VirualAlloc and ReadFile

csrss

So, got no idea why this is not working but it simply not working.

    SIZE\_T BASE\_BUFFER\_SIZE = 512;

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

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

GetLAstError() returns Invalid access to memory location. I am dealing with virtual alloc for a first time :P What am i doing wrong? Thanks

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Rajesh R Subramanian

csrss wrote:

VirtualAlloc(0, BASE_BUFFER_SIZE, MEM_RESERVE, PAGE_READWRITE);

Just a hunch. The doc says that MEM_RESERVE flag Reserves a range of the process's virtual address space without allocating any actual physical storage in memory or in the paging file on disk. If you're actually going to do something with the memory, MEM_COMMIT may be a good option (you could combine flags, like MEM_RESERVE | MEM_COMMIT). At least the doc[^] says so. You also failed to tell what's "not working". I'm assuming that your ReadFile call failed, and GetLastError returns invalid access to memory location. Reading that, I'm assuming that since you're just reserving memory with your VirtualAlloc call, and not committing it before using, you're getting an invalid access error.

"Real men drive manual transmission" - Rajesh.

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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