config -- kernel configuration file format
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 [Toc] [Back]
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 [Toc] [Back]
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
device NAME [count]
Configures device NAME for inclusion into the kernel image. If
count is specified, the device is configured for count instances.
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>.
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 instead.
This directive is useful for setting kernel tunables in embedded
environments that do not start from loader(8).
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).
Set the kernel name to NAME. At least one ident directive is
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 include:
alpha The DEC Alpha architecture.
amd64 The AMD x86-64 architecture.
i386 The Intel x86 based PC architecture.
ia64 The Intel IA64 architecture.
pc98 The PC98 architecture.
powerpc The IBM PowerPC architecture.
sparc64 The Sun Sparc64 architecture.
A kernel configuration file may have only one machine directive.
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
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
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).
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.
Remove kernel option kerneloptionname from the list of previously
defined options. This directive can be used to cancel the
effects of options directives in files included using include.
Add compile time kernel options to the kernel build. The argument
optionspecs is a comma separated list of option specifications.
Each option specification has the form
If OptionValue 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>.
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
Obsolete Directives [Toc] [Back]
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).
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.
kenv(1), make(1), device.hints(5), loader.conf(5), config(8), kldload(8),
Samuel J. Leffler and Michael J. Karels, Building 4.4BSD Kernels with
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.
FreeBSD 5.2.1 July 3, 2003 FreeBSD 5.2.1 [ Back ]