How do I make my program watch for file modification in C++?

There are a lot of programs, Visual Studio for instance, that can detect when an outside program modifies a file and then reload the file if the user wants chooses. Is there a relatively easy way to do this sort of thing in C++ (doesn't necessarily have to be platform independent)?

There are several ways to do this depending on the platform. I would choose from the following choices:

Cross Platform

Trolltech's Qt has an object called QFileSystemWatcher which allows you to monitor files and directories. I'm sure there are other cross platform frameworks that give you this sort of capability too, but this one works fairly well in my experience.

Windows (Win32)

There is a Win32 api called FindFirstChangeNotification which does the job. There is a nice article which a small wrapper class for the api called How to get a notification if change occurs in a specified directory which will get you started.

Windows (.NET Framework)

If you are ok using C++/CLI with the .NET Framework then System.IO.FileSystemWatcher is your class of choice. Microsoft has a nice article on how to monitor file system changes using this class.

OS X

The FSEvents API is new for OS X 10.5 and very full-featured.

Linux

Use inotify as Alex mentioned in his answer.

If you don't need to be platform-independent, an approach on Linux that may be less of a machine load than "polling" (checking periodically) is inotify, see http://en.wikipedia.org/wiki/Inotify and the many links from it for example. For Windows, see http://msdn.microsoft.com/en-us/library/aa365261(VS.85).aspx .

SimpleFileWatcher might be what you are looking for. But of course it is an external dependency - maybe that is no option for you.

Sure, just like VC++ does. You get the last modified time when you open the file, and you periodically check it while you have the file open. If last_mod_time > saved_mod_time, it happened.

A working exemple for WinCE

void FileInfoHelper::WatchFileChanges( TCHAR *ptcFileBaseDir, TCHAR *ptcFileName ){
static int iCount = 0;
DWORD dwWaitStatus; 
HANDLE dwChangeHandles; 

if( ! ptcFileBaseDir || ! ptcFileName ) return;

wstring wszFileNameToWatch = ptcFileName;

dwChangeHandles = FindFirstChangeNotification(
	ptcFileBaseDir,
	FALSE,
	FILE_NOTIFY_CHANGE_FILE_NAME |
	FILE_NOTIFY_CHANGE_DIR_NAME |
	FILE_NOTIFY_CHANGE_ATTRIBUTES |
	FILE_NOTIFY_CHANGE_SIZE |
	FILE_NOTIFY_CHANGE_LAST_WRITE |
	FILE_NOTIFY_CHANGE_LAST_ACCESS |
	FILE_NOTIFY_CHANGE_CREATION |
	FILE_NOTIFY_CHANGE_SECURITY |
	FILE_NOTIFY_CHANGE_CEGETINFO
	);

if (dwChangeHandles == INVALID_HANDLE_VALUE) 
{
	printf("\n ERROR: FindFirstChangeNotification function failed [%d].\n", GetLastError());
	return;
}

while (TRUE) 
{ 
	// Wait for notification.
	printf("\n\n[%d] Waiting for notification...\n", iCount);
	iCount++;

	dwWaitStatus = WaitForSingleObject(dwChangeHandles, INFINITE); 
	switch (dwWaitStatus) 
	{ 
		case WAIT_OBJECT_0: 
			
			printf( "Change detected\n" );
			
			DWORD iBytesReturned, iBytesAvaible;
			if( CeGetFileNotificationInfo( dwChangeHandles, 0, NULL, 0, &iBytesReturned, &iBytesAvaible) != 0 ) 
			{
				std::vector< BYTE > vecBuffer( iBytesAvaible );
				
				if( CeGetFileNotificationInfo( dwChangeHandles, 0, &vecBuffer.front(), vecBuffer.size(), &iBytesReturned, &iBytesAvaible) != 0 ) {
					BYTE* p_bCurrent = &vecBuffer.front();
					PFILE_NOTIFY_INFORMATION info = NULL;
					
					do {
						info = reinterpret_cast<PFILE_NOTIFY_INFORMATION>( p_bCurrent );
						p_bCurrent += info->NextEntryOffset;

						if( wszFileNameToWatch.compare( info->FileName ) == 0 )
						{
							wcout << "\n\t[" << info->FileName << "]: 0x" << ::hex << info->Action;

							switch(info->Action) {
								case FILE_ACTION_ADDED:
									break;
								case FILE_ACTION_MODIFIED:
									break;
								case FILE_ACTION_REMOVED:
									break;
								case FILE_ACTION_RENAMED_NEW_NAME:
									break;
								case FILE_ACTION_RENAMED_OLD_NAME:
									break;
							}
						}
					}while (info->NextEntryOffset != 0);
				}
			}

			if ( FindNextChangeNotification( dwChangeHandles ) == FALSE )
			{
				printf("\n ERROR: FindNextChangeNotification function failed [%d].\n", GetLastError());
				return;
			}

			break; 

		case WAIT_TIMEOUT:
			printf("\nNo changes in the timeout period.\n");
			break;

		default: 
			printf("\n ERROR: Unhandled dwWaitStatus [%d].\n", GetLastError());
			return;
			break;
	}
}

FindCloseChangeNotification( dwChangeHandles );
}

Add an answer for libuv (though it's written in C), it has support for both Windows and Linux with system-specific APIs:

> inotify on Linux, FSEvents on Darwin, kqueue on BSDs, > ReadDirectoryChangesW on Windows, event ports on Solaris, unsupported > on Cygwin

You may check the document here, beware that the document says that the notification related APIs are not very consistent.