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INTRO(9) FreeBSD Kernel Developer's Manual INTRO(9)
intro – introduction to kernel programming interfaces
Welcome to the FreeBSD kernel documentation. Outside of the source code
itself, this set of man(1) pages is the primary resource for information
on usage of the numerous programming interfaces available within the
kernel. In some cases, it is also a source of truth for the
implementation details and/or design decisions behind a particular
subsystem or piece of code.
The intended audience of this documentation is developers, and the
primary authors are also developers. It is written assuming a certain
familiarity 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
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severely outdated, or missing altogether. 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(1) pages in this section most frequently describe functions, but may
also describe types, global variables, macros, or high-level concepts.
Code written for the FreeBSD kernel is expected to conform to the
established style and coding conventions. Please see style(9) for a
detailed set of rules and guidelines.
Below is presented various subsystems.
Data Structures
There are implementations for many well-known data structures available
in the kernel.
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).
Data output in hexadecimal format: hexdump(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)
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 register 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
by acl(9). Also vaccess(9).
I/O and Storage
The GEOM framework represents I/O requests using the bio(9) structure.
Disk drivers connect themselves to GEOM using the disk(9) API.
The devstat(9) facility provides an interface for recording device
statistics in disk drivers.
Much of the networking stack uses the mbuf(9), a flexible memory
management unit commonly used to store network packets.
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 register themselves with
Virtualization of the network stack is provided by VNET(9).
The front-end for interfacing with network sockets from within the kernel
is described by socket(9). The back-end interface for socket
implementations is domain(9).
The low-level packet filter interface is described by pfil(9).
The bpf(9) interface provides a mechanism to redirect packets to
The subsystem for IEEE 802.11 wireless networking is described by
A framework for modular TCP implementations is described by
A framework for modular congestion control algorithms is described by
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),
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) API.
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
Clocks and Timekeeping
The kernel clock frequency and overall system time model is described by
A few global time variables, such as system up-time, are described by
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) interfaces, 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
Dynamic handlers for pre-defined event hooks are registered and invoked
using the EVENTHANDLER(9) API.
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 deliberately 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
search 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 by psignal(9).
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 privileged 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, supporting dynamically-registered security modules; see mac(9).
Cryptographic services are provided by the OpenCrypto framework. This
API provides and interface for both consumers and crypto drivers; see
For information on random number generation, see random(9) and prng(9).
Kernel Modules
The interfaces for declaring loadable kernel modules are described by
The machine-independent portion of the interrupt framework supporting the
registration and execution of interrupt handlers is described by
Software interrupts are provided by swi(9).
Device drivers register their 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 configured and
enabled, see 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 by
Static probes for DTrace are defined using the SDT(9) macros.
Stack traces can be captured and printed with the stack(9) API.
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
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) programming API.
pwmbus(9) Pulse-Width Modulation (PWM) bus interface methods.
usbdi(9) Universal Serial Bus programming interface.
superio(9) Functions for Super I/O controller devices.
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)
A facility for asynchronous logging to files from within the kernel is
provided by alq(9).
The osd(9) framework provides a mechanism 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).
man(1), style(9)
The FreeBSD Architecture Handbook,
FreeBSD 14.0-CURRENT July 31, 2023 FreeBSD 14.0-CURRENT