C++ bindings for libpmemobj (part 4) - pool handle wrapper

One of the necessary steps in developing the C++ libpmemobj bindings was the introduction of an abstraction of the C pool handle. We decided to do a very simple hierarchy where the pool template inherits from a generic pool_base. This was necessary to be able to have functions/methods which do not depend on the pool’s template argument. Please note that this makes both of these handles impossible to keep in persistent memory, due to the presence of a vtable. It wouldn’t make much sense either way, because the pool handle is runtime and not persistent data.

Basic usage

The pool is a class template, where the single template parameter is the type of the root object. So if your root object was a struct root, you would use a similar definition to the following:

pool<root> pop;

There are three basic operations you can do on a pool, that is

  • open which opens an existing pmemobj pool
  • create which creates a new pmemobj pool
  • close which closes an already opened/created pool

An example usage would be:

if (access(path.c_str(), F_OK) != 0) {
    pop = pool<root>::create(path, "some_layout", PMEMOBJ_MIN_POOL, S_IRWXU);
} else {
    pop = pool<root>::open(path, "some_layout");

And that’s pretty much it, you now have an opened and mapped pmemobj pool file ready to use. You can also perform a consistency check by calling the check() method. When you are done with the pmemobj pool you can close it with the close() method.

The next step you probably want to do is get the pool’s root object.

auto q = pop.get_root();

There is one thing I have to mention here that is of utmost importance. The root object’s constructor will NOT be called. This is a design decision we have made, because the root object might be reallocated and that might lead to unresolvable complications and possibly to memory leaks. This means that it is the responsibility of the user to properly initialize the root object.

Advanced features

The pool class also has a set of functions for fine-grained low-level management of persistent memory. So you can:

  • flush - flush persistent objects/regions in persistent memory
  • persist - flush and persist objects/regions in persistent memory
  • drain - issue a persistent memory drain
  • memcpy_persist - do a memcpy followed by a persist
  • memset_persist - do a memset followed by a persist

Should you ever need to interface with the C API, you can get the pmemobj pool handle by calling the get_handle() method.

These are all of the features of the C++ pool wrapper. As always, any and all comments and suggestions are welcome.

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