CONFIG(5)

NAME
config - kernel configuration file format

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

  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). Environment variables specified in the loader(8) 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) environment. Disabling the loader(8) 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) environment to disable use of the static environment. This option has no effect if specified in any environment after the loader(8) 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).

    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)). 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) 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) environment to disable use of these hints files. This option has no effect if specified in any environment after the loader(8) environment is processed.

    The file filename must conform to the syntax specified by 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 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) 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) variable definition being inserted into the generated makefile. If only the name of the make(1) 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), this tunable can also be set at boot time using loader(8).

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) 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), make(1), device.hints(5), loader.conf(5), config(8), kldload(8), loader(8) Building 4.4BSD Kernels with Config, Samuel J. Leffler, Michael J. Karels.

HISTORY
The config(8) 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.