printf("\n");
printf("%02X ",buffer[i]&0xff);
	}



printf("\n");

for (int i=0; i<512; i++)
	{
buffer[i]=12;
}


SetFilePointer (hDevice, (sector*512), NULL, FILE_BEGIN); 
LPDWORD bytesN=0;BOOL h;


try{

h=WriteFile(hDevice,buffer,512,bytesN,NULL);   //this is the part where i am trying to write plz help 

}catch(CException *e){printf("exception");}



printf("\n\nWritten\n");


	for (int i=0; i<512; i++)
	{
		if(i%16==0)
			printf("\n");
printf("%02X ",buffer[i]&0xff);

	}
getch();
return 0;

}

EXCEPTION IT GIVES IS:


Unhandled exception at 0x7c811024 in cod.exe: 0xC0000005: Access violation writing location 0x00000000.



if it is unable to write then how can data be still written..


i had also attached the source file and the exception i get screenshot


help me out guys plzzzzzz

[Lindley]

Ah, good old null pointer exception. I know thee well. However, in this case I think it's due to confusion regarding the WinAPI typedefs rather than anything tricky.

LPDWORD bytesN=0;
h=WriteFile(hDevice,buffer,512,bytesN,NULL);

An LPDWORD stands for "long pointer to double word", which is a fancy way of saying int*. In other words, bytesN needs to be a pointer to int. The WriteFile documentation indicates that this is an out parameter, so clearly the function is going to dereference the pointer and write a value into the int there.

You, however, are providing a null pointer. Hence the error.

The proper way to use that sort of interface is to do like such:

DWORD bytesN=0;
h=WriteFile(hDevice,buffer,512,&bytesN,NULL);

Do you see and understand the difference?

I haven't looked at the other parameters in depth, so it's possible there may be more problems.

[hoxsiew]

WriteFile(hDevice,buffer,512,bytesN,NULL)

should be:

WriteFile(hDevice,buffer,512,&bytesN,NULL)

I'm not sure how it compiled.

[vikrant3mahajan]

hey man....thanx a hell lot ...you are the saviour ..thnx a lot..u solved the prob

[hoxsiew]

Lindley beat me to it.