.\" $NetBSD: config.5,v 1.47 2021/10/04 14:35:20 andvar Exp $
.\"
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.Dd July 19, 2016
.Dt CONFIG 5
.Os
.Sh NAME
.Nm config
.Nd kernel configuration file syntax
.Sh DESCRIPTION
The kernel configuration file specifies the way the kernel should be compiled
by the rest of the toolchain.
It is processed by
.Xr config 1
to produce a number of files that will allow the user to compile a possibly
customised kernel.
One compilation can issue several kernel binaries, with different root and
dump devices configurations, or with full debugging information.
.Pp
This manual page is intended to serve as a complete reference of all aspects
of the syntax used in the many files processed by
.Xr config 1 .
The novice user will prefer looking at the examples given in
.Xr config.samples 5
in order to understand better how the default configuration can be changed,
and how all of its elements interact with each other.
.Pp
The kernel configuration file actually contains the description of all the
options, drivers and source files involved in the kernel compilation, and the
logic that binds them.
The
.Ic machine
statement, usually found in the
.Pa std.${MACHINE}
file, hides this from the user by automatically including all the descriptive
files spread all around the kernel source tree, the main one being
.Pa conf/files .
.Pp
Thus, the kernel configuration file contains two parts:
the description of the compilation options, and the selection of those options.
However, it begins with a small preamble that controls a couple of options of
.Xr config 1 ,
and a few statements belong to any of the two sections.
.Pp
The user controls the options selection part, which is located in a file
commonly referenced as the
.Em main configuration file
or simply the
.Em kernel configuration file .
The developer is responsible for describing the options in the relevant files
from the kernel source tree.
.Pp
Statements are separated by new-line characters.
However, new-line characters can appear in the middle of a given statement,
with the value of a space character.
.\"
.\"
.Ss OBJECTS AND NAMES
.\"
.Xr config 1
is a rather complicated piece of software that tries to comply with any
configuration the user might think of.
Quite a few different objects are manipulated through the kernel configuration
file, therefore some definitions are needed.
.\"
.\"
.Ss Options and attributes
.\"
The basic objects driving the kernel compilation are
.Em options ,
and are called
.Em attributes
in some contexts.
An attribute
usually refers to a feature a given piece of hardware might have.
However, the scope of an attribute is rather wide and can just be a place
holder to group some source files together.
.Pp
There is a special class of attribute, named
.Em interface attribute ,
which represents a hook that allows a device to attach to (i.e., be a child of)
another device.
An interface attribute
has a (possibly empty) list of
.Em locators
to match the actual location of a device.
For example, on a PCI bus, devices are located by a device number
that is fixed by the wiring of the motherboard.
Additionally, each of those devices can appear through several interfaces named
functions.
A single PCI device entity is a unique function number of a given device from
the considered PCI bus.
Therefore, the locators for a
.Xr pci 4
device are
.Ql dev
(for device), and
.Ql function .
.Pp
A
.Em locator
can either be a single integer value, or an array of integer values.
It can have a default value, in which case it can be wildcarded with a
.Ql \&?
in the options selection section of the configuration file.
A single locator
definition can take one of the following forms:
.Pp
.Bl -bullet -offset indent -compact
.It
.Ar locator
.It
.Ar locator\| Ns Li =\| Ns Ar value
.It
.Li "\&[" Ns Ar locator\| Ns Li =\| Ns Ar value\| Ns Li "\&]"
.It
.Ar locator Ns Li "\&[" Ns Ar length Ns Li "\&]"
.It
.Ar locator Ns Li "\&[" Ns Ar length Ns Li "\&]={" \
        Ns Ar value\| Ns Li \&, Ar ... Ns Li "\&}"
.It
.Li "\&[" Ns Ar locator Ns Li "\&[" Ns Ar length Ns Li "\&]={" \
        Ns Ar value\| Ns Li \&, Ar ... Ns Li "\&}]"
.El
.Pp
The variants that specify a default value can be enclosed into square brackets,
in which case the locator will not have to be specified later in the options
selection section of the configuration file.
.Pp
In the options selection section, the locators are specified when declaring an
instance as a space-separated list of
.Dq Ao Ar locator\| Ac \~ Ao Ar value\| Ac
where value can be the
.Ql \&?
wildcard if the locator allows it.
.\"
.\"
.Ss Devices, instances and attachments
.\"
The main benefit of the kernel configuration file is to allow the user to avoid
compiling some drivers, and wire down the configuration of some others.
We have already seen that devices attach to each other through
interface attributes,
but not everything can attach to anything.
Furthermore, the user has the ability to define precise instances for the
devices.
An instance
is simply the reality of a device when it is probed and attached by the kernel.
.Pp
Each driver has a name for its devices.
It is called the
.Em base device name
and is found as
.Dq Ar base
in this documentation.
An
.Em instance
is the concatenation of a base device name and a number.
In the kernel configuration file, instances can sometimes be wildcarded
(i.e., the number is replaced by a
.Ql *
or a
.Ql \&? )
in order to match all the possible instances of a device.
.Pp
The usual
.Ql *
becomes a
.Ql \&?
when the instance name is used as an
.Em attachment name .
In the options selection part of the kernel configuration files, an
.Em attachment
is an interface attribute
concatenated with a number or the wildcard
.Ql \&? .
.\"
.\"
.Ss Pseudo-devices
.\"
Some components of the kernel behave like a device although they don't have
any actual reality in the hardware.
For example, this is the case for special network devices, such as
.Xr tun 4
and
.Xr tap 4 .
They are integrated in the kernel as
.Em pseudo-devices ,
and can have several
instances and even children, just like normal devices.
.\"
.\"
.Ss Dependencies
.\"
The options description part of the kernel configuration file contains all the
logic that ties the source files together, and it is done first through writing
down dependencies between
.Xr config 1
objects.
.Pp
In this documentation, the syntax for
.Em dependencies
is a comma-separated list of options and attributes .
.Pp
For example, the use of an Ethernet network card requires the source files that
handle the specificities of that protocol.
Therefore, all Ethernet network card drivers depend on the
.Ql ether
attribute.
.\"
.\"
.Ss Conditions
.\"
Finally, source file selection is possible through the help of
.Em conditionals ,
referred to as
.Dq Ar condition
later in this documentation.
The syntax for those conditions uses well-known operators
.Pf ( Ql & ,
.Ql \(or ,
and
.Ql \&! )
to combine options and attributes .
.\"
.\"
.Ss CONTEXT NEUTRAL STATEMENTS
.\"
.Bl -tag -width Ic -compact
.\"
.Pp
.It Ic version Ar yyyymmdd
Indicates the syntax version used by the rest of the file, or until the next
.Ic version
statement.
The argument is an ISO date.
A given
.Xr config 1
binary might only be compatible with a limited range of version numbers.
.\"
.Pp
.It Ic include Ar path
Includes a file.
The path is relative to the top of the kernel source tree, or the inner-most
defined
.Ic prefix .
.\"
.Pp
.It Ic cinclude Ar path
Conditionally includes a file.
Contrary to
.Ic include ,
it will not produce an error if the file does not exist.
The argument obeys the same rules as for
.Ic include .
.\"
.Pp
.It Ic prefix Op Ar path
If
.Ar path
is given, it pushes a new prefix for
.Ic file ,
.Ic include
and
.Ic cinclude .
.Ic prefix
statements act like a stack, and an empty
.Ar path
argument has the latest prefix popped out.
The
.Ar path
argument is either absolute or relative to the current defined prefix, which
defaults to the top of the kernel source tree.
.\"
.Pp
.It Ic buildprefix Op Ar path
If
.Ar path
is given, it pushes a new build prefix for
.Ic file .
.Ic buildprefix
statements act like a stack, and an empty
.Ar path
argument has the latest prefix popped out.
The
.Ar path
argument is relative to the current defined buildprefix, which
defaults to the top of the kernel build directory.
When prefix is either absolute or relative out of the kernel source tree
.Pq Pa \&../ ,
buildprefix must be defined.
.\"
.Pp
.It Ic ifdef Ar attribute
.It Ic ifndef Ar attribute
.It Ic elifdef Ar attribute
.It Ic elifndef Ar attribute
.It Ic else
.It Ic endif
Conditionally interprets portions of the current file.
Those statements depend on whether or not the given
.Ar attribute
has been previously defined, through
.Ic define
or any other statement that implicitly defines attributes such as
.Ic device .
.El
.\"
.\"
.Ss PREAMBLE
.\"
In addition to
.Ic include , cinclude ,
and
.Ic prefix ,
the preamble may contain the following optional statements:
.Bl -tag -width Ic
.\"
.It Ic build Ar path
Defines the build directory for the compilation of the kernel.
It replaces the default of
.Pa ../compile/ Ns Aq Ar config-file
and is superseded by the
.Fl b
parameter of
.Xr config 1 .
.\"
.It Ic source Ar path
Defines the directory in which the source of the kernel lives.
It replaces the default of
.Pa ../../../..
and is superseded by the
.Fl s
parameter of
.Xr config 1 .
.El
.\"
.\"
.Ss OPTIONS DESCRIPTION
.\"
The user will not usually have to use descriptive statements, as they are meant
for the developer to tie a given piece of code to the rest of the kernel.
However, third parties may provide sources to add to the kernel compilation,
and the logic that binds them to the
.Nx
kernel will have to be added to the user-edited configuration file.
.Pp
.Bl -tag -width Ic -compact
.\"
.Pp
.It Ic devclass Ar class
Defines a special attribute, named
.Em device class .
A given device cannot belong to more than one device class.
.Xr config 1
translates that property by the rule that a device cannot depend on more than
one device class, and will properly fill the configuration information file it
generates according to that value.
.\"
.Pp
.It Ic defflag \
    Oo Ar file Oc \
    Ar option \
    Oo Ar option Oo Ar ... Oc Oc \
    Op Ic \&: Ar dependencies
Defines a boolean
.Ar option ,
that can either be selected or be un-selected by the user with the
.Ic options
statement.
The optional
.Ar file
argument names a header file that will contain the C pre-processor definition
for the option.
If no file name is given, it will default to
.Li opt_ Ns Ao Ar option Ac Ns Li \&.h .
.Xr config 1
will always create the header file, but if the user choose not to select the
option, it will be empty.
Several options can be combined in one header file, for convenience.
The header file is created in the compilation directory, making them directly
accessible by source files.
.\"
.Pp
.It Ic defparam \
    Oo Ar file Oc \
    Ar option Ns Oo Ns Ic = Ns Ar value\^ Oc \
    Oo Ns Ic \&:= Ns Ar lint-value Oc \
    Oo Ar option Oo Ar ... Oc Oc \
    Op Ic \&:\~ Ns Ar dependencies
Behaves like
.Ic defflag ,
except the defined option must have a value.
Such options are not typed:
they can have either a numeric or a string value.
If a
.Ar value
is specified, it is treated as a default, and the option is
always defined in the corresponding header file.
If a
.Ar lint-value
is specified,
.Xr config 1
will use it as a value when generating a lint configuration with
.Fl L ,
and ignore it in all other cases.
.\"
.Pp
.It Ic deffs Ar name ...
Defines a file-system
.Ar name .
It is no more than a regular option, as defined by
.Ic defflag ,
but it allows the user to select the
file-systems to be compiled in the kernel with the
.Ic file-system
statement instead of the
.Ic options
statement.
.\"
.Pp
.It Ic obsolete defflag \
    Oo Ar file Oc \
    Ar option ...
.It Ic obsolete defparam \
    Oo Ar file Oc \
    Ar option ...
Those two statements are identical and mark the listed option names as
obsolete.
If the user selects one of the listed options in the kernel configuration
file,
.Xr config 1
will emit a warning and ignore the option.
The optional
.Ar file
argument should match the original definition of the option.
.\"
.Pp
.It Ic define \
    Ar attribute \
    Oo Ic \&{ Ar locators Ic \&} Oc \
    Op Ic \&: Ar dependencies
Defines an
.Ar attribute .
The
.Ar locators
list is optional, and can be empty.
If the pair of braces are present, the locator list is defined and the
declared attribute becomes an
.Em interface attribute ,
on which devices can attach.
.\"
.Pp
.It Ic maxpartitions Ar number
Defines the maximum number of partitions the disklabels for the considered
architecture can hold.
This statement cannot be repeated and should only appear in the
.Pa "std.${ARCH}"
file.
.\"
.Pp
.It Ic maxusers Ar min default max
Indicates the range of values that will later be accepted by
.Xr config 1
for the
.Ic maxusers
statement in the options selection part of the configuration file.
In case the user doesn't include a
.Ic maxusers
statement in the configuration file, the value
.Ar default
is used instead.
.\"
.Pp
.It Ic device \
    Ar base \
    Oo Ic \&{ Ar locators Ic \&} Oc \
    Op Ic \&: Ar dependencies
Declares a device of name
.Ar base .
The optional list of
.Ar locators ,
which can also be empty, indicates the device can have children attached
directly to it.
Internally, that means
.Ar base
becomes an
.Em interface attribute .
For every device the user selects,
.Xr config 1
will add the matching
.Fn CFDRIVER_DECL
statement to
.Pa ioconf.c .
However, it is the responsibility of the developer to add the relevant
.Fn CFATTACH_DECL_NEW
line to the source of the device's driver.
.\"
.Pp
.It Ic attach \
    Ar base \
    Ic at Ar attr\^ \
      Ns Oo Ic \&, Ar attr\^ \
      Ns Oo Ic \&, Ar ... Oc Oc \
    Oo Ic with Ar name Oc \
    Op Ic \&: dependencies
All devices must have at least one declared attachment.
Otherwise, they will never be found in the
.Xr autoconf 9
process.
The attributes on which an instance of device
.Ar base
can attach must be interface attributes, or
.Ic root
in case the device is at the top-level, which is usually the case of e.g.,
.Xr mainbus 4 .
The instances of device
.Ar base
will later attach to one interface attribute from the specified list.
.Pp
Different
.Ic attach
definitions must use different names using the
.Ic with
option.
It is then possible to use the associated
.Ar name
as a conditional element in a
.Ic file
statement.
.\"
.Pp
.It Ic defpseudo Ar base Op Ic \&: Ar dependencies
Declares a pseudo-device.
Those devices don't need an attachment to be declared, they will always be
attached if they were selected by the user.
.\"
.Pp
.It Ic defpseudodev Ar base \
    Oo Ic \&{ Ar locators Ic \&} Oc \
    Op Ic \&: Ar dependencies
Declares a pseudo-device.
Those devices don't need an attachment to be declared, they will always be
attached if they were selected by the user.
This declaration should be used if the pseudodevice uses
.Xr autoconf 9
functions to manage its instances or attach children.
As for normal devices, an optional list of
.Ar locators
can be defined, which implies an interface attribute named
.Ar base ,
allowing the pseudo-device to have children.
Interface attributes can also be defined in the
.Ar dependencies
list.
.\"
.Pp
.It Ic file Ar path \
    Oo Ar condition Oc \
    Oo Ic needs-count Oc \
    Oo Ic needs-flag Oc \
    Op Ic compile with Ar rule
Adds a source file to the list of files to be compiled into the kernel, if the
.Ar condition
is met.
The
.Ic needs-count
option indicates that the source file requires the number of all the countable
objects it depends on (through the
.Ar condition )
to be defined.
It is usually used for pseudo-devices
whose number can be specified by the user in the
.Ic pseudo-device
statement.
Countable objects are devices and pseudo-devices.
For the former, the count is the number of declared instances.
For the latter, it is the number specified by the user, defaulting to 1.
The
.Ic needs-flag
options requires that a flag indicating the selection of an attribute to
be created, but the precise number isn't needed.
This is useful for source files that only partly depend on the attribute,
and thus need to add pre-processor statements for it.
.Pp
Both
.Ic needs-count
and
.Ic needs-flag
produce a header file for each of the considered attributes.
The name of that file is
.Ao Ns Ar attribute Ns Ac Ns Pa \&.h .
It contains one pre-processor definition of
.Dv NATTRIBUTE
set to 0 if the attribute was not selected by the user, or to the number of
instances of the device in the
.Ic needs-count
case, or to 1 in all the other cases.
.Pp
The
.Ar rule
argument specifies the
.Xr make 1
rule that will be used to compile the source file.
If it is not given, the default rule for the type of the file will be used.
For a given file, there can be more than one
.Ic file
statement, but not from the same configuration source file, and all later
statements can only specify a
.Ar rule
argument, and no
.Ar condition
or flags.
This is useful when a file needs special consideration from one particular
architecture.
.Pp
The path is relative to the top of the kernel source tree, or the inner-most
defined
.Ic prefix .
.\"
.Pp
.It Ic object Ar path Op Ar condition
Adds an object file to the list of objects to be linked into the kernel, if the
.Ar conditions
are met.
This is most useful for third parties providing binary-only components.
.Pp
The path is relative to the top of the kernel source tree, or the inner-most
defined
.Ic prefix .
.\"
.Pp
.It Ic device-major Ar base Oo Ic char Ar number Oc Oo Ic block Ar number Oc \
    Op Ar condition
Associates a major device number with the device
.Ar base .
A device can be a character device, a block device, or both, and can have
different numbers for each.
The
.Ar condition
indicates when the relevant line should be added to
.Pa ioconf.c ,
and works just like the
.Ic file
statement.
.\"
.Pp
.It Ic makeoptions \
    Ar condition name Ns Ic += Ns Ar value \
    Ns Op Ic \&, Ar condition name Ns Ic += Ns Ar value \
    Ns Op Ic \&, Ar ...
Appends to a definition in the generated
.Pa Makefile .
.Pp
This variant of
.Ic makeoptions
belongs to the options description section.
The
.Ar condition
is mandatory and only
.Ic +=
can be used.
Not to be confused with the confusingly similar variant of
.Ic makeoptions
used in the selections section.
.El
.\"
.\"
.Ss OPTIONS SELECTION
.\"
.Bl -tag -width Ic
.\"
.It Ic machine Ar machine Op Ar arch Op Ar subarch Op Ar ...
The
.Ic machine
statement should appear first in the kernel configuration file, with the
exception of context-neutral statements.
It makes
.Xr config 1
include, in that order, the following files:
.Bl -enum
.It
.Pa conf/files
.It
.Pa arch/${ARCH}/conf/files.${ARCH}
if defined
.It
.Pa arch/${SUBARCH}/conf/files.${SUBARCH}
for each defined sub-architecture
.It
.Pa arch/${MACHINE}/conf/files.${MACHINE}
.El
.Pp
It also defines an attribute for the
.Ar machine ,
the
.Ar arch
and each of the
.Ar subarch .
.\"
.It Ic package Ar path
Simpler version of:
.Bd -literal -offset indent
prefix DIR
include FILE
prefix
.Ed
.\"
.It Ic ident Ar string
Defines the identification string of the kernel.
This statement is optional, and the name of the main configuration file will be
used as a default value.
.\"
.It Ic no ident
Deletes any pre-existing identification string of the kernel.
.\"
.It Ic maxusers Ar number
Despite its name, this statement does not limit the maximum number of users on
the system.
There is no such limit, actually.
However, some kernel structures need to be adjusted to accommodate with more
users, and the
.Ic maxusers
parameter is used for example to compute the maximum number of opened files,
and the maximum number of processes, which itself is used to adjust a few
other parameters.
.\"
.It Ic options \
    Ar name Ns Oo Ic = Ns Ar value\^ Oc \
    Ns Op Ic \&, Ar name Ns Oo Ic = Ns Ar value\^ Oc \
    Ns Op Ic \&, Ar ...
Selects the option
.Ar name ,
affecting it a
.Ar value
if the options requires it (see the
.Ic defflag
and
.Ic defparam
statements).
.Pp
If the option has not been declared in the options description part of the
kernel configuration machinery, it will be added as a pre-processor definition
when source files are compiled.
If the option has previously been selected, the statement produces a
warning, and the new
.Ic options
statement replaces the original.
.\"
.It Ic no options \
    Ar name \
    Ns Op Ic \&, Ar name \
    Ns Op Ic \&, Ar ...
Un-selects the option
.Ar name .
If option
.Ar name
has not previously been selected, the statement produces a warning.
.\"
.It Ic file-system \
    Ar name \
    Ns Op Ic \&, Ar name \
    Ns Op Ic \&, Ar ...
Adds support for all the listed file-systems.
.\"
.It Ic no file-system \
    Ar name \
    Ns Op Ic \&, Ar name \
    Ns Op Ic \&, Ar ...
Removes support for all the listed file-systems.
.\"
.It Ic config Ar name \
    Ic root on Ar device \
    Oo Ic type Ar fs Oc \
    Op Ic dumps on Ar device
Adds
.Ar name
to the list of kernel binaries to compile from the configuration file, using
the specified root and dump devices information.
.Pp
Any of the
.Ar device
and
.Ar fs
parameters can be wildcarded with
.Ql \&?
to let the kernel automatically discover those values.
The
.Ar device
can also be specified as a quoted specification string.
The kernel interprets this string like the console input
when prompting for a root device.
E.g.,
.Dq Li wedge: Ns Ar NAME\|
specifies a named disk wedge.
.Pp
At least one
.Ic config
statement must appear in the configuration file.
.\"
.It Ic no config Ar name
Removes
.Ar name
from the list of kernel binaries to compile from the configuration file.
.\"
.It Ar instance Ic at Ar attachment Op Ar locator-specifications ...
Configures an instance of a device attaching at a specific location in the
device tree.
All parameters can be wildcarded, with a
.Ql *
for
.Ar instance ,
and a
.Ql \&?
for
.Ar attachment
and the locators.
.\"
.It Ic no Ar instance Op Ic at Ar attachment
Removes the previously configured instances of a device that exactly match the
given specification.
If two instances differ only by their locators, both are removed.
If no
.Ar attachment
is specified, all matching instances are removed.
.Pp
If
.Ar instance
is a bare device name, all the previously defined instances of that device,
regardless of the numbers or wildcard, are removed.
.\"
.It Ic no device at Ar attachment
Removes all previously configured instances that attach to the specified
attachment.
If
.Ar attachment
ends with a
.Ql * ,
all instances attaching to all the variants of
.Ar attachment
are removed.
.\"
.It Ic pseudo-device Ar device Op Ar number
Adds support for the specified pseudo-device.
The parameter
.Ar number
is passed to the initialisation function of the pseudo-device, usually to
indicate how many instances should be created.
It defaults to 1, and some pseudo-devices ignore that parameter.
.\"
.It Ic no pseudo-device Ar name
Removes support for the specified pseudo-device.
.\"
.It Ic makeoptions \
    Ar name Ns Ic = Ns Ar value \
    Ns Op Ic \&, Ar name Ns Ic += Ns Ar value \
    Ns Op Ic \&, Ar ...
Adds or appends to a definition in the generated
.Pa Makefile .
A definition cannot be overridden, it must be removed before it can be added
again.
Optionally, if an option
.Li makeoptions_ Ns Aq Ar name
is defined with
.Ic defparam ,
the
.Ar value
is defined as an option too.
.Pp
This variant of
.Ic makeoptions
belongs to the options selection section.
Both
.Ic =
and
.Ic +=
can be used.
Not to be confused with the confusingly similar variant of
.Ic makeoptions
used in the descriptions section.
.\"
.It Ic no makeoptions \
    Ar name \
    Ns Op Ic \&, Ar name \
    Ns Op Ic \&, Ar ...
Removes one or more definitions from the generated
.Pa Makefile .
.\"
.It Ic select Ar name
Adds the specified attribute and its dependencies.
.\"
.It Ic no select Ar name
Removes the specified attribute and all the attributes which depend on it.
.El
.Sh FILES
The files are relative to the kernel source top directory (e.g.,
.Pa /usr/src/sys ) .
.Pp
.Bl -tag -width ".Pa conf/files"
.It Pa arch/${MACHINE}/conf/std.${MACHINE}
Standard configuration for the given architecture.
This file should always be included.
.It Pa arch/${MACHINE}/conf/GENERIC
Standard options selection file for the given architecture.
Users should always start changing their main kernel configuration file by
editing a copy of this file.
.It Pa conf/files
Main options description file.
.El
.Sh EXAMPLES
.Xr config.samples 5
uses several examples to cover all the practical aspects of writing or
modifying a kernel configuration file.
.Sh SEE ALSO
.Xr config 1 ,
.Xr options 4 ,
.Xr config.samples 5 ,
.Xr config 9