Simple message queue for Windows C application


I'm hoping someone can provide a reference or example.

I have a multi-threaded application in C.  I'd like to use message queues to communicate between two of the threads and be able to pass messages of ~16 bytes.  I'm developing this under Windows right now but will port this to a different embedded operating system in the future.  I realize the thread and message queue functions will be different under a different operating system.

What I'm looking for is the simple way to do a message queue under Windows.  The MSDN literature immediately starts talking about GUI windows and system messages.  I'm not interested in either.

All I want to do is create the queue, pend a thread listening on the queue, and be able to write to it.

Any pointers or examples would be appreciated.

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I think you're talking about message queues for IPC (Inter-Process Communication).
According to this page,
Windows does not provide messages queues as a way of IPC.

You can try with user defined Windows messages, which are mainly for GUI by the way, but it's quite different from IPC message queue.
>> I'm developing this under Windows right now but will port this to a different embedded operating system in the future

In case you want to send a message to the thread, you can do it with PostThreadMessage:

For example :
PostThreadMessage(nThreadId, WM_MY_MESSAGE, wData, lData);

I'm sure, you can simply put these 16 bytes somewhere in a global variable protected with the critical section and call SetEvent in order to notify the thread about the new data coming.

About thread synchronization:
CodeProject. Thread Synchronization for Beginners

You could just use an STL queue if it is between threads.  Just do a mutex lock when performing the push and pop.

Since it is threads, they share the same address space so anything global will work.
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If you are looking for a platform independent messaging library in C++, here is it -

There are others as well, so depending on your requirement, need to select the right one.
JohnSantaFeAuthor Commented:
Hmmm.  I've used four other operating system that all had an inter-thread message queue as a fundamental component which other threads pend and wait on.  I'm surprised how difficult this seems under windows.

PostThreadMessage is close to what I'm use to, however, my receiving thread can't pend and wait for the message.

Maybe I'm not thinking of this in Windows terms.  My design problem is I have one thread listening to an ethernet port.  I have a second thread responding to commands that came in on the ethernet port and sending data out a slow serial link.  Because the serial link is slow, I don't want to tie up a single thread and not listen to the ethernet port for too long.  It is possible (and even likely) that while I'm sending data out the serial port additional commands will come in the ethernet port.

I could use an event, however if the serial link is busy and/or multiple events come in, the serial thread will not know it.  A counting semaphore might be possible to keep track that there are more things for the serial link thread to perform...? A queue of some sort is needed so the ethernet listening thread can load it while the other is busy.

Any other comments or suggestions would be appreciated.

1. Implement this queue yourself.
2. Try to use boost. It's a kind of a fashion now, but seems like this is exactly what you need.

MSDN. Message Queuing (MSMQ)
I'd recommend thinking about using 'PostThreadMessage()' together 'GlobalAlloc()'. This way, the data chinks will be placed in the receiving thread's message queue where you can process them one after one. I.e.
#define WM_DATAREADY (WM_USER + 0)

// thread on ethernet port

LPVOID pv = GlobalAlloc(GPTR,BUF_SIZE);



// thread on serial port

MSG msg;

while(GetMessage(&msg,NULL,0,0)) {

  if(WM_DATAREADY == msg.message) {

    LPVOID pv = (LPVOID) msg.lParam;



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JohnSantaFeAuthor Commented:

Will the thread suspend when using GetMessage()  ?
If not, would using WaitMessage() followed by GetMessage() make sense so the thread isn't constantly spinning?
The thread will be suspended, as 'GetMessage()' is a blocking call and will not return until at least one message is received, see ("GetMessage")
JohnSantaFeAuthor Commented:
Thanks the wording in the MSDN article was a little unclear to me.
I'll give this a try and post back later.
Check POCO library, it has pretty decent NotificationQueu and NotificationCenter classes that will do exactly what you want to in a portable manner.
Oh and it also has SocketReactor pattern implemented so that would be another useful goodie.
Sorry for multiple comments, but im a dizzy at the moment. 
JohnSantaFeAuthor Commented:
I decided to go with JKR's suggestion of using PostThreadMessage and GetMessage.  

The libraries, especially Poco look pretty interesting, but for now I just want something simple.

Thanks for the help.
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