CONFIG(8) FreeBSD System Manager's Manual CONFIG(8)


configbuild system configuration files


config [ -CVgp][ -I path][ -d destdir] SYSTEM_NAME

config [ -x kernel]


The config utility builds a set of system configuration files from the file SYSTEM_NAME which describes the system to configure. A second file tells config what files are needed to generate a system and can be augmented by configuration specific set of files that give alternate files for a specific machine (see the FILES section below).

Available options and operands:

Print the config version number.
If the INCLUDE_CONFIG_FILE is present in a configuration file, kernel image will contain full configuration files included literally (preserving comments). This flag is kept for backward compatibility.
-I path
Search in path for any file included by the include directive. This option may be specified more than once.
-d destdir
Use destdir as the output directory, instead of the default one. Note that config does not append SYSTEM_NAME to the directory given.
Print the MACHINE and MACHINE_ARCH values for this kernel and exit.
Configure a system for debugging.
-x kernel
Print kernel configuration file embedded into a kernel file. This option makes sense only if options INCLUDE_CONFIG_FILE entry was present in your configuration file.
Configure a system for profiling; for example, kgmon(8) and gprof(1). If two or more -p options are supplied, config configures a system for high resolution profiling.
Specify the name of the system configuration file containing device specifications, configuration options and other system parameters for one system configuration.

The config utility should be run from the conf subdirectory of the system source (usually /sys/ARCH/conf), where ARCH represents one of the architectures supported by FreeBSD. The config utility creates the directory ../compile/SYSTEM_NAME or the one given with the -d option as necessary and places all output files there. The output of config consists of a number of files; for the i386, they are: Makefile, used by make(1) in building the system; header files, definitions of the number of various devices that will be compiled into the system.

After running config, it is necessary to run “ make depend” in the directory where the new makefile was created. The config utility prints a reminder of this when it completes.

If any other error messages are produced by config, the problems in the configuration file should be corrected and config should be run again. Attempts to compile a system that had configuration errors are likely to fail.


Traditional BSD kernels are compiled without symbols due to the heavy load on the system when compiling a “debug” kernel. A debug kernel contains complete symbols for all the source files, and enables an experienced kernel programmer to analyse the cause of a problem. The debuggers available prior to 4.4BSD-Lite were able to find some information from a normal kernel; gdb(1) provides very little support for normal kernels, and a debug kernel is needed for any meaningful analysis.

For reasons of history, time and space, building a debug kernel is not the default with FreeBSD: a debug kernel takes up to 30% longer to build and requires about 30 MB of disk storage in the build directory, compared to about 6 MB for a non-debug kernel. A debug kernel is about 11 MB in size, compared to about 2 MB for a non-debug kernel. This space is used both in the root file system and at run time in memory. Use the -g option to build a debug kernel. With this option, config causes two kernel files to be built in the kernel build directory:

  • kernel.debug is the complete debug kernel.
  • kernel is a copy of the kernel with the debug symbols stripped off. This is equivalent to the normal non-debug kernel.

There is currently little sense in installing and booting from a debug kernel, since the only tools available which use the symbols do not run on-line. There are therefore two options for installing a debug kernel:

  • make install” installs kernel in the root file system.
  • make install.debug” installs kernel.debug in the root file system.


list of common files system is built from
/sys/conf/Makefile. ARCH
generic makefile for the ARCH
/sys/conf/files. ARCH
list of ARCH specific files
/sys/ ARCH /compile/ SYSTEM_NAME
default kernel build directory for system SYSTEM_NAME on ARCH.



The SYNOPSIS portion of each device in section 4.

Building 4.3 BSD UNIX System with Config.


The config utility appeared in 4.1BSD.

Before support for -x was introduced, options INCLUDE_CONFIG_FILE included entire configuration file that used to be embedded in the new kernel. This meant that strings(1) could be used to extract it from a kernel: to extract the configuration information, you had to use the command:

strings -n 3 kernel | sed -n 's/^___//p'


The line numbers reported in error messages are usually off by one.
May 8, 2007 FreeBSD