# Compiling the kernel
Most NetBSD users will sooner or later want to recompile their kernel, or
compile a customized kernel. This might be for several reasons:
* you can install bug-fixes, security updates, or new functionality by
rebuilding the kernel from updated sources.
* by removing unused device drivers and kernel sub-systems from your
configuration, you can dramatically reduce kernel size and, therefore, memory
* by enabling optimisations more specific to your hardware, or tuning the
system to match your specific sizing and workload, you can improve
* you can access additional features by enabling kernel options or sub-systems,
some of which are experimental or disabled by default.
* you can solve problems of detection/conflicts of peripherals.
* you can customize some options (for example keyboard layout, BIOS clock
* you can get a deeper knowledge of the system.
## Requirements and procedure
To recompile the kernel you must have installed the compiler set (`comp.tgz`).
The basic steps to an updated or customised kernel then are:
1. Install or update the kernel sources
2. Create or modify the kernel configuration file
3. Building the kernel from the configuration file, either manually or using
4. Install the kernel
## Installing the kernel sources
You can get the kernel sources from AnonCVS (see [[Obtaining the
sources|guide/fetch]]), or from the `syssrc.tgz` tarball that is located in the
`source/sets/` directory of the release that you are using.
If you chose to use AnonCVS to fetch the entire source tree, be patient, the
operation can last many minutes, because the repository contains thousands of
If you have a source tarball, you can extract it as root:
# cd /
# tar zxf /path/to/syssrc.tgz
Even if you used the tarball from the release, you may wish to use AnonCVS to
update the sources with changes that have been applied since the release. This
might be especially relevant if you are updating the kernel to include the fix
for a specific bug, including a vulnerability described in a NetBSD Security
Advisory. You might want to get the latest sources on the relevant release or
critical updates branch for your version, or Security Advisories will usually
contain information on the dates or revisions of the files containing the
specific fixes concerned. See [[Fetching by CVS|guide/fetch#cvs]] for more
details on the CVS commands used to update sources from these branches.
Once you have the sources available, you can create a custom kernel: this is not
as difficult as you might think. In fact, a new kernel can be created in a few
steps which will be described in the following sections.
## Creating the kernel configuration file
The directories described in this section are i386 specific. Users of other
architectures must substitute the appropriate directories, see the
subdirectories of `src/sys/arch` for a list.
The kernel configuration file defines the type, the number and the
characteristics of the devices supported by the kernel as well as several kernel
configuration options. For the i386 port, kernel configuration files are located
in the `/usr/src/sys/arch/i386/conf` directory.
Please note that the names of the kernel configuration files are historically in
all uppercase, so they are easy to distinguish from other files in that
$ cd /usr/src/sys/arch/i386/conf/
CARDBUS GENERIC_PS2TINY NET4501
CVS GENERIC_TINY SWINGER
DELPHI GENERIC_VERIEXEC SWINGER.MP
DISKLESS INSTALL VIRTUALPC
GENERIC INSTALL.MP files.i386
GENERIC.FAST_IPSEC INSTALL_LAPTOP kern.ldscript
GENERIC.MP INSTALL_PS2 kern.ldscript.4MB
GENERIC.MPDEBUG INSTALL_SMALL largepages.inc
GENERIC.local INSTALL_TINY majors.i386
GENERIC_DIAGNOSTIC IOPENER std.i386
The easiest way to create a new file is to copy an existing one and modify it.
Usually the best choice on most platforms is the GENERIC configuration, as it
contains most drivers and options. In the configuration file there are comments
describing the options; a more detailed description is found in the
man page. So, the usual procedure is:
$ cp GENERIC MYKERNEL
$ vi MYKERNEL
The modification of a kernel configuration file basically involves three operations:
1. support for hardware devices is included/excluded in the kernel (for
example, SCSI support can be removed if it is not needed.)
2. support for kernel features is enabled/disabled (for example, enable NFS
client support, enable Linux compatibility, ...)
3. tuning kernel parameters.
Lines beginning with `#` are comments; lines are disabled by commenting them
and enabled by removing the comment character. It is better to comment lines
instead of deleting them; it is always possible uncomment them later.
The output of the
command can be used to determine which lines can be disabled. For each line of
XXX at YYY
both `XXX` and `YYY` must be active in the kernel configuration file. You'll
probably have to experiment a bit before achieving a minimal configuration but
on a desktop system without SCSI and PCMCIA you can halve the kernel size.
You should also examine the options in the configuration file and disable the
ones that you don't need. Each option has a short comment describing it, which
is normally sufficient to understand what the option does. Many options have a
longer and more detailed description in the
man page. While you are at it you should set correctly the options for local
time on the CMOS clock. For example:
## Building the kernel manually
Based on your kernel configuration file, either one of the standard
configurations or your customised configuration, a new kernel must be built.
These steps can either be performed manually, or using the `build.sh` command
that was introduced in section [Chapter 31, *Crosscompiling NetBSD with
`build.sh`*](chap-build.html "Chapter 31. Crosscompiling NetBSD with build.sh").
This section will give instructions on how to build a native kernel using manual
steps, the following section
[[Building the kernel using build.sh|guide/kernel#building_the_kernel_using_build.sh]]
describes how to use **build.sh** to do the same.
* Configure the kernel
* Generate dependencies
* Compile the kernel
### Configuring the kernel manually
When you've finished modifying the kernel configuration file (which we'll call
`MYKERNEL`), you should issue the following command:
$ config MYKERNEL
If `MYKERNEL` contains no errors, the
program will create the necessary files for the compilation of the kernel,
otherwise it will be necessary to correct the errors before running
### Notes for crosscompilings
program used to create header files and Makefile for a kernel build is platform
specific, it is necessary to use the `nbconfig` program that's part of a newly
created toolchain (created for example with
/usr/src/build.sh -m sparc64 tools/
). That aside, the procedure is just as like compiling a "native" NetBSD kernel.
The command is for example:
% /usr/src/tooldir.NetBSD-4.0-i386/bin/nbconfig MYKERNEL
This command has created a directory `../compile/MYKERNEL` with a number of
header files defining information about devices to compile into the kernel, a
Makefile that is setup to build all the needed files for the kernel, and link
### Generating dependencies and recompiling manually
Dependencies generation and kernel compilation is performed by the following
$ cd ../compile/MYKERNEL
$ make depend
It can happen that the compilation stops with errors; there can be a variety of
reasons but the most common cause is an error in the configuration file which
didn't get caught by
Sometimes the failure is caused by a hardware problem (often faulty RAM chips):
the compilation puts a higher stress on the system than most applications do.
Another typical error is the following: option B, active, requires option A
which is not active. A full compilation of the kernel can last from some minutes
to several hours, depending on the hardware.
The result of a successful make command is the `netbsd` file in the compile
directory, ready to be installed.
### Notes for crosscompilings
For crosscompiling a sparc64 kernel, it is necessary to use the crosscompiler
toolchain's `nbmake-sparc64` shell wrapper, which calls
all the necessary settings for crosscompiling for a sparc64 platform:
% cd ../compile/MYKERNEL/
% /usr/src/tooldir.NetBSD-4.0-i386/bin/nbmake-sparc64 depend
This will churn away a bit, then spit out a kernel:
text data bss dec hex filename
5016899 163728 628752 5809379 58a4e3 netbsd
% ls -l netbsd
-rwxr-xr-x 1 feyrer 666 5874663 Dec 2 23:17 netbsd
% file netbsd
netbsd: ELF 64-bit MSB executable, SPARC V9, version 1 (SYSV), statically linked, not stripped
Now the kernel in the file `netbsd` can either be transferred to an UltraSPARC
machine (via NFS, FTP, scp, etc.) and booted from a possible harddisk, or
directly from the cross-development machine using NFS.
## Building the kernel using build.sh
After creating and possibly editing the kernel config file, the manual steps of
configuring the kernel, generating dependencies and recompiling can also be done
using the `src/build.sh` script, all in one go:
$ cd /usr/src
$ ./build.sh kernel=MYKERNEL
This will perform the same steps as above, with one small difference: before
compiling, all old object files will be removed, to start with a fresh build.
This is usually overkill, and it's fine to keep the old file and only rebuild
the ones whose dependencies have changed. To do this, add the `-u` option to
$ cd /usr/src
$ ./build.sh -u kernel=MYKERNEL
At the end of its job, `build.sh` will print out the location where the new
compiled kernel can be found. It can then be installed.
## Installing the new kernel
Whichever method was used to produce the new kernel file, it must now be
installed. The new kernel file should be copied to the root directory, after
saving the previous version.
# mv /netbsd /netbsd.old
# mv netbsd /
Customization can considerably reduce the kernel's size. In the following
example `netbsd.old` is the install kernel and `netbsd` is the new kernel.
-rwxr-xr-x 3 root wheel 3523098 Dec 10 00:13 /netbsd
-rwxr-xr-x 3 root wheel 7566271 Dec 10 00:13 /netbsd.old
The new kernel is activated after rebooting:
# shutdown -r now
## If something went wrong
When the computer is restarted it can happen that the new kernel doesn't work as
expected or even doesn't boot at all. Don't worry: if this happens, just reboot
with the previously saved kernel and remove the new one (it is better to reboot
* Reboot the machine
* Press the space bar at the boot prompt during the 5 seconds countdown
> boot netbsd.old -s
* Now issue the following commands to restore the previous version of the kernel:
# fsck /
# mount /
# mv netbsd.old netbsd
This will give you back the working system you started with, and you can revise
your custom kernel config file to resolve the problem. In general, it's wise to
start with a GENERIC kernel first, and then make gradual changes.
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