VirualAlloc and ReadFile

[csrss]

Yep, you are right. But here with this stuff i got more and more problems. For example, i cannot return bytes from function, (this code is a function, which should read a file into BYTE array and return this array among with nr of bytes read).

BYTE *ReadBytes(HANDLE hDevice, LPDWORD lpdwBytesRead)
{
SIZE_T BASE_BUFFER_SIZE = 512;

BYTE \* bInitialBuffer = NULL;
bInitialBuffer = (BYTE \*)VirtualAlloc(0, 
		BASE\_BUFFER\_SIZE, MEM\_COMMIT | MEM\_RESERVE, PAGE\_READWRITE);

if(bInitialBuffer == NULL)
{
	// handle this shit
	TRACE("VirtualAlloc failed!");
	return NULL;
}
DWORD dwReadBytes;
while (FALSE != ReadFile(hDevice, bInitialBuffer, BASE\_BUFFER\_SIZE, 
	(LPDWORD)&dwReadBytes, 0) && dwReadBytes > 0)
{
	TRACE1("got some buffer! %d", dwReadBytes);
	lpdwBytesRead += dwReadBytes;
}
return bInitialBuffer;

}

and if i call it:

DWORD read = 0;
BYTE *DataBuffer = ReadBytes(hFile, &read);

read is always zero, and i got no bytes returned from function... And i cannot use just defined size (skip memory allocation). Help ;(

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[Rajesh R Subramanian]

One step at a time. Have you progressed from your previous error, or not? With the MEM_RESERVE flag, do you still get an invalid access error while calling ReadFile?

"Real men drive manual transmission" - Rajesh.

[Rajesh R Subramanian]

csrss wrote:

i cannot return bytes from function, (this code is a function, which should read a file into BYTE array and return this array among with nr of bytes read).

This has nothing to do with VirtualAlloc.

"Real men drive manual transmission" - Rajesh.

[csrss]

At this moment:

while (FALSE != ReadFile(hDevice, bInitialBuffer, BASE_BUFFER_SIZE,
(LPDWORD)&dwReadBytes, 0) && dwReadBytes > 0)
{
TRACE1("got some buffer! %d", dwReadBytes);

dwReadBytes contains right value, which is 106 bytes, that is exactly the size of my file (just a text file). Here :

lpdwBytesRead += dwReadBytes;

It tries to append them, but lpdwBytesRead contain some trash, even if it has been initialized outside the function to zero:

DWORD read = 0;
BYTE *DataBuffer = ReadBytes(hFile, &read);

When function returns, read is zero and there is no buffer returned :(

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[csrss]

I know it is not perfect :P (the code) I have never worked with unsigned type allocation before. (the only proper way for returning unsigned type from function i know is to declare buffer size and make this, BYTE, static) If i pass a pointer to BYTE to this function from calling function, it still returns empty BYTE buffer. I am out of ideas. Somebody, help? :P

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[csrss]

I got it!!! YEah!

*lpdwBytesRead += dwReadBytes;

Its just too much on my head, too much...

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[David Crow]

csrss wrote:

lpdwBytesRead += dwReadBytes;

This looks suspect. You are incrementing the pointer instead of what the pointer points to. Is that your intent?

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[Rajesh R Subramanian]

This piece of code works (you had trouble bringing the buffer to the calling function, which is resolved here)

#define BASE_BUFFER_SIZE 512

BOOL ReadBytes(HANDLE hFile, PBYTE bInitialBuffer)
{
DWORD dwReadBytes;
while (FALSE != ReadFile(hFile, bInitialBuffer, BASE_BUFFER_SIZE,
(LPDWORD)&dwReadBytes, 0) && dwReadBytes > 0)
{
/*CString str;
str.Format(L"got buffer! %d",dwReadBytes);
AfxMessageBox(str);*/
}
if(dwReadBytes <= 0)
{
return FALSE;
}
return TRUE;
}

void CMTest1Dlg::OnBnClickedOk()
{
HANDLE hFile = NULL;

hFile = CreateFile(L"C:\\\\temp\\\\myfile.txt", GENERIC\_READ, 0, 0, OPEN\_EXISTING, 0, 0);
if(INVALID\_HANDLE\_VALUE == hFile){
	AfxMessageBox(L"CreateFile failed");
	return;
}

BYTE \*buffer = NULL;
buffer = (BYTE \*)VirtualAlloc(0, BASE\_BUFFER\_SIZE, MEM\_COMMIT | MEM\_RESERVE, PAGE\_READWRITE);

if(NULL == buffer)
{
	AfxMessageBox(L"VirtualAlloc failed!");
	return;
}
BOOL bRead = ReadBytes(hFile, buffer);
//use buffer here

}

"Real men drive manual transmission" - Rajesh.

[Rajesh R Subramanian]

Everything's good now? :) The piece of code I've given above in my other reply doesn't involve any pointer arithmetic.

"Real men drive manual transmission" - Rajesh.

[csrss]

For now i have solved the issue with simple text file and one time allocation :) Now, it is time to read mp4 file into buffer and make a copy of it :X

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[csrss]

It was very suspicious in fact! Yes, i have made a disaster mistake and it is corrected by now :)

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