I have implemented multithreaded shared library in C++ (For Linux and Windows). I would like to add logging mechanism inside the library itself. The caller of the library is not aware of that. The log file would be same so I am wondering how could I design the thread safe logging if multiple process is using my library and trying to open and log into the same log file. Any suggestions?
You can try using log4cpp library.
Use file locking. I believe fcntl is POSIX compliant so should work on Windows too. Does your code use Posix calls?
With fcntl, you should be able to lock a specific range of bytes. So if you seek to end and try to lock out the amount of bytes you are about to write, it should be pretty fast. To obtain the lock, you can probably spin, relinquishing the CPU for a small amount of time, if you don't obtain the lock.
Your library shares the log file with the client application? If so, there is absolutely no way to do thread-safe logging. The client could just create a thread and log while calling you.
Otherwise, you have two options:
Use a mutex. Simplest solution.
Have a logging thread with a lock-free (you'll probably be able to get away with a mutex) FIFO queue of messages that is created/destroyed when your library is created/destroyed. I hope you have an init/deinit function...
The difference b/w 1 and 2 is that for 1, you hold a mutex for the full I/O operation. In the second, you only hold a mutex for as long as it takes to push a message at the back of the queue, which can be a constant operation if you do it right.
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Could you help me to understand how to use mutexes in multithread Linux application, where:
during data writing it is need to lock variable on write and read
during data reading from the variable it is need to lock it on write.
So it is possible to read simultaneously, but writing opertion is a single opertaion in the same time. During writing, all other operation should wait before it finishes.
You're asking about something that is a bit higher level than mutexes. A mutex is a simple, low-level device. When you lock a thread with a mutex, the CPU is either executing code in the thread that obtained the lock or it is executing some other process entirely. In other words, the mutex has locked out all other threads that belong to the same (heavyweight) process.
You are asking about a read-write lock. Read-write locks use mutexes underneath the hood. The POSIX functions that deal with read-write locks start with pthread_rwlock_. Since you are on a Linux machine, just type man pthread and look for the section marked "READ/WRITE LOCK ROUTINES".
You need a reader/writer lock to allow multiple readers/single writer.
Boost.Thread has one of these (boost::shared_mutex), if you have no other preferred threading library. This uses PThreads primitives under the covers, and will probably save you time in wrapping the raw APIs yourself.
I would not recommend implementing this yourself - it's easy to get something that appears to work, but under load either crashes or kills performance or (worst of all) silently modifies your data in a way it should not be, so you get bad results.
A simple boost::mutex can also be used here as noted by #Als, but won't allow multiple concurrent reads. That is simpler to implement, and may be sufficient for your needs, depending on your read/write access profile.
You will need to use mutexes, if you have global or static objects which are being accessed(read and written to) from different threads.
I don't want to reinvent the wheel, and what I'm looking for most likely already exist in the FOSS world.
I'm looking for a pthread bases utility library that implements often used primitives to do communication between threads.
My main need is some kind of blocking queue for fixed size messages and the ability to wait for data to arrive on multiple queues at the same time (what you usually do using poll and select with file-handles).
Does something like this exist?
Programming language is C++ but I'm fine with a C library. OS is Linux but anything posix will do.
EDIT
I'm not looking for a thin wrapper around pthreads (like boost::thread or so). I already have this up and running. I'm looking for higher level primitives. Basically What java.util.concurrancey offers for the java guys.
Your requirements are already baked into POSIX Message Queues.
Instead of using select() you can do it in reverse. Rather than waiting in a select() you can use mq_notify() to tell you when there is something to read. MQs give you the option of having a signal delivered or having them spawn a new thread to read the queue.
If you are really intent on using select(), Linux makes this painless since the mqd_t type is actually a file descriptor. You can simply use the mqd_t returned from mq_open() like any other FD in select().
Note that use of a mqd_t in select() is not portable. In theory you should be able to do something similar on other systems but I have never tested it. Since POSIX MQs have a path to an entry to the filesystem you should be able to do a straight open() on the path and use the returned file descriptor in the select(), mapping it to the mqd_t used in mq_open() to determine which queue to read. Again, I have never tried it.
There's always boost::thread.
You could try OpenMP, though I'm not sure whether it's based on the pthread API or not.
For what programming language / environment?
Some options:
C: c-pthread-queue, APR queue
Python: queue module
I'm looking for a way to do asynchronous and thread-safe logging in my C++ project, if possible to one file. I'm currently using cerr and clog for the task, but since they are synchronous, execution shortly pauses every time something is logged. It's a relatively graphics-heavy app, so this kind of thing is quite annoying.
The new logger should use asynchronous I/O to get rid of these pauses. Thread-safety would also be desirable as I intend to add some basic multithreading soon.
I considered a one-file-per-thread approach, but that seemed like it would make managing the logs a nightmare. Any suggestions?
I noticed this 1 year+ old thread. Maybe the asynchronous logger I wrote could be of interest.
http://www.codeproject.com/KB/library/g2log.aspx
G2log uses a protected message queue to forward log entries to a background worker that the slow disk accesses.
I have tried it with a lock-free queue which increased the average time for a LOG call but decreased the worst case time, however I am using the protected queue now as it is cross-platform. It's tested on Windows/Visual Studio 2010 and Ubuntu 11.10/gcc4.6.
It's released as public domain so you can do with it what you want with no strings attached.
This is VERY possible and practical. How do I know? I wrote exactly that at my last job. Unfortunately (for us), they now own the code. :-) Sadly, they don't even use it.
I intend on writing an open source version in the near future. Meanwhile, I can give you some hints.
I/O manipulators are really just function names. You can implement them for your own logging class so that your logger is cout/cin compatible.
Your manipulator functions can tokenize the operations and store them into a queue.
A thread can be blocked on that queue waiting for chunks of log to come flying through. It then processes the string operations and generates the actual log.
This is intrinsically thread compatible since you are using a queue. However, you still would want to put some mutex-like protection around writing to the queue so that a given log << "stuff" << "more stuff"; type operation remains line-atomic.
Have fun!
I think the proper approach is not one-file-per-thread, but one-thread-per-file. If any one file (or resource in general) in your system is only ever accessed by one thread, thread-safe programming becomes so much easier.
So why not make Logger a dedicated thread (or several threads, one per file, if you're logging different things in different files), and in all other threads, writing to log would place the message on the input queue in the appropriate Logger thread, which would get to it after it's done writing the previous message. All it takes is a mutex to protect the queue from adding an event while Logger is reading an event, and a condvar for Logger to wait on when its queue is empty.
Have you considered using a log library.
There are several available, I discovered Pantheios recently and it really seems to be quite incredible.
It's more a front-end logger, you can customize which system is used. It can interact with ACE or log4cxx for example and it seems really easy to use and configure. The main advantage is that it use typesafe operators, which is always great.
If you just want a barebone logging library:
ACE
log4c*
Boost.Log
Pick any :)
I should note that it's possible to implement lock-free queues in C++ and that they are great for logging.
I had the same issue and I believe I have found the perfect solution. I present to you, a single-header library called loguru: https://github.com/emilk/loguru
It's simple to use, portable, configurable, macro-based and by default doesn't #include anything (for that sweet, sweet compilation times).
I am writing an application needs to use large audio multi-samples, usually around 50 mb in size. One file contains approximately 80 individual short sound recordings, which can get played back by my application at any time. For this reason all the audio data gets loaded into memory for quick access.
However, when loading one of these files, it can take many seconds to put into memory because I need to read a large amount of data with ifstream, meaning my program GUI is temporarily frozen. I have tried memory mapping my file but this causes huge CPU spikes and a mess of audio every time I need to jump to a different area of the file, which is not acceptable.
So this has led me to think that performing an Asynchronous file read will solve my problem, that is the data gets read in a different process and calls a function on completion. This needs to be both compatible for Mac OS X and Windows and in C++.
EDIT: Don't want to use the Boost library because I want to keep a small code base.
boost has an asio library, which I've not used before (it's not on NASA's list of approved third-party libraries).
My own approach has been to write the file reading code twice, once for Windows, once for the POSIX aio API, and then just pick the right one to link with.
For Windows, use OVERLAPPED (you have to enable it in the CreateFile call, then pass an OVERLAPPED structure when you read). You can either have it set an event on completion (ReadFile) or call a completion callback (ReadFileEx). You'll probably need to change your main event loop to use MsgWaitForMultipleObjectsEx so you can either wait for the I/O events or allow callbacks to run, in addition to receiving WM_ window messages. MSDN has documentation for these functions.
For Linux, there's either fadvise and epoll, which will use the readahead cache, or aio_read which will allow actual async read requests. You'll get a signal when the request completes, which you should use to post an XWindows message and wake up your event processing loop.
Both are a little different in the details, but the net effect is the same -- you request a read which completes in the background, then your event dispatch loop gets woken up when the I/O finishes.
Boost.Asio library has limited implementation of asynchronous file I/O operations (only Windows wrapper for HANDLE) therefore it not suitable for you. See this question also.
You could easily implement your own asynchronous reading using standard streams and Boost.Thread library (or platform specific threads support).
I have a main process that uses a single thread library and I can only the library functions from the main process. I have a thread spawned by the parent process that puts info it receives from the network into a queue.
I need to able to tell the main process that something is on the queue. Then it can access the queue and process the objects. The thread cannot process those objects because the library can only be called by one process.
I guess I need to use pipes and signals. I also read from various newsgroups that I need to use a 'self-trick' pipe.
How should this scenario be implemented?
A more specific case of the following post:
How can unix pipes be used between main process and thread?
Why not use a simple FIFO (named pipe)? The main process will automatically block until it can read something.
If it shouldn't block, it must be possible to poll instead, but maybe it will suck CPU. There probably exists an efficient library for this purpose.
I wouldn't recommend using signals because they are easy to get wrong. If you want to use them anyway, the easiest way I've found is:
Mask all signals in every thread,
A special thread handles signals with sigwait(). It may have to wake up another thread which will handle the signal, e.g. using condition variables.
The advantage is that you don't have to worry anymore about which function is safe to call from the handler.
The "optimal" solution depends quite a bit on your concrete setup. Do you have one process with a main thread and a child thread or do you have one parent process and a child process? Which OS and which thread library do you use?
The reason for the last question is that the current C++03 standard has no notion of a 'thread'. This means in particular that whatever solution your OS and your thread library offer are platform specific. The most portable solutions will only hide these specifics from you in their implementation.
In particular, C++ has no notion of threads in its memory model, nor does it have a notion of atomic operations, synchronization, ordered memory accesses, race conditions etc.
Chances are, however, that whatever library you are using already provides a solution for your problem on your platform.
I highly suggest you used a thread-safe queue such as this one (article and source code). I have personally used it and it's very simple to use. The API consist in simple methods such as push(), try_pop(), wait_and_pop() and empty().
Note that it is based on Boost.Thread.