Go: How Does defer Statement Work?

Illustration created for “A Journey With Go”, made from the original Go Gopher, created by Renee French.

ℹ️ This article is based on Go 1.12.

defer statement is a convenient way to execute a piece of code before a function returns, as explained in Golang specification:

Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred.

Here is an example of the LIFO (last-in-first-out) implementation:

func main() {
   defer func() {
      println(`defer 1`)
   }()
   defer func() {
      println(`defer 2`)
   }()
}
defer 2 <- Last in, first to go out
defer 1

Let’s have a look at the internals and then a more complex case.

Internals implementation

Go runtime implements the LIFO with a linked list. Indeed, a defer struct has a link to the next one to be executed:

type _defer struct {
   siz     int32
   started bool
   sp      uintptr
   pc      uintptr
   fn      *funcval
   _panic  *_panic
   link    *_defer // next deferred function to be executed
}

When a new defer method is created, it is attached to the current Goroutine and the previous one is linked to the new one as the next function to be executed:

func newdefer(siz int32) *_defer {
   var d…