if n == 0 {
		return Data{f, nil}, nil
	}
	mmapLength := int(((size + pagesize - 1) / pagesize) * pagesize) // round up to page size
	data, err := syscall.Mmap(int(f.Fd()), 0, mmapLength, syscall.PROT_READ, syscall.MAP_SHARED)
	if err != nil {
		return Data{}, &fs.PathError{Op: "mmap", Path: f.Name(), Err: err}
	}
	return Data{f, data[:n]}, nil

	O_TRUNC    = C.O_TRUNC
	O_CLOEXEC  = C.O_CLOEXEC

	PROT_NONE  = C.PROT_NONE
	PROT_READ  = C.PROT_READ
	PROT_WRITE = C.PROT_WRITE
	PROT_EXEC  = C.PROT_EXEC

	MAP_ANON    = C.MAP_ANON
	MAP_SHARED  = C.MAP_SHARED
	MAP_PRIVATE = C.MAP_PRIVATE
	MAP_FIXED   = C.MAP_FIXED

	MADV_DONTNEED = C.MADV_DONTNEED
	MADV_FREE     = C.MADV_FREE

	SA_SIGINFO = C.SA_SIGINFO
	SA_RESTART = C.SA_RESTART

	if runtime.GOOS == "netbsd" && runtime.GOARCH == "arm" {
		t.Skip("netbsd-arm doesn't provide fault address (golang.org/issue/45026)")
	}
	m, err := syscall.Mmap(-1, 0, 0x1000, syscall.PROT_READ /* Note: no PROT_WRITE */, syscall.MAP_SHARED|syscall.MAP_ANON)
	if err != nil {
		t.Fatalf("can't map anonymous memory: %s", err)
	}
	defer syscall.Munmap(m)
	old := debug.SetPanicOnFault(true)
	defer debug.SetPanicOnFault(old)
	const lowBits = 0x3e7

		return Data{f, nil, nil}, nil
	}
	mmapLength := int(((size + pagesize - 1) / pagesize) * pagesize) // round up to page size
	data, err := syscall.Mmap(int(f.Fd()), 0, mmapLength, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
	if err != nil {
		return Data{}, &fs.PathError{Op: "mmap", Path: f.Name(), Err: err}
	}
	return Data{f, data[:n], nil}, nil
}

func munmapFile(d Data) error {
	if len(d.Data) == 0 {
		return nil
	}

	// Page-align the offset.
	off := r.Offset()
	align := syscall.Getpagesize()
	aoff := off &^ int64(align-1)

	data, err := syscall.Mmap(int(r.f.Fd()), aoff, int(length+uint64(off-aoff)), syscall.PROT_READ, syscall.MAP_SHARED|syscall.MAP_FILE)
	if err != nil {
		return nil, false
	}

	data = data[off-aoff:]
	r.MustSeek(int64(length), 1)
	return data, true

	}
	err = out.f.Truncate(int64(filesize))
	if err != nil {
		Exitf("resize output file failed: %v", err)
	}
	out.buf, err = syscall.Mmap(int(out.f.Fd()), 0, int(filesize), syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED|syscall.MAP_FILE)
	if err != nil {
		return err
	}

	// copy heap to new mapping
	if uint64(oldlen+len(out.heap)) > filesize {
		panic("mmap size too small")
	}
	copy(out.buf[oldlen:], out.heap)

	syscall.Syscall(syscall.SYS_MUNMAP, base, 3<<30, 0)

	for off := uintptr(0); off < 3<<30; off += 65536 {
		_, _, errno := syscall.Syscall6(syscall.SYS_MMAP,
			base+off, 65536, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED|syscall.MAP_FIXED, tmp.Fd(), 0)
		if errno != 0 {
			t.Skipf("could not map a page at requested 0x%x: %s", base+off, errno)
		}
		defer syscall.Syscall(syscall.SYS_MUNMAP, base+off, 65536, 0)
	}

pkg syscall (netbsd-386), const MAP_PRIVATE ideal-int
pkg syscall (netbsd-386), const MAP_RENAME = 32
pkg syscall (netbsd-386), const MAP_RENAME ideal-int
pkg syscall (netbsd-386), const MAP_SHARED = 1
pkg syscall (netbsd-386), const MAP_SHARED ideal-int
pkg syscall (netbsd-386), const MAP_STACK = 8192
pkg syscall (netbsd-386), const MAP_STACK ideal-int
pkg syscall (netbsd-386), const MAP_TRYFIXED = 1024

	"syscall"
)

type sharedMemSys struct{}

func sharedMemMapFile(f *os.File, size int, removeOnClose bool) (*sharedMem, error) {
	prot := syscall.PROT_READ | syscall.PROT_WRITE
	flags := syscall.MAP_FILE | syscall.MAP_SHARED
	region, err := syscall.Mmap(int(f.Fd()), 0, size, prot, flags)
	if err != nil {
		return nil, err
	}

	return &sharedMem{f: f, region: region, removeOnClose: removeOnClose}, nil
}

	MAP_NOEXTEND                      = 0x100
	MAP_NORESERVE                     = 0x40
	MAP_PRIVATE                       = 0x2
	MAP_RENAME                        = 0x20
	MAP_RESERVED0080                  = 0x80
	MAP_SHARED                        = 0x1
	MCL_CURRENT                       = 0x1
	MCL_FUTURE                        = 0x2
	MSG_CTRUNC                        = 0x20
	MSG_DONTROUTE                     = 0x4