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INTRO(9) Kernel Developer’s Manual INTRO(9)
NAME
intro — introduction to kernel programming interfaces
DESCRIPTION
Welcome to the FreeBSD kernel documentation. Outside of the source code itself,
this set of man pages is the primary resource for information on usage of the numer‐
ous programming APIs (KPIs) available to the kernel. In some cases it is also a
source of truth for the implementation details and/or design decisions behind a par‐
ticular subsystem or piece of code.
The intended audience of this documentation is developers, and the primary authors
are also developers. Kernel documentation may be written assuming a certain famil‐
iarity with common programming or OS‐level concepts and practices. However, this
documentation should also attempt to provide enough background information that
readers approaching a particular subsystem or interface for the first time will be
able to understand. Naturally, what is considered "enough" background information
will vary from person to person.
To further set expectations, we acknowledge that kernel documentation, like the
source code itself, is forever a work‐in‐progress. There will be large sections of
the codebase whose documentation is subtly or severely outdated, or missing alto‐
gether. This documentation is a supplement to the source code, and can not always
be taken at face value.
At its best, section 9 documentation will provide a description of a particular
piece of code that, paired with its implementation, fully informs the reader of the
intended and realized effects.
Man pages in this section most frequently describe functions, but may also describe
types, global variables, macros, or high‐level concepts.
CODING GUIDELINES
Code written for the FreeBSD kernel is expected to conform to the established style
and coding conventions. Please see style(9) for the detailed set of rules and
guidelines.
OVERVIEW
Below is presented various subsystems.
Data Structures
There are implementations for many well‐known data structures available in the ker‐
nel.
bitstring(3) Simple bitmap implementation.
counter(9) An SMP‐safe general‐purpose counter implementation.
hash(9) Hash map implementation.
nv(9) Name/value pairs.
queue(3) Singly‐linked and doubly‐linked lists, and queues.
refcount(9) An SMP‐safe implementation of reference counts.
sbuf(9) Dynamic string composition.
sglist(9) A scatter/gather list implementation.
Utility Functions
Functions or facilities of general usefulness or convenience. See also the “Testing
and Debugging Tools” or “Miscellaneous” sub‐sections below.
Formatted output and logging functions are described by printf(9).
A rich set of macros for declaring sysctl(8) variables and functions is described by
sysctl(9).
Non‐recoverable errors in the kernel should trigger a panic(9). Run‐time assertions
can be verified using the KASSERT(9) macros. Compile‐time assertions should use
_Static_assert().
Endian‐swapping functions: byteorder(9).
Deprecation messages may be emitted with gone_in(9).
A unit number facility is provided by unr(9).
Synchronization Primitives
The locking(9) man page gives an overview of the various types of locks available in
the kernel and advice on their usage.
Atomic primitives are described by atomic(9).
The epoch(9) and smr(9) facilities are used to create lock‐free data structures.
There is also seqc(9).
Memory Management
Dynamic memory allocations inside the kernel are generally done using malloc(9).
Frequently allocated objects may prefer to use uma(9).
Much of the virtual memory system operates on vm_page_t structures. The following
functions are documented:
vm_page_advise(9), vm_page_alloc(9), vm_page_bits(9), vm_page_aflag(9),
vm_page_alloc(9), vm_page_bits(9), vm_page_busy(9), vm_page_deactivate(9),
vm_page_free(9), vm_page_grab(9), vm_page_insert(9), vm_page_lookup(9),
vm_page_rename(9), vm_page_sbusy(9), vm_page_wire(9)
Virtual address space maps are managed with the vm_map(9) API.
The machine‐dependent portion of the virtual memory stack is the pmap(9) module.
Allocation policies for NUMA memory domains are managed with the domainset(9) API.
File Systems
The kernel interface for file systems is VFS(9). File system implementations regis‐
ter themselves with vfsconf(9).
The abstract and filesystem‐independent representation of a file, directory, or
other file‐like entity within the kernel is the vnode(9).
The implementation of access control lists for filesystems is described in acl(9).
Also vaccess(9).
I/O and Storage
The GEOM framework represents I/O requests using the bio(9) structure.
disk(9).
Networking
Much of the networking stack uses the mbuf(9), a flexible memory management unit.
Network interfaces are implemented using the ifnet(9) API, which has functions for
drivers and consumers.
A framework for managing packet output queues is described by altq(9).
To receive incoming packets, network protocols can register themselves with
netisr(9)
Virtualization of the network stack is provided by VNET(9).
The kernel socket interface is described in socket(9).
The low‐level packet filter interface is described by pfil(9).
The subsystem for IEEE 802.11 wireless networking is described in ieee80211(9).
A framework for modular TCP implementations is described by tcp_functions(9).
A framework for modular congestion control algorithms is described by mod_cc(9).
Device Drivers
Consult the device(9) and driver(9) pages first.
Most drivers act as devices, and provide a set of methods implementing the device
interface. This includes methods such as DEVICE_PROBE(9), DEVICE_ATTACH(9), and
DEVICE_DETACH(9).
In addition to devices, there are buses. Buses may have children, in the form of
devices or other buses. Bus drivers will implement additional methods, such as
BUS_ADD_CHILD(9), BUS_READ_IVAR(9), or BUS_RESCAN(9).
Buses often perform resource accounting on behalf of their children. For this there
is the rman(9) KPI.
Drivers can request and manage their resources (e.g. memory‐space or IRQ number)
from their parent using the following sets of functions:
bus_alloc_resource(9), bus_adjust_resource(9), bus_get_resource(9),
bus_map_resource(9), bus_release_resource(9), bus_set_resource(9)
Direct Memory Access (DMA) is handled using the busdma(9) framework.
Functions for accessing bus space (i.e. read/write) are provided by bus_space(9).
Clocks and Timekeeping
The kernel clock frequency and overall system time model is described in hz(9).
A few global time variables, such as system up‐time, are described by time(9).
Raw CPU cycles are provided by get_cyclecount(9).
Userspace Memory Access
Read/write access to userspace memory from the kernel is not guaranteed, and some
architectures prevent this by default. Therefore, memory transactions that cross
the kernel/user boundary must go through one of several interfaces built for this
task.
Most device drivers use the uiomove(9) set of routines.
Simpler primitives for reading or writing smaller chunks of memory are described by
casuword(9), copy(9), fetch(9), and store(9).
Threads, Tasks, and Callbacks
Kernel threads and processes are created using the kthread(9) and kproc(9) inter‐
faces, respectively.
Where dedicated kernel threads are too heavyweight, there is also the taskqueue(9)
interface.
For low‐latency callback handling, the callout(9) framework should be used.
Dynamic handlers for pre‐defined event hooks are registered and invoked using the
EVENTHANDLER(9) KPI.
Thread Switching and Scheduling
The machine‐independent interface to a context switch is mi_switch(9).
To prevent preemption, use a critical(9) section.
To voluntarily yield the processor, kern_yield(9).
The various functions which will put a thread to sleep are described by sleep(9).
Sleeping threads are removed from the scheduler and placed on a sleepqueue(9).
Processes and Signals
To locate a process or process group by its identifier, use pfind(9) and pgfind(9).
Alternatively, the pget(9) function provides additional specificity.
The "hold count" of a process can be manipulated with PHOLD(9).
The kernel interface for signals is described by signal(9).
Signals can be sent to processes or process groups using the functions described in
psignal(9).
Security
See the generic security overview in security(7).
The basic structure for user credentials is struct ucred, managed by the ucred(9)
API. Thread credentials are verified using priv(9) to allow or deny certain privi‐
leged actions.
Policies influenced by kern.securelevel must use the securelevel_gt(9) or
securelevel_ge(9) functions.
The Mandatory Access Control (MAC) framework provides a wide set of hooks, support‐
ing dynamically‐registered security modules. See mac(9).
Cryptographic services are provided by the OpenCrypto framework. This KPI provides
and interface for both consumers and crypto drivers. See crypto(9).
For information on random number generation, see random(9) and prng(9).
Kernel Modules
The interfaces for declaring loadable kernel modules are described in module(9).
Interrupts
The machine‐independent portion of the interrupt framework supporting the registra‐
tion and execution of interrupt handlers is described in intr_event(9).
Software interrupts are provided by swi(9).
Device drivers register interrupt handlers using the bus_setup_intr(9) function.
Start‐up and Shutdown
The SYSINIT framework provides macros to dynamically register start‐up and shutdown
functions. See SYSINIT(9).
For start‐up functions which require interrupts to be started, config_intrhook(9).
For details on the shutdown/reboot procedure and available shutdown hooks, see
reboot(9).
Testing and Debugging Tools
A kernel test framework: kern_testfrwk(9)
A facility for defining configurable fail points is described by fail(9).
Commands for the ddb(4) kernel debugger are defined with the DB_COMMAND(9) family of
macros.
The ktr(4) tracing facility adds static tracepoints to many areas of the kernel.
These tracepoints are defined using the macros described in ktr(9).
Static probes for DTrace are defined using the SDT(9) macros.
Stack traces can be captured and printed with the stack(9) KPI.
Kernel sanitizers can perform additional compiler‐assisted checks against memory
use/access. These runtimes are capable of detecting difficult‐to‐identify classes
of bugs, at the cost of a large overhead. Supported is the Kernel Address Sanitizer
KASAN(9), and the Kernel Memory Sanitizer KMSAN(9).
The LOCK_PROFILING kernel config option enables extra code to assist with profiling
and/or debugging lock performance. See LOCK_PROFILING(9).
Driver Tools
Defined functions/APIs for specific types of devices.
iflib(9) Programming interface for iflib(4) based network drivers.
pci(9) Peripheral Component Interconnect (PCI) and PCI Express (PCIe) program‐
ming API.
usbdi(9) Universal Serial Bus programming interface.
superio(9) Functions for Super I/O controller devices.
Miscellaneous
Dynamic per‐CPU variables: dpcpu(9).
CPU bitmap management: cpuset(9).
Kernel environment management: getenv(9).
Contexts for CPU floating‐point registers are managed by the fpu_kern(9) facility.
A facility for asynchronous logging to files from within the kernel is provided by
alq(9).
The osd(9) framework provides a simple way to dynamically extend core structures in
a way that preserves KBI. See the hhook(9) and khelp(9) APIs for information on how
this is used.
The kernel object implementation is described by kobj(9).
SEE ALSO
style(9)
The FreeBSD Architecture Handbook, https://docs.freebsd.org/en/books/arch‐handbook/.
GNU July 10, 2023 INTRO(9)

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