What does a function without body mean?

Solution 1:

1. The function is defined here:

   // startTimer adds t to the timer heap.
   //go:linkname startTimer time.startTimer
   func startTimer(t *timer) {
       if raceenabled {
           racerelease(unsafe.Pointer(t))
       }
       addtimer(t)
   }
   

2. Function declarations:

   A function declaration may omit the body. Such a declaration provides the signature for a function implemented outside Go, such as an assembly routine.

3. Not every programming language can express its own runtime entirely (C can, for example). Parts of the Go runtime and the standard library are in C, parts are in assembly while some other are in .goc, which is a not well documented hybrid of Go and C.