Using the unsafe package, we can also convert a numeric variable pointer to its C counterpart. This allows us to edit it directly in the C code:

package main

/*
void half(double* f) {
    *f = *f/2;
}
*/
import "C"

import (
    "fmt"
    "math"
    "unsafe"
)

func main() {
    a := float64(math.Pi)
    fmt.Println(a)
    C.half((*C.double)(unsafe.Pointer(&a)))
    fmt.Println(a)
}

The preceding example does exactly that; it halves the value of a in a C function, without copying and assigning the new value in Go.