Tag Archives: C

Introducing libbytesize

Problem area

Many project have to deal with representing sizes of storage or memory. In general, sizes in bytes. What may seem to be a trivial thing turns into hundreds of lines of of code if the following things are to be covered properly:

  • using binary (GiB,…) and decimal (GB) units correctly
  • handling sizes bigger than MAXUINT64 (which is 16 EiB – 1)
  • parsing users’ input correctly with:
    • binary and decimal units
    • numeric values in various formats (traditional, scientific)
  • handling localization and internationalization correctly
    • different radix characters used in different languages
    • units being translated and typed in by users in their native format (even with non-latin scripts)
  • handling negative sizes
    • it sometimes make sense to work with these for example when some storage space is missing somewhere

Of course, not all projects working with sizes in bytes have hundreds of lines for dealing with the above points, but the result is a bad user experience. In some cases, valid localized inputs are not accepted and correctly parsed or no matter what the current locale and language configuration is the users always get the English format and unit. One of the biggest problems I see in many projects is that binary and decimal units are not used and differentiated correctly. If something shows the value 10 G, does it mean 10 GiB and thus 10240 MiB or is it 10 GB and thus 10000 MB? Sometimes one can find this piece of information in the documentation (e.g. man pages), but often one just have to guess and try. Fortunately quite rarely one can be really surprised with the documented behaviour. For example in case of the lvm utilities where g means GiB and G means GB. We should generally be doing a much better job in handling sizes right and consistently in all projects, that have to handle those. However, it’s obvious that having a few hundreds of lines of code in every such project is nonsense.

An existing solution

One of the projects that I can gladly call a good example of how to deal with sizes in bytes is the Blivet python package used mainly by the Anaconda OS (Fedora, RHEL,…) installer. It has all the concerns mentioned above addressed in a proper and well-tested way in its class called simply Size. As the title of this post reveals, I’m trying to introduce a new library here so the obvious question is: Why to invent and write something new when a good and well-tested solution already exists? The answer lies in the description of Blivet and it is the fact that it is written in Python which makes its implementation of the Size class hardly usable from any other language/environment.

One step further

The obvious solution to move further towards a widely reusable solution was to rewrite the Blivet’s Size class in C so that it can be used from this low-level language and many other languages that very often facilitate use of C libraries. However, again what may seem to be an easy thing to do is not at all that simple. The Blivet’s Python implementation is based on the Python’s type Decimal which is a numeric type supporting unlimitted precision and arbitrarily big numbers. Also, dealing with strings and their processing is way simpler in Python than in C.

Nevertheless, C also has some nice libraries for working with big and highly precise numbers, namely the GMP and MPFR libraries that were created as part of the GNU project and which are for example used by many tools and libraries doing some serious maths. So it soon became clear, that writing a C implementation of the Size class shouldn’t be an overly complicated task. And it turned out be the case.

Here it is

The result is the libbytesize library that uses GMP and MPFR together with GObject Introspection to provide a nice object-oriented API facilitating the work with sizes in bytes. It properly takes care of all the potential issues mentioned in the beginning of this post and is widely usable due to the broad support of GObject Introspection in many high-level languages. The library provides a single class called (warning: here comes the surprise) Size which right now is basically a very thin wrapper around the mpz_t type provided by the GMP library for arbitrarily big integer numbers and thus it actually stores byte sizes as numbers of bytes. That is actually the precision limitation, but since no storage provides or works with fractions of bytes, it’s no real limitation at all.

There are (at this point) four constructors 1:

  • bs_size_new() which creates a new instance initialized to 0 B,
  • bs_size_new_from_bytes() which creates a new instance initialized to a given number of bytes,
  • bs_size_new_from_str() which creates a new instance initialized to the number of bytes the given string (e.g. "10 GiB") represents,
  • bs_size_new_from_size() which is a copy constructor.

Then there are some query functions the most important of which are the following two:

  • bs_size_convert_to() which can be used to convert a given size to some particular unit and
  • bs_size_human_readable() which gives a human-readable representation of a given size – i.e. with such unit that the the resulting number is not too big nor too small

Last but not least there are many methods for doing arithmetic and logical operations with sizes in bytes. It’s probably wise to mention here that not all arithmetic operations implemented for the mpz_t type are implemented for sizes. Some of them just don’t make sense – multiplication of size by size (what is GiB**2?), the raising operation, (square) root and others. However, there are some extra ones that don’t really make much sense for generic numbers, but are quite useful when working with sizes namely the bs_size_round_to_nearest() which rounds a given size (up or down) to a nearest multiple of another size. Like for example if you need to know how much space an LVM LV of requested size will take in a VG with some particular extent size.

Since the GObject Introspection allows for having overrides and the new library is expected to be used by Blivet instead of its own Python-only implementation of the Size class, there already are Python overrides making the work with the libbytesize’s Size class really simple. Here as example python interpret session demostrating the simplicity of use:

>>> from gi.repository.ByteSize import Size
>>> s = Size("10 GiB")
>>> str(s)
'10 GiB'
>>> repr(s)
'Size (10 GiB)'
>>> s2 = Size(10 * 1024**3)
>>> s2
Size (10 GiB)
>>> s + s2
Size (20 GiB)
>>> s - s2
Size (0 B)
>>> s3 = Size(s2)
>>> sum([s, s2, s3])
Size (30 GiB)
>>> -s2
Size (-10 GiB)
>>> abs(-s2)
Size (10 GiB)

And here come the dogs

I mean docs. The project is hosted on GitHub together with its documentation. The current release is 0.2 where the zero in the beginning means that it is not a stable release yet. The API is unlikely to change in any significant way for the (stable) release 1.0, but since the library is not being used in any big project right now, we are leaving us with some "manipulation space" for potential changes. So if you find the API of the library wrong, feel free to let us know and we might change it according to your favor! If you want to get a quick but still quite comprehensive overview of the library’s API, have a look at the header file it provides.

The last thing I’d like to mention here is that the library is packaged for the Fedora GNU/Linux distribution so if you happen to be using this distribution, you can easily start playing with the library by typing this into your shell:

$ sudo dnf install libbytesize python-libbytesize ipython
$ ipython

Using ipython also gives you the TAB-completion. See the above intepret session example to get a better idea about what to type in then. Have fun and don’t forget to share your ideas in the comments!


  1. bs is the "namespace" prefix and size is the class prefix

libblockdev reaches the 1.0 milestone!

A year ago, I started working on a new storage library for low-level operations with various types of block devices — libblockdev. Today, I’m happy to announce that the library reached the 1.0 milestone which means that it covers all the functionality that has been stated in the initial goals and it’s going to keep the API stable.

A little bit of a background

Are you asking the question: "Why yet another code implementing what’s already been implemented in many other places?" That’s, of course, a very good and probably crucial question. The answer is that I and people who were at the birth of the idea think that this is for the first time such thing is implemented in a way that it is usable for a wide range of tools, applications, libraries, etc. Let’s start with the requirements every widely usable implementation should meet:

  1. it should be written in C so that it is usable for code written in low-level languages
  2. it should be a library as DBus is not usable together with chroot() and things like that and running subprocesses is suboptimal (slow, eating lot of random data entropy, need to parse the output, etc.)
  3. it should provide bindings for as many languages as possible, in particular the widely used high-level languages like Python, Ruby, etc.
  4. it shouldn’t be a single monolithic piece required by every user code no matter how much of the library it actually needs
  5. it should have a stable API
  6. it should support all major storage technologies (LVM, MD RAID, BTRFS, LUKS,…)

If we take the candidates potentially covering the low-level operations with blockdev devices — Blivet, ssm and udisks2 (now being replaced by storaged) — we can easily come to a conclusion that none of them meets the requirements above. Blivet 1 covers the functionality in a great way, but it’s written in Python and thus hardly usable from code written in other languages. The same applies to ssm 2 is also written in Python, it’s an application and it doesn’t cover all the technologies (it doesn’t try to). udisks2 3 and now storaged 4 provide a DBus API and don’t provide for example functions related to BTRFS (and even LVM in case of udisks2).

The libblockdev library is:
  • written in C,
  • using GLib and providing bindings for all languages supporting GObject instrospection (Python, Perl, Ruby, Haskell*,…),
  • modular — using separate plugins for all technologies (LVM, Btrfs,…),
  • covering all technologies Blivet supports 5 plus some more,

by which it fulfills all the requirements mentioned above. It’s only a wish, but a strong one, that every new piece of code written for low-level manipulation with block devices 6, should be written as part of the libblockdev library, tested and reused in as many places as possible instead of writing it again and again in many, many places with new, old, weird and surprising and custom bugs.

Architecture

As mentioned above, the library loads plugins that provide the functionality, each related to one storage technology. Right now, there are lvm, btrfs, swap, loop, crypto, mpath, dm, mdraid, kbd and s390 plugins. 7 The library itself basically only provides a thin wrapper around its plugins so that it can all be easily used via GObject introspection and so that it is easy to setup logging (and probably more in the future). However, each of the plugins can be used as a standalone shared library in case that’s desired. The plugins are loaded when the bd_init() function is called 8 and changes (loading more/less plugins) can later be done with the bd_reinit() function. It is also possible to reload a plugin in a long-running process if it gets updated, for example. If a function provided by a plugin that was not loaded is called, the call fails with an error, but doesn’t crash and thus it is up to the caller code to deal with such situation.

The libblockdev library is stateless from the perspective of the block device manipulations. I.e., it has some internal state (like tracking if the library has been initialized or not), but it doesn’t hold any state information about the block devices. So if you e.g. use it to create some LVM volume groups and then try to create a logical volume in a different, non-existing VG, it just fails creating it at the point where LVM realizes that such volume group doesn’t exist. That makes the library a lot simpler and "almost thread-safe" with the word "almost" being there just because some of the technologies doesn’t provide any other API than running various utilities as subprocesses which cannot generally be considered thread-safe. 9

Scope (provided functionality)

The first goal for the library was to replace the Blivet’s devicelibs subpackage that provided all the low-level functions for manipulations with block devices. That fact also defined the original scope of the library. Later, we realized that we would like to add the LVM cache and bcache support to Blivet and the scope of the library got extended to the current state. The supported technologies are defined by the list of plugins the library uses (see above) and the full list of the functions can be seen either in the project’s features.rst file or by browsing the documentation.

Tests and reliability

Right now, there are 135 tests run manually and by a Jenkins instance hooked up to the project’s Git repository. The tests use loop devices to test vast majority of the functions the library provides 10. They must be run as root, but that’s unavoidable if they should really test the functionality and not just some mocked up stubs that we would believe behave like a real system.

The library is used by Fedora 22’s installation process as F22’s Blivet has been ported to use libblockdev before the Beta release. There have been few bugs reported against the library (majority of them were related to FW RAID setups) with all bugs being fixed and covered by tests for those particular use cases (based on data gathered from the logs in bug reports).

Future plans

Although the initial goals are all covered by the version 1.0 of the library there are already many suggestions for additional functionality and also extensions for some of the functions that are already implemented (extra arguments, etc.). The most important goal for the near future is to fix reported bugs in the current version and promote the library as much as possible so that the wish mentioned above gets fulfilled. The plan for a bit further future (let’s say 6-8 months) is to work on additional functionality targetting version 2.0 that will break the API for the purpose of extending and improving it.

To be more concrete, for example one of the planned new plugins is the fs plugin that will provide various functions related to file systems. One of such functions will definitely be the mkfs() function that will take a list (or dictionary) of extra options passed to the particular mkfs utility on top of the options constructed by the implementation of the function. The reason for that is the fact that some file systems support many configuration options during their creation and it would be cumbersome to cover them all with function parameters. In relation to that, at least some (if not all) of the LVM functions will also get such extra argument so that they are useful even in very specific use cases that require fine-tuning of the parameters not covered by functions’ arguments.

Another potential feature is to add some clever and nice way of progress reporting to some functions that are expected to take a lot of time to finish –like lvresize(), pvmove(), resizefs() and others. It’s not always possible to track the progress because even the underlying tools/libraries don’t report it, but where possible, libblockdev should be able to pass that information to its callers ideally in some unified way.

So a lot of work behind, much more ahead. It’s a challenging world, but I like taking challenges.


  1. a python package used by the Anaconda installer as a storage backend

  2. System Storage Manager

  3. daemon used by e.g. gnome-disks and the whole GNOME "storage stack"

  4. a fork of udisks2 adding an LVM API and being actively developed

  5. the first goal for the library was to replace Blivet’s devicelibs subpackage

  6. at higher than the most low-level layers, of course

  7. I hope that with the exception of kbd which stands for Kernel Block Devices the related technologies are clear, but don’t hesitate to ask in the comments if not.

  8. or e.g. BlockDev.init(plugins) in Python over the GObject introspection

  9. use Google and "fork shared library" for further reading

  10. 119 out of 132 to be more precise

(Not so) GObject introspection

Why?

One of the nice features of the C programming language is that all the other languages are doing their best to allow easy creation of bindings for a code written in C. Thus when I was thinking about the choice of programming language for a new library (the libblockdev library) C was the answer because I wanted to make the new library usable from as many programming languages as possible. 1 The next question was how to write to code so that it would be really easy to create bindings for other languages and since I’ve now been fighting with Gtk for more than 3 years at work I knew that GObject introspection is a powerful mechanism providing number of bindings basically for free.

How does it work?

I’m not going to describe the details of how GObject introspection works, but the basic idea is that if one writes nice GObject-based C code and adds some little pieces of metadata in few specific places, two tools can be used to produce an XML and a .typelib (binary) files providing enough information for many other languages to use the compiled C code just like the native code. That is achieved by the existence of those XML and .typelib files in connection with a code that can parse and use them. You may remark that many programming languages have tools for generating bindings for C code. The difference here is that with GObject introspection it’s only necessary to run two tools to generate two files that can then be used by numbers of languages. The best thing is that the resulting XML and .typelib files (let’s call them introspection data) are declarative and contain only metadata for the compiled C code. So no additional tweaks of any "native" code is required.

Can I tweak the details?

On the other hand, the GObject introspection and introspection data were designed in a way that they could be used from a wide variety of programming languages ranging from compiled, strong-typed functional programming language Haskell to interpreted, dynamic and very benevolent language Python and many others. For this reason languages supporting GObject introspection usually also support a way to define thin wrappers (so called overrides) providing even easier usage of the C code. For example Python has a nice feature of defining default parameter values allowing the caller code to omit such parameters and let the callee use the default values. However, this is still quite rare in the area of programming languages so the GObject introspection doesn’t build on this principle (even though it is planned for some future releases ). By providing an easy way for defining overrides Python allows creation of simple file defining new functions/classes/methods wrapping the C/GObject functions/classes adding the default parameter values. More complex examples are overrides allowing native iteration over items in containers, raising exceptions per return values, etc.

But…., or not?

So far so good, right? Looks like there really is an easy way to write code reusable from many other programming languages. But…, there always is some "but", you know. But (pun intended) this time it’s only a "first glance but" –all this is GObject introspection and writing proper GObject code is a real pain, although there are some tools generating the basic structure, it’s still a lot of work to do. However, if you are curious enough, at least as much as I am, you can find out that the g-ir-scanner tool that is used to generate the XML file doesn’t complain if you run it on a non-GObject, "plain C", code! Although the GObject introspection developers will argue, say that it is not supported and nobody uses it this way (which can be proven wrong) and explain that it’s really easy and much better to write GObject code with all that boilerplate code it requires, generating introspection data for non-GObject code works just fine. One needs to give all introspected symbols a common project prefix, write gtk-doc documentation comments (similar to doxygen) with some introspection-related additional fields (e.g. documenting whether the caller should free the returned value or it is owned by the callee), but that is required for proper GObject-based code anyway. Running g-ir-scanner then produces nice XML edible by g-ir-compile producing the .typelib file. By moving them to their right places or telling e.g. Python where to find them, it’s super-easy to use the C code from Python: from gi.repository import Foo and here you go using your libfoo.so as Foo.bar(), etc.!

To be continued…

Things might get a bit tricky on the way if you want to use GObject introspection in some more or really complex situations, but I haven’t encountered any such that I’d decide to give it up and stop using GObject introspection or that I’d have to resign on functionality or API of the library and I’ll share the tips and tricks I’ve discovered so far here on this blog in following articles.


  1. Not that I haven’t cursed myself many times for this decision.

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