---
name: "freebsd-config"
section: "5"
platform: "linux"
package: "freebsd-buildutils"
version: "10.3~svn296373-7"
release: "Debian 10 (Buster)"
platforms: ["linux"]
url: "https://new.carta.tech/freebsd-config/5"
---

# freebsd-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 normally uses an environment prepared for it at boot time by [loader(8)](/loader/8.md). This directive makes the kernel ignore the boot environment and use the compiled-in environment instead.

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

- **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*. 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 the order of appearance.

- **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.

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

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

  - **alpha**

    The DEC Alpha architecture.

  - **arm**

    The ARM architecture.

  - **amd64**

    The AMD x86-64 architecture.

  - **i386**

    The Intel x86 based PC architecture.

  - **ia64**

    The Intel IA64 architecture.

  - **mips**

    The MIPS architecture.

  - **pc98**

    The PC98 architecture.

  - **powerpc**

    The IBM PowerPC architecture.

  - **sparc64**

    The Sun Sparc64 architecture.

    If argument *cpuarch* is specified, it points [config(8)](/config/8.md) to the cpu architecture of the machine. Currently the **pc98** architecture requires its cpu architecture to be set to **i386**. 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.

- **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.

  Example:

  ```
  makeoptions MYMAKEOPTION="foo"
  makeoptions MYMAKEOPTION+="bar"
  makeoptions MYNULLMAKEOPTION
  ```

- **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.

  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*⟩.

- **profile** *number*

  Enables kernel profiling if *number* is non-zero. If *number* is 2 or greater, the kernel is configured for high-resolution profiling. Kernels can also be built for profiling using the **-p** option to [config(8)](/config/8.md).

### Obsolete Directives

The following kernel configuration directives are obsolete.

- **config**

  This directive was used to specify the device to be used for the root file system. From FreeBSD 4.0 onwards, this information is passed to a booting kernel by [loader(8)](/loader/8.md).

## 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.