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

		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)

	PROT_NONE                       = 0x4 // can't be accessed
	PROT_EXEC                       = 0x8 // can be executed
	MAP_PRIVATE                     = 0x1 // changes are private
	MAP_SHARED                      = 0x2 // changes are shared
	MAP_FIXED                       = 0x4 // place exactly
	__MAP_MEGA                      = 0x8
	__MAP_64                        = 0x10
	MAP_ANON                        = 0x20

	MAP_FIXED                     = 0x10
	MAP_INITDATA                  = 0x800
	MAP_NORESERVE                 = 0x40
	MAP_PRIVATE                   = 0x2
	MAP_RENAME                    = 0x20
	MAP_SHARED                    = 0x1
	MAP_TEXT                      = 0x400
	MAP_TYPE                      = 0xf
	MCL_CURRENT                   = 0x1
	MCL_FUTURE                    = 0x2
	MSG_CTRUNC                    = 0x10

	MAP_FIXED_NOREPLACE                         = 0x100000
	MAP_HUGE_MASK                               = 0x3f
	MAP_HUGE_SHIFT                              = 0x1a
	MAP_PRIVATE                                 = 0x2
	MAP_SHARED                                  = 0x1
	MAP_SHARED_VALIDATE                         = 0x3
	MAP_TYPE                                    = 0xf
	MCAST_BLOCK_SOURCE                          = 0x2b

		{"MAP_POPULATE", Const, 0},
		{"MAP_PREFAULT_READ", Const, 1},
		{"MAP_PRIVATE", Const, 0},
		{"MAP_RENAME", Const, 0},
		{"MAP_RESERVED0080", Const, 0},
		{"MAP_RESERVED0100", Const, 1},
		{"MAP_SHARED", Const, 0},
		{"MAP_STACK", Const, 0},
		{"MAP_TRYFIXED", Const, 3},
		{"MAP_TYPE", Const, 0},
		{"MAP_WIRED", Const, 3},
		{"MAXIMUM_REPARSE_DATA_BUFFER_SIZE", Const, 4},