Asynchronous communication in c

Question

I have to send a command over serial and receive back an answer based on the command and do something based on the message received. I was told that I have to use callbacks as this is an asynchronous operation.

I have a 2 threads, one that can send messages and one that receives the messages.

Example:

//Thread 1
sendMessage("Initialize");

//Thread 2 
while(1)
{
    checkForMessages();
}

How can I write a function that is initialized for a specific message and handles the message recieved.

Example:

CommHandle(Command,MsgReceived)
{
  if(command)
  {
    if(MsgReceived == ok)
    ...
    if(MsgReceived == error)
    ...
  }

}

Answer 1

I was told that I have to use callbacks as this is an asynchronous operation.

Not necessarily. There is something in Windows called "asynchronous I/O", this is to be regarded as an internal Windows term, which is synonymous with "overlapped I/O" (explanation here). When you are using overlapped I/O, you will get a callback when the transmission is finished. This is nice, since it reduces CPU load, but it is not really necessary if your program has nothing better to do while waiting. So it depends on the nature of your application.

But no matter the nature of your application, you should indeed handle all serial communication through threads, so that you won't cause the main GUI thread to freeze up in embarrassing ways.

Having one rx and one tx thread gives you a dilemma though: they are using the same port handle and they cannot freely access it, because that wouldn't be thread-safe. The solution is to either make one single super-thread handling all transmissions, or to protect the port handle through a mutex.

I'm not sure which method that is best, I have no recommendation. I have only used the "super-thread" one myself: one obvious advantage was that I could centralize WaitFor instructions like "kill thread", "port is open", "port is closed" at one place. But at the same time the code turned out rather complex.

How can I write a function that is initialized for a specific message and handles the message recieved.

Let your thread(s) shovel their received data into some buffers. A tx buffer and a rx buffer. Depending on your serial protocol and performance, you might have to use double buffers: one that is used for the current transmission and one that contains the most recently received data.

Then from main, pick up the data from the buffers. They need to be thread-safe. Once you have gotten that far, simply write a parser like you would with any form of data and take actions from there