[](https://fuchsia.dev/docs/glossary#zircon) is the core that powers Fuchsia. It is composed of a kernel and a small set of userspace services, drivers, and libraries necessary for core system functions such as booting. Although [](https://fuchsia.dev/docs/glossary#zircon) applies many of the concepts popularized by microkernels, it does not strive to be minimal. Instead, the microkernel-like architecture of Zircon enables Fuchsia to reduce the amount of trusted code running in the system to a few core functions: - Memory management - Scheduling - Inter-process communication ![Data table showing a comparison between kernel services in Fuchsia and a typical operating system, indicating Fuchsia includes fewer services in its kernel.](https://fuchsia.dev/static/docs/get-started/images/intro/kernel-services.png) ## System calls User space code interacts with the objects in kernel space using **system calls**. Zircon has system calls to perform low-level operations such as: - Memory management - Task and process management - Inter-process communication (IPC) and synchronization - Exception handling - Hardware support services (clocks, entropy, device I/O) > [!IMPORTANT] > Zircon has fewer system calls than POSIX-oriented operating systems due to > services like filesystems and drivers being hosted outside the kernel. See the > full list of available Zircon system calls in the > [reference documentation.](https://fuchsia.dev/docs/reference/syscalls) User space processes access system calls through `libzircon.so` --- a [](https://fuchsia.dev/docs/glossary#virtual-dynamic-shared-object) . The Zircon vDSO is a shared library in ELF format that the kernel maps into the address space of each new process. This library is considered "virtual" because it is exposed directly by the kernel image rather than being loaded from a file. Most system calls operate directly with one or more [](https://fuchsia.dev/docs/glossary#handle) --- process-local references to objects living in kernel space represented as a 32-bit integer (`zx_handle_t`). Each handle declares the privileges, or **rights**, the holder has to perform actions on the handle itself or the referenced object. > [!IMPORTANT] > **Handles vs. file descriptors** > > Similar to POSIX file descriptors, handles are references to a specific > kernel object and they play a role in granting capabilities. However, Zircon > handles are slightly more flexible with rights applied to the handle rather > than the calling process. It is possible for a single process to have two > different handles to the same kernel object with different rights. > > In addition, handles cannot be referenced by name and Zircon does not > reserve any identifiers for common streams like stdin and stdout. > > For more details, see [Zircon handles](https://fuchsia.dev/docs/concepts/kernel/handles). ## Jobs, processes and threads Zircon exposes three main kernel objects for running code: - [Thread](https://fuchsia.dev/docs/reference/kernel_objects/thread): Thread of execution within a given address space. - [Process](https://fuchsia.dev/docs/reference/kernel_objects/process): Set of executable instructions run in a private, isolated address space. - [Job](https://fuchsia.dev/docs/reference/kernel_objects/job): Group of related processes and jobs. All jobs form a single rooted tree. ![Tree diagram illustrating Fuchsia's process hierarchy. Processes are grouped into jobs, which are ultimately owned by the Root Job.](https://fuchsia.dev/static/docs/get-started/images/intro/processes-jobs.png) Processes form the basis for system capabilities. Each process is granted a set of capabilities through the various handles it holds. Fuchsia software may or may not run within the confines of a single process. Jobs allow "applications" that are composed of more than one process to be controlled as a single entity. ## Inter-process communication Since processes are isolated by default, the kernel needs to provide a way for them to securely communicate with each other. Zircon includes the following kernel object types for inter-process communication (IPC): - [Event](https://fuchsia.dev/docs/reference/kernel_objects/event): Signaling interface between two processes. - [Socket](https://fuchsia.dev/docs/reference/kernel_objects/socket): Streaming data transport, similar to a pipe. - [Stream](https://fuchsia.dev/docs/reference/kernel_objects/stream): Streaming data transport that is seekable, like a file. - [Channel](https://fuchsia.dev/docs/reference/kernel_objects/channel): Message-based transport capable of passing both data and a set of handles. - [FIFO](https://fuchsia.dev/docs/reference/kernel_objects/fifo): Control plane for shared memory access, optimized for small data payloads. Among these objects, channels are uniquely suited to assist in launching new processes because they are capable of transferring handles (and therefore, capabilities) across to another process. Channels have exactly two endpoint handles, each owned by a separate process. Only the owners may read or write messages, but ownership of an endpoint may be transferred from one process to another. When handles are written into a channel, they are removed from the sending process. When a message with handles is read from a channel, the handles are added to the receiving process. ![Diagram showing how processes communicate through shared objects found in the kernel. The most common of these connections is the channel.](https://fuchsia.dev/static/docs/get-started/images/intro/ipc.png) > [!NOTE] > **Note:** You can find more of Zircon's deep technical details in the [kernel documentation](https://fuchsia.dev/docs/concepts/kernel/README). Zircon channels are the basis for service-level IPC protocols described by the [](https://fuchsia.dev/docs/glossary#fidl) . FIDL protocols are the primary method of IPC used by Fuchsia programs. You will explore creating and consuming FIDL protocols in more detail later on. ## Exercise: Jobs and processes Let's explore some of these fundamental concepts on a running system. In this exercise, you'll see how jobs and processes interact to form a tree. ### Start the emulator If you do not already have an instance running, start the emulator: ffx emu start --headless When startup is complete, the emulator prints the following message and returns: Logging to "$HOME/.local/share/Fuchsia/ffx/emu/instances/fuchsia-emulator/emulator.log" Waiting for Fuchsia to start (up to 60 seconds)........ Emulator is ready. ### Dump the process list Connect to a device shell prompt and use the `ps` command to dump the list of running jobs and processes. fx shell ps Below is a trimmed example of what the output looks like: TASK PSS PRIVATE SHARED STATE NAME j: 1027 507.8M 507.4M root p: 1061 564.4k 564k 36k bin/bootsvc p: 1150 4264.4k 4264k 36k bin/component_manager j: 1479 228.4k 228k p: 1583 228.4k 228k 36k pwrbtn-monitor.cm j: 1484 532.4k 532k p: 1599 532.4k 532k 36k svchost.cm j: 1544 402.4k 304k p: 1633 402.4k 304k 232k netsvc.cm j: 1681 296.4k 296k p: 1733 296.4k 296k 36k console-launcher.cm j: 1799 7232.4k 7232k p: 1825 7232.4k 7232k 36k archivist.cm j: 1927 660.4k 660k p: 1955 660.4k 660k 36k base-resolver.cm j: 2072 1016.4k 1016k p: 2088 1016.4k 1016k 36k driver_manager.cm j: 2239 348.4k 348k p: 2252 348.4k 348k 36k device-name-provider.cm j: 2364 275.3M 275.3M p: 2380 1012.4k 1012k 36k fshost.cm p: 6544 252.1M 252.1M 36k /pkg/bin/blobfs p: 10205 9744.4k 9744k 36k /pkg/bin/minfs p: 10475 12.8M 12.8M 36k pkgfs Let's focus on two columns in the output for now: - **TASK** : This tells you whether each entry is a job (`j`) or process (`p`) followed by their unique id. - **NAME**: This provides a little more detail about what piece of the system is running there. Let's break down some interesting things here based on what we've discussed so far: 1. Every process is connected to a parent job. Some jobs have multiple processes. 2. All jobs trace back to the `root` job as the ultimate parent, forming a tree. 3. During startup, the system launches a few processes directly into the `root` job. Most other processes are launched under their own parent jobs. 4. After the initial startup work, many of the entries have a `.cm` extension. These refer to **components**, and you will learn more about them later on. 5. Some of these components are core services like filesystems (`fshost.cm`) and drivers (`driver_manager.cm`) that live in user space separate from the kernel. Next, we'll explore how the Zircon enables the fundamentals of Fuchsia's security model. > [!IMPORTANT] > **Extra Credit: Fuchsia Startup** > > Exploring the tree of running processes is also a great way to learn about > the startup process of a Fuchsia device. Take a moment to review > [device startup](https://fuchsia.dev/docs/concepts/process/everything_between_power_on_and_your_component) and map how the initial processes align with the > `ps` output on the emulator. [](https://issues.fuchsia.dev/issues/new?component=1439230&template=1881640&description=Issue+on:+https://fuchsia.dev/docs/get-started/learn/intro/zircon.md "Report a bug or suggest improvements on this page") [](https://cs.opensource.google/fuchsia/fuchsia/+/main:docs/get-started/learn/intro/zircon.md "View the source code of this page") [](https://ci.android.com/edit?repo=fuchsia/fuchsia/main&file=docs/get-started/learn/intro/zircon.md&cloneRepo=fuchsia/fuchsia:refs/heads/main&fullPath=fuchsia/docs/get-started/learn/intro/zircon.md "Edit the source code of this page")