diff --git a/Makefile b/Makefile
index 3a763620..791fb1c8 100644
--- a/Makefile
+++ b/Makefile
@@ -221,8 +221,8 @@ CCOPTS_FEATURES += -DDUK_OPT_DEBUG_BUFSIZE=512
 #CCOPTS_FEATURES += -DDUK_OPT_BUFLEN16
 #CCOPTS_FEATURES += -DDUK_OPT_OBJSIZES16
 #CCOPTS_FEATURES += -DDUK_OPT_HEAPPTR16
-#CCOPTS_FEATURES += '-DDUK_OPT_HEAPPTR_ENC16(p)=XXX'
-#CCOPTS_FEATURES += '-DDUK_OPT_HEAPPTR_DEC16(x)=XXX'
+#CCOPTS_FEATURES += '-DDUK_OPT_HEAPPTR_ENC16(ud,p)=XXX'
+#CCOPTS_FEATURES += '-DDUK_OPT_HEAPPTR_DEC16(ud,x)=XXX'
 #CCOPTS_FEATURES += '-DDUK_OPT_DECLARE=XXX'
 CCOPTS_FEATURES += -DDUK_CMDLINE_FANCY
 CCOPTS_FEATURES += -DDUK_CMDLINE_ALLOC_LOGGING
@@ -840,16 +840,16 @@ CCOPTS_AJDUK += -DDUK_OPT_OBJSIZES16
 CCOPTS_AJDUK += -DDUK_OPT_STRTAB_CHAIN
 CCOPTS_AJDUK += -DDUK_OPT_STRTAB_CHAIN_SIZE=128
 CCOPTS_AJDUK += -DDUK_OPT_HEAPPTR16
-CCOPTS_AJDUK += '-DDUK_OPT_HEAPPTR_ENC16(p)=ajsheap_enc16(p)'
-CCOPTS_AJDUK += '-DDUK_OPT_HEAPPTR_DEC16(x)=ajsheap_dec16(x)'
+CCOPTS_AJDUK += '-DDUK_OPT_HEAPPTR_ENC16(ud,p)=ajsheap_enc16((ud),(p))'
+CCOPTS_AJDUK += '-DDUK_OPT_HEAPPTR_DEC16(ud,x)=ajsheap_dec16((ud),(x))'
 CCOPTS_AJDUK += -DDUK_OPT_EXTERNAL_STRINGS
-#CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_INTERN_CHECK(ptr,len)=ajsheap_extstr_check_1((ptr),(len))'
-#CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_FREE(ptr)=ajsheap_extstr_free_1((ptr))'
-CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_INTERN_CHECK(ptr,len)=ajsheap_extstr_check_2((ptr),(len))'
-CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_FREE(ptr)=ajsheap_extstr_free_2((ptr))'
-#CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_INTERN_CHECK(ptr,len)=ajsheap_extstr_check_3((ptr),(len))'
-#CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_FREE(ptr)=ajsheap_extstr_free_3((ptr))'
-CCOPTS_AJDUK += '-DDUK_OPT_DECLARE=extern uint8_t *ajsheap_ram; extern duk_uint16_t ajsheap_enc16(void *p); extern void *ajsheap_dec16(duk_uint16_t x); extern const void *ajsheap_extstr_check_1(const void *ptr, duk_size_t len); extern const void *ajsheap_extstr_check_2(const void *ptr, duk_size_t len); extern const void *ajsheap_extstr_check_3(const void *ptr, duk_size_t len);  extern void ajsheap_extstr_free_1(const void *ptr); extern void ajsheap_extstr_free_2(const void *ptr); extern void ajsheap_extstr_free_3(const void *ptr);'
+#CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_INTERN_CHECK(ud,ptr,len)=ajsheap_extstr_check_1((ptr),(len))'
+#CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_FREE(ud,ptr)=ajsheap_extstr_free_1((ptr))'
+CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_INTERN_CHECK(ud,ptr,len)=ajsheap_extstr_check_2((ptr),(len))'
+CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_FREE(ud,ptr)=ajsheap_extstr_free_2((ptr))'
+#CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_INTERN_CHECK(ud,ptr,len)=ajsheap_extstr_check_3((ptr),(len))'
+#CCOPTS_AJDUK += '-DDUK_OPT_EXTSTR_FREE(ud,ptr)=ajsheap_extstr_free_3((ptr))'
+CCOPTS_AJDUK += '-DDUK_OPT_DECLARE=extern uint8_t *ajsheap_ram; extern duk_uint16_t ajsheap_enc16(void *ud, void *p); extern void *ajsheap_dec16(void *ud, duk_uint16_t x); extern const void *ajsheap_extstr_check_1(const void *ptr, duk_size_t len); extern const void *ajsheap_extstr_check_2(const void *ptr, duk_size_t len); extern const void *ajsheap_extstr_check_3(const void *ptr, duk_size_t len); extern void ajsheap_extstr_free_1(const void *ptr); extern void ajsheap_extstr_free_2(const void *ptr); extern void ajsheap_extstr_free_3(const void *ptr);'
 #CCOPTS_AJDUK += -DDUK_OPT_DEBUG -DDUK_OPT_DPRINT
 #CCOPTS_AJDUK += -DDUK_OPT_DEBUG -DDUK_OPT_DPRINT -DDUK_OPT_DDPRINT -DDUK_OPT_DDDPRINT
 
diff --git a/examples/cmdline/duk_cmdline.c b/examples/cmdline/duk_cmdline.c
index 4387209d..1afde017 100644
--- a/examples/cmdline/duk_cmdline.c
+++ b/examples/cmdline/duk_cmdline.c
@@ -551,8 +551,8 @@ int main(int argc, char *argv[]) {
 		ctx = duk_create_heap(AJS_Alloc,
 		                      AJS_Realloc,
 		                      AJS_Free,
-		                      (void *) &ctx,  /* alloc_udata */
-		                      NULL);          /* fatal_handler */
+		                      (void *) 0xdeadbeef,  /* heap_udata: ignored by AjsHeap, use as marker */
+		                      NULL);                /* fatal_handler */
 #else
 		fprintf(stderr, "Warning: option --alloc-ajsheap ignored, no ajsheap allocator support\n");
 		fflush(stderr);
diff --git a/examples/cmdline/duk_cmdline_ajduk.c b/examples/cmdline/duk_cmdline_ajduk.c
index d53cdf48..40e9499c 100644
--- a/examples/cmdline/duk_cmdline_ajduk.c
+++ b/examples/cmdline/duk_cmdline_ajduk.c
@@ -145,10 +145,31 @@ void ajsheap_register(duk_context *ctx) {
  *  which is reserved for NULL pointers.
  */
 
-duk_uint16_t ajsheap_enc16(void *p) {
+duk_uint16_t ajsheap_enc16(void *ud, void *p) {
 	duk_uint32_t ret;
 	char *base = (char *) ajsheap_ram - 4;
 
+	/* Userdata is not needed in this case but would be useful if heap
+	 * pointer compression were used for multiple heaps.  The userdata
+	 * allows the callback to distinguish between heaps and their base
+	 * pointers.
+	 *
+	 * If not needed, the userdata can be left out during compilation
+	 * by simply ignoring the userdata argument of the pointer encode
+	 * and decode macros.  It is kept here so that any bugs in actually
+	 * providing the value inside Duktape are revealed during compilation.
+	 */
+	(void) ud;
+#if 1
+	/* Ensure that we always get the heap_udata given in heap creation.
+	 * (Useful for Duktape development, not needed for user programs.)
+	 */
+	if (ud != (void *) 0xdeadbeef) {
+		fprintf(stderr, "invalid udata for ajsheap_enc16: %p\n", ud);
+		fflush(stderr);
+	}
+#endif
+
 	if (p == NULL) {
 		ret = 0;
 	} else {
@@ -164,10 +185,20 @@ duk_uint16_t ajsheap_enc16(void *p) {
 	}
 	return (duk_uint16_t) ret;
 }
-void *ajsheap_dec16(duk_uint16_t x) {
+void *ajsheap_dec16(void *ud, duk_uint16_t x) {
 	void *ret;
 	char *base = (char *) ajsheap_ram - 4;
 
+	/* See userdata discussion in ajsheap_enc16(). */
+	(void) ud;
+#if 1
+	/* Ensure that we always get the heap_udata given in heap creation. */
+	if (ud != (void *) 0xdeadbeef) {
+		fprintf(stderr, "invalid udata for ajsheap_dec16: %p\n", ud);
+		fflush(stderr);
+	}
+#endif
+
 	if (x == 0) {
 		ret = NULL;
 	} else {