---
name: "config"
section: "5"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/config/5"
---

# config(5)

kernel configuration file format

## NAME

**config** – kernel configuration file format

## DESCRIPTION

A kernel configuration file specifies the configuration of a FreeBSD kernel. It is processed by [config(8)](/config/8.md) to create a build environment where a kernel may be built using [make(1)](/make/1.md).

### Lexical Structure

A kernel configuration file comprises a sequence of specification directives.

A specification directive starts with a keyword at the beginning of the line and is followed by additional parameters.

A specification directive may be terminated by a semicolon ‘;’ 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.

Case is significant, “machine” and “MACHINE” are different tokens.

A double quote character ‘"’ starts a quoted string. All characters up to the next quote character form the value of the quoted string. A ‘"’ character may be inserted into a quoted string by using the sequence ‘\\"’.

Numbers are specified using C-style syntax.

A ‘#’ character starts a comment; all characters from the ‘#’ character till the end of the current line are ignored.

Whitespace between tokens is ignored, except inside quoted strings. Whitespace following a comment line is ignored.

### 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.

The list of keywords and their meanings are as follows:

- **cpu** *cputype*

  Specify the CPU this kernel will run on. There can be more than one **cpu** directive in a configuration file. The allowed list of CPU names is architecture specific and is defined in the file *sys/conf/options*.⟨*arch*⟩.

- **device** *name* \[, *name* \[...\]\]

  - **devices** *name* \[, *name* \[...\]\]

  Configures the specified devices for inclusion into the kernel image. Devices that are common to all architectures are defined in the file *sys/conf/files*. Devices that are specific to architecture *arch* are defined in the file *sys/conf/files*.⟨*arch*⟩.

- **env** *filename*

  Specifies a filename containing a kernel environment definition.

  The kernel will augment this compiled-in environment with the environment prepared for it at boot time by [loader(8)](/loader/8.md). Environment variables specified in the [loader(8)](/loader/8.md) environment will take precedence over environment variables specified in *filename*, and environment variables specified in the dynamic environment take precedence over both of these.

  *loader\_env.disabled=1* may be specified in the static environment to disable the [loader(8)](/loader/8.md) environment. Disabling the [loader(8)](/loader/8.md) should be done with caution and due consideration for whether or not it supplies environment variables needed for properly booting the system.

  *static\_env.disabled=1* may be specified in the [loader(8)](/loader/8.md) environment to disable use of the static environment. This option has no effect if specified in any environment after the [loader(8)](/loader/8.md) environment is processed. This option is not usable in conjunction with *loader\_env.disabled*.

  This directive is useful for setting kernel tunables in embedded environments that do not start from [loader(8)](/loader/8.md).

  All **env** and **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 *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 *filename*.

- **envvar** *setting*

  Specifies an individual environment setting to be added to the kernel's compiled-in environment. *setting* must be of the form “*name=value*”. Optional quotes are supported in both name and value.

  All **env** and **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** *filename*

  Specifies a file containing a list of files specific to that kernel configuration file (a la *files*.⟨*arch*⟩).

- **hints** *filename*

  Specifies a file to load a static device configuration specification from. From FreeBSD 5.0 onwards, the kernel reads the system's device configuration at boot time (see [device.hints(5)](/device.hints/5.md)). This directive configures the kernel to use the static device configuration listed in *filename*.

  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 [loader(8)](/loader/8.md) takes precedence over hints in the compiled-in environment.

  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.

  *static\_hints.disabled=1* may be specified in either a compiled-in environment or the [loader(8)](/loader/8.md) environment to disable use of these hints files. This option has no effect if specified in any environment after the [loader(8)](/loader/8.md) environment is processed.

  The file *filename* must conform to the syntax specified by [device.hints(5)](/device.hints/5.md). 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** *name*

  Set the kernel name to *name*. At least one **ident** directive is required.

- **include** *filename*

  Read subsequent text from file *filename* and return to the current file after *filename* is successfully processed.

- **includeoptions** *filename*

  Specifies a file containing a list of additional options specific to that kernel configuration file. This is useful for build environments that need to add custom options and is often used in conjunction with the **makeoption** directive.

- **machine** *arch* \[*cpuarch*\]

  Specifies the architecture of the machine the kernel is being compiled for. Legal values for *arch* include:

  - **arm64**

    The 64-bit ARM application architecture.

  - **arm**

    The ARM architecture

  - **amd64**

    The AMD x86-64 architecture.

  - **i386**

    The Intel x86 based PC architecture.

  - **powerpc**

    The IBM PowerPC architecture.

  - **riscv**

    The RISC-V architecture.

    If argument *cpuarch* is specified, it points [config(8)](/config/8.md) to the cpu architecture of the machine. When *cpuarch* is not specified, it is assumed to be the same as *arch*. *arch* corresponds to MACHINE. *cpuarch* corresponds to MACHINE\_ARCH.

    A kernel configuration file may have only one **machine** directive, unless the second one matches the machine argument in the first one exactly.

- **makeoption** *options*

  - **makeoptions** *options*

  Add *options* to the generated makefile.

  The *options* argument is a comma separated list of one or more option specifications. Each option specification has the form

  *MakeVariableName*\[=*Value*\]

  *MakeVariableName*+=*Value*

  and results in the appropriate [make(1)](/make/1.md) variable definition being inserted into the generated makefile. If only the name of the [make(1)](/make/1.md) variable is specified, *value* is assumed to be the empty string.

  Note that, as the common makefiles overwrite the *CFLAGS* variable after having processed the configuration file, customizing *CFLAGS* directly via **makeoptions** is not possible. Nonetheless, custom compiler flags can be specified using the *CONF\_CFLAGS* variable instead. Its content is appended to *CFLAGS* after the common makefiles have set the latter, allowing to override their compilation flags.

  Example:

  ```
  makeoptions MYMAKEOPTION="foo"
  makeoptions MYMAKEOPTION+="bar"
  makeoptions MYNULLMAKEOPTION
  makeoptions CONF_CFLAGS+="-DSOME_CONTROLLING_MACRO"
  ```

- **maxusers** *number*

  This optional directive is used to configure the size of some kernel data structures. The parameter *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.

  As explained in [tuning(7)](/tuning/7.md), this tunable can also be set at boot time using [loader(8)](/loader/8.md).

- **nocpu** *cputype*

  Remove the specified CPU from the list of previously selected CPUs. This directive can be used to cancel the effect of **cpu** directives in files included using **include**.

- **nodevice** *name* \[, *name* \[...\]\]

  - **nodevices** *name* \[, *name* \[...\]\]

  Remove the specified devices from the list of previously selected devices. This directive can be used to cancel the effects of **device** or **devices** directives in files included using **include**.

- **nomakeoption** *name*

  - **nomakeoptions** *name*

  Removes previously defined [make(1)](/make/1.md) option *name* from the kernel build. This directive can be used to cancel the effects of **makeoption** directives in files included using **include**.

- **nooption** *name* \[, *name* \[...\]\]

  - **nooptions** *name* \[, *name* \[...\]\]

  Remove the specified kernel options from the list of previously defined options. This directive can be used to cancel the effects of **option** or **options** directives in files included using **include**.

- **option** *optionspec* \[, *optionspec* \[...\]\]

  - **options** *optionspec* \[, *optionspec* \[...\]\]

  Add compile time kernel options to the kernel build. Each option specification has the form

  *name*\[=*value*\]

  If *value* is not specified, it is assumed to be NULL. Options common to all architectures are specified in the file *sys/conf/options*. Options specific to architecture *arch* are specified in the file *sys/conf/options*.⟨*arch*⟩.

## FILES

- *sys/compile/**NAME*

  Compile directory created from a kernel configuration.

- *sys/conf/Makefile*.*arch*

  *Makefile* fragments for architecture *arch*.

- *sys/conf/files*

  Devices common to all architectures.

- *sys/conf/files*.*arch*

  Devices for architecture *arch*.

- *sys/conf/options*

  Options common to all architectures.

- *sys/conf/options*.*arch*

  Options for architecture *arch*.

## SEE ALSO

[kenv(1)](/kenv/1.md), [make(1)](/make/1.md), [device.hints(5)](/device.hints/5.md), [loader.conf(5)](/loader.conf/5.md), [config(8)](/config/8.md), [kldload(8)](/kldload/8.md), [loader(8)](/loader/8.md) Samuel J. Leffler and Michael J. Karels, *Building 4.4BSD Kernels with Config*.

## HISTORY

The [config(8)](/config/8.md) utility first appeared in 4.1BSD, and was subsequently revised in 4.4BSD.

The kernel configuration mechanism changed further in FreeBSD 4.0 and FreeBSD 5.0, moving toward an architecture supporting dynamic kernel configuration.