Index: constraint.c
===================================================================
--- constraint.c	(revision 36870)
+++ constraint.c	(revision 36871)
@@ -313,7 +313,14 @@
 			break;
 
 		case PCB_ORDC_ERROR:
-			printf("error!\n");
+			if (ctx->error_cb == NULL)
+				break;
+
+			pcb_ordc_exec_node(ctx, &a, node->ch_first);
+			pcb_ordc_exec_node(ctx, &b, node->ch_first->next);
+			ctx->error_cb(ctx, a.val.s, b.val.s); /* no need to convert; grammar ensures static strings */
+			val_free(&a);
+			val_free(&b);
 			break;
 
 		case PCB_ORDC_CINT:
Index: constraint.h
===================================================================
--- constraint.h	(revision 36870)
+++ constraint.h	(revision 36871)
@@ -61,11 +61,15 @@
 	} val;
 } pcb_ordc_val_t;
 
-typedef struct pcb_ordc_ctx_s {
-	int dummy;
+typedef struct pcb_ordc_ctx_s pcb_ordc_ctx_t;
+struct pcb_ordc_ctx_s {
+	void *user_data;
 	pcb_ordc_node_t *root;
-} pcb_ordc_ctx_t;
 
+	/*** application provided callbacks ***/
+	void (*error_cb)(pcb_ordc_ctx_t *ctx, const char *varname, const char *msg);
+};
+
 int pcb_ordc_parse_str(pcb_ordc_ctx_t *ctx, const char *script);