Chapter 5. The SYSINIT Framework

SYSINIT is the framework for a generic call sort and dispatch mechanism. FreeBSD currently uses it for the dynamic initialization of the kernel. SYSINIT allows FreeBSD’s kernel subsystems to be reordered, and added, removed, and replaced at kernel link time when the kernel or one of its modules is loaded without having to edit a statically ordered initialization routing and recompile the kernel. This system also allows kernel modules, currently called KLD’s, to be separately compiled, linked, and initialized at boot time and loaded even later while the system is already running. This is accomplished using the "kernel linker" and "linker sets".

5.1. Terminology

Linker Set

A linker technique in which the linker gathers statically declared data throughout a program’s source files into a single contiguously addressable unit of data.

5.2. SYSINIT Operation

SYSINIT relies on the ability of the linker to take static data declared at multiple locations throughout a program’s source and group it together as a single contiguous chunk of data. This linker technique is called a "linker set". SYSINIT uses two linker sets to maintain two data sets containing each consumer’s call order, function, and a pointer to the data to pass to that function.

SYSINIT uses two priorities when ordering the functions for execution. The first priority is a subsystem ID giving an overall order for SYSINIT’s dispatch of functions. Current predeclared ID’s are in <sys/kernel.h> in the enum list sysinit_sub_id. The second priority used is an element order within the subsystem. Current predeclared subsystem element orders are in <sys/kernel.h> in the enum list sysinit_elem_order.

There are currently two uses for SYSINIT. Function dispatch at system startup and kernel module loads, and function dispatch at system shutdown and kernel module unload. Kernel subsystems often use system startup SYSINIT’s to initialize data structures, for example the process scheduling subsystem uses a SYSINIT to initialize the run queue data structure. Device drivers should avoid using SYSINIT() directly. Instead drivers for real devices that are part of a bus structure should use DRIVER_MODULE() to provide a function that detects the device and, if it is present, initializes the device. It will do a few things specific to devices and then call SYSINIT() itself. For pseudo-devices, which are not part of a bus structure, use DEV_MODULE().

5.3. Using SYSINIT

5.3.1. Interface Headers

<sys/kernel.h> Macros

SYSINIT(uniquifier, subsystem, order, func, ident)
SYSUNINIT(uniquifier, subsystem, order, func, ident)

5.3.2. Startup

The SYSINIT() macro creates the necessary SYSINIT data in SYSINIT’s startup data set for SYSINIT to sort and dispatch a function at system startup and module load. SYSINIT() takes a uniquifier that SYSINIT uses to identify the particular function dispatch data, the subsystem order, the subsystem element order, the function to call, and the data to pass the function. All functions must take a constant pointer argument.

Example 1. Example of a SYSINIT()
#include <sys/kernel.h>

void foo_null(void *unused)

struct foo foo_voodoo = {

void foo_arg(void *vdata)
        struct foo *foo = (struct foo *)vdata;
SYSINIT(bar, SI_SUB_FOO, SI_ORDER_FOO, foo_arg, &foo_voodoo);

Note that SI_SUB_FOO and SI_ORDER_FOO need to be in the sysinit_sub_id and sysinit_elem_order enum’s as mentioned above. Either use existing ones or add your own to the enum’s. You can also use math for fine-tuning the order a SYSINIT will run in. This example shows a SYSINIT that needs to be run just barely before the SYSINIT’s that handle tuning kernel parameters.

Example 2. Example of Adjusting SYSINIT() Order
static void
mptable_register(void *dummy __unused)


    mptable_register, NULL);

5.3.3. Shutdown

The SYSUNINIT() macro behaves similarly to the SYSINIT() macro except that it adds the SYSINIT data to SYSINIT’s shutdown data set.

Example 3. Example of a SYSUNINIT()
#include <sys/kernel.h>

void foo_cleanup(void *unused)
SYSUNINIT(foobar, SI_SUB_FOO, SI_ORDER_FOO, foo_cleanup, NULL);

struct foo_stack foo_stack = {

void foo_flush(void *vdata)
SYSUNINIT(barfoo, SI_SUB_FOO, SI_ORDER_FOO, foo_flush, &foo_stack);

Last modified on: March 9, 2024 by Danilo G. Baio