Difference between pages "Clang" and "Installation Troubleshooting"

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==Introduction==
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This page shows you how to troubleshoot issues with your Funtoo Linux installation.
LLVM can be used as an alternative to GNU's compiler, GCC. The main benefit of using LLVM compilers instead of GCC is their lower memory usage, faster compile time and better diagnostics. There are some Benchmarks on the [http://clang.llvm.org/features.html#performance Clang] and [http://www.phoronix.com/scan.php?page=article&item=llvm3_gcc_open64 Phoronix] homepages.
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It may happen that some programs do not compile (like glibc) because they depend on GCC-specific language extensions [http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html] (this is why the whole BSD code can be compiled with LLVM but some GNU code cannot) or segfault after successful compilation with LLVM (like xorg-server) but after following this guide, the system will still be able to compile packages with gcc. So if something goes wrong, it can be switched back to gcc for the particular package by uncommenting lines in /etc/make.conf and the bug should be reported.
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== Boot failure ==
  
LLVM's C/C++ frontends clang and clang++ version 3.0 are stable enough to be self-hosting [http://blog.llvm.org/2010/02/clang-successfully-self-hosts.html] and compile Boost [http://blog.llvm.org/2010/05/clang-builds-boost.html], Qt [http://labs.qt.nokia.com/2010/10/29/compiling-qt-with-clang/], LibreOffice [http://wiki.documentfoundation.org/Development/Building_LibreOffice_with_Clang], FreeBSD [http://wiki.freebsd.org/BuildingFreeBSDWithClang], some parts of the Linux kernel [http://lwn.net/Articles/411654/] and more.
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If you did not get a <tt>login:</tt> prompt, it's possible that your install was not successful. You can reboot from the live CD and bind-mount your existing installation without repartitioning or recreating filesystems, chroot inside, and then check your system to ensure everything is configured properly:
  
Further, using LLVM 3.0 and up, there is a third way to compile with LLVM: the dragonegg package creates a gcc-plugin, that uses LLVM's optimizers but parses the code and creates binaries with gcc, which means that everything that compiles and works with gcc should work with dragonegg also. This plugin can be enabled by using a single CFLAG. Since LLVM 3.0 the old llvm-gcc package is deprecated and replaced by dragonegg, so it will disappear from portage with llvm version 2.9.
 
 
==LLVM Frontends==
 
To be able to compile some sourcecode of a specific language, LLVM needs an appropriate frontend. There are clang, llvm-gcc and dragonegg in portage.
 
 
The goal of the Clang project is to create a new C, C++, Objective C and Objective C++ front-end for the LLVM compiler.
 
 
llvm-gcc is a modified version of gcc that compiles C/ObjC programs into native objects, LLVM bitcode or LLVM assembly language, depending upon the options. As written in the previous section, dragonegg replaced llvm-gcc in version 3.0.
 
 
So after installing llvm, clang and dragonegg, you will be able to choose between gcc and llvm whenever you like or use them both at the same time.
 
 
== Install LLVM and its Frontends ==
 
Simply emerge the packages on ~arch systems. On arch systems you have to unmask some packages first. dragonegg requires gcc's ''lto'' USE-flag to be set and works with gcc 4.5 and gcc 4.6.
 
 
<console>
 
<console>
###i## emerge llvm clang dragonegg
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# ##i##mkdir /mnt/funtoo
 +
# ##i##mount /dev/sda4 /mnt/funtoo
 +
# ##i##mount /dev/sda1 /mnt/funtoo/boot
 +
# ##i##cd /mnt/funtoo
 +
# ##i##mount --bind /proc proc
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# ##i##mount --bind /dev dev
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# ##i##cp /etc/resolv.conf etc/
 +
# ##i##env -i HOME=/root TERM=$TERM PATH='/bin:/sbin:/usr/bin:/usr/sbin' chroot /mnt/funtoo
 
</console>
 
</console>
Note, that for clang++ the C++ headers search path is hardcoded to the active gcc profile.
 
If you change the active gcc profile, or update gcc to a new version, you will have to remerge clang to update the search path.
 
  
To use dragonegg, run gcc as usual, with an extra command line argument "-fplugin=/usr/lib/llvm/dragonegg.so"
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Once you are done making changes, leave the chroot, unmount mounted filesystems, and reboot:
If you change the active gcc profile, or update gcc to a new version, you will have to remerge dragonegg to update the plugin.
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After the installation, check which CPUs are supported by using the command
 
 
<console>
 
<console>
###i## llvm-as < /dev/null | llc -mcpu=help
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(chroot) # ##i##exit
 +
# ##i##umount /mnt/funtoo/boot /mnt/funtoo/dev /mnt/funtoo/proc /mnt/funtoo
 +
# ##i##reboot
 
</console>
 
</console>
and then add the following lines to ''<code>/etc/make.conf</code>'' (uncommenting the lines you need) to enable compilation via LLVM, adapting the march-option according to the previous command:
 
  
in ''<code>/etc/make.conf</code>'':
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If you continue to experience difficulties getting Funtoo Linux to boot properly, please ask for help in the [http://forums.funtoo.org/ forums].
<console>
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###i## nano /etc/portage/make.conf
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# LLVM
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#CC="/usr/bin/clang"
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#CXX="/usr/bin/clang++"
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# llvm-gcc for C++ code and fortran
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== Debugging system startup ==
# llvm-gcc is deprecated and only used with LLVM 2.9
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#CC="/usr/bin/llvm-gcc"
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#CXX="/usr/bin/llvm-g++"
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#CPP="/usr/bin/llvm-cpp"
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#F77="/usr/bin/llvm-gfortran"
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# Flags for clang: Insert your arch here instead of k8 and have a look at the manpage of clang for flag descriptions.
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<tt>/etc/rc.conf</tt> contains system settings related to the system initialization scripts. If you are experiencing problems with the system startup scripts, you may find it beneficial to set <tt>rc_logger</tt> to <tt>YES</tt>. This will instruct OpenRC to launch a logging daemon to log the entire rc process to <tt>/var/log/rc.log</tt>.
# Some gcc flags like -pipe and -pthread also work, though they might be ignored by clang.
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#CFLAGS="-march=k8 -O2"
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# Flags for dragonegg; just use all the gcc flags you like and append -fplugin=/path/to/dragonegg.so
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== Graphics Corruption at boot ==
#CFLAGS="-march=k8 -O2 -fplugin=/usr/lib64/llvm/dragonegg.so"
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</console>
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{{Note}} Have a look at clang's manpages for additional information. If you get errors that your compiler cannot produce code, you should check your flags, e.g. don't use -O4 -flto -S or stuff like that; the examples above will work.
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== Using clang with portage ==
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Although Gentoo package tree is not designed to be used with compiler other than GCC, clang can be enforced on most of the packages through ''CC'' and ''CXX'' variables.
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Please note, however, that many of Gentoo packages still don't build with clang and a few don't work correctly after being built. That's why we suggest using ''/etc/portage/env'' file to enable the use of clang per-package.
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In order to do that, first create a new environment override to use:
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in ''<code>/etc/portage/env/clang</code>'':
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<console>
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###i## nano /etc/portage/env/clang
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CC=clang
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CXX=clang++
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</console>
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Then you can enable use of clang for packages using ''[[:/etc/portage/env|/etc/portage/package.env]]'' file:
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in ''<code>/etc/portage/package.env</code>'':
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<console>
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###i## nano /etc/portage/package.env
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app-foo/bar clang
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app-bar/baz clang
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</console>
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If you want to use clang by default you can and need to specify some core packages. Here is small list of core packages that are currently failing on clang, but not that could be outdated:
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You need to add ''<code>/etc/portage/env/gcc</code>'':
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<console>
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###i## nano /etc/portage/env/gcc
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CC=gcc
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CXX=g++
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</console>
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in addition, it is recommend to add compiler flags there:
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<console>
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###i## nano /etc/portage/env/gcc
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CFLAGS="-O2 -march=native -mtune=native -pipe"
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CXXFLAGS="-O2 -march=native -mtune=native -pipe"
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LDFLAGS="-Wl,--as-needed"
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#You can disable gold link here
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#EXTRA_ECONF="--enable-gold=default"
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</console>
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And in ''<code>/etc/portage/package.env</code>'':
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<console>
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###i## nano /etc/portage/package.env
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#---------------CORE PACKAGES TO BUILD WITH GCC:
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sys-apps/which gcc
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sys-fs/reiserfsprogs gcc
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sys-libs/ncurses gcc
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sys-libs/zlib gcc
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sys-apps/busybox gcc
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sys-fs/e2fsprogs gcc
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sys-devel/binutils gcc
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sys-libs/glibc gcc
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sys-devel/dragonegg gcc
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dev-libs/openssl gcc
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sys-boot/grub gcc
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#---------------USER PACKAGES TO BUILD WITH GCC:
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sys-apps/pacman gcc
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www-client/firefox gcc
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x11-libs/cairo gcc
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media-libs/mesa gcc
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</console>
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If you have {{Package|app-portage/flaggie}} installed, you can modify ''<code>/etc/portage/package.env</code>'' by running the following:
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<console>
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###i## flaggie app-foo/bar app-bar/baz +clang
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</console>
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== Enabling link-time optimizations ==
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The ''link-time optimization'' feature defers optimizing the resulting executables to linking phase. This can result in better optimization of packages but is unsupported in Gentoo, and many packages simply fail to build.
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When using LTO, clang compiles units into LLVM byte-code rather than machine code. In order to support linking such object files, the [[gold]] linker must be installed and set as the default linker, as it does support plugins.
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Similarly, ''ar'' needs plugin support as well. Sadly, binutils ar doesn't support passing '--plugin'' option before the actual command. Thus, we need to create a wrapper for it:
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in ''<code>/usr/local/bin/clang-ar</code>'':
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<console>
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###i## nano /usr/local/bin/clang-ar
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#!/bin/sh
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firstarg=${1}
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shift
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exec /usr/bin/ar "${firstarg}" --plugin /usr/lib/llvm/LLVMgold.so "${@}"
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</console>
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If that's done, you can create a new environment override profile for LTO-enabled clang:
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in ''<code>/etc/portage/env/clang-lt</code>'':
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<console>
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CC='clang'
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CXX='clang++'
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CFLAGS="${CFLAGS} -O4"
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CXXFLAGS="${CXXFLAGS} -O4"
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LDFLAGS="${LDFLAGS} -O4 -Wl,-plugin,/usr/lib/llvm/LLVMgold.so"
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AR='/usr/local/bin/clang-ar'
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RANLIB=':'
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NM='nm --plugin /usr/lib64/llvm/LLVMgold.so'
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</console>
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Note that the link-time optimizations were indirectly enabled here via ''-O4''. If you don't want to enable other optimizations enforced by ''-O3'', please use ''-flto'' instead. You need to also pass optimization flags when linking because that's where clang needs them.
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If you experience graphics corruption issues at boot time, you may be experiencing an issue with the [http://fedoraproject.org/wiki/Features/KernelModesetting Kernel Mode Setting subsystem], known as KMS. To resolve this problem, boot from the live CD again [[#boot failure|following the steps above]]. Then, from within the chroot, edit <tt>/etc/boot.conf</tt> and add the line "<tt>params += nomodeset</tt>" to the "<tt>Funtoo Linux genkernel</tt>" section. Your <tt>/etc/boot.conf</tt> should now look like this:
  
You may also need to adjust the libdir path to plugin. Newer (live) versions of clang add `-plugin` when linking automatically, so `-Wl,-plugin`… is no longer necessary.
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<pre>
 +
"Funtoo Linux genkernel" {
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        kernel kernel[-v]
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        initrd initramfs[-v]
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        params += real_root=auto
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        params += nomodeset
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}
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</pre>
  
== Using clang with distcc ==
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Now, save your changes and run <tt>boot-update</tt>:
  
In order to use clang on distcc client, additional symlinks have to be created in ''<code>/usr/lib*/distcc/bin</code>'':
 
 
<console>
 
<console>
###i## ln -s /usr/bin/distcc /usr/lib/distcc/bin/clang
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(chroot) # ##i##boot-update
###i## ln -s /usr/bin/distcc /usr/lib/distcc/bin/clang++
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</console>
 
</console>
  
{{GLW|src=http://wiki.gentoo.org/wiki/Clang}}
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Now, exit the chroot, unmount filesystems and reboot following the [[#boot failure|boot failure]] instructions.
  
 
[[Category:HOWTO]]
 
[[Category:HOWTO]]
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[[Category:Install]]
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[[Category:Official Documentation]]

Latest revision as of 00:26, March 13, 2014

This page shows you how to troubleshoot issues with your Funtoo Linux installation.

Boot failure

If you did not get a login: prompt, it's possible that your install was not successful. You can reboot from the live CD and bind-mount your existing installation without repartitioning or recreating filesystems, chroot inside, and then check your system to ensure everything is configured properly:

# mkdir /mnt/funtoo
# mount /dev/sda4 /mnt/funtoo
# mount /dev/sda1 /mnt/funtoo/boot
# cd /mnt/funtoo
# mount --bind /proc proc
# mount --bind /dev dev
# cp /etc/resolv.conf etc/
# env -i HOME=/root TERM=$TERM PATH='/bin:/sbin:/usr/bin:/usr/sbin' chroot /mnt/funtoo

Once you are done making changes, leave the chroot, unmount mounted filesystems, and reboot:

(chroot) # exit
# umount /mnt/funtoo/boot /mnt/funtoo/dev /mnt/funtoo/proc /mnt/funtoo 
# reboot

If you continue to experience difficulties getting Funtoo Linux to boot properly, please ask for help in the forums.

Debugging system startup

/etc/rc.conf contains system settings related to the system initialization scripts. If you are experiencing problems with the system startup scripts, you may find it beneficial to set rc_logger to YES. This will instruct OpenRC to launch a logging daemon to log the entire rc process to /var/log/rc.log.

Graphics Corruption at boot

If you experience graphics corruption issues at boot time, you may be experiencing an issue with the Kernel Mode Setting subsystem, known as KMS. To resolve this problem, boot from the live CD again following the steps above. Then, from within the chroot, edit /etc/boot.conf and add the line "params += nomodeset" to the "Funtoo Linux genkernel" section. Your /etc/boot.conf should now look like this:

"Funtoo Linux genkernel" {
        kernel kernel[-v]
        initrd initramfs[-v]
        params += real_root=auto 
        params += nomodeset
} 

Now, save your changes and run boot-update:

(chroot) # boot-update

Now, exit the chroot, unmount filesystems and reboot following the boot failure instructions.