An introduction to pmemobj (part 7) - persistent lists

The pmemobj library provides non-transactional persistent atomic circular doubly-linked lists (or NTPACDLL for short) API with an interface familiar to anyone who have ever included sys/queue.h header file - it’s in fact so similar that I considered not writing this post at all, you can just search the web for CIRCLEQ example.

Fun fact: The exact same list code is used internally by libpmemobj in the transaction undo log implementation.

Declaring a list

Let’s start by defining the structure of the thing we want on the list:

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struct note {
	time_t date_created;
	POBJ_LIST_ENTRY(struct note) notes;
	char msg[];
};

It’s a pmem notepad :) All of the notes are going to be stored in the root object:

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struct my_root {
	POBJ_LIST_HEAD(plist, struct note) head;
};

The head of this list does not have to be initialized (maybe you have noticed that this is a recurring theme throughout the library, not initializing things that is).

Inserting new list entries

You can either insert an existing object to a list (POBJ_LIST_INSERT) or allocate directly to the list (POBJ_LIST_INSERT_NEW). You can also insert the entries anywhere in the list you want thanks to the self-explanatory _HEAD, _TAIL, _AFTER and _BEFORE macro variants.

Continuing our example, we want to add a new entry to the head of the list:

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int note_construct(PMEMobjpool *pop, void *ptr, void *arg) {
	struct note *n = ptr;
	char *msg = arg;

	pmemobj_memcpy_persist(pop, n->msg, msg, strlen(msg));

	time(&n->date_created);
	pmemobj_persist(pop, &n->date_created, sizeof time_t);

	return 0;
}

void create_note(char *msg) {
	TOID(struct my_root) root = POBJ_ROOT(pop, struct root);

	size_t nlen = strlen(msg);
	POBJ_LIST_INSERT_NEW_HEAD(pop, &D_RW(root)->head, notes,
				  sizeof(struct note) + nlen,
				  note_construct, msg);
}

Quite a lot happening here. You should remember the constructor from the non-transactional allocation API. Also worth noting is the allocation of flexible array in the struct note.

Accessing list entries

Reading things from the list is done using: POBJ_LIST_FIRST, POBJ_LIST_LAST, POBJ_LIST_EMPTY, POBJ_LIST_NEXT and POBJ_LIST_PREV. They pretty much do what it says in the name.

Let’s write functions to read our notes:

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static TOID(struct note) current_note;

void note_read_init() {
	TOID(struct my_root) root = POBJ_ROOT(pop, struct root);

	current_note = POBJ_LIST_FIRST(&D_RO(root)->head);
}

void note_read_next() {
	current_note = POBJ_LIST_NEXT(current_note, notes);
}

void note_read_prev() {
	current_note = POBJ_LIST_PREV(current_note, notes);
}

void note_print(const TOID(struct note) note) {
	printf("Created at %s: \n %s\n",
	       ctime(D_RO(note)->date_created),
	       D_RO(note)->msg);
}

I’ll leave it up to the reader to correctly use these functions in a CLI/GUI.

Iterating through the list

You can iterate a list in a similar fashion it’s done for internal collections. There are two macros: POBJ_LIST_FOREACH and POBJ_LIST_FOREACH_REVERSE. No magic here.

Reading all the notes:

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void note_print_all() {
	TOID(struct my_root) root = POBJ_ROOT(pop, struct root);

	TOID(struct note) iter;
	POBJ_LIST_FOREACH(iter, &D_RO(root)->head, notes) {
		note_print(iter);
	}
}

Removing entries

Just like you can insert an existing object or a new one, you can just remove entry from the list (POBJ_LIST_REMOVE) or remove and free it (POBJ_LIST_REMOVE_FREE).

Removing current note:

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void note_remove() {
	TOID(struct my_root) root = POBJ_ROOT(pop, struct root);

	POBJ_LIST_REMOVE_FREE(pop, &D_RO(root)->head,
			      current_note, notes);
}

Moving entries between lists

This is something new, POBJ_LIST_MOVE_ELEMENT allows you to move an object from one list to another. A good use case example can be found in the PI code here. It’s used there as a task queue and once worker thread completes a task is then moved from todo list to done list.

[This entry was edited on 2017-12-11 to reflect the name change from NVML to PMDK.]