Index: trunk/src/attrib.c
===================================================================
--- trunk/src/attrib.c	(revision 28776)
+++ trunk/src/attrib.c	(revision 28777)
@@ -50,6 +50,15 @@
 	return NULL;
 }
 
+char **pcb_attribute_get_ptr(const pcb_attribute_list_t *list, const char *name)
+{
+	int i;
+	for (i = 0; i < list->Number; i++)
+		if (strcmp(name, list->List[i].name) == 0)
+			return &list->List[i].value;
+	return NULL;
+}
+
 int pcb_attribute_put(pcb_attribute_list_t * list, const char *name, const char *value)
 {
 	int i;
Index: trunk/src/attrib.h
===================================================================
--- trunk/src/attrib.h	(revision 28776)
+++ trunk/src/attrib.h	(revision 28777)
@@ -46,9 +46,11 @@
 };
 
 /* Returns NULL if the name isn't found, else the value for that named
-   attribute.  */
+   attribute. The ptr version returns the address of the value str in the slot */
 char *pcb_attribute_get(const pcb_attribute_list_t *list, const char *name);
+char **pcb_attribute_get_ptr(const pcb_attribute_list_t *list, const char *name);
 
+
 /* Adds an attribute to the list.  If the attribute already exists, the value
    is replaced. Returns non-zero if an existing attribute was replaced.  */
 int pcb_attribute_put(pcb_attribute_list_t * list, const char *name, const char *value);
Index: trunk/src/change.c
===================================================================
--- trunk/src/change.c	(revision 28776)
+++ trunk/src/change.c	(revision 28777)
@@ -32,6 +32,7 @@
 #include "conf_core.h"
 
 #include "change.h"
+#include "compat_misc.h"
 #include "board.h"
 #include "data.h"
 #include "draw.h"
@@ -48,6 +49,8 @@
 #include "obj_text_op.h"
 #include "obj_subc_op.h"
 
+static const char core_chg_cookie[] = "core: change.c";
+
 int defer_updates = 0;
 int defer_needs_update = 0;
 
@@ -820,3 +823,101 @@
 	ctx.noarg.pcb = PCB;
 	return pcb_object_operation(&InvalLabelFunctions, &ctx, Type, Ptr1, Ptr2, Ptr3);
 }
+
+
+/*** undoable attribute change */
+typedef struct {
+	pcb_any_obj_t *obj;
+	char *value;
+	int delete;
+	char key[1]; /* must be the last item, spans longer than 1 */
+} chg_attr_t;
+
+static int undo_chg_attr_swap(void *udata)
+{
+	chg_attr_t *ca = udata;
+	int curr_delete = 0;
+	char *curr_value = NULL, **slot;
+
+	slot = pcb_attribute_get_ptr(&ca->obj->Attributes, ca->key);
+
+	/* temp save current state */
+	if (slot == NULL)
+		curr_delete = 1;
+	else
+		curr_value = *slot;
+
+	/* install ca in the slot */
+	if (!ca->delete) {
+		if (curr_delete) {
+			pcb_attribute_put(&ca->obj->Attributes, ca->key, NULL);
+			slot = pcb_attribute_get_ptr(&ca->obj->Attributes, ca->key);
+		}
+		*slot = ca->value;
+	}
+	else {
+		*slot = NULL;
+		pcb_attribute_remove(&ca->obj->Attributes, ca->key);
+	}
+
+
+	/* save current state in ca */
+	ca->delete = curr_delete;
+	ca->value = curr_value;
+	return 0;
+}
+
+static void undo_chg_attr_print(void *udata, char *dst, size_t dst_len)
+{
+	chg_attr_t *ca = udata;
+	pcb_snprintf(dst, dst_len, "chg_attr: #%ld/%s to '%s'\n",
+		ca->obj->ID, ca->key, ca->delete ? "<delete>" : ca->value);
+}
+
+static void undo_chg_attr_free(void *udata)
+{
+	chg_attr_t *ca = udata;
+	free(ca->value);
+}
+
+static const uundo_oper_t undo_chg_attr = {
+	core_chg_cookie,
+	undo_chg_attr_free,
+	undo_chg_attr_swap,
+	undo_chg_attr_swap,
+	undo_chg_attr_print
+};
+
+int pcb_uchg_attr(pcb_board_t *pcb, pcb_any_obj_t *obj, const char *key, const char *new_value)
+{
+	int len;
+	chg_attr_t *ca;
+	const char *curr;
+
+	if (key == NULL)
+		return -1;
+
+	curr = pcb_attribute_get(&obj->Attributes, key);
+	if ((curr == NULL) && (new_value == NULL))
+		return 0; /* nothing to do: both delete */
+	if ((curr != NULL) && (new_value != NULL) && (strcmp(curr, new_value) == 0))
+		return 0; /* nothing to do: value match */
+
+	len = strlen(key); /* +1 for the terminator is implied by sizeof(->str) */
+
+	ca = pcb_undo_alloc(pcb, &undo_chg_attr, sizeof(chg_attr_t) + len);
+	ca->obj = obj;
+	memcpy(ca->key, key, len+1);
+	if (new_value == NULL) {
+		ca->value = NULL;
+		ca->delete = 1;
+	}
+	else {
+		ca->value = pcb_strdup(new_value);
+		ca->delete = 0;
+	}
+	undo_chg_attr_swap(ca);
+
+	pcb_undo_inc_serial();
+	return 0;
+}
Index: trunk/src/change.h
===================================================================
--- trunk/src/change.h	(revision 28776)
+++ trunk/src/change.h	(revision 28777)
@@ -108,4 +108,8 @@
 /* Invalidate the term label of an object */
 void *pcb_obj_invalidate_label(pcb_objtype_t Type, void *Ptr1, void *Ptr2, void *Ptr3);
 
+/* Undoable obj attribute change; if new_value is NULL, delete the attribute */
+int pcb_uchg_attr(pcb_board_t *pcb, pcb_any_obj_t *obj, const char *key, const char *new_value);
+
+
 #endif