diff --git a/cgo.c b/cgo.c
new file mode 100644
index 0000000..d50841e
--- /dev/null
+++ b/cgo.c
@@ -0,0 +1,13 @@
+#include "cgo.h"
+
+int engine_config_err_log(ph7 *pEngine, const char **pzPtr, int *pLen) {
+	return ph7_config(pEngine, PH7_CONFIG_ERR_LOG, pzPtr, pLen);
+}
+
+int vm_extract_output(ph7_vm *pVm, const char **pzPtr, int *pLen) {
+	return ph7_vm_config(pVm, PH7_VM_CONFIG_EXTRACT_OUTPUT, pzPtr, pLen);
+}
+
+int vm_set_output_callback(ph7_vm *pVm, void *pUserData) {
+	return ph7_vm_config(pVm, PH7_VM_CONFIG_OUTPUT, &write_vm_output, pUserData);
+}
diff --git a/cgo.h b/cgo.h
new file mode 100644
index 0000000..cf529a7
--- /dev/null
+++ b/cgo.h
@@ -0,0 +1,8 @@
+#include <stdlib.h>
+#include "ph7.h"
+
+int engine_config_err_log(ph7 *pEngine, const char **pzPtr, int *pLen);
+int vm_extract_output(ph7_vm *pVm, const char **pzPtr, int *pLen);
+
+extern int write_vm_output(void*, unsigned int, void*);
+int vm_set_output_callback(ph7_vm *pVm, void *pUserData);
diff --git a/go.mod b/go.mod
index ca422a0..3662698 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module git.codemonkeysoftware.net/b/go-ph7
 
 go 1.19
+
+require github.com/mattn/go-pointer v0.0.1
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..1fdefea
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+github.com/mattn/go-pointer v0.0.1 h1:n+XhsuGeVO6MEAp7xyEukFINEa+Quek5psIR/ylA6o0=
+github.com/mattn/go-pointer v0.0.1/go.mod h1:2zXcozF6qYGgmsG+SeTZz3oAbFLdD3OWqnUbNvJZAlc=
diff --git a/ph7.go b/ph7.go
index c99a908..efea127 100644
--- a/ph7.go
+++ b/ph7.go
@@ -1,21 +1,16 @@
 package ph7
 
 /*
-#include <stdlib.h>
+#include "cgo.h"
 #include "ph7.h"
-
-int engine_config_err_log(ph7 *pEngine, const char **pzPtr, int *pLen) {
-	return ph7_config(pEngine, PH7_CONFIG_ERR_LOG, pzPtr, pLen);
-}
-
-int vm_extract_output(ph7_vm *pVm, const char **pzPtr, int *pLen) {
-	return ph7_vm_config(pVm, PH7_VM_CONFIG_EXTRACT_OUTPUT, pzPtr, pLen);
-}
 */
 import "C"
 import (
 	"fmt"
 	"io"
+	"unsafe"
+
+	pointer "github.com/mattn/go-pointer"
 )
 
 type ResultCode int
@@ -100,17 +95,17 @@ func (e Engine) ErrLog() (string, error) {
 func (e Engine) Compile(source []byte, phpOnly bool) (*VM, error) {
 	csource := C.CBytes(source)
 	defer C.free(csource)
-	vm := new(VM)
 	var flags C.int
 	if phpOnly {
 		flags |= C.PH7_PHP_ONLY
 	}
-	err := newError(C.ph7_compile_v2(e.ptr, (*C.char)(csource), C.int(len(source)), &vm.ptr, flags))
+	var ptr *C.ph7_vm
+	err := newError(C.ph7_compile_v2(e.ptr, (*C.char)(csource), C.int(len(source)), &ptr, flags))
 	if err != nil {
 		return nil, err
 	}
 
-	return vm, nil
+	return newVM(ptr), nil
 }
 
 func (e Engine) Close() error {
@@ -118,14 +113,41 @@ func (e Engine) Close() error {
 }
 
 type VM struct {
-	ptr *C.ph7_vm
+	ptr          *C.ph7_vm
+	pointerKey   unsafe.Pointer
+	outputWriter io.Writer
+	// TODO clear outputWriteError when resetting VM
+	outputWriteErr error
 }
 
-func (vm VM) SetOutputWriter(w io.Writer) error {
-	return nil
+func newVM(cvm *C.ph7_vm) *VM {
+	vm := new(VM)
+	vm.ptr = cvm
+	vm.pointerKey = pointer.Save(vm)
+	return vm
 }
 
-func (vm VM) ExtractOutput() (string, error) {
+//export write_vm_output
+func write_vm_output(buf unsafe.Pointer, bufLen C.uint, userData unsafe.Pointer) C.int {
+	vm := pointer.Restore(userData).(*VM)
+	gobuf := C.GoBytes(buf, C.int(bufLen))
+	_, vm.outputWriteErr = vm.outputWriter.Write(gobuf)
+	if vm.outputWriteErr != nil {
+		return C.int(ResultCodeAbort)
+	}
+	return C.int(ResultCodeOK)
+}
+
+func (vm *VM) SetOutputWriter(w io.Writer) error {
+	vm.outputWriter = w
+	return newError(C.vm_set_output_callback(vm.ptr, vm.pointerKey))
+}
+
+func (vm *VM) OutputWriteError() error {
+	return vm.outputWriteErr
+}
+
+func (vm *VM) ExtractOutput() (string, error) {
 	var s *C.char
 	var n C.int
 
@@ -137,10 +159,11 @@ func (vm VM) ExtractOutput() (string, error) {
 	return C.GoStringN(s, n), nil
 }
 
-func (v VM) Exec() error {
-	return newError(C.ph7_vm_exec(v.ptr, (*C.int)(nil)))
+func (vm *VM) Exec() error {
+	return newError(C.ph7_vm_exec(vm.ptr, (*C.int)(nil)))
 }
 
-func (v VM) Close() error {
-	return newError(C.ph7_vm_release(v.ptr))
+func (vm *VM) Close() error {
+	pointer.Unref(vm.pointerKey)
+	return newError(C.ph7_vm_release(vm.ptr))
 }
diff --git a/ph7_test.go b/ph7_test.go
index a1fde66..a180d88 100644
--- a/ph7_test.go
+++ b/ph7_test.go
@@ -1,6 +1,8 @@
 package ph7_test
 
 import (
+	"bytes"
+	"fmt"
 	"testing"
 
 	ph7 "git.codemonkeysoftware.net/b/go-ph7"
@@ -59,3 +61,37 @@ func TestCompileError(t *testing.T) {
 		t.Fatalf("expected error message, got nothing")
 	}
 }
+
+func TestOutputWriter(t *testing.T) {
+	const source = `<?php
+		for ($x = 0; $x <= 1000; $x++) {
+			echo $x;
+		}
+		?>`
+	var expected bytes.Buffer
+	for i := 0; i <= 1000; i++ {
+		_, err := fmt.Fprint(&expected, i)
+		mustSucceed(t, err)
+	}
+
+	engine, err := ph7.NewEngine()
+	mustSucceed(t, err)
+	defer mustSucceedF(t, engine.Close)
+
+	vm, err := engine.Compile([]byte(source), false)
+	mustSucceed(t, err)
+	defer vm.Close()
+
+	var actual bytes.Buffer
+	err = vm.SetOutputWriter(&actual)
+	mustSucceed(t, err)
+
+	mustSucceed(t, vm.Exec())
+
+	if !bytes.Equal(expected.Bytes(), actual.Bytes()) {
+		t.Fatalf("expected %s, got %s", expected.String(), actual.String())
+	}
+}
+
+// TODO Test failing writer, including vm.OutputWriteError
+// TODO Test setting writer twice