I'm writing a multipart downloader in Qt. Multiple QNetWorkRequest with http header "Range" are used to download a file. Now I write data received in each part(QNetworkReply) to file.part1, file.part2, etc.
Is it possible to write data to the same file simultaneously? Do I have to implement a lock for it and which is the best way to save data in my application?
Any suggestions?
Why not just merge the file parts when you are finished? You can write the parts to a QFile easily. Perhaps something about the approach or the data keeps you from doing this, but if you can, it's probably the approach I would take before dealing with treating a QFile as a shared resource.
If you want multiple concurrent replies to be able to write to and access the QFile, then yes, the QFile becomes a shared resource. As far as I know, you're going to need to lock it. At that point, you have several options. You can have the slot(s) handling the replies attempt to acquire a QSemaphore, you can use QMutex and QMutexLocker if you'd prefer to lock on a mutex. You could treat it as a multiple producer (the various QNetworkReplys) single consumer (whatever is writing to the file) problem (here's a Stack Overflow post that provides some useful links) if you want to go that route. In short, there are numerous approaches here, all of which I think are more of a hassle than simply merging the file.part's at the end if you're able to go that route.
In terms of merging to a single QFile concurrently, there may be an easier Qt way of doing it, but I've never found it. Perhaps someone else can chime in if such a method exists.
I'm not sure what you mean by "which is the best way to save data in my application?" Are you referring to saving application specific settings in a persistent manner? If so, look into QSettings. If you're referring to saving the data you're downloading, I'd probably write it to a QFile, just like you appear to be doing. Although it's hard to know for sure without knowing more about what you're downloading.
