freebsd/usr.sbin/config/config.5

.\" Copyright (c) 2003 Joseph Koshy
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.Dd October 17, 2022
.Dt CONFIG 5
.Os
.Sh NAME
.Nm config
.Nd kernel configuration file format
.Sh DESCRIPTION
A kernel configuration file specifies the configuration of a
.Fx
kernel.
It is processed by
.Xr config 8
to create a build environment where a kernel may be built using
.Xr make 1 .
.Ss Lexical Structure
A kernel configuration file comprises a sequence of specification
directives.
.Pp
A specification directive starts with a keyword at the beginning
of the line and is followed by additional parameters.
.Pp
A specification directive may be terminated by a semicolon
.Ql \&;
or by a newline.
Long input lines may be broken into shorter lines by starting the
second and subsequent lines with a white space character.
.Pp
Case is significant,
.Dq Li machine
and
.Dq Li MACHINE
are different tokens.
.Pp
A double quote character
.Ql \[dq]
starts a quoted string.
All characters up to the next quote character form the value
of the quoted string.
A
.Ql \[dq]
character may be inserted into a quoted string by
using the sequence
.Ql \e\[dq] .
.Pp
Numbers are specified using
.Tn C Ns -style
syntax.
.Pp
A
.Ql #
character starts a comment; all characters from the
.Ql #
character till the end of the current line are ignored.
.Pp
Whitespace between tokens is ignored, except inside quoted strings.
Whitespace following a comment line is ignored.
.Ss Configuration Directives
Kernel configuration directives may appear in any order
in a kernel configuration file.
Directives are processed in order of appearance with subsequent
directive lines overriding the effect of prior ones.
.Pp
The list of keywords and their meanings are as follows:
.Pp
.Bl -tag -width indent -compact
.\" -------- CPU --------
.It Ic cpu Ar cputype
Specify the CPU this kernel will run on.
There can be more than one
.Ic cpu
directive in a configuration file.
The allowed list of CPU names is architecture specific and is
defined in the file
.Pa sys/conf/options . Ns Aq Ar arch .
.\" -------- DEVICE --------
.Pp
.It Ic device Ar name Op , Ar name Op ...
.It Ic devices Ar name Op , Ar name Op ...
Configures the specified devices
for inclusion into the kernel image.
Devices that are common to all architectures are
defined in the file
.Pa sys/conf/files .
Devices that are specific to architecture
.Ar arch
are defined in the file
.Pa sys/conf/files . Ns Aq Ar arch .
.\" -------- ENV --------
.Pp
.It Ic env Ar filename
Specifies a filename containing a kernel environment definition.
.Pp
The kernel will augment this compiled-in environment with the environment
prepared for it at boot time by
.Xr loader 8 .
Environment variables specified in the
.Xr loader 8
environment will take precedence over environment variables specified in
.Ar filename ,
and environment variables specified in the dynamic environment take precedence
over both of these.
.Pp
.Va loader_env.disabled=1
may be specified in the static environment to disable the
.Xr loader 8
environment.
Disabling the
.Xr loader 8
should be done with caution and due consideration for whether or not it supplies
environment variables needed for properly booting the system.
.Pp
.Va static_env.disabled=1
may be specified in the
.Xr loader 8
environment to disable use of the static environment.
This option has no effect if specified in any environment after the
.Xr loader 8
environment is processed.
This option is not usable in conjunction with
.Va loader_env.disabled .
.Pp
This directive is useful for setting kernel tunables in
embedded environments that do not start from
.Xr loader 8 .
.Pp
All
.Ic env
and
.Ic envvar
directives will be processed and added to the static environment in reversed
order of appearance so that later specified variables properly override earlier
specified variables.
Note that within
.Ar filename ,
the first appearance of a given variable will be the first one seen by the
kernel, effectively shadowing any later appearances of the same variable within
.Ar filename .
.\" -------- ENVVAR --------
.Pp
.It Ic envvar Ar setting
Specifies an individual environment setting to be added to the kernel's
compiled-in environment.
.Ar setting
must be of the form
.Dq Va name=value .
Optional quotes are supported in both name and value.
.Pp
All
.Ic env
and
.Ic envvar
directives will be processed and added to the static environment in reversed
order of appearance so that later specified variables properly override earlier
specified variables.
.\" -------- FILES --------
.Pp
.It Ic files Ar filename
Specifies a file containing a list of files specific to that kernel
configuration file (a la
.Pa files . Ns Aq Ar arch ) .
.\" -------- HINTS --------
.Pp
.It Ic hints Ar filename
Specifies a file to load a static device configuration specification
from.
From
.Fx 5.0
onwards, the kernel reads the system's device configuration at boot
time (see
.Xr device.hints 5 ) .
This directive configures the kernel to use the static device configuration
listed in
.Ar filename .
.Pp
Hints provided in this static device configuration will be overwritten in the
order in which they're encountered.
Hints in the compiled-in environment takes precedence over compiled-in hints,
and hints in the environment prepared for the kernel by
.Xr loader 8
takes precedence over hints in the compiled-in environment.
.Pp
Once the dynamic environment becomes available, all compiled-in hints will be
added to the dynamic environment if they do not already have an override in
the dynamic environment.
The dynamic environment will then be used for all searches of hints.
.Pp
.Va static_hints.disabled=1
may be specified in either a compiled-in environment or the
.Xr loader 8
environment to disable use of these hints files.
This option has no effect if specified in any environment after the
.Xr loader 8
environment is processed.
.Pp
The file
.Ar filename
must conform to the syntax specified by
.Xr device.hints 5 .
Multiple hints lines are allowed.
The resulting hints will be the files concatenated in reverse order of
appearance so that hints in later files properly override hints in earlier
files.
.\" -------- IDENT --------
.Pp
.It Ic ident Ar name
Set the kernel name to
.Ar name .
At least one
.Ic ident
directive is required.
.\" -------- INCLUDE --------
.Pp
.It Ic include Ar filename
Read subsequent text from file
.Ar filename
and return to the current file after
.Ar filename
is successfully processed.
.\" -------- MACHINE --------
.Pp
.It Ic machine Ar arch Op Ar cpuarch
Specifies the architecture of the machine the kernel is being
compiled for.
Legal values for
.Ar arch
include:
.Pp
.Bl -tag -width ".Cm powerpc" -compact
.It Cm arm64
The 64-bit ARM application architecture.
.It Cm arm
The ARM architecture
.It Cm amd64
The AMD x86-64 architecture.
.It Cm i386
The Intel x86 based PC architecture.
.It Cm powerpc
The IBM PowerPC architecture.
.It Cm riscv
The RISC-V architecture.
.El
.Pp
If argument
.Ar cpuarch
is specified, it points
.Xr config 8
to the cpu architecture of the machine.
When
.Ar cpuarch
is not specified, it is assumed to be the same as
.Ar arch .
.Ar arch
corresponds to MACHINE.
.Ar cpuarch
corresponds to MACHINE_ARCH.
.Pp
A kernel configuration file may have only one
.Ic machine
directive, unless the second one matches the
machine argument in the first one exactly.
.\" -------- MAKEOPTION --------
.Pp
.It Ic makeoption Ar options
.It Ic makeoptions Ar options
Add
.Ar options
to the generated makefile.
.Pp
The
.Ar options
argument is a comma separated list of one or more option
specifications.
Each option specification has the form
.Pp
.D1 Ar MakeVariableName Ns Op = Ns Ar Value
.D1 Ar MakeVariableName Ns += Ns Ar Value
.Pp
and results in the appropriate
.Xr make 1
variable definition being inserted into the generated makefile.
If only the name of the
.Xr make 1
variable is specified,
.Ar value
is assumed to be the empty string.
.Pp
Example:
.Bd -literal -offset indent -compact
makeoptions MYMAKEOPTION="foo"
makeoptions MYMAKEOPTION+="bar"
makeoptions MYNULLMAKEOPTION
.Ed
.\" -------- MAXUSERS --------
.Pp
.It Ic maxusers Ar number
This optional directive is used to configure the size
of some kernel data structures.
The parameter
.Ar number
can be 0 (the default) or an integer greater than or equal to 2.
A value of 0 indicates that the kernel should configure
its data structures according to the size of available
physical memory.
If auto configuration is requested, the kernel will set
this tunable to a value between 32 and 384 for 32-bit systems,
or scale the value higher based on available memory for 64-bit
systems.
.Pp
As explained in
.Xr tuning 7 ,
this tunable can also be set at boot time using
.Xr loader 8 .
.\" -------- NOCPU --------
.Pp
.It Ic nocpu Ar cputype
Remove the specified CPU
from the list of previously selected CPUs.
This directive can be used to cancel the effect of
.Ic cpu
directives in files included using
.Ic include .
.\" -------- NODEVICE --------
.Pp
.It Ic nodevice Ar name Op , Ar name Op ...
.It Ic nodevices Ar name Op , Ar name Op ...
Remove the specified devices
from the list of previously selected devices.
This directive can be used to cancel the effects of
.Ic device
or
.Ic devices
directives in files included using
.Ic include .
.\" -------- NOMAKEOPTION --------
.Pp
.It Ic nomakeoption Ar name
.It Ic nomakeoptions Ar name
Removes previously defined
.Xr make 1
option
.Ar name
from the kernel build.
This directive can be used to cancel the effects of
.Ic makeoption
directives in files included using
.Ic include .
.\" -------- NOOPTION --------
.Pp
.It Ic nooption Ar name Op , Ar name Op ...
.It Ic nooptions Ar name Op , Ar name Op ...
Remove the specified kernel options
from the list of previously defined options.
This directive can be used to cancel the effects of
.Ic option
or
.Ic options
directives in files included using
.Ic include .
.\" -------- OPTIONS --------
.Pp
.It Ic option Ar optionspec Op , Ar optionspec Op ...
.It Ic options Ar optionspec Op , Ar optionspec Op ...
Add compile time kernel options to the kernel build.
Each option specification has the form
.Pp
.D1 Ar name Ns Op = Ns Ar value
.Pp
If
.Ar value
is not specified, it is assumed to be
.Dv NULL .
Options common to all architectures are specified in
the file
.Pa sys/conf/options .
Options specific to architecture
.Ar arch
are specified in the file
.Pa sys/conf/options . Ns Aq Ar arch .
.El
.Sh FILES
.Bl -tag -width ".Pa sys/conf/Makefile. Ns Ar arch" -compact
.It Pa sys/compile/ Ns Ar NAME
Compile directory created from a kernel configuration.
.It Pa sys/conf/Makefile . Ns Ar arch
.Pa Makefile
fragments for architecture
.Ar arch .
.It Pa sys/conf/files
Devices common to all architectures.
.It Pa sys/conf/files . Ns Ar arch
Devices for architecture
.Ar arch .
.It Pa sys/conf/options
Options common to all architectures.
.It Pa sys/conf/options . Ns Ar arch
Options for architecture
.Ar arch .
.El
.Sh SEE ALSO
.Xr kenv 1 ,
.Xr make 1 ,
.Xr device.hints 5 ,
.Xr loader.conf 5 ,
.Xr config 8 ,
.Xr kldload 8 ,
.Xr loader 8
.Rs
.%T "Building 4.4BSD Kernels with Config"
.%A "Samuel J. Leffler"
.%A "Michael J. Karels"
.Re
.Sh HISTORY
The
.Xr config 8
utility first appeared in
.Bx 4.1 ,
and was subsequently revised in
.Bx 4.4 .
.Pp
The kernel configuration mechanism changed further in
.Fx 4.0
and
.Fx 5.0 ,
moving toward an architecture supporting dynamic kernel
configuration.