I have an application which uses inotify and std::thread to check if file is modified outside the application. The problem is that when the file is modified inside the application, inotify works and it is considered as updated outside. My question is, how can I check if file is modified only outside the application. Almost every text editor works that way (when file is modified outside, it asks to reload. But when you modify a text inside, it does not ask anything like that). How are those applications designed?
I don't KNOW how the editors do this, but I expect that when the application writes to the file itself, it "knows" that it did the writing, so either turns of the notifications whilst doing this, or just says "Sure, I know I wrote to the file, so I'll just ignore it".
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I'm writing a Qt/C++ program that does long-running simulations, and to guard against data loss, I wrote some simple autosave behaviour. The program periodically saves to the user's temp directory (using QDir::temp()), and if the program closes gracefully, this file is deleted. If the program starts up and sees the file in that directory, it assumes a previous instance crashed or was forcibly ended, and it prompts the user about loading it.
Now here is the complication - I'd like this functionality to work properly even if multiple instances of the program are used at once. So when the program loads, it can't just look for the presence of an autosave file. If it finds one, it needs to determine if that file was created by a running instance (in which case, there's nothing wrong and nothing to be done) or if it has been left over by a instance that crashed or was forcibly ended (in which case it should prompt the user about loading it).
My program is for Windows/Mac/Linux, so what would be the best way to implement this using Qt or otherwise in a cross-platform fashion?
Edit:
The comments suggested the use of the process identifier, which I can get using QCoreApplication::applicationPid(). I like this idea, but when the program loads and sees a file with a certain PID in the name, how can it look at the other running instances (if any) to see if there is a match?
You can simply use QSaveFile which, as the documentation states:-
The QSaveFile class provides an interface for safely writing to files.
QSaveFile is an I/O device for writing text and binary files, without losing existing data if the writing operation fails.
While writing, the contents will be written to a temporary file, and if no error happened, commit() will move it to the final file. This ensures that no data at the final file is lost in case an error happens while writing, and no partially-written file is ever present at the final location. Always use QSaveFile when saving entire documents to disk.
As for multiple instances, you just need to reflect that in the filename.
I am not exactly sure if the title captures what exactly I want to ask so here it is:
I just made my first desktop application that uses a XML file to store data. The XML File is stored somewhere in C:/Users/myname/Documents/adjsklf/asdjfklasd/.... on my own machine.
How do I go about creating this file in a specific location C:/Users/theirname/Documents/myAppName/data.xml? Or more specifically, how do I get the "theirname" file name? For machines that have multiple users, how do I get the filename that belongs to the user who actually is using the app?
Also, when I first started this application, I wasn't really thinking of deploying this application. So I made an explicit constructor of a class, dataManipulation, that manipulates all the data in my XML file. What happens is that when my program runs, it executes MainWindow, which at the same time constructs my dataManipulation object with the path I want. However, since now I have a few friends who want to try out this app, I need to be able to detect whether the file exists first using the path I mentioned earlier. What's the best way to achieve that?
Thank you so much!
Use QStandardPaths::writableLocation.
There are many ways to perform file opening checks. If you want to do it in Qt-like way, take a look at QFile and do similarily. In constructor it only sets the path, but the file is opened only in QFile::open, and its return value indicates whether it was successfully opened. You can create init() method in your data manipulation class, check its return value and show a message to user if needed.
I'm trying to fulfill a client request here, and I'm not entirely sure I can actually do it. I have an MFC application that relies upon ShellExecute to open files in their appropriate viewer, which spawns multiple viewers if you try to open multiple files one after the other. If you open one .txt document, and then open another, two copies of notepad appear as expected.
The client wants us to change this functionality - Windows' functionality - to just pass file locations to any viewers that might already be opening. The first image clicked should open Image Viewer, but any other should just be opened in that existing process.
Is that kind of inter-application control/communication possible? Can I generically "pass" files to existing processes? I don't think I can. Executing a program with a file as a parameter is one thing, but passing a file to a running process is something else altogether. I'm not sure you can do that generically, I don't think that kind of functionality is anywhere in the Windows APIs.
I could be wrong, though.
This isn't possible if the viewer don't support multiple open files in same instance.
in your example: notepad will launch a new version with each file, while Notepad++ (a free editor) will open in same instance in a new tab.
The first thing you should try is calling the program again with the new parameters. If the program is written in such a way it will delegate the new parameter to the existing instance. Notepad doesn't do this, image viewer may though.
The next thing you can try is managing the life of the application by keeping track of the handle yourself. You call CreateProcess, so you create and own the handle to this process. On the next call to CreateProcess, enumerate the open windows and try to find your last handle. If the handle is found, close it and continue with your open process. You should only get one open application. For the most reliable solution, put this in a separate thread and wait for the handle (as well as a new request event) to avoid any race conditions.
I wrote an IE plugin using MFC activex. The plugin actually creates a file in CLSID_APPDATA folder and writes some data inside it. But the problem is that file which is created cannot be seen (i mean i cant see any file in CLSID_APPDATA folder on windows vista) whereas I am actually writing data inside it. The plugin is not signed.
I have the code from http://support.microsoft.com/kb/161873 to mark my activex component as safe.
Please let me know if I need to do something more to make it possible for file creation. I hope I was clear. Do let me know if I need to provide more details.
regards,
Pradip.B
It sounds like you're falling foul of IE's "Protected Mode" which redirects file writes made from IE from \Users\UserName\Local\ to \Users\Username\LocalLow (or something very similar to that).
Take a look at the following links as they should point you in the right direction:
More details on Protected Mode IE in Windows Vista
The difference between Local and LocalLow folders
It is possible to move a .sqlite file somewhere while not corrupting it in C or C++?
Somewhere could be another folder or something.
If so could you give me some tips/pointers.
Yes, of course. It is a regular file that can be moved around just like any other file.
The sqlite engine itself will make sure that it does not corrupt. Either it is in a clean state, or locked when being written to (with journaling).
One thing to remember that you should not use the database file on filesystems that do not have reliable locking, e.g. on network disks.
Well, it depends a little. It is just a regular file that (assuming nothing is connected to it, writing to it at the time) can be copied without problem. Welcome to the world of open source systems where things really are that simple (ie you don't need to update the registry, a config file or two, reapply installer settings or anything like that).
However, things are tricky if something is using the file. When an app opens a sqlite file it can tell the OS to lock it - the default is open with exclusive access for the opening app (using a certain flag). In such a case, you will not be able to move the file as the OS will prevent it until the app that has it locked is stopped.