The Fuchsia System Interface is the binary interface that the Fuchsia operating system presents to software it runs. The foundation of the interface is the vDSO, which provides access to the system calls. Programs are not allowed to issue system calls directly (e.g., by trapping into the kernel). Instead, programs use the vDSO to interface with the kernel.
The bulk of the system interface is provided through inter-process communication protocols, typically defined using FIDL. These protocols are spoken over various kernel primitives, including channels and sockets.
fuchsia.io FIDL library provides protocols for file and directory
operations. Fuchsia uses the
fuchsia.io protocols to provide a namespace to
components through which components can access system services and resources.
The names in these namespaces follow certain conventions, which are part of the
system ABI. See namespaces for more details.
Packages themselves also provide an interface to the system in terms of directory structure and file formats. The system uses this information to initialize processes in which components stored in these packages execute.
The Application Programming Interface (API) for a system is the source-level interface to the system. You typically write software that uses this interface directly. Changes to the system API might require you to update your source code to account for the changes to the API.
The Application Binary Interface (ABI) for a system is the binary-level interface to the system. Typically you don't write software that uses the system ABI directly. Instead, you write software against the system API. When the software is compiled, the binary artifact created by the compiler interfaces with the system through the ABI. Changes to the system ABI may require you to recompile your source code to account for the changes in the ABI.
This section describes the various ABI surfaces for Fuchsia components.
The vDSO is a virtual shared library that provides access to the kernel.
Concretely, the vDSO is an ELF shared library, called
exports a number of symbols with a C calling convention. The source of truth for
these symbols is //zircon/vdso. Their semantics are
described in the documentation.
The bulk of the system interfaces are defined in the Fuchsia Interface Definition Language (FIDL). The FIDL compiler produce language specific APIs and runtime libraries, referred to as FIDL bindings, for a variety of target languages. These bindings provide an idiomatic interface for sending and receiving interprocess communication messages over Zircon channels (and other primitives).
The FIDL protocol definitions themselves and the language-specific bindings generated by the compiler are part of the system API but not part of the system ABI. Instead, the format of the serialized messages, called the wire format, comprises the ABI. The FIDL wire format is defined by the specification.
In addition to the messages sent, some FIDL protocols make use of user signals on the underlying kernel objects. Currently, these signals are not declared in FIDL. Typically, the semantics of any associated user signals are documented in prose in comments in the FIDL definitions.
A component's namespace is provided during startup and lets the component interact with capabilities published by the rest of the system.
For example, components can discover implementations of FIDL
/svc entry in this namespace, which lists them under well-known names.
Similarly, by convention, the
pkg namespace entry is mapped to the package
from which the component was resolved.
A component serves an outgoing directory to publish capabilities to the rest of the system.
Some namespaces include files with data. The data format used by these files is
also part of the system ABI.
For example, components access the root certificates through a namespace entry
that contains a
certs.pem file. The
pem data format is therefore part of the
Fuchsia packages have a directory structure that follows certain naming conventions. These conventions are also part of the system ABI. This section gives two examples of important packaging conventions.
By convention, the
meta directory in a package contains metadata files that
describe the package. The structure of this metadata, including the data formats
used by these files such as
is part of the system ABI.
By convention, the
lib directory in a package contains the shared libraries
used by components in the package. When the system runs an executable from the
package, requests for shared libraries are resolved relative to this
One important difference between Fuchsia and other operating systems is that the
shared libraries themselves are provided by the package creator rather than the
system itself. For that reason, the shared libraries themselves (including
libc) are not part of the system ABI.
Processes on Fuchsia are fairly flexible and largely under the control of the executable running in the process, but some of the initial structure of the process is controlled by the system and part of the system ABI.
For additional details, see Program Loading.
Fuchsia uses the ELF data format for executables. When loading an executable
into a process, the loader parses contents of the executable as ELF. The loader
INTERP directive from the executable and resolves that name as a
file in the
lib directory of the package that contained the executable. The
loader then parses the contents of the
INTERP file as an ELF shared library,
relocates the library, and maps the library into the newly created process.
As part of starting a process, the creator of the process supplies the process
with a message that contains, for example, the command line arguments, the
environ, the initial handles, and the namespace for the process.
(The outgoing directory is included in the set of initial handles for the
The format of this message, including length limitations on fields such as the
command line arguments and the
environ, are part of the system ABI, as are
the conventions around the contents of the message. For example, by convention,
PWD environment variable is the name the creator suggests the process use
as its current working directory.
The initial handles are associated with numerical identifiers. The conventions
around these identifiers are part of the system ABI. For example, by convention,
PA_PROC_SELF handle is a handle to the process object for the newly
created process. In addition to the types of these handles, the rights
associated with these handles are also part of the system ABI.
Before starting a process, the creator modifies the root VMAR for the process. For example, the creator maps the vDSO and allocates a stack for the initial thread. The structure of the VMAR when the process is started is part of the system ABI.
Processes are run in jobs, which can apply policy to the processes and jobs they
contain. The job policy applied to processes is part of the system ABI. For
example, components run in processes with
ZX_POL_NEW_PROCESS set to
ZX_POL_ACTION_DENY. This enforces that the component use the
fuchsia.process.Launcher protocol to create processes rather than issuing the
zx_process_create system call directly.
Components that use the Vulkan API for hardware accelerated graphics link
libvulkan.so and specify the
vulkan feature in their manifests. This
library is provided by the package that contains the component and therefore is
not part of the system ABI. However,
libvulkan.so loads another shared
library, called the Vulkan Installable Client Driver (Vulkan ICD). The Vulkan
ICD is loaded using
fuchsia.vulkan.loader.Loader, which means the library is
provided by the system itself rather than the package that contains the
component. For this reason, the Vulkan ICD is part of the system ABI.
The Vulkan ICD is an ELF shared library that exports exactly three symbols. These symbols are reserved for use by the Vulkan ICD and should not be used directly.
In addition, the Vulkan ICD shared library has a
NEEDED section that lists
several shared libraries upon which the Vulkan ICD depends. The package
containing the component is required to provide these shared libraries.
The Vulkan ICD also imports a number of symbols. The conventions around these imported symbols, for example their parameters and semantics, are part of the system ABI.
NEEDED section and the list of imported symbols for the Vulkan
ICD are both larger than we desire. Hopefully we will be able to minimize these
aspects of the system ABI.
Datagram sockets used for networking include a frame that specifies the network address associated with the datagram. This frame is also part of the system ABI.
Programs that run in the terminal communicate with the terminal using the
Fuchsia Terminal Protocol, which is a text-based protocol similar to
This protocol is also exposed over the network through
ssh, both by clients
that expect incoming
ssh connections to support this protocol and by servers
that expect outgoing
ssh connections to support this protocol.