Regarding Locking The File
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A QSystemSemaphore can protect a resource. A resource can almost anything: a block of memory, access to a piece of hardware, or access to a file. In your case, you allow only one access to your file at the same time. So, you create a semaphore with an appropriate key (perhaps "com.your.domain.your.app/yourFile" or something unique like that), and use an initial value of 1. Then, when you want to start accessing your file, you call the acquire() method on the semaphore, and check the return value. If the return value is true, then you have successfully acquired the resource (access to the file), and can proceed to modify the file. If you get a false, then something else is already accessing the file and you can not modify it.
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I had a similar problem. My application is watching a directory using QFileSystemWatcher. If a new file is stored in the directory i want to read that file. The file is written to the directory by another application which is in no way connected to my app. The problem I encountered was that sometimes the FileSystemWatcher notices a new file and I start reading it with QFile.readAll() before it is acctually completely written, thus getting an empty or incomplete String. My dirty solution was checking if the file was modified within the last 2 seconds and if not start reading. This works in my case since the file is only once written and afterwards will only be accessed or modified by my application. But in the case mentioned by the OP this wont work.
@Andre: Will QSystemSemaphore be an adequate solution for my problem as well ?
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[quote author="KA51O" date="1309335683"]@Andre: Will QSystemSemaphore be an adequate solution for my problem as well ?[/quote]
No, it will not.
The use of QSystemSemaphore assumes that the process all use that same semaphore to get access to the resource. If they don't (and your case, you can be sure of that), then the semaphore is useless. -
No you cannot read while someone else has aquired the semaphore and is writing.
How about using "QReadWriteLock":http://developer.qt.nokia.com/doc/qt-4.7/qreadwritelock.html ?
I think its more intuitive to use. -
@Rajveer:
QReadWriteLock does not work across processes, so it is useless for this case.It is up to you to figure out if you want to allow reading while the file is being written to too. It depends on your application if that is a problem or not. You can look at QReadWriteLock to see what kind of problems you may expect though, and what kind of algorithm you could use.
By default, if you only use one semaphore for all file access, then both reading and writing block the file completely. You could implement something more intelligent than that based on using one or more semaphores though. QReadWriteLock can be an inspiration for that.
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[quote author="KA51O" date="1309336526"]Too bad. Thanks for the reply anyways.
Is there maybe another way to deal with my problem in a more "professional" way? I cannot use any OS specific methods like 'fuser' or 'lsof'.[/quote]Why cannot you use those? You could implement a solution based on OS specific methods for each OS you support.
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Hi Andre
Below is example code for Read & write
Read:
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QFile myfile(filename);
if (myfile.open(QIODevice::ReadOnly | QIODevice::Text)) // Open the file
{
Reading code;
myfile.close();
}
@Write:
@
QFile myfile(filename);
if (myfile.open(QIODevice::WriteOnly)) // Open the file
{
writing code;
myfile.close();
}
@Andre can you please show me how to use the semaphore with above two codes.
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[quote author="Rajveer" date="1309338534"]Hi KA51O
So if i take a WriteLock while writing to the file,so can any other process can read at that time.
[/quote]
Any other process that takes part in the (system) semaphore game will most probably not read the file. Any process which does not use a semaphore or mutex can happily read and write the file as long as it has the OS permission to do so.
You can not avoid that situation entirely, you have to rely on the good behavior of all parties joining the party.
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[quote author="Andre" date="1309338341"]@Rajveer:
QReadWriteLock does not work across processes, so it is useless for this case. [/quote]
Thanks for the hint.I hadn't stumbled upon "QSystemSemaphore's":http://developer.qt.nokia.com/doc/qt-4.7/qsystemsemaphore.html up until now. Thanks for that hint too.