C++ – Logging safely from a worker thread

cmultithreading

In one of my worker threads I want to do some logging. The log messages are channeled to a GUI textarea, which should only be accessed from the main thread. So the problem is: how do I log messages safely from a worker thread?

My current solution is to have the logging function check whether we are currently in the main thread. If yes, then just log. If no, then add the message to a queue of pending messages (this queue is protected by a mutex). The main thread also has a timer who's callback function (which also executes in the main thread) takes care of dequeueing and logging any pending messages.

Above solution is just my own little invention. Are there better or more standard solutions to this problem?

Best Answer

If you have one rule for logging in one thread ("just do it now") and another rule for other threads ("add it to a queue for later") then your logging will get out of order. I can't imagine this being a good thing. Have one rule for all your logging - add it to the queue.

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C# – Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

As per Prerak K's update comment (since deleted):

I guess I have not presented the question properly.

Situation is this: I want to load data into a global variable based on the value of a control. I don't want to change the value of a control from the child thread. I'm not going to do it ever from a child thread.

So only accessing the value so that corresponding data can be fetched from the database.

The solution you want then should look like:

UserContrl1_LOadDataMethod()
{
    string name = "";
    if(textbox1.InvokeRequired)
    {
        textbox1.Invoke(new MethodInvoker(delegate { name = textbox1.text; }));
    }
    if(name == "MyName")
    {
        // do whatever
    }
}

Do your serious processing in the separate thread before you attempt to switch back to the control's thread. For example:

UserContrl1_LOadDataMethod()
{
    if(textbox1.text=="MyName") //<<======Now it wont give exception**
    {
        //Load data correspondin to "MyName"
        //Populate a globale variable List<string> which will be
        //bound to grid at some later stage
        if(InvokeRequired)
        {
            // after we've done all the processing, 
            this.Invoke(new MethodInvoker(delegate {
                // load the control with the appropriate data
            }));
            return;
        }
    }
}

The difference between a process and a thread

Both processes and threads are independent sequences of execution. The typical difference is that threads (of the same process) run in a shared memory space, while processes run in separate memory spaces.

I'm not sure what "hardware" vs "software" threads you might be referring to. Threads are an operating environment feature, rather than a CPU feature (though the CPU typically has operations that make threads efficient).

Erlang uses the term "process" because it does not expose a shared-memory multiprogramming model. Calling them "threads" would imply that they have shared memory.

Best Answer

Related Solutions

C# – Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

The difference between a process and a thread

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