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How to write in a single file with multiple threads?
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Closed 8 years ago.
I have a bi-threaded c# application where both threads write to the same log and I'm getting an error.
'The process cannot access the file 'logfile.log' because it is being used by another process.'
I'm somewhat new to C# and multi-threading in general so if anyone can point me in the right direction here it would be much appreciated.
The error message says that the file is held open by another process, not thread. Are you sure the file is being closed properly each time you run your app ?
Generally speaking, you should use a lock to control access to a shared resource inside your application. For example, if you have a LogWriter class with a Log() function, a minimal implementation with no queuing might look like this:
public class LogWriter
{
private readonly object _lock = new object;
public void Log(string message)
{
lock(_lock)
{
//write message to log file
string appName = Path.GetFileNameWithoutExtension(Environment.GetCommandLineArgs()[0]);
StreamWriter sw = new StreamWriter(Environment.CurrentDirectory + Path.DirectorySeparatorChar + appName + ".log", true);
sw.WriteLine(string.Format("{0:u} {1}", DateTime.Now, message));
sw.Flush();
sw.Close();
}
}
The lock(_lock) ensures that only one thread accesses the file at a time. The sw.Close() ensures that the file is not left open when the process terminates ;-)
As the error message states, you can't create a writer from multiple different threads. You'll need to find some way of either synchronizing the access to the file between threads, which is to say ensuring that one thread waits to try to access the file until all other threads are done with it, or you could designate a single thread as being the thread responsible for accessing that file, requiring all other threads that want to access that file to request the other thread to do it for them.
You need to create a list(or whatever data structure you like) that is accessible to parallel threads. Then add record to that list in each thread
lock (yourList) {
// here you can add items to the List
}
When finished, dump your list to a file.
Or create a new List per each thread, return them all back, and then join all lists into one.
Or use a Database and add records to a table (the most logical solution)
As alternative solution
Create a global List
pass that list to all threads and lock it(example above) and write to it; unlock it
Create another infinite loop thread with delay
Inside infinite loop, after delay, lock the list, get all the data, write to file, empty the list
release the list
Related
Hello and thanks for your help.
This time I am having a curious problem with a program (C#) I am writing and would like to hear your advice.
I am writing a normal program (not multithreaded) but then added a timer (System.Timers.Timer)
Also I am using a StreamWriter to write on a file. I open this like this
StreamWriter logStream=new StreamWriter(filename, true);
meaning that if the file exists, it appends , if not it creates.
Later I write in the file like this
logStream.WriteLine(message);
However, I write to the stream from both the main function and from the function that is called by the timer.
the problem symptoms
My program is throwing an error sometimes when I flush or write the stream saying that "Can not access a closed file" and other times "Can not access a closed TextWriter... (What is a "TextWriter"?)
However curiously, the file keeps being written without problem. (Even the "can not access a closed file" message is written in the supposed closed file)
I am not familiar with the inner workings of a Timer. (I suppose it runs a separate thread?)
My question is
Is it possible to use a StreamWriter from several threads? (in this case the main one and the Timer one)
Is it possible that there is happening a race condition or some problem like that?
One more thing: I made a logic mistake and close and reopen the file every time I want to write on it. Yes, it is a mistake and I should correct it. But maybe if I correct this the error I described above will disappear masking a more serious flaw.
My suspicions is that since I am closing and opening the file every time I write on it, maybe the both threads try to access them on a wrong time
Any help will be greatly appreciated
Closing and opening you file under this scenario will create a race condition like you suspect. You cannot keep the stream open and pass the object to the thread because you might end up with a similar issue if you call from different thread. Your best solution remain using a thread safe method that will write what you send to it.
the methods are static because the lock has to be accessible from all instance of the class.
private static ReaderWriterLockSlim readerWriterLockSlim = new ReaderWriterLockSlim();
public static void AppendToFile(string path, string text)
{
// Set to locked (other thread will freeze here until object is unlocked
readerWriterLockSlim.EnterWriteLock();
try
{
// Write that will append to the file
using (StreamWriter sw = File.AppendText(path))
{
// append the text
sw.WriteLine(text);
sw.Close();
}
}
finally
{
// Clear the lock
readerWriterLockSlim.ExitWriteLock();
}
}
We currently have one application that monitors a folder for new files. To make it fault tolerant and be able to process more files at once, we want to be able to run multiple instances of this application on different machines. We use File.Move to "lock" a file and make sure that only one thread can process a file at a time.
To test that only one application and/or thread can perform a File.Move on a file, I created a simple application (based on the original application's code), which created 10 threads per application and monitored a folder, when each thread detects a new file, it performs File.Move on it and changes the file's extension, to try and stop other thread's from doing the same.
I have seen an issue when running multiple copies of this application (and it running on its own), whereby 2 threads (either in the same application or different ones), both successfully perform File.Move with no exception thrown, but the thread that performed it last (I change the file's extension to include the DateTime.Now.ToFileTime()), successfully renamed the file.
I have looked at what File.Move does and it checks to see if the file exists before it performs the operation, then it calls out to Win32Native.MoveFile to perform the move.
All the other threads/applications throw an exception, as I would expect.
The reasons why this is an issue are:
I thought only 1 thread can perform a File.Move on a file at a time.
I need to reliably have only one application/thread be able to process a file at a time.
Here is the code that performs the File.Move:
public bool TryLock(string originalFile, out string changedFileName)
{
FileInfo fileInfo = new FileInfo(originalFile);
changedFileName = Path.ChangeExtension(originalFile, ".original." + DateTime.Now.ToFileTime());
try
{
File.Move(originalFile, changedFileName);
}
catch (IOException ex)
{
Console.WriteLine("{3} - Thread {1}-{2} File {0} is already in use", fileInfo.Name, Thread.CurrentThread.ManagedThreadId, id, DateTime.Now.ToLongTimeString());
return false;
}
catch (Exception ex)
{
Console.WriteLine("{3} - Thread {1}-{2} File {0} error {4}", fileInfo.Name, Thread.CurrentThread.ManagedThreadId, id, DateTime.Now.ToLongTimeString(), ex);
return false;
}
return true;
}
Note - id is just a sequential number I assigned to each thread for logging.
I am running Windows 7 Enterprise SP1 on a SSD with NTFS.
From the MSDN description I assume that File.Move does not open the file in exclusive mode.
If you try to move a file across disk volumes and that file is in use,
the file is copied to the destination, but it is not deleted from the
source.
Anyway, I think you are better off to create your own move mechanism and have it open the file in exclusive mode prior to copying it (and then deleting it):
File.Open(pathToYourFile, FileMode.Open, FileAccess.Read, FileShare.None);
Other threads won't be able to open it if the move operation is already in progress. You might have race condition issues between the moment the copy is finalized (thus you need to dispose of the file handle) and deleting it.
Using File.Move as a lock isn't going to work. As stated in #marceln's answer, it won't delete the source file it is already in use elsewhere and doesn't have a "locking" behavior, you can't relay on it.
What i would suggest is to use a BlockingCollection<T> to manage the processing of your files:
// Assuming this BlockingCollection is already filled with all string file paths
private BlockingCollection<string> _blockingFileCollection = new BlockingCollection<string>();
public bool TryProcessFile(string originalFile, out string changedFileName)
{
FileInfo fileInfo = new FileInfo(originalFile);
changedFileName = Path.ChangeExtension(originalFile, ".original." + DateTime.Now.ToFileTime());
string itemToProcess;
if (_blockingFileCollection.TryTake(out itemToProcess))
{
return false;
}
// The file should exclusively be moved by one thread,
// all other should return false.
File.Move(originalFile, changedFileName);
return true;
}
Are you moving across volumes or within a volume? In the latter case no copying is necessary.
.
#usr In production, once a thread has "locked" a file, we will be moving it across network shares
I'm not sure whether that is a true move or a copy operation. In any case, you could:
open the file exclusively
copy the data
delete the source by handle (Deleting or Renaming a file using an open handle)
That allows you to lock other processes out of that file for the duration of the move. It is more of a workaround than a real solution. Hope it helps.
Note, that for the duration of the move the file is unavailable and other processes will receive an error accessing it. You might need a retry loop with a time delay between operations.
Here's an alternative:
Copy the file to the target folder with a different extension that is being ignored by readers
Atomically rename the file to remove the extension
Renaming on the same volume is always atomic. Readers might receive a sharing violation error for a very short period of time. Again, you need a retry loop or tolerate a very small window of unavailability.
Based on #marceln and #YuvalItzchakov answer/comments, I tried the following, which seems to give more reliable results:
using (var readFileStream = File.Open(originalFile, FileMode.Open, FileAccess.Read, FileShare.Delete))
{
readFileStream.Lock(0, readFileStream.Length - 1);
File.Move(originalFile, changedFileName);
readFileStream.Unlock(0, readFileStream.Length - 1);
}
I want to use Windows's own file copying as it should be more efficient than copying the stream and in production we will be moving the files from one network share to another.
I have a txt file ABC.txt which will be read and wrote by multi processes. So when one process is reading from or writing to file ABC.txt, file ABC.txt must be locked so that any other processes can not reading from or writing to it. I know the enum System.IO.FileShare may be the right way to handle this problem. But I used another way which I'm not sure if it is right. The following is my solution.
I added another file Lock.txt to the folder. Before I can read from or write to file ABC.txt, I must have the capability to read from file Lock.txt. And after I have read from or written to file ABC.txt, I have to release that capability. The following is the code.
#region Enter the lock
FileStream lockFileStream = null;
bool lockEntered = false;
while (lockEntered == false)
{
try
{
lockFileStream = File.Open("Lock.txt", FileMode.Open, FileAccess.Read, FileShare.None);
lockEntered = true;
}
catch (Exception)
{
Thread.Sleep(500);
}
}
#endregion
#region Do the work
// Read from or write to File ABC.txt
// Read from or write to other files
#endregion
#region Release the lock
try
{
if (lockFileStream != null)
{
lockFileStream.Dispose();
}
}
catch
{
}
#endregion
On my computer, it seems that this solution works well, but I still can not make sure if it is appropriate..
Edit: Multi processes, not multi threads in the same process.
C#'s named EventWaitHandle is the way to go here. Create an instance of wait handle in every process which wants to use that file and give it a name which is shared by all such processes.
EventWaitHandle waitHandle = new EventWaitHandle(true, EventResetMode.AutoReset, "SHARED_BY_ALL_PROCESSES");
Then when accessing the file wait on waitHandle and when finished processing file, set it so the next process in the queue may access it.
waitHandle.WaitOne();
/* process file*/
waitHandle.Set();
When you name an event wait handle then that name is shared across all processes in the operating system. Therefore in order to avoid possibility of collisions, use a guid for name ("SHARED_BY_ALL_PROCESSES" above).
A mutex in C# may be shared across multiple processes. Here is an example for multiple processes writing to a single file:
using (var mutex = new Mutex(false, "Strand www.jakemdrew.com"))
{
mutex.WaitOne();
File.AppendAllText(outputFilePath,theFileText);
mutex.ReleaseMutex();
}
You need to make sure that the mutex is given a unique name that will be shared across the entire system.
Additional reading here:
http://www.albahari.com/threading/part2.aspx#_Mutex
Your solution is error prone. You've basically implemented double-checked locking (http://en.wikipedia.org/wiki/Double-checked_locking) which can be very unsafe.
A better solution would be to either introduce thread isolation, whereby only one thread ever accesses the file and does so by reading from a queue upon which requests to read or write are placed by other threads (and of course the queue is protected by mutually exclusive access by threads) or where the threads synchronize themselves either by synchronization devices (lock sections, mutices, whatever) or by using some other file access logic (for example, System.IO.FileShare came up in a few reponses here.)
If it was me, I would install something like SQL Server Compact Edition for reading/writing this data.
However, if you want to be able to lock access to a resource that is shared between multiple processes, you need to use a Mutex or a Semaphore.
The Mutex class is a .Net wrapper around an OS Level locking mechanism.
Overview of Synchronization Primitives
I'm taking over a C# project, and when testing it out I'm getting errors. The error is that the log file cannot be written to because it is in use by another process. Here's the code:
public void WriteToLog(string msg)
{
if (!_LogExists)
{
this.VerifyOrCreateLogFile(); // Creates log file if it does not already exist.
}
// do the actual writing on its own thread so execution control can immediately return to the calling routine.
Thread t = new Thread(new ParameterizedThreadStart(WriteToLog));
t.Start((object)msg);
}
private void WriteToLog(object msg)
{
lock (_LogLock)
{
string message = msg as string;
using (StreamWriter sw = File.AppendText(LogFile))
{
sw.Write(message);
sw.Close();
}
}
}
_LogLock is defined as a class variable:
private object _LogLock = 0;
Based on my research and the fact that this has been working fine in a production system for a few years now, I don't know what the problem could be. The lock should prevent another thread from attempting to write to the log file.
The changes I've made that need to be tested are a lot more log usage. We're basically adding a debug mode to save much more info to the log than used to be saved.
Thanks for any help!
EDIT:
Thanks for the quick answers! The code for VerifyOrCreateLogFile() does use the _LogLock, so that shouldn't be an issue. It does do some writing to the log before it errors out, so it gets past creating the file just fine.
What seems to be the problem is that previously only one class created an instance of the log class, and now I've added instances to other classes. It makes sense that this would create problems. Changing the _LogLock field to be static fixes the issue.
Thanks again!
The lock should prevent another thread from attempting to write to the log file.
This is only true if you're using a single instance of this class.
If each (or even some) of the log requests use a separate instance, then the lock will not protect you.
You can easily "correct" this by making the _LogLock field static:
private static object _LogLock = 0;
This way, all instances will share the same lock.
I see 2 problems with the code:
Lock must be the same among all "users" of ths Log class, easiest solution is to make either _LogLock or the complete class static
VerifyOrCreateLogFile could pose a problem if 2 or more parallel threads call WriteToLog when _LogExists is false...
One possibility is that the OS isn't releasing the file lock quickly enough before you exit the lock in WriteToLog and another thread that was blocked waiting for the lock tried to open it before the OS finished releasing the file lock. Yes, it can happen. You either need to sleep for a little before trying to open the file, centralize the writing to the log to a dedicated object (so that he and only he has access to this file and you don't have to worry about file lock contentions).
Another possibility is that you need to lock around
if (!_LogExists) {
this.VerifyOrCreateLogFile(); // Creates log file if it does not already exist.
}
The third possibility is that you have multiple instances of whatever class is housing these methods. The lock object won't be shared across instances (make it static to solve this).
At the end of the day, unless you're an expert in writing safe multi-threaded code, just let someone else worry about this stuff for you. Use a framework that handles these issues for you (log4net?).
you can do the code executable by simply
removing sw.Close(); from your code ...
do it....
it will work fine.....
I have the same BackgroundWorker code piece in two simultaneously running applications. Will this code avoid the problem of same resource getting access by two processes and run smoothly?
void bw_DoWork(object sender, DoWorkEventArgs e)
{
bool flag = false;
System.Threading.Thread.Sleep(1000);
while (flag.Equals(false))
{
string dir = #"C:\ProgramData\Msgs";
try
{
if (Directory.GetFiles(smsdir).Length > 0)
{
flag = true;
}
}
catch (Exception exc)
{
Logger.Log("Dir Access Exception: " + exc.Message);
System.Threading.Thread.Sleep(10);
}
}
On one level, depending on what you're doing, there's nothing wrong with having multiple applications accessing the same directory or file. If it's just read access, then by all means, both can access it at once.
If you've got identical code in multiple applications, then a Boolean isn't going to cut it for synchronization, no matter what you do: Each application has its own copy of the Boolean, and cannot modify the other.
For cross application synhronization, I'd use the Mutex class. There's a constructor that takes a string parameter, specifying the name of the Mutex. Mutex names are unique across all of Windows, not just your application. You can do Mutex m = new Mutex(false, "MySpecialMutex"); in two different applications, and each object will be referring to the same thing.
No, it won't solve the issue because setting the boolean's value and checking it is not an atomic function and is thus not thread safe. You have to use either a Mutex or a Monitor object.
Check this link for more info: Monitor vs Mutex in c#
No, it will not -- at least, the code you have pasted will not accomplish any sort of meaningful process synchronization.
If you want a more detailed and helpful answer, you are going to need to be more specific about what you are doing.
You must come up with some kind of cross-process synchronization scheme - any locking mechanism you use in that code is irrelevant if you're trying to prevent collisions between two processes as opposed to two threads running on the same process.
A good way to do locking across processes like this is to use a file. First process in creates a file and opens it with exclusive access, and then deletes it when its done. The second process in will either see that the file exists and have to wait till it doesn't or it will fail when attempting to open the file exclusively.
no, 'flag' is local to the scope of the method, which is local to the scope of the thread. In other words, it will also equal false.
This is what the lock function is for. Use it like this
In your class, declare a private object called gothread.
in your method write it like this
lock(gothread)
{
// put your code in here, one thread will not be able to enter when another thread is already
// in here
}