23.6. Rebuilding World

Once the local source tree is synchronized against a particular version of FreeBSD such as FreeBSD-STABLE or FreeBSD-CURRENT, the source tree can be used to rebuild the system. This process is known as rebuilding world.

Before rebuilding world, be sure to perform the following tasks:

Procedure 23.1. Perform These Tasks Before Building World
  1. Backup all important data to another system or removable media, verify the integrity of the backup, and have a bootable installation media at hand. It cannot be stressed enough how important it is to make a backup of the system before rebuilding the system. While rebuilding world is an easy task, there will inevitably be times when mistakes in the source tree render the system unbootable. You will probably never have to use the backup, but it is better to be safe than sorry!

  2. Review the recent freebsd-stable or freebsd-current entries, depending upon the branch being tracked. Be aware of any known problems and which systems are affected. If a known issue affects the version of synchronized code, wait for an all clear announcement to be posted stating that the problem has been solved. Resynchronize the sources to ensure that the local version of source has the needed fix.

  3. Read /usr/src/UPDATING for any extra steps necessary for that version of the source. This file contains important information about potential problems and may specify the order to run certain commands. Many upgrades require specific additional steps such as renaming or deleting specific files prior to installing the new world. These will be listed at the end of this file where the currently recommended upgrade sequence is explicitly spelled out. If UPDATING contradicts any steps in this chapter, the instructions in UPDATING take precedence and should be followed.

Do Not Use make world:

Some older documentation recommends using make world. However, that command skips some important steps and should only be used by experts. For almost all circumstances make world is the wrong thing to do, and the procedure described here should be used instead.

23.6.1. Overview of Process

The build world process assumes an upgrade from an older FreeBSD version using the source of a newer version that was obtained using the instructions in Section 23.5, “Synchronizing Source”.

In FreeBSD, the term world includes the kernel, core system binaries, libraries, programming files, and built-in compiler. The order in which these components are built and installed is important.

For example, the old compiler might have a bug and not be able to compile the new kernel. Since the new kernel should be built with the new compiler, the new compiler must be built, but not necessarily installed, before the new kernel is built.

The new world might rely on new kernel features, so the new kernel must be installed before the new world is installed. The old world might not run correctly on the new kernel, so the new world must be installed immediately upon installing the new kernel.

Some configuration changes must be made before the new world is installed, but others might break the old world. Hence, two different configuration upgrade steps are used. For the most part, the update process only replaces or adds files and existing old files are not deleted. Since this can cause problems, /usr/src/UPDATING will indicate if any files need to be manually deleted and at which step to do so.

These concerns have led to the recommended upgrade sequence described in the following procedure.


It is a good idea to save the output from running make to a file. If something goes wrong, a copy of the error message can be posted to one of the FreeBSD mailing lists.

The easiest way to do this is to use script with a parameter that specifies the name of the file to save all output to. Do not save the output to /tmp as this directory may be cleared at next reboot. A better place to save the file is /var/tmp. Run this command immediately before rebuilding the world, and then type exit when the process has finished:

# script /var/tmp/mw.out
Script started, output file is /var/tmp/mw.out
Procedure 23.2. Overview of Build World Process

The commands used in the build world process should be run in the order specified here. This section summarizes the function of each command.

  1. If the build world process has previously been run on this system, a copy of the previous build may still exist in /usr/obj. To speed up the new build world process, and possibly save some dependency headaches, remove this directory if it already exists:

    # chflags -R noschg /usr/obj/*
    # rm -rf /usr/obj
  2. Compile the new compiler and a few related tools, then use the new compiler to compile the rest of the new world. The result is saved to /usr/obj.

    # cd /usr/src
    # make buildworld
  3. Use the new compiler residing in /usr/obj to build the new kernel, in order to protect against compiler-kernel mismatches. This is necessary, as certain memory structures may have changed, and programs like ps and top will fail to work if the kernel and source code versions are not the same.

    # make buildkernel
  4. Install the new kernel and kernel modules, making it possible to boot with the newly updated kernel. If kern.securelevel has been raised above 1 and noschg or similar flags have been set on the kernel binary, drop the system into single-user mode first. Otherwise, this command can be run from multi-user mode without problems. See init(8) for details about kern.securelevel and chflags(1) for details about the various file flags.

    # make installkernel
  5. Drop the system into single-user mode in order to minimize problems from updating any binaries that are already running. It also minimizes any problems from running the old world on a new kernel.

    # shutdown now

    Once in single-user mode, run these commands if the system is formatted with UFS:

    # mount -u /
    # mount -a -t ufs
    # swapon -a

    If the system is instead formatted with ZFS, run these two commands. This example assumes a zpool name of zroot:

    # zfs set readonly=off zroot
    # zfs mount -a
  6. Optional: If a keyboard mapping other than the default US English is desired, it can be changed with kbdmap(1):

    # kbdmap
  7. Then, for either file system, if the CMOS clock is set to local time (this is true if the output of date(1) does not show the correct time and zone), run:

    # adjkerntz -i
  8. Remaking the world will not update certain directories, such as /etc, /var and /usr, with new or changed configuration files. The next step is to perform some initial configuration file updates to /etc in preparation for the new world. The following command compares only those files that are essential for the success of installworld. For instance, this step may add new groups, system accounts, or startup scripts which have been added to FreeBSD since the last update. This is necessary so that the installworld step will be able to use any new system accounts, groups, and scripts. Refer to Section 23.6.4, “Merging Configuration Files” for more detailed instructions about this command:

    # mergemaster -p
  9. Install the new world and system binaries from /usr/obj.

    # cd /usr/src
    # make installworld
  10. Update any remaining configuration files.

    # mergemaster -iF
  11. Delete any obsolete files. This is important as they may cause problems if left on the disk.

    # make delete-old
  12. A full reboot is now needed to load the new kernel and new world with the new configuration files.

    # reboot
  13. Make sure that all installed ports have first been rebuilt before old libraries are removed using the instructions in Section 4.5.3, “Upgrading Ports”. When finished, remove any obsolete libraries to avoid conflicts with newer ones. For a more detailed description of this step, refer to Section 23.6.5, “Deleting Obsolete Files and Libraries”.

    # make delete-old-libs

If the system can have a window of down-time, consider compiling the system in single-user mode instead of compiling the system in multi-user mode, and then dropping into single-user mode for the installation. Reinstalling the system touches a lot of important system files, all the standard system binaries, libraries, and include files. Changing these on a running system, particularly one with active users, is asking for trouble.

23.6.2. Configuration Files

This build world process uses several configuration files.

The Makefile located in /usr/src describes how the programs that comprise FreeBSD should be built and the order in which they should be built.

The options available to make are described in make.conf(5) and some common examples are included in /usr/share/examples/etc/make.conf. Any options which are added to /etc/make.conf will control the how make runs and builds programs. These options take effect every time make is used, including compiling applications from the Ports Collection, compiling custom C programs, or building the FreeBSD operating system. Changes to some settings can have far-reaching and potentially surprising effects. Read the comments in both locations and keep in mind that the defaults have been chosen for a combination of performance and safety.

How the operating system is built from source code is controlled by /etc/src.conf. Unlike /etc/make.conf, the contents of /etc/src.conf only take effect when the FreeBSD operating system itself is being built. Descriptions of the many options available for this file are shown in src.conf(5). Be cautious about disabling seemingly unneeded kernel modules and build options. Sometimes there are unexpected or subtle interactions.

23.6.3. Variables and Targets

The general format for using make is as follows:

# make -x -DVARIABLE target

In this example, -x is an option passed to make. Refer to make(1) for examples of the available options.

To pass a variable, specify the variable name with -DVARIABLE. The behavior of the Makefile is controlled by variables. These can either be set in /etc/make.conf or they can be specified when using make. For example, this variable specifies that profiled libraries should not be built:

# make -DNO_PROFILE target

It corresponds with this setting in /etc/make.conf:

NO_PROFILE=    true     #    Avoid compiling profiled libraries

The target tells make what to do and the Makefile defines the available targets. Some targets are used by the build process to break out the steps necessary to rebuild the system into a number of sub-steps.

Having separate options is useful for two reasons. First, it allows for a build that does not affect any components of a running system. Because of this, buildworld can be safely run on a machine running in multi-user mode. It is still recommended that installworld be run in part in single-user mode, though.

Secondly, it allows NFS mounts to be used to upgrade multiple machines on a network, as described in Section 23.7, “Tracking for Multiple Machines”.

It is possible to specify -j which will cause make to spawn several simultaneous processes. Since much of the compiling process is I/O-bound rather than CPU-bound, this is useful on both single CPU and multi-CPU machines.

On a single-CPU machine, run the following command to have up to 4 processes running at any one time. Empirical evidence posted to the mailing lists shows this generally gives the best performance benefit.

# make -j4 buildworld

On a multi-CPU machine, try values between 6 and 10 to see how they speed things up.


If any variables were specified to make buildworld, specify the same variables to make installworld. However, -j must never be used with installworld.

For example, if this command was used:

# make -DNO_PROFILE buildworld

Install the results with:

# make -DNO_PROFILE installworld

Otherwise, the second command will try to install profiled libraries that were not built during the make buildworld phase.

23.6.4. Merging Configuration Files

Contributed by Tom Rhodes.

FreeBSD provides the mergemaster(8) Bourne script to aid in determining the differences between the configuration files in /etc, and the configuration files in /usr/src/etc. This is the recommended solution for keeping the system configuration files up to date with those located in the source tree.

Before using mergemaster, it is recommended to first copy the existing /etc somewhere safe. Include -R which does a recursive copy and -p which preserves times and the ownerships on files:

# cp -Rp /etc /etc.old

When run, mergemaster builds a temporary root environment, from / down, and populates it with various system configuration files. Those files are then compared to the ones currently installed in the system. Files that differ will be shown in diff(1) format, with the + sign representing added or modified lines, and - representing lines that will be either removed completely or replaced with a new file. Refer to diff(1) for more information about how file differences are shown.

Next, mergemaster will display each file that differs, and present options to: delete the new file, referred to as the temporary file, install the temporary file in its unmodified state, merge the temporary file with the currently installed file, or view the results again.

Choosing to delete the temporary file will tell mergemaster to keep the current file unchanged and to delete the new version. This option is not recommended. To get help at any time, type ? at the mergemaster prompt. If the user chooses to skip a file, it will be presented again after all other files have been dealt with.

Choosing to install the unmodified temporary file will replace the current file with the new one. For most unmodified files, this is the best option.

Choosing to merge the file will present a text editor with the contents of both files open. The files can be merged by reviewing both files side by side on the screen and choosing parts from both to create a finished product. When the files are compared side by side, l selects the left contents and r selects contents from the right. The final output will be a file consisting of both parts, which can then be installed. This option is customarily used for files where settings have been modified by the user.

Choosing to view the results again will redisplay the file differences.

After mergemaster is done with the system files, it will prompt for other options. It may prompt to rebuild the password file and will finish up with an option to remove left-over temporary files.

23.6.5. Deleting Obsolete Files and Libraries

Based on notes provided by Anton Shterenlikht.

As a part of the FreeBSD development lifecycle, files and their contents occasionally become obsolete. This may be because functionality is implemented elsewhere, the version number of the library has changed, or it was removed from the system entirely. These obsoleted files, libraries, and directories should be removed when updating the system. This ensures that the system is not cluttered with old files which take up unnecessary space on the storage and backup media. Additionally, if the old library has a security or stability issue, the system should be updated to the newer library to keep it safe and to prevent crashes caused by the old library. Files, directories, and libraries which are considered obsolete are listed in /usr/src/ObsoleteFiles.inc. The following instructions should be used to remove obsolete files during the system upgrade process.

After the make installworld and the subsequent mergemaster have finished successfully, check for obsolete files and libraries:

# cd /usr/src
# make check-old

If any obsolete files are found, they can be deleted using the following command:

# make delete-old

A prompt is displayed before deleting each obsolete file. To skip the prompt and let the system remove these files automatically, use BATCH_DELETE_OLD_FILES:

# make -DBATCH_DELETE_OLD_FILES delete-old

The same goal can be achieved by piping these commands through yes:

# yes|make delete-old


Deleting obsolete files will break applications that still depend on those obsolete files. This is especially true for old libraries. In most cases, the programs, ports, or libraries that used the old library need to be recompiled before make delete-old-libs is executed.

Utilities for checking shared library dependencies include sysutils/libchk and sysutils/bsdadminscripts.

Obsolete shared libraries can conflict with newer libraries, causing messages like these:

/usr/bin/ld: warning: libz.so.4, needed by /usr/local/lib/libtiff.so, may conflict with libz.so.5
/usr/bin/ld: warning: librpcsvc.so.4, needed by /usr/local/lib/libXext.so, may conflict with librpcsvc.so.5

To solve these problems, determine which port installed the library:

# pkg which /usr/local/lib/libtiff.so
  /usr/local/lib/libtiff.so was installed by package tiff-3.9.4
# pkg which /usr/local/lib/libXext.so
  /usr/local/lib/libXext.so was installed by package libXext-1.1.1,1

Then deinstall, rebuild, and reinstall the port. To automate this process, ports-mgmt/portmaster can be used. After all ports are rebuilt and no longer use the old libraries, delete the old libraries using the following command:

# make delete-old-libs

If something goes wrong, it is easy to rebuild a particular piece of the system. For example, if /etc/magic was accidentally deleted as part of the upgrade or merge of /etc, file will stop working. To fix this, run:

# cd /usr/src/usr.bin/file
# make all install

23.6.6. Common Questions

Do I need to re-make the world for every change?

It depends upon the nature of the change. For example, if svn only shows the following files as being updated:


it probably is not worth rebuilding the entire world. Instead, go into the appropriate sub-directories and run make all install. But if something major changes, such as src/lib/libc/stdlib, a complete rebuild of world is highly recommended.

Some users rebuild world every fortnight and let changes accumulate over that fortnight. Others only re-make those things that have changed and are careful to spot all the dependencies. It all depends on how often a user wants to upgrade and whether they are tracking FreeBSD-STABLE or FreeBSD-CURRENT.

What would cause a compile to fail with lots of signal 11 (or other signal number) errors?

This normally indicates a hardware problem. Building world is an effective way to stress test hardware, especially memory. A sure indicator of a hardware issue is when make is restarted and it dies at a different point in the process.

To resolve this error, swap out the components in the machine, starting with RAM, to determine which component is failing.

Can /usr/obj be removed when finished?

This directory contains all the object files that were produced during the compilation phase. Normally, one of the first steps in the make buildworld process is to remove this directory and start afresh. Keeping /usr/obj around when finished makes little sense, and its removal frees up a approximately 2GB of disk space.

Can interrupted builds be resumed?

This depends on how far into the process the problem occurs. In general, make buildworld builds new copies of essential tools and the system libraries. These tools and libraries are then installed, used to rebuild themselves, and are installed again. The rest of the system is then rebuilt with the new system tools.

During the last stage, it is fairly safe to run these commands as they will not undo the work of the previous make buildworld:

# cd /usr/src
# make -DNO_CLEAN all

If this message appears:

Building everything..

in the make buildworld output, it is probably fairly safe to do so.

If that message is not displayed, it is always better to be safe than sorry and to restart the build from scratch.

Is it possible to speed up making the world?

Several actions can speed up the build world process. For example, the entire process can be run from single-user mode. However, this will prevent users from having access to the system until the process is complete.

Careful file system design or the use of ZFS datasets can make a difference. Consider putting /usr/src and /usr/obj on separate file systems. If possible, place the file systems on separate disks on separate disk controllers. When mounting /usr/src, use noatime which prevents the file system from recording the file access time. If /usr/src is not on its own file system, consider remounting /usr with noatime.

The file system holding /usr/obj can be mounted or remounted with async so that disk writes happen asynchronously. The write completes immediately, and the data is written to the disk a few seconds later. This allows writes to be clustered together, and can provide a dramatic performance boost.


Keep in mind that this option makes the file system more fragile. With this option, there is an increased chance that, should power fail, the file system will be in an unrecoverable state when the machine restarts.

If /usr/obj is the only directory on this file system, this is not a problem. If you have other, valuable data on the same file system, ensure that there are verified backups before enabling this option.

Turn off profiling by setting NO_PROFILE=true in /etc/make.conf.

Pass -jn to make(1) to run multiple processes in parallel. This usually helps on both single- and multi-processor machines.

What if something goes wrong?

First, make absolutely sure that the environment has no extraneous cruft from earlier builds:

# chflags -R noschg /usr/obj/usr
# rm -rf /usr/obj/usr
# cd /usr/src
# make cleandir
# make cleandir

Yes, make cleandir really should be run twice.

Then, restart the whole process, starting with make buildworld.

If problems persist, send the error and the output of uname -a to FreeBSD general questions mailing list. Be prepared to answer other questions about the setup!

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.