Overview

Length, as well as capacity only, applies to the buffered channel.  The length of a channel is the number of elements that are already there in the channel whereas the capacity of a buffered channel is the number of elements which that channel can hold.  So length actually represents the number of elements queued in the buffer of the channel while capacity refers to the size of the buffer of the channel. Hence the length of a channel is always less than or equal to the capacity of the channel.

• len() function can be used to get the length of the channel

• cap() function can be used to get the capacity of the channel

Length and capacity of unbuffered channel is always zero

Length and capacity of a buffered channel

package main

import "fmt"

func main() {
	ch := make(chan int, 3)
	ch <- 5
	fmt.Printf("Len: %d\n", len(ch))
	fmt.Printf("Capacity: %d\n", cap(ch))

	ch <- 6
	fmt.Printf("Len: %d\n", len(ch))
	fmt.Printf("Capacity: %d\n", cap(ch))

	ch <- 7
	fmt.Printf("Len: %d\n", len(ch))
	fmt.Printf("Capacity: %d\n", cap(ch))
}

Output

Len: 1
Capacity: 3
Len: 2
Capacity: 3
Len: 3
Capacity: 3

In the above code, the first created a channel of capacity 3.  After that, we keep sending some value to the channel. As you can notice from your output that after each send operation to the length of channel increases by one while capacity is always the same which is 3.

Length and capacity of a unbuffered channel

Length and capacity of unbuffered channel is always zero

package main

import "fmt"

func main() {
    ch := make(chan int)
    fmt.Printf("Len: %d\n", len(ch))
    fmt.Printf("Capacity: %d\n", cap(ch))
}

Output

Len: 0
Capacity: 0

Length and capacity of nil channel

Length and capacity of nil channel is always zero

package main

import "fmt"

func main() {
	var ch chan int

	fmt.Printf("Len: %d\n", len(ch))
	fmt.Printf("Capacity: %d\n", cap(ch))
}

Output

Len: 0
Capacity: 0

Below is the summary table of result of len() and cap() on different types of channel

Command	Unbuffered Channel(Not Closed and not nil)	Buffered Channel(Not Closed and not nil)	Closed Channel	Nil Channel
Length	0	Number of elements queued in the buffer of the channel	-0 if unbuffered channel-Number of elements queued in the buffer if buffered channel	0
Capacity	0	Size of the buffer of the channel	-0 if unbuffered channel-Size of the buffer if buffered channel	0