Function signature with no function body

Question

When viewing the source for the math.Ceil method, I found this syntax where there's an exported function signature with no body, and a non-exported version of the same signature that includes the implementation:

// Ceil returns the least integer value greater than or equal to x.
//
// Special cases are:
//  Ceil(±0) = ±0
//  Ceil(±Inf) = ±Inf
//  Ceil(NaN) = NaN
func Ceil(x float64) float64

func ceil(x float64) float64 {
    return -Floor(-x)
}

I assume this is some syntax which allows you to easily export a local function. Is that correct? And why would one do this instead of just having a single exported function and using it within the package?

Answer 1

According to the Go language specification.

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

In this case, the Ceil function is implemented by an architecture specific assembly file for 386 in floor_386.s. Both the amd64 and arm architectures each have an assembly file that implements Ceil() as well, but those assembly files are just glue to call the unexported ceil() function.