> I have a MessageHandler() function for a process to handle incoming QNX 
> messages, that will immediately respond to any messages sent with a MsgReply()
> , then actually pull the data out and process the message. I'm using this to 
> make a QNX message similar to an asynchronous pulse, but able to send an 
> arbitrary amount of data. I understand that message data is copied between 
> process address spaces, and the message itself should hang around for the 
> scope of the MessageHandler() function. If I then received another message 
> before I finished processing the first one could I run into problems with the 
> second message getting copied over the memory of the first one?
> 
> Here's an example:
> 
> void MessageHandler(void *Msg, int Type, int ReceiveID)
> {
>    MsgReply(ReceiveID, EOK, NULL, 0); // reply immediately
> 
>    MyMsgType *foo = (MyMsgType*)Msg;
> 
>    int a = foo->GetAValue();
> 
>    // if I do more stuff here, foo pointer should still be valid for the
>    // scope of this function, but if I immediately get another message
>    // will Msg (and foo) get overwritten?

Not until you a the MsgReceive.  If that is a problem in your design, just pass MsgReceive a different pointer, a 
circular queue type of design.

By the way have you looked at the async_ stuff which are ok as long as you don't need networking.

> }