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2011-11-27
09:38:15
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How to use the same thread function for multiple threads (safely)
Introduction
My recent forray into threads and multithreaded applications has taught me a few
lessons which I thought I'd pass on. As I suspect that most people know this already
(it was just me taking a while to catch up), the target audience is beginners to the
world of multithreading. If you're not a beginner, then please read this anyway and
let me know if I've got it all wrong!
The Problem
My problem was that I wanted several threads to basically do the same thing. Being
the OO developer that I am I wanted to be able to wrap this up in a class and just
create several instances of that class and Bob's your uncle. Unfortunately, the
function that you pass to
AfxBeginThread must be either declared outside a class
or be a static class member function. Being OO-type people who want to try to
reduce namespace pollution we'll be using the static class member function
approach, unfortunately this means that all of our class instances will be using the
same function, which is exactly what I'm trying to avoid.
Racing for the Handle
By way of an introduction I'll start off with a brief description of what
AfxBeginThread does. Essentially AfxBeginThread is a helper function that
handles setting up and starting a
CWinThread object for us. To do this it goes
through (broadly speaking) four steps:
1. Allocate a new
CWinThread object on the heap.
2. Call
CWinThread::CreateThread() setting it to initially start suspended.
3. Set the thread's priority.
4. Call
CWinThread::ResumeThread().
The default usage of
CWinThread and AfxBeginThread has a slight problem. By
default, as soon as the thread has completed it is deleted and so if our thread
doesn't take much time to complete, by the time that
AfxBeginThread returns our
CWinThread pointer may already have been deleted and so dereferencing this
pointer to get at the thread handle for a
WaitForSingleObject() call will cause
our application to crash.
Fortunately
CWinThread provides a simple answer to this by way of the
m_bAutoDelete member variable. As described above, this is TRUE by default.
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