Fuchsia Core Libraries

This document describes the core libraries in the Fuchsia system, starting from the bottom of the dependency chain.

Zircon libraries


This library defines the Zircon system ABI.

TODO(kulakowski) Talk about how this is not quite the kernel syscall interface, since the VDSO abstracts that.


libzircon defines C types and function calls acting on those objects. libzx is a light C++ wrapper around those. It adds type safety beyond zx_handle_t, so that every kernel object type has a corresponding C++ type, and adds ownership semantics to those handles. It otherwise takes no opinions around naming or policy.

For more information about libzx, see its documentation.


Much of Zircon is written in C++, both in kernel and in userspace. Linking against the C++ standard library is not especially well suited to this environment (it is too easy to allocate, throw exceptions, etc., and the library itself is large). There are a number of useful constructs in the standard libary that we would wish to use, like type traits and unique pointers. However, C++ standard libraries are not really to be consumed piecemeal like this. So we built a library which provides similar constructs named fdl. This library also includes constructs not present in the standard library but which are useful library code for kernel and device driver environments (for instance, slab allocation).

For more information about FBL, read its overview.


FXL is a platform-independent library containing basic C++ building blocks, such as logging and reference counting. FXL depends on the C++ standard library but not on any Zircon- or Fuchsia-specific libraries. We build FXL both for target (Fuchsia) and for host (Linux, Mac) systems.

Generally speaking, we try to use the C++ standard library for basic building blocks, but in some cases the C++ standard library either doesn‘t have something we need (e.g., a featureful logging system) or has a version of what we need doesn’t meet our requirements (e.g., std::shared_ptr is twice as large as fxl::RefPtr).


FSL is a Zircon-specific library containing high-level C++ concepts for working with the Zircon system calls. For example, FSL provides an fsl::MessageLoop abstraction on top of Zircon's underlying waiting primitives. FSL also contains helpers for working with Zircon primitives asynchronously that build upon fsl::MessageLoop (e.g., for draining a socket asynchronously).

FSL depends on FXL and implements some interfaces defined in FXL, such as fxl::TaskRunner. FSL also depends on libfidl and implements FIDL's asynchronous waiter mechanism using fsl::MessageLoop.