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


configbuild kernel compilation directories or modify a kernel


config [-p] [-b builddir] [-s srcdir] [config-file]

config [-u] [-f | -o outfile] -e infile


In the first synopsis form, the config program creates a kernel build directory from the kernel configuration file specified by config-file.
In the second synopsis form, config allows editing of the kernel binary specified by infile. Devices may be enabled, disabled, or modified without recompiling, by editing the kernel executable. Similarly, the same editing can be done at boot-time, using the in-kernel editor, as described in boot_config(8).
For kernel building, the options are as follows:
-b builddir
Create the build directory in the path specified by builddir instead of the default ../compile/SYSTEMNAME.
Configure for a system that includes profiling code; see kgmon(8) and gprof(1). When this option is specified, config acts as if the lines “makeoptions PROF="-pg"” and “option GPROF” appeared in the specified kernel configuration file. In addition, “.PROF” is appended to the default compilation directory name.
The -p flag is expected to be used for “one-shot” profiles of existing systems; for regular profiling, it is probably wiser to make a separate configuration containing the makeoptions line.
-s srcdir
Use srcdir as the top-level kernel source directory instead of the default (four directories above the build directory).
For kernel modification, the options are as follows:
Allows the modification of kernel device configuration (see boot_config(8)). Temporary changes can be made to the running kernel's configuration or a new kernel binary may be written for permanent changes between system reboots. See the section KERNEL MODIFICATION below for more details.
Overwrite the infile kernel binary with the modified kernel. Otherwise, -o should be given to specify an alternate output file.
-o outfile
Write the modified kernel to outfile.
Check to see if the kernel configuration was modified at boot-time (i.e. boot -c was used). If so, compare the running kernel with the kernel to be edited (infile). If they seem to be the same, apply all configuration changes performed at boot.


The output of config consists of a number of files, principally ioconf.c (a description of I/O devices that may be attached to the system) and a Makefile, used by make(1) when building the kernel.
If config stops due to errors, the problems reported should be corrected and config should be run again. config attempts to avoid changing the compilation directory if there are configuration errors, but this code is not well-tested and some problems (such as running out of disk space) are unrecoverable.
If config-file is not specified, config uses the current directory as the build directory, and looks in it for a file called CONFIG. If config is run this way, the location of the top-level kernel source directory must be specified using the -s option or by using the “source” directive at the beginning of the system configuration file.
The configuration files consists of various statements which include the following:
machine var
Required. Specifies the machine architecture.
include file
Include another configuration file.
option name
Set a kernel option. Kernel options may take either the form NAME or the form NAME=value. These options are passed to the compiler with the -D flag.
rmoption name
Delete a previously set option. This is useful when including another kernel configuration file. A typical use is to include the GENERIC kernel provided with each release and remove options that are unwanted, thus allowing for automatic inclusion of new device drivers.
maxusers number
Required. Used to size various system tables and maximum operating conditions in an approximate fashion. Multiple instances of this keyword may be specified. The number provided in the last instance will be used, and warnings will be printed for each duplicate value. This is convenient when used with the include directive.
config bsd root on dev [swap on dev [and dev ...]] [dumps on dev [and dev ...]]
Required. Specifies the swap and dump devices which the system should use.
config bsd swap generic
Otherwise, if generic is specified, the system follows generic routines to decide what should happen.
To debug kernels and their crash dumps with gdb, add “makeoptions DEBUG="-g"” to the kernel configuration file, along with (typically) “option KGDB”. Refer to options(4) for further details.
Many other statements exist, and the file format is fairly rich; for more information see the various configuration files included in the system, as well as files.conf(5) for the config rules base.


When -e is specified, device parameters that are normally hard-coded into the kernel may be changed. This is useful to avoid the need for kernel recompilation or rebooting. Modifications are made to the currently running kernel and can be written to a new kernel binary so changes are preserved during subsequent system restarts.
When invoked, the kernel identification is first shown.
# config -e -o bsd.new /bsd 
OpenBSD 5.3-current (GENERIC.MP) #91: Mon Mar 25 16:43:17 MDT 2013 
Enter 'help' for information 
One or more warnings may be printed before the ukc> prompt.
warning: no output file specified
Neither the -f nor -o option has been specified. Changes will be ignored.
WARNING kernel mismatch. -u ignored. 
WARNING the running kernel version:
config does not believe the running kernel is the same as the infile specified. Since the log of changes (from boot -c) in the running kernel is kernel-specific, the -u option is ignored.
The commands are as follows:
add dev
Add a device through copying another.
base 8 | 10 | 16
Change the base of numbers displayed and entered.
bufcachepercent [number]
Change the BUFCACHEPERCENT value. Without arguments, displays its current value.
change devno | dev
Modify one or more devices.
disable attr val | devno | dev
Disable one or more devices.
enable attr val | devno | dev
Enable one or more devices.
Exit without saving changes.
find devno | dev
Find one or more devices.
Give a short summary of all commands and their arguments.
lines [count]
Set the number of rows per page.
Show all known devices, a screen at a time.
nkmempg [number]
Change the NKMEMPAGES value. Without arguments, displays its current value.
Exit and save changes.
show [attr [val]]
Show all devices for which attribute attr has the value val.
timezone [minuteswest [dst]]
Change the tz timezone structure. minuteswest is the number of minutes west of GMT and dst is non-zero if Daylight Saving Time is in effect. Without arguments, displays its current value.

EXAMPLES (kernel building)

Note: The standard OpenBSD kernel configuration (GENERIC or GENERIC.MP) is suitable for most purposes. Use of an alternative kernel configuration is not recommended. A custom kernel is built in the following way.
To compile a kernel from a non-writable media (such as a CD-ROM) mounted on /usr/src, do the following:
# cd /somedir 
# cp /usr/src/sys/arch/somearch/conf/SOMEFILE . 
# vi SOMEFILE	(to make any changes) 
# config -s /usr/src/sys -b . SOMEFILE 
# make
To compile a kernel inside a writable source tree, do the following:
# cd /usr/src/sys/arch/somearch/conf 
# vi SOMEFILE	(to make any changes) 
# config SOMEFILE 
# cd ../compile/SOMEFILE 
# make
In the examples above, somedir is a writable directory, somearch is the architecture (e.g. i386), and SOMEFILE should be a name indicative of a particular configuration (often that of the hostname). config will warn if a “make clean” is required.
The new kernel, called bsd, can be copied to /bsd and the system will boot it next time. Most people save their backup kernels as /bsd.1, /bsd.2, etc.

EXAMPLES (kernel modification)

The Ethernet card is not detected at boot because the kernel configuration does not match the physical hardware configuration, e.g. wrong IRQ in OpenBSD/i386. The Ethernet card is supposed to use the ne(4) driver.
ukc> find ne 
24 ne0 at isa0 port 0x240 size 0 iomem 0xd8000 iosiz 0 irq 9 drq -1 drq2 -1 flags 0x0 
25 ne1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0 
26 ne* at isapnp0 port -1 size 0 iomem -1 iosiz 0 irq -1 drq -1 flags 0x0 
27 ne* at pci* dev -1 function -1 flags 0x0 
28 ne* at pcmcia* function -1 irq -1 flags 0x0 
ne1 seems to match the configuration except it uses IRQ 10 instead of IRQ 5. So the irq on ne1 should be changed via the change command. The device can be specified by either name or number.
ukc> change ne1 
25 ne1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 10 drq -1 drq2 -1 
change (y/n) ? y 
port [0x300] ? 
size [0] ? 
iomem [-1] ? 
iosiz [0] ? 
irq [10] ? 5 
drq [-1] ? 
drq2 [-1] ? 
flags [0] ? 
25 ne1 changed 
25 ne1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 5 drq -1 drq2 -1 flags 0x0 
It's also possible to disable all devices with a common attribute. For example:
ukc> disable port 0x300 
 25 ne1 disabled 
 72 we1 disabled 
 75 el0 disabled 
 77 ie1 disabled
The show command is useful for finding which devices have a certain attribute. It can also be used to find those devices with a particular value for an attribute.
ukc> show slot 
  2 ahc* at eisa0 slot -1 
 10 uha* at eisa0 slot -1 
 12 ep0 at eisa0 slot -1 
 17 ep* at eisa0 slot -1 
102 ahb* at eisa0 slot -1 
103 fea* at eisa0 slot -1 
ukc> show port 0x300 
 25 ne1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0 
 72 we1 at isa0 port 0x300 size 0 iomem 0xcc000 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0 
 75 el0 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 9 drq -1 drq2 -1 flags 0x0 
 77 ie1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0 
It is possible to add new devices, but only devices that were linked into the kernel. If a new device is added, following devices will be renumbered.
ukc> find ep 
 11 ep0 at isa0 port -1 size 0 iomem -1 iosiz 0 irq -1 drq -1 drq2 -1 flags 0x0 
 12 ep0 at eisa0 slot -1 flags 0x0 
 13 ep0 at pci* dev -1 function -1 flags 0x0 
 14 ep* at isapnp0 port -1 size 0 iomem -1 iosiz 0 irq -1 drq -1 flags 0x0 
 15 ep* at isa0 port -1 size 0 iomem -1 iosiz 0 irq -1 drq -1 drq2 -1 flags 0x0 
 16 ep* at eisa0 slot -1 flags 0x0 
 17 ep* at pci* dev -1 function -1 flags 0x0 
 18 ep* at pcmcia* dev -1 irq -1 flags 0x0 
ukc> add ep1 
Clone Device (DevNo, 'q' or '?') ? 13 
Insert before Device (DevNo, 'q' or '?') 14 
 14 ep1 at pci* dev -1 function -1 
ukc> change 14 
 14 ep1 at pci* dev -1 function -1 
change (y/n) ? y 
dev [-1] ? 14 
function [-1] ? 
flags [0] ? 18 
 14 ep1 changed 
 14 ep1 at pci* dev 14 function -1 flags 0x12 
When done, exit the program with the quit or exit commands. exit will ignore any changes while quit writes the changes to outfile (if -o or -f was given, else ignore changes).
ukc> quit


options(4), files.conf(5), boot.conf(8), boot_config(8)
The SYNOPSIS portion of each device in section 4 of the manual.
Building 4.4 BSD Systems with Config.


The config program appeared in 4.1BSD and was completely revised in 4.4BSD. The -e option appeared in OpenBSD 2.6.


Included files should start with an empty line or comment.
March 13, 2016 OpenBSD-current