The kernel provides low-level facilities for creating and setting up processes. However, these facilities are difficult to use because they involve directly mapping memory for executables, shared libraries, and stacks. Instead, you should use one of the higher-level mechanisms for creating processes.
Fuchsia provides a service, fuchsia.process.Launcher, that does the low-level work of constructing processes for you. You provide this service with all the kernel objects needed to construct the process (e.g., the job object in which the process should be created, the executable image, and the standard input and output handles), and the service does the work of parsing the ELF executable format, configuring the address space for the process, and sending the process the startup message.
Most clients do not need to use this service directly. Instead, most clients can use the simple C frontend in the FDIO library called fdio_spawn. This function, and its more advanced fdio_spawn_etc and fdio_spawn_vmo companions, connect to the fuchsia.process.Launcher service and send the service the appropriate messages to create the process. The fdio_spawn_action_t array passed to fdio_spawn_etc can be used to customize the created process.
Regardless of whether you use the fuchsia.process.Launcher service directly or the fdio_spawn frontend, this approach to creating processes is most appropriate for creating processes within your own namespace because you need to supply all the kernel objects for the new process.
To create a process in its own namespace, Fuchsia provides the fuchsia.sys.Launcher service. Rather than providing this process all the kernel objects needed to construct the new process, you simply provide the service a high-level description of the process you wish to create and the fuchsia.sys.Launcher implementation supplies the new process with the appropriate kernel objects. For example, if you provide the URL of a component within a package, fuchsia.sys.Launcher will create a process for that component in a namespace appropriate for that component with access to its own package and whatever other resources are declared in the sandbox section of its manifest.
Rather than returning a zx::process handle directly, fuchsia.sys.Launcher returns a fuchsia.sys.ComponentController interface. This layer of abstraction lets fuchsia.sys.Launcher create components that are not backed by individual processes. For example, if you launch a component written in Dart, the component might run in an instance of the Dart VM that is shared between a number of components with compatible security constraints.
Early on in the boot process, the system does create a number of processes manually. For example, the kernel manually creates the first userspace process, userboot.
Userboot's most important job is to load the next process from the bootfs image in the ZBI, which by default is component_manager.
Direct construction of processes (such as how userboot loads component_manager) is prohibited in the fuchsia job tree using a job policy. Libraries or programs that might be used from the fuchsia job tree may use fdio_spawn (or its companions) to create processes while conforming to the security policy.
