How are Go channels implemented internally?

A channel is a typed, goroutine-safe FIFO queue managed by the runtime. Internally it is a hchan struct containing a circular ring buffer (for buffered channels), send and receive queues of waiting goroutines, a mutex, and metadata like element type, capacity, and current length.

// Unbuffered channel — synchronous rendezvous
ch := make(chan int) // cap=0, buf=nil
// Send blocks until a goroutine receives; receive blocks until send

// Buffered channel — asynchronous up to capacity
ch2 := make(chan int, 5) // cap=5, ring buffer of 5 ints
ch2 <- 1 // does not block (buffer not full)
ch2 <- 2
v := <-ch2 // 1 (FIFO)

// Select — multiplexed channel operations
select {
case msg := <-ch:
    fmt.Println("received", msg)
case ch2 <- 42:
    fmt.Println("sent 42")
case <-time.After(1 * time.Second):
    fmt.Println("timeout")
default:
    fmt.Println("non-blocking — no case ready")
}

// Closing a channel — signals receivers: no more data
close(ch2)
val, ok := <-ch2 // ok=false means channel closed and empty

// Range over channel — reads until closed
for v := range ch2 {
    fmt.Println(v)
}

// Common rules:
// - Sending to a closed channel panics
// - Receiving from a closed, empty channel returns zero value, ok=false
// - Closing a nil channel panics
// - Only the SENDER should close (receiver cannot know when sender is done)

Goroutine parking: when a send finds the buffer full (or an unbuffered channel has no receiver), the goroutine is added to the sendq (a linked list of goroutines in the hchan) and parked. When a receiver arrives, it directly copies the data from the parked sender's stack — a zero-copy optimisation for unbuffered channels — and unparks the sender goroutine.

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