configuration file format
A kernel configuration file specifies the configuration of a
kernel. It is processed by
to create a build environment where a
kernel may be built using make(1)
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, “
” 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 ‘
may be inserted into a quoted string by using the sequence
Numbers are specified using C-style syntax.
’ 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.
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:
- 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
- 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
- Specifies a filename containing a kernel environment
definition. The kernel normally uses an environment prepared for it at
boot time by loader(8). This directive makes
the kernel ignore the boot environment and use the compiled-in environment
This directive is useful for setting kernel tunables in embedded
environments that do not start from
- Specifies a file containing a list of files specific to
that kernel configuration file (a la
- 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. 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
the order of appearance.
- Set the kernel name to
name. At least one
ident directive is required.
- Read subsequent text from file
filename and return to the current file
after filename is successfully processed.
- Specifies the architecture of the machine the kernel is
being compiled for. Legal values for arch
If argument cpuarch is specified, it points
config(8) to the cpu architecture of the
machine. Currently the pc98 architecture
requires its cpu architecture to be set to
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
- The DEC Alpha architecture.
- The ARM architecture.
- The AMD x86-64 architecture.
- The Intel x86 based PC architecture.
- The Intel IA64 architecture.
- The MIPS architecture.
- The PC98 architecture.
- The IBM PowerPC architecture.
- The Sun Sparc64 architecture.
- Add options to the
The options argument is a comma separated
list of one or more option specifications. Each option specification has
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.
- 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), this tunable can
also be set at boot time using loader(8).
- 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
- Remove the specified devices from the list of previously
selected devices. This directive can be used to cancel the effects of
devices directives in files included using
- Removes previously defined
name from the kernel build. This
directive can be used to cancel the effects of
makeoption directives in files included using
- 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
- Add compile time kernel options to the kernel build. Each
option specification has the form
If value is not specified, it is assumed to
NULL. Options common to all
architectures are specified in the file
sys/conf/options. Options specific to
architecture arch are specified in the
- 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
The following kernel configuration directives are obsolete.
- 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
- Compile directory created from a kernel configuration.
- Makefile fragments for
- Devices common to all architectures.
- Devices for architecture
- Options common to all architectures.
- Options for architecture
Samuel J. Leffler and
Michael J. Karels, Building 4.4BSD
Kernels with Config.
utility first appeared in
, and was subsequently revised in
The kernel configuration mechanism changed further in FreeBSD
and FreeBSD 5.0
, moving toward an
architecture supporting dynamic kernel configuration.