Init

Purpose

init/ is the boot-time entry point of the kernel proper. It contains start_kernel() (the function called from architecture-specific assembly after the MMU and stack are set up), the initramfs unpacker that materializes the first user-space root file system, and a small amount of bookkeeping (version stamping, calibration loops).

When this directory is done, the system is running PID 1 in user space.

Directory layout

init/
├── main.c                # start_kernel(), do_initcalls(), kernel_init()
├── calibrate.c           # BogoMIPS / loops_per_jiffy calibration
├── do_mounts.c           # Root filesystem mount logic
├── do_mounts_initrd.c    # Old-style initrd handling
├── do_mounts_rd.c        # ramdisk image handling
├── initramfs.c           # cpio-archive unpacker
├── initramfs_internal.h
├── initramfs_test.c      # KUnit tests
├── init_task.c           # The very first task_struct (PID 0)
├── noinitramfs.c         # Stub when no initramfs is built in
├── version.c             # UTS string, banner
├── version-timestamp.c
└── Kconfig

Key abstractions

Symbol	File	Purpose
start_kernel()	init/main.c	The single C entry point of the kernel. Calls every early subsystem initializer in order.
kernel_init()	init/main.c	The kernel thread that runs do_initcalls() and then execs /init.
do_initcalls()	init/main.c	Walks the initcall sections in the linker script and invokes each registered initializer.
init_task	init/init_task.c	The first task_struct (PID 0, the swapper/idle task).
unpack_to_rootfs()	init/initramfs.c	Decodes a cpio image into the root rootfs.

How it works

graph TD
    A["arch/<arch>/kernel/head_*.S"] -->|calls| B["start_kernel()"]
    B --> C["setup_arch()"]
    C --> D["mm_init(), sched_init()"]
    D --> E["rcu_init()"]
    E --> F["init_IRQ(), time_init()"]
    F --> G["rest_init()"]
    G -->|spawns| H["kernel_init thread"]
    G -->|enters| I["cpu_idle_loop()"]
    H --> J["do_initcalls() across<br/>all initcall levels"]
    J --> K["prepare_namespace()<br/>mount initramfs"]
    K --> L["run_init_process('/init')"]
    L -->|execs| M["PID 1 in user space"]

The initcall mechanism is a key piece. Macros like subsys_initcall(foo_init) in subsystem code put a function pointer into a section in the kernel image (e.g. .initcall4.init). At boot, do_initcalls() walks each section in order:

• early_initcall — runs before SMP is brought up.
• core_initcall, postcore_initcall, arch_initcall, subsys_initcall, fs_initcall, device_initcall, late_initcall — run in increasing order during normal boot.

This ordering is how the kernel guarantees, e.g., that the slab allocator is up before file systems try to allocate.

Integration points

• arch/: every architecture's boot stub eventually jumps to start_kernel().
• mm/: mm_init() brings up the page allocator and slab.
• kernel/sched/: sched_init() and init_idle() create the idle task and put it on every CPU.
• kernel/rcu/: rcu_init() brings up RCU before any subsystem can use it.
• drivers/base/: most drivers register through device_initcall().
• fs/: file systems register through fs_initcall() so they exist by the time the root is mounted.
• usr/: the initramfs cpio image is generated by usr/ and embedded into vmlinux. init/initramfs.c decodes it.

Key source files

File	Purpose
init/main.c	start_kernel(), the initcall walker, the kernel_init thread.
init/init_task.c	Static definition of the first task.
init/initramfs.c	cpio unpacker for the embedded or loaded initramfs.
init/do_mounts.c	The prepare_namespace() machinery that picks and mounts the root.
init/calibrate.c	loops_per_jiffy calibration ("BogoMIPS").
init/version.c	UTS namespace strings (uname output).

Entry points for modification

• To add a boot-time message: insert pr_info() in init/main.c.
• To add an early kernel command-line option: use early_param() and add a parser in your subsystem; it will be called before start_kernel() finishes.
• To add a new initcall level: rare. Look at include/linux/init.h and the linker scripts in include/asm-generic/vmlinux.lds.h.
• To change the root-mount logic: init/do_mounts.c. This is sensitive — many user-space setups depend on it.

Related pages

• Architecture support — what runs before start_kernel().
• Kernel core — what start_kernel() brings up.
• usr — how the initramfs is generated and embedded.