Why am I not able to get handle of file?

SherTeks

Hello, I've a code snippet. Please follow the comments.

	//Create a directory if it doesn't exist
	if ( !PathFileExists("C:\\\\TestDir") )
	{
	CreateDirectory("C:\\\\TestDir", NULL);
	}

	//Create a batch file in the directory
	CFile cfBatchFile( "C:\\\\TestDir\\\\TestBatch.bat", CFile::modeCreate| CFile::modeRead | CFile::modeWrite );

	//Write 'C:\\\\windows\\\\system32\\cmd.exe" ipconfig>C:\\\\TestDir\\\\Test.txt in the batch file 
	cfBatchFile.Write( "\\"C:\\\\windows\\\\system32\\cmd.exe\\" ipconfig>C:\\\\TestDir\\\\Test.txt", MAX\_PATH );

	cfBatchFile.Close();

	//ShellExecute runs "C:\\\\TestDir\\\\TestBatch.bat" and generates output for 'ipconfig' and redirects it to text file "C:\\\\TestDir\\\\Test.txt"
	if ( ShellExecute(NULL, "open", "C:\\\\TestDir\\\\TestBatch.bat", NULL, NULL, SW\_HIDE) > (HINSTANCE)32)
	{
		CFile\* pcfTxt = NULL;

		//Trying to open 'C:\\\\TestDir\\\\Test.txt' for reading BUT CRASH HAPPENS HERE (unhandled Exception)
		pcfTxt = new CFile(\_T("C:\\\\TestDir\\\\Test.txt"), CFile::modeRead | CFile::shareDenyNone);

		INT nLogTxtLen = pcfTxt->GetLength();
	}

In Debug mode (without a Debug point), the crash is shown at pcfTxt = new CFile(_T("C:\\TestDir\\Test.txt"), CFile::modeRead | CFile::shareDenyNone); But if I place a debug point at : pcfTxt = new CFile(_T("C:\\TestDir\\Test.txt"), CFile::modeRead | CFile::shareDenyNone); and then continue, no crash arises. I guess the issue is something to do with the handle of 'C:\\TestDir\\Test.txt' file. How could I rectify this. Is some Sleep() required to wait for the handle to be available. Replies would be appreciated. Thanks Edit/Delete Message

SandipG

You can try using CreateProcess[^]instead of ShellExecute and then Call WaitForSingleObject[^]on the handle for process obtained from CreateProcess. Introducing Sleep might work but it will not be a good solution.

Regards, Sandip.

David Crow

SherTeks wrote:

In Debug mode (without a Debug point), the crash is shown at...

Please explain this "crash" in more detail. Is an assertion firing? Is an exception being thrown?

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ky_rerun

looks like this is related to delayed write. Just because you close a file does not mean the OS will write it the folowing is from the FlushFileBuffers documentation Due to disk caching interactions within the system, the FlushFileBuffers function can be inefficient when used after every write to a disk drive device when many writes are being performed separately. If an application is performing multiple writes to disk and also needs to ensure critical data is written to persistent media, the application should use unbuffered I/O instead of frequently calling FlushFileBuffers. To open a file for unbuffered I/O, call the CreateFile function with the FILE_FLAG_NO_BUFFERING and FILE_FLAG_WRITE_THROUGH flags. This prevents the file contents from being cached and flushes the metadata to disk with each write. For more information, see CreateFile. to me it looks like you need to open it in non buffering mode.

a programmer traped in a thugs body

SandipG

I think you wanted to reply to OP :)

Regards, Sandip.

ky_rerun

ooks like this is related to delayed write. Just because you close a file does not mean the OS will write it the folowing is from the FlushFileBuffers documentation Due to disk caching interactions within the system, the FlushFileBuffers function can be inefficient when used after every write to a disk drive device when many writes are being performed separately. If an application is performing multiple writes to disk and also needs to ensure critical data is written to persistent media, the application should use unbuffered I/O instead of frequently calling FlushFileBuffers. To open a file for unbuffered I/O, call the CreateFile function with the FILE_FLAG_NO_BUFFERING and FILE_FLAG_WRITE_THROUGH flags. This prevents the file contents from being cached and flushes the metadata to disk with each write. For more information, see CreateFile. to me it looks like you need to open it in non buffering mode.

a programmer traped in a thugs body

Stuart Dootson

I would suggest you a) use CreateProcess to run ipconfig (on its own, not in a batch file), so that you can b) redirect the standard output and standard error handles[^] of the created process to a string within your parent process. That way, your program is using the exact same mechanism as CMD to redirect ipconfig's output to where you want to make use of it. And you don't need to mess with files and stuff.

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