multiply thread access to physical file.

[banburybug]

Evening all been looking into an issue where multiply threads access a single file.

So basically I have one thread which open, writes and then closes the file. Then another thread which copies the file and deletes the file. From reading up by default fileIO is Synchronous.

However on occasion CopyFile fails with last error code of ERROR_SHARING_VIOLATION.

Just to make sure my understanding is correct of Synchronous is if one thread is writing to that file and the other thread has to wait for that write to finish?

Thanks in advance all :)

[kbw]

The Windows file system, NTFS, is very slow. It's a transactional file system and the OS can be still messing around with the file long after the app has stopped using it.

If it says it's still in use, it's still in use. It's just an error you have to cope with.

[banburybug]

HEY KBW thank you for your help.

Well what I am thinking is using LockFile() So that I can do other things to the file then once finished let the other thread do it bit.

[modoran]

You use a mutex, making all the worker threads waiting until the file is written.
In your particular case a semaphone is more appropiate to use (only one thread which write data needs exclusive access, readers does not need to wait each other)

[banburybug]

Hey modoran thank you for your response, currently im trying to understand everything.

Just found when one thread is copying that file and another thread tries to write to that file at the same time it fail to write. I thought it being asynchronous it would wait at the write command till it has finished coping in the other thread.

Been reading this,
http://msdn.microsoft.com/en-gb/library/windows/desktop/aa365683(v=vs.85).aspx

[modoran]

That article is NOT what you seek.

[banburybug]

well the article explains fileIO Synchronous and Asynchronous, I put this link in to say this is where i am getting information on the subject.

[banburybug]

Gone down the root of a named mutex does the job. Was playing around with them and found if I have one thread with waitforsingleobject with INFINITE set, eventually that thread halts as its waiting for the mutex to be the other thread which uses the same named mutex has released it and closed the handle and the thread still going.

Can the OS get backlogged has it has not released it?

[modoran]

You really should understand how mutexes work, that's why I said your problem has nothing to do with syncronous/asyncronous mode to access a file.

Un that particular case, I's use a critical section instead of a mutex (a mutex consume more CPU cycles, but works also between different processes).


In main thread call:

CRITICAL_SECTION cs;
InitializeCriticalSection (&cs);

Then create as many threads as you want ...

When you want tyo access the file, put this code inside any threads, including the main thread:

EnterCriticalSection (&cs); // all other threads will be automatically blocked here

// access the file here, only one thread can run at this point


LeaveCriticalSection (&cs); // do not forget to release it or other threads will wait forever 

[banburybug]

Hey modoran thank you again for your help, what I like about the mutex is that if one thread cannot get the named mutex it jumps over that code and carrys on with something else instead of waiting, as that thread does other things.

[banburybug]

okay never mind reading TryEnterCriticalSection function :)

[jackdark]

Read This to complete short example about this functions:
http://strongcpp.blogspot.ru/2013/04/blog-post.html