In this demo we will show the message passing mechanism. Since messages are not sharable to other threads, this is a way for a thread to communicate with others privately.
To explain this demo, we will first introduce the following:
send_msg(insrc/user/thread.c)get_msg(insrc/user/thread.c)
In Badger Kernel, each thread owns a ring buffer to store pending messages. When this function is called, the kernel will check the corresponding buffer of the destination thread.
If the buffer is not full, the message will be copied into the destination thread's buffer according to the destination and message we specified and remains there until the thread takes it out.
If the buffer is full, the kernel will refuse to send the message, and thus the return value of this function will be false.
When a thread calls this function, the kernel checks whether there is a message in the buffer.
If the buffer is non-empty, the kernel will copy the information of the message to the variables we gave and remove it from the message buffer. So we can view the content and src variables to know the content and which thread sent this message.
First, the setup thread creates two threads sender and receiver then it jumps to the spammer function.
spammer doesn't end or give away the use of the CPU, so it repeatedly sends messages to receiver, which makes receiver's message buffer full. As a result, when sender runs, it fails to send a message to receiver.
When receiver runs, it drops the messages which are not from sender. Since the message buffer only contains the messages from spammer, they will all be dropped and receiver will yield.
spammer will clog receiver's buffer again and the process repeats. Finally spammer will send only one message, then exit. At this time, sender can send the message to receiver as the buffer is not full. So receiver will get the message sent from sender and end the demo.