CONFIG(5) | File Formats Manual | CONFIG(5)

# NAME

`config` — kernel configuration file syntax

# DESCRIPTION

The kernel configuration file specifies the way the kernel should be compiled by the rest of the toolchain. It is processed by 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.

This manual page is intended to serve as a complete reference of all aspects of the syntax used in the many files processed by config(1). The novice user will prefer looking at the examples given in 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.

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 `machine` statement, usually found in the 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 conf/files.

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 config(1), and a few statements belong to any of the two sections.

The user controls the options selection part, which is located in a file commonly referenced as the main configuration file or simply the kernel configuration file. The developer is responsible for describing the options in the relevant files from the kernel source tree.

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.

## OBJECTS AND NAMES

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.

## Options and attributes

The basic objects driving the kernel compilation are options, and are called 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.

There is a special class of attribute, named 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 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 pci(4) device are ‘`dev`’ (for device), and ‘`function`’.

A 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 ‘`?`’ in the options selection section of the configuration file. A single locator definition can take one of the following forms:

- locator
- locator`=`value
- `[`locator`=`value`]`
- locator`[`length`]`
- locator`[`length`]={`value`,` ...`}`
- `[`locator`[`length`]={`value`,` ...`}]`

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.

In the options selection section, the locators are specified when declaring an instance as a space-separated list of “⟨locator⟩ ⟨value⟩” where value can be the ‘`?`’ wildcard if the locator allows it.

## 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.

Each driver has a name for its devices. It is called the base device name and is found as “base” in this documentation. An 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 ‘`*`’ or a ‘`?`’) in order to match all the possible instances of a device.

The usual ‘`*`’ becomes a ‘`?`’ when the instance name is used as an attachment name. In the options selection part of the kernel configuration files, an attachment is an interface attribute concatenated with a number or the wildcard ‘`?`’.

## 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 tun(4) and tap(4). They are integrated in the kernel as pseudo-devices, and can have several instances and even children, just like normal devices.

## 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 config(1) objects.

In this documentation, the syntax for dependencies is a comma-separated list of options and attributes .

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 ‘`ether`’ attribute.

## Conditions

Finally, source file selection is possible through the help of conditionals, referred to as “condition” later in this documentation. The syntax for those conditions uses well-known operators (‘`&`’, ‘`|`’, and ‘`!`’) to combine options and attributes .

## CONTEXT NEUTRAL STATEMENTS

`version` yyyymmdd
  Indicates the syntax version used by the rest of the file, or until the next `version` statement. The argument is an ISO date. A given config(1) binary might only be compatible with a limited range of version numbers.

`include` path
  Includes a file. The path is relative to the top of the kernel source tree, or the inner-most defined `prefix`.

`cinclude` path
  Conditionally includes a file. Contrary to `include`, it will not produce an error if the file does not exist. The argument obeys the same rules as for `include`.

`prefix` [path]
  If path is given, it pushes a new prefix for `file`, `include` and `cinclude`. `prefix` statements act like a stack, and an empty path argument has the latest prefix popped out. The path argument is either absolute or relative to the current defined prefix, which defaults to the top of the kernel source tree.

`buildprefix` [path]
  If path is given, it pushes a new build prefix for `file`. `buildprefix` statements act like a stack, and an empty path argument has the latest prefix popped out. The 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 (../), buildprefix must be defined.

`ifdef` attribute
`ifndef` attribute
`elifdef` attribute
`elifndef` attribute
`else`
`endif`
  Conditionally interprets portions of the current file. Those statements depend on whether or not the given attribute has been previously defined, through `define` or any other statement that implicitly defines attributes such as `device`.

## PREAMBLE

In addition to `include`, `cinclude`, and `prefix`, the preamble may contain the following optional statements:

`build` path
  Defines the build directory for the compilation of the kernel. It replaces the default of ../compile/⟨config-file⟩ and is superseded by the `-b` parameter of config(1).
`source` path
  Defines the directory in which the source of the kernel lives. It replaces the default of ../../../.. and is superseded by the `-s` parameter of config(1).

## 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 NetBSD kernel will have to be added to the user-edited configuration file.

`devclass` class
  Defines a special attribute, named device class. A given device cannot belong to more than one device class. 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.

`defflag` [file] option [option [...]] [`:` dependencies]
  Defines a boolean option, that can either be selected or be un-selected by the user with the `options` statement. The optional 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 `opt_`⟨option⟩`.h`. 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.

`defparam` [file] option[`=`value] [`:=`lint-value] [option [...]] [`:` dependencies]
  Behaves like `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 value is specified, it is treated as a default, and the option is always defined in the corresponding header file. If a lint-value is specified, config(1) will use it as a value when generating a lint configuration with `-L`, and ignore it in all other cases.

`deffs` name ...
  Defines a file-system name. It is no more than a regular option, as defined by `defflag`, but it allows the user to select the file-systems to be compiled in the kernel with the `file-system` statement instead of the `options` statement.

`obsolete defflag` [file] option ...
`obsolete defparam` [file] 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, config(1) will emit a warning and ignore the option. The optional file argument should match the original definition of the option.

`mkflagvar` option ...
  Specifes that an option previously defined with `defflag` should have a variable created in the kernel Makefile if the option is selection with an `options` statement. No variable is created if the option is not selected. The Makefile variable will have the name `KERNEL_OPT_`⟨option⟩ and, because options defined with `defflag` are boolean, the variable will have the value “1”.

`define` attribute [`{` locators `}`] [`:` dependencies]
  Defines an attribute. The 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 interface attribute, on which devices can attach.

`maxpartitions` 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 std.${ARCH} file.

`maxusers` min default max
  Indicates the range of values that will later be accepted by config(1) for the `maxusers` statement in the options selection part of the configuration file. In case the user doesn't include a `maxusers` statement in the configuration file, the value default is used instead.

`device` base [`{` locators `}`] [`:` dependencies]
  Declares a device of name base. The optional list of locators, which can also be empty, indicates the device can have children attached directly to it. Internally, that means base becomes an interface attribute. For every device the user selects, config(1) will add the matching `CFDRIVER_DECL`() statement to ioconf.c. However, it is the responsibility of the developer to add the relevant `CFATTACH_DECL_NEW`() line to the source of the device's driver.

`attach` base `at` attr[`,` attr[`,` ...]] [`with` name] [`: dependencies`]
  All devices must have at least one declared attachment. Otherwise, they will never be found in the autoconf(9) process. The attributes on which an instance of device base can attach must be interface attributes, or `root` in case the device is at the top-level, which is usually the case of e.g., mainbus(4). The instances of device base will later attach to one interface attribute from the specified list.

  Different `attach` definitions must use different names using the `with` option. It is then possible to use the associated name as a conditional element in a `file` statement.

`defpseudo` base [`:` 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.

`defpseudodev` base [`{` locators `}`] [`:` 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 autoconf(9) functions to manage its instances or attach children. As for normal devices, an optional list of locators can be defined, which implies an interface attribute named base, allowing the pseudo-device to have children. Interface attributes can also be defined in the dependencies list.

`file` path [condition] [`needs-count`] [`needs-flag`] [`compile with` rule]
  Adds a source file to the list of files to be compiled into the kernel, if the condition is met. The `needs-count` option indicates that the source file requires the number of all the countable objects it depends on (through the condition) to be defined. It is usually used for pseudo-devices whose number can be specified by the user in the `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 `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.

  Both `needs-count` and `needs-flag` produce a header file for each of the considered attributes. The name of that file is ⟨attribute⟩.h. It contains one pre-processor definition of `NATTRIBUTE` set to 0 if the attribute was not selected by the user, or to the number of instances of the device in the `needs-count` case, or to 1 in all the other cases.

  The rule argument specifies the 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 `file` statement, but not from the same configuration source file, and all later statements can only specify a rule argument, and no condition or flags. This is useful when a file needs special consideration from one particular architecture.

  The path is relative to the top of the kernel source tree, or the inner-most defined `prefix`.

`object` path [condition]
  Adds an object file to the list of objects to be linked into the kernel, if the conditions are met. This is most useful for third parties providing binary-only components.

  The path is relative to the top of the kernel source tree, or the inner-most defined `prefix`.

`device-major` base [`char` number] [`block` number] [condition]
  Associates a major device number with the device base. A device can be a character device, a block device, or both, and can have different numbers for each. The condition indicates when the relevant line should be added to ioconf.c, and works just like the `file` statement.

`makeoptions` condition name`+=`value[`,` condition name`+=`value[`,` ...]]
  Appends to a definition in the generated Makefile.

  This variant of `makeoptions` belongs to the options description section. The condition is mandatory and only `+=` can be used. Not to be confused with the confusingly similar variant of `makeoptions` used in the selections section.

## OPTIONS SELECTION

`machine` machine [arch [subarch [...]]]
  The `machine` statement should appear first in the kernel configuration file, with the exception of context-neutral statements. It makes config(1) include, in that order, the following files:
  1. conf/files
  2. arch/${ARCH}/conf/files.${ARCH} if defined
  3. arch/${SUBARCH}/conf/files.${SUBARCH} for each defined sub-architecture
  4. arch/${MACHINE}/conf/files.${MACHINE}

  It also defines an attribute for the machine, the arch and each of the subarch.

`ioconf` name
  Used instead of `machine` in config file snippets for kernel modules to generate just the ioconf.c definitions. The generated file contains subtrees that start with attachments declared with `pseudo-root`.
`package` path
  Simpler version of:

      prefix DIR
      include FILE
      prefix

`ident` 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.
`no ident`
  Deletes any pre-existing identification string of the kernel.
`maxusers` 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 `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.
`options` name[`=`value][`,` name[`=`value][`,` ...]]
  Selects the option name, affecting it a value if the options requires it (see the `defflag` and `defparam` statements).

  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 `options` statement replaces the original.

`no options` name[`,` name[`,` ...]]
  Un-selects the option name. If option name has not previously been selected, the statement produces a warning.
`file-system` name[`,` name[`,` ...]]
  Adds support for all the listed file-systems.
`no file-system` name[`,` name[`,` ...]]
  Removes support for all the listed file-systems.
`config` name `root on` device [`type` fs] [`dumps on` device]
  Adds name to the list of kernel binaries to compile from the configuration file, using the specified root and dump devices information.

  Any of the device and fs parameters can be wildcarded with ‘`?`’ to let the kernel automatically discover those values. The 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., “`wedge:`NAME” specifies a named disk wedge.

  At least one `config` statement must appear in the configuration file.

`no config` name
  Removes name from the list of kernel binaries to compile from the configuration file.
`pseudo-root` instance
  Used in conjunction with `ioconf` in kernel modules. Declares an instance of a device that serves as the root for a subtree described in the module config. The declared instance can then be used as an attachment for devices in the kernel module. Multiple such attachment points may be declared in the config file. instance may name a device (e.g. ‘`pci*`’) an interface attribute (e.g. ‘`audiobus*`’)
instance `at` attachment [locator-specifications ...]
  Configures an instance of a device attaching at a specific location in the device tree. All parameters can be wildcarded, with a ‘`*`’ for instance, and a ‘`?`’ for attachment and the locators.
`no` instance [`at` 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 attachment is specified, all matching instances are removed.

  If instance is a bare device name, all the previously defined instances of that device, regardless of the numbers or wildcard, are removed.

`no device at` attachment
  Removes all previously configured instances that attach to the specified attachment. If attachment ends with a ‘`*`’, all instances attaching to all the variants of attachment are removed.
`pseudo-device` device [number]
  Adds support for the specified pseudo-device. The parameter 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.
`no pseudo-device` name
  Removes support for the specified pseudo-device.
`makeoptions` name`=`value[`,` name`+=`value[`,` ...]]
  Adds or appends to a definition in the generated Makefile. A definition cannot be overridden, it must be removed before it can be added again. Optionally, if an option `makeoptions_`⟨name⟩ is defined with `defparam`, the value is defined as an option too.

  This variant of `makeoptions` belongs to the options selection section. Both `=` and `+=` can be used. Not to be confused with the confusingly similar variant of `makeoptions` used in the descriptions section.

`no makeoptions` name[`,` name[`,` ...]]
  Removes one or more definitions from the generated Makefile.
`select` name
  Adds the specified attribute and its dependencies.
`no select` name
  Removes the specified attribute and all the attributes which depend on it.

# FILES

The files are relative to the kernel source top directory (e.g., /usr/src/sys).

arch/${MACHINE}/conf/std.${MACHINE}
  Standard configuration for the given architecture. This file should always be included.
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.
conf/files
  Main options description file.

# EXAMPLES

config.samples(5) uses several examples to cover all the practical aspects of writing or modifying a kernel configuration file.

# SEE ALSO

config(1), options(4), config.samples(5), config(9)

August 13, 2024 | NetBSD 11.0