Test code for investigating Golang's mmap behaviour

what-mmap-does.go

package main

import (
	"fmt"
	"syscall"
	"unsafe"
)

type slice struct {
	addr uintptr
	len  int
	cap  int
}

func check(b []byte) (uintptr, int, int) {
	info := (*slice)(unsafe.Pointer(&b))
	return info.addr, info.len, info.cap
}

func newSlice(addr uintptr, len int, cap int) []byte {
	var b []byte
	ps := (*slice)(unsafe.Pointer(&b))
	ps.addr = addr
	ps.len = len
	ps.cap = cap
	return b
}

func confirm(b []byte) {
	for i := range b {
		b[i] = byte(255)
	}
}

func main() {
	b, err := syscall.Mmap(0, 0, 4096,
		syscall.PROT_READ|syscall.PROT_WRITE,
		syscall.MAP_ANON|syscall.MAP_PRIVATE)
	if err != nil {
		panic("syscall.Mmap: " + err.Error())
	}

	confirm(b)
	addr, length, capacity := check(b)
	fmt.Printf("0x%016x %d %d\n", addr, length, capacity)

	bb := append(b, byte(0))
	confirm(bb)
	addr, length, capacity = check(bb)
	fmt.Printf("0x%016x %d %d\n", addr, length, capacity)

	confirm(b)
	addr, length, capacity = check(b)
	fmt.Printf("0x%016x %d %d\n", addr, length, capacity)

	newb := newSlice(addr, length, capacity)
	err = syscall.Munmap(newb)
	if err != nil {
		panic("syscall.Munmap: " + err.Error())
	}

	println("Ok until now.")

	// Panic
	confirm(b)
}