Question

Win32 Read/Write a Failing using SetFilePointer()

Asked by: mjgardne

Hello,

I am struggling with something really stupid!  I need to create a temporary file for reading and writing.  I use the Win32 API to generate an appropriate temporary file name, and call CreateFile(), as in the following code.  If successful, I use WriteFile() to fill it with data.  Now, is the issue...  I have been trying to rewind the file to its beginning, but SetFilePointer() always says that my handle, which I just used and have not changed, is invalid.  After hours of struggling, I think that it may have something to do with my CreateFile parameters. All I want to do is create a temporary file, write data to it, rewind, read the data from it, and then have Windows dispose of the file when it is closed.  According the the docs, what I am doing is okay, but I have not been able to find anything on the Internet showing using a temporary file for READING and WRITING.  I hope that someone can help.

Thanks,

Mike

HANDLE hFile = CreateFile(tempName, (GENERIC_WRITE | GENERIC_READ), 0, NULL, CREATE_ALWAYS, 
                         (FILE_ATTRIBUTE_NORMAL | FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_DELETE_ON_CLOSE), 
                         NULL); 
..............
// Return to BOF.
DWORD result = SetFilePointer(hFile, 0, NULL, FILE_BEGIN);
 
This code always returns 6, which is "Handle is invalid).

                                  
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Asked On
2009-10-17 at 22:53:52ID24821273
Tags

Win32 ReadFile WriteFile SetFilePointer

Topics

Windows Programming

,

Microsoft Visual C++

,

Microsoft Visual C++.Net

Participating Experts
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Comments
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Answers

 

by: pgnatyukPosted on 2009-10-17 at 23:23:52ID: 25598909

Please check the code I attached. It is your code with few modifications done to illustrate the API. SetFilePointer works with hFile handle. Please compare with your code.

You use FILE_FLAG_DELETE_ON_CLOSE flag, so the file is deleted on close. 

#include <windows.h>
#include <wchar.h>
 
int main()
{
	HANDLE hFile = CreateFile(L"temp.tmp", 
		GENERIC_WRITE | GENERIC_READ, 0, 
		NULL, CREATE_ALWAYS, 
		FILE_ATTRIBUTE_NORMAL|FILE_ATTRIBUTE_TEMPORARY|FILE_FLAG_DELETE_ON_CLOSE, 
		NULL); 
 
	if (hFile != INVALID_HANDLE_VALUE)
	{
		SYSTEMTIME st = { 0 };
		GetLocalTime(&st);
		WCHAR szText[MAX_PATH] = { 0 };
		_snwprintf(szText, MAX_PATH, L"Time: %02d h:%02d min\n", st.wHour, st.wMinute);
 
		DWORD nBytes = wcslen(szText) * sizeof(WCHAR);
		DWORD nWritten = 0;
		WriteFile(hFile, szText, nBytes, &nWritten, NULL);
 
 
		DWORD nFilePos = SetFilePointer(hFile, 0, NULL, FILE_BEGIN);
 
		WCHAR szBuf[MAX_PATH] = { 0 };
		DWORD nRead = 0;
		ReadFile(hFile, szBuf, nBytes, &nRead, NULL);
 
		wprintf(szBuf);
 
		CloseHandle(hFile);
	}
 
	return 0;
}
                                              
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by: mjgardnePosted on 2009-10-18 at 00:31:48ID: 25599029

Hello pgnatyuk,

Thank you for your posting, but, unfortunately, it did not help.  :(  I understand the API and the code that I have, can create and write to a file.  The issue is that SetFilePointer() is returning "invalid file handle" even just after the file has been successfully created.  However, the same handle successfully permits writing to the file, which results in files that are not empty when I exit (when removing FILE_FLAG_DELETE_ON_CLOSE from CreateFile()).  So, I still believe that something is not quite correct with the CreateFile() when using it for Reading and Writing to a created and opened file.  The MSDN docs say that "The file handle must be created with the GENERIC_READ OR GENERIC_WRITE access right."  ORRRRRRRRRRRRRRRRRRRRRR      So, I wonder whether I cannot use SetFilePointer() on a file that has been opened for (GENERIC_READ | GENERIC_WRITE), which would be really strange.  Anyway, I hope that this info helps and we can figure out what is going on... Worst case, I can create the file, write it, close it... open the file, read it, close it... Delete it... But, according to what I've read, my current code should work...  Arggg... Very frustrating... Win32 never gives up it secrets easily... :P

 

by: pgnatyukPosted on 2009-10-18 at 00:42:07ID: 25599049

Check the code I posted - I attached it again. There is a line creating the temporary file and this file will be deleted in the end.

In the example I posted, I write to the file, then use SetFilePointer to 0 and then read from the file.

Probably, you have something else in your code that you didn't post and don't pay attention on it.

Please check all places you have CloseHandle in your code and set hFile = INVALID_HANDLE_VALUE after this CloseHandle. If SetFilePointer says that hFile is invalid, so you did something wrong with it.

Let's check SetFilePointer. Do you say that the pointer is not moving 0? GetLastError says "Invalid handle"?

HANDLE hFile = CreateFile(L"temp.tmp", 
		GENERIC_WRITE | GENERIC_READ, 0, 
		NULL, CREATE_ALWAYS, 
	FILE_ATTRIBUTE_NORMAL|FILE_ATTRIBUTE_TEMPORARY|FILE_FLAG_DELETE_ON_CLOSE, 
		NULL); 
                                              
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by: mjgardnePosted on 2009-10-18 at 15:17:40ID: 31642709

Hi pgnatyuk,

I followed your advice, after getting a good night of sleep, and found my issue.  First, I created a simple program that did the steps that I needed, and everything worked properly.  Next, I carefully reviewed my code and... POOF!  There it was...  My file handle is an object variable, and in the method that was not working, I accidentally had another variable with the same name, so it masked it.  Arggg!  Three hours I looked for this issue!!!  Anyway, thank you for your advice.  Everything is working, now! :)

Mike

 

by: mjgardnePosted on 2009-10-18 at 15:21:38ID: 25601681

For others, here is the simple test that I performed to verify that the basic elements of my code were working correctly...

	void EFSRaw::Test()
	{
		HANDLE hMe = NULL;
 
		hMe = CreateFile(L"c:\\temp\\blah.txt", (GENERIC_WRITE | GENERIC_READ), 0, NULL, CREATE_ALWAYS, 
                                   (FILE_ATTRIBUTE_NORMAL | FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_DELETE_ON_CLOSE), NULL); 
		if (hMe != INVALID_HANDLE_VALUE)
		{
			static TCHAR *msg = L"This is a message to Me.";
			DWORD wrote = 0;
			if (WriteFile(hMe, msg, 51, &wrote, NULL))
			{
				// Go to BOF
				Int64 pos = FileSeek(hMe, 0, FILE_BEGIN);
				if (pos != INVALID_SET_FILE_POINTER)
				{
					TCHAR buf[512];
					memset(buf, 0x0, 512);
					DWORD read = 0;
					if (ReadFile(hMe, buf, wrote, &read, NULL))
					{
						String ^s = gcnew String(buf);
						CloseHandle(hMe);
					}
					else
					{
						DWORD result = GetLastError();
						String ^win32ErrMsg = MCUtilities::MyFormatMessage(result);
					}
				}
				else
				{
					DWORD result = GetLastError();
					String ^win32ErrMsg = MCUtilities::MyFormatMessage(result);
				}
 
			}
			else
			{
				DWORD result = GetLastError();
				String ^win32ErrMsg = MCUtilities::MyFormatMessage(result);
			}
		}
		else
		{
			DWORD result = GetLastError();
			String ^win32ErrMsg = MCUtilities::MyFormatMessage(result);
		}
	}

                                              
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