diff --git a/internal/unsafeheader/unsafeheader.go b/internal/unsafeheader/unsafeheader.go
new file mode 100644
index 0000000..bfc5d3c
--- /dev/null
+++ b/internal/unsafeheader/unsafeheader.go
@@ -0,0 +1,30 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package unsafeheader contains header declarations for the Go runtime's
+// slice and struct implementations.
+//
+// This package allows x/sys to use types equivalent to
+// reflect.SliceHeader and reflect.StructHeader without introducing
+// a dependency on the (relatively heavy) "reflect" package.
+package unsafeheader
+
+import (
+	"unsafe"
+)
+
+// Slice is the runtime representation of a slice.
+// It cannot be used safely or portably and its representation may change in a later release.
+type Slice struct {
+	Data unsafe.Pointer
+	Len  int
+	Cap  int
+}
+
+// StringHeader is the runtime representation of a string.
+// It cannot be used safely or portably and its representation may change in a later release.
+type String struct {
+	Data unsafe.Pointer
+	Len  int
+}
diff --git a/internal/unsafeheader/unsafeheader_test.go b/internal/unsafeheader/unsafeheader_test.go
new file mode 100644
index 0000000..2793c1f
--- /dev/null
+++ b/internal/unsafeheader/unsafeheader_test.go
@@ -0,0 +1,101 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unsafeheader_test
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/internal/unsafeheader"
+)
+
+// TestTypeMatchesReflectType ensures that the name and layout of the
+// unsafeheader types matches the corresponding Header types in the reflect
+// package.
+func TestTypeMatchesReflectType(t *testing.T) {
+	t.Run("Slice", func(t *testing.T) {
+		testHeaderMatchesReflect(t, unsafeheader.Slice{}, reflect.SliceHeader{})
+	})
+
+	t.Run("String", func(t *testing.T) {
+		testHeaderMatchesReflect(t, unsafeheader.String{}, reflect.StringHeader{})
+	})
+}
+
+func testHeaderMatchesReflect(t *testing.T, header, reflectHeader interface{}) {
+	h := reflect.TypeOf(header)
+	rh := reflect.TypeOf(reflectHeader)
+
+	for i := 0; i < h.NumField(); i++ {
+		f := h.Field(i)
+		rf, ok := rh.FieldByName(f.Name)
+		if !ok {
+			t.Errorf("Field %d of %v is named %s, but no such field exists in %v", i, h, f.Name, rh)
+			continue
+		}
+		if !typeCompatible(f.Type, rf.Type) {
+			t.Errorf("%v.%s has type %v, but %v.%s has type %v", h, f.Name, f.Type, rh, rf.Name, rf.Type)
+		}
+		if f.Offset != rf.Offset {
+			t.Errorf("%v.%s has offset %d, but %v.%s has offset %d", h, f.Name, f.Offset, rh, rf.Name, rf.Offset)
+		}
+	}
+
+	if h.NumField() != rh.NumField() {
+		t.Errorf("%v has %d fields, but %v has %d", h, h.NumField(), rh, rh.NumField())
+	}
+	if h.Align() != rh.Align() {
+		t.Errorf("%v has alignment %d, but %v has alignment %d", h, h.Align(), rh, rh.Align())
+	}
+}
+
+var (
+	unsafePointerType = reflect.TypeOf(unsafe.Pointer(nil))
+	uintptrType       = reflect.TypeOf(uintptr(0))
+)
+
+func typeCompatible(t, rt reflect.Type) bool {
+	return t == rt || (t == unsafePointerType && rt == uintptrType)
+}
+
+// TestWriteThroughHeader ensures that the headers in the unsafeheader package
+// can successfully mutate variables of the corresponding built-in types.
+//
+// This test is expected to fail under -race (which implicitly enables
+// -d=checkptr) if the runtime views the header types as incompatible with the
+// underlying built-in types.
+func TestWriteThroughHeader(t *testing.T) {
+	t.Run("Slice", func(t *testing.T) {
+		s := []byte("Hello, checkptr!")[:5]
+
+		var alias []byte
+		hdr := (*unsafeheader.Slice)(unsafe.Pointer(&alias))
+		hdr.Data = unsafe.Pointer(&s[0])
+		hdr.Cap = cap(s)
+		hdr.Len = len(s)
+
+		if !bytes.Equal(alias, s) {
+			t.Errorf("alias of %T(%q) constructed via Slice = %T(%q)", s, s, alias, alias)
+		}
+		if cap(alias) != cap(s) {
+			t.Errorf("alias of %T with cap %d has cap %d", s, cap(s), cap(alias))
+		}
+	})
+
+	t.Run("String", func(t *testing.T) {
+		s := "Hello, checkptr!"
+
+		var alias string
+		hdr := (*unsafeheader.String)(unsafe.Pointer(&alias))
+		hdr.Data = (*unsafeheader.String)(unsafe.Pointer(&s)).Data
+		hdr.Len = len(s)
+
+		if alias != s {
+			t.Errorf("alias of %q constructed via String = %q", s, alias)
+		}
+	})
+}
diff --git a/unix/syscall_darwin.1_13.go b/unix/syscall_darwin.1_13.go
index f911617..dc0befe 100644
--- a/unix/syscall_darwin.1_13.go
+++ b/unix/syscall_darwin.1_13.go
@@ -6,7 +6,11 @@
 
 package unix
 
-import "unsafe"
+import (
+	"unsafe"
+
+	"golang.org/x/sys/internal/unsafeheader"
+)
 
 //sys	closedir(dir uintptr) (err error)
 //sys	readdir_r(dir uintptr, entry *Dirent, result **Dirent) (res Errno)
@@ -71,6 +75,7 @@
 			cnt++
 			continue
 		}
+
 		reclen := int(entry.Reclen)
 		if reclen > len(buf) {
 			// Not enough room. Return for now.
@@ -79,13 +84,15 @@
 			// restarting is O(n^2) in the length of the directory. Oh well.
 			break
 		}
+
 		// Copy entry into return buffer.
-		s := struct {
-			ptr unsafe.Pointer
-			siz int
-			cap int
-		}{ptr: unsafe.Pointer(&entry), siz: reclen, cap: reclen}
-		copy(buf, *(*[]byte)(unsafe.Pointer(&s)))
+		var s []byte
+		hdr := (*unsafeheader.Slice)(unsafe.Pointer(&s))
+		hdr.Data = unsafe.Pointer(&entry)
+		hdr.Cap = reclen
+		hdr.Len = reclen
+		copy(buf, s)
+
 		buf = buf[reclen:]
 		n += reclen
 		cnt++
diff --git a/unix/syscall_unix.go b/unix/syscall_unix.go
index 8f710d0..400ba9f 100644
--- a/unix/syscall_unix.go
+++ b/unix/syscall_unix.go
@@ -12,6 +12,8 @@
 	"sync"
 	"syscall"
 	"unsafe"
+
+	"golang.org/x/sys/internal/unsafeheader"
 )
 
 var (
@@ -113,15 +115,12 @@
 		return nil, errno
 	}
 
-	// Slice memory layout
-	var sl = struct {
-		addr uintptr
-		len  int
-		cap  int
-	}{addr, length, length}
-
-	// Use unsafe to turn sl into a []byte.
-	b := *(*[]byte)(unsafe.Pointer(&sl))
+	// Use unsafe to convert addr into a []byte.
+	var b []byte
+	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&b))
+	hdr.Data = unsafe.Pointer(addr)
+	hdr.Cap = length
+	hdr.Len = length
 
 	// Register mapping in m and return it.
 	p := &b[cap(b)-1]