A channel can be used as a short fifo, to synchronise two, otherwise asynchronous, processes. To use it as a short fifo, output single bytes or words to the channel end using the outuchar(), outuint(), inuchar(), and inuint() primitives.
The uchar primitives send 8-bit data values through the channel. The input primitive will block until data is available, and the output primitive will block until space is available. A channel can hold at least 8 bytes; there is no guaranteed upperlimit on the number of bytes it can store.
The uint primitives send words through the channel. The input primitive will block until data is available, and the output primitive will block until space is available. A channel can hold at least 64 bits (32-bit words) ; there is no guaranteed upperlimit on the number of words it can store.
A usage model is where two threads should stay loosely coupled, in that B() should be no further than two iterations ahead of A():
A(chanend x) { B(chanend x) {
unsigned char i = 0 ; unsigned char i = 0;
outuchar(x, i++); while(1) {
outuchar(x, i++); inuchar(x);
while (1) { // ... do work
inuchar(x); outuchar(x, i);
// ... do work }
outuchar(x, i++); }
}
}
Initially, A puts two tokens in the FIFO to B. This indicates that B can complete two rounds of work without having to synchronise. Every time that B completes an iteration, it puts a token in the FIFO to A indicating that it has completed it, A will not start its work until B has completed, and will signal its iteration.
When a FIFO is empty, it indicates that the thread on the receiving side of that thread cannot progress. When a FIFO contains tokens, it means that the thread at the receiving end can continue. The FIFOs never contain more than two tokens.