Using the toolchain
Using the toolchain is as simple as adding the toolchain’s bin directory in
your PATH
, such as:
export PATH="${PATH}:/your/toolchain/path/bin"
Depending on the project being compiled, there may be different ways to specify the toolchain.
If the software uses GNU autotools or a similar configure script, you should
use the --host
tuple to tell the build system to use your toolchain
(if the software package uses the autotools system you should also
pass --build
, for completeness):
./configure --host=your-host-tuple --build=your-build-tuple
Other systems may need, for example:
make CC=your-host-tuple-gcc
make CROSS_COMPILE=your-host-tuple-
make CHOST=your-host-tuple
and so on. Please read the documentation of the package being compiled as to how it can be cross-compiled.
Note
in the above example,
host
refers to the host of your program (i.e. the target of the toolchain), not the host of the toolchain; andbuild
refers to the machine where you build your program, that is the host of the toolchain.
Assembling a root filesystem
Assembling a root filesystem for a target device requires the successive
building of a set of software packages for the target architecture.
Building a package potentially requires artifacts which were generated as
part of an earlier build. Note that not all artifacts which are installed as
part of a package are desirable on a target’s root filesystem (e.g.,
man/info files, include files, etc.).
Therefore we must distinguish between a staging
directory and a rootfs
directory.
A staging
directory is a location into which we install all the build
artifacts. We can then point future builds to this location so they can
find the appropriate header and library files. A rootfs
directory is a
location into which we place only the files we want to have on our target.
There are four schools of thought here:
-
Option 1: Install directly into the sysroot of the toolchain.
By default (i.e., if you don’t pass any arguments to the tools which would change this behaviour) the toolchain that is built by crosstool-NG will only look in its toolchain directories for system header and library files:
#include "..." search starts here: #include <...> search starts here: <ct-ng install path>/lib/gcc/<host tuple>/4.5.2/include <ct-ng install path>/lib/gcc/<host tuple>/4.5.2/include-fixed <ct-ng install path>/lib/gcc/<host tuple>/4.5.2/../../../../<host tuple>/include <ct-ng install path>/<host tuple>/sysroot/usr/include
In other words, the compiler will automagically find headers and libraries without extra flags if they are installed under the toolchain’s sysroot directory.
However, this is bad because the toolchain gets poluted, and can not be re-used.
$ ./configure --build=<build tuple> --host=<host tuple> \ --prefix=/usr --enable-foo-bar... $ make $ make DESTDIR=/<ct-ng install path>/<host tuple>/sysroot install
-
Option 2: Copy the toolchain’s sysroot to the
staging
area.If you start off by copying the toolchain’s sysroot directory to your staging area, you can simply proceed to install all your packages’ artifacts to the same staging area. You then only need to specify a
--sysroot=<staging area>
option to the compiler of any subsequent builds and all your required header and library files will be found/used.This is a viable option, but requires the user to always specify
CFLAGS
in order to include--sysroot=<staging area>
, or requires the use of a wrapper to a few select tools (gcc, ld…) to pass this flag.Instead of polluting the toolchain’s sysroot you are copying its contents to a new location and polluting the contents in that new location. By specifying the
--sysroot
option you’re effectively abandoning the default sysroot in favour of your own.Incidentally this is what buildroot does using a wrapper, when using an external toolchain.
$ cp -a $(<host tuple>-gcc --your-cflags-except-sysroot -print-sysroot) \ /path/to/staging $ ./configure --build=<build tuple> --host=<host tuple> \ --prefix=/usr --enable-foo-bar... \ CC="<host tuple>-gcc --sysroot=/path/to/staging" \ CXX="<host tuple>-g++ --sysroot=/path/to/staging" \ LD="<host tuple>-ld --sysroot=/path/to/staging" \ AND_SO_ON="tuple-andsoon --sysroot=/path/to/staging" $ make $ make DESTDIR=/path/to/staging install
-
Option 3: Use separate staging and sysroot directories.
In this scenario you use a staging area to install programs, but you do not pre-fill that staging area with the toolchain’s sysroot. In this case the compiler will find the system includes and libraries in its sysroot area but you have to pass appropriate
CPPFLAGS
andLDFLAGS
to tell it where to find your headers and libraries from your staging area (or use a wrapper).$ ./configure --build=<build tuple> --host=<host tuple> \ --prefix=/usr --enable-foo-bar... \ CPPFLAGS="-I/path/to/staging/usr/include" \ LDFLAGS="-L/path/to/staging/lib -L/path/to/staging/usr/lib" $ make $ make DESTDIR=/path/to/staging install
-
Option 4: A mix of options 2 and 3, using carefully crafted union mounts.
The staging area is a union mount of:
-
the sysroot as a read-only branch
-
the real staging area as a read-write branch
This also requires passing
--sysroot
to point to the union mount, but has other advantages, such as allowing per-package staging, and a few more obscure pros. It also has its disadvantages, as it potentially requires non-root users to create union mounts. Additionally, union mounts are not yet mainstream in the Linux kernel, so it requires patching. There is a FUSE-based unionfs implementation, but development is almost stalled, and there are a few gotchas.$ (good luck!)
-
It is strongly advised not to use the toolchain sysroot directory as an install directory (i.e., option 1) for your programs/packages. If you do so, you will not be able to use your toolchain for another project. It is even strongly advised that your toolchain is chmod-ed to read-only once successfully install, so that you don’t go polluting your toolchain with your programs’/packages’ files. This can be achieved by selecting the “Render the toolchain read-only” from crosstool-NG’s “Paths and misc options” configuration page.
Thus, when you build a program/package, install it in a separate, staging, directory and let the cross-toolchain continue to use its own, pristine, sysroot directory.
When you are done building and want to assemble your rootfs you could
simply take the full contents of your staging directory and use the
populate
script to add in the necessary files from the sysroot.
However, the staging area you have created will include lots of build
artifacts that you won’t necessarily want/need on your target. For
example: static libraries, header files, linking helper files, man/info
pages. You’ll also need to add various configuration files, scripts, and
directories to the rootfs so it will boot.
Therefore you’ll probably end up creating a separate rootfs directory which you will populate from the staging area, necessary extras, and then use crosstool-NG’s populate script to add the necessary sysroot libraries.
The populate
script
When your root directory is ready, it is still missing some important bits: the toolchain’s libraries. To populate your root directory with those libs, just run:
your-target-tuple-populate -s /your/root -d /your/root-populated
This will copy /your/root
into /your/root-populated
, and put the
needed and only the needed libraries there. Thus you don’t pollute
/your/root with any cruft that would no longer be needed should you have
to remove stuff. /your/root
always contains only those things you
install in it.
You can then use /your/root-populated
to build up your file system
image, a tarball, or to NFS-mount it from your target, or whatever you
need.
The populate script accepts the following options:
-
-s src_dir
: Usesrc_dir
as the un-populated root directory. -
-d dst_dir
: Put the populated root directory indst_dir
. -
-l lib1 [...]
: Always add specified libraries. -
-L file
: Always add libraries listed infile
. -
-f
: Removedst_dir
if it previously existed; continue even if any library specified with-l
or-L
is missing. -
-v
: Be verbose, and tell what’s going on (you can see exactly where libs are coming from). -
-h
: Print the help.
See your-target-tuple-populate -h
for more information on the options.
Here is how populate works:
- Perform some sanity checks:
src_dir
anddst_dir
are specified,src_dir
exists,- unless forced,
dst_dir
does not exist, src_dir
!=dst_dir
.
-
Copy
src_dir
todst_dir
. - Add forced libraries to
dst_dir
:- build the list from
-l
and-L
options, - get forced libraries from the sysroot (see below for heuristics)
- abort on the first missing library, unless
-f
is specified.
- abort on the first missing library, unless
- build the list from
- Add all missing libraries to
dst_dir
:- scan
dst_dir
for every ELF files that are executable or shared object - list the “NEEDED Shared library” fields
- check if the library is already in
dst_dir/lib
ordst_dir/usr/lib
- if not, get the library from the sysroot
- if it’s in
sysroot/lib
, copy it todst_dir/lib
- if it’s in
sysroot/usr/lib
, copy it todst_dir/usr/lib
- in both cases, use the
SONAME
of the library to create the file indst_dir
- if it was not found in the sysroot, this is an error.
- if it’s in
- check if the library is already in
- scan
The cross-ldd script
There is another script provided by crosstool-NG to work with the sysroot
on the host. A dynamically linked application will load certain shared
libraries at runtime. These libraries in turn may require some more shared
libraries as dependencies. The search paths for each of these dynamic
objects differ, and finding the shared libraries required for a given
application is not always trivial. Crosstool-NG attempts to solve this
task by providing a ${CT_TARGET}-ldd
script in the generated toolchain
(optionally, if the “Install a cross ldd-like helper” option is selected
in the configuration).
This script recursively resolves all the dynamic library dependencies
and outputs the list of libraries in a format compatible with that of
GNU libc’s ldd
script. It needs to have the system root specified,
using either the --root
option or the CT_XLDD_ROOT
environment variable.
For example:
PATH=~/x-tools/powerpc64-multilib-linux-gnu/bin:$PATH
powerpc64-multilib-linux-gnu-gcc -o example example.c
powerpc64-multilib-linux-gnu-ldd --root=`powerpc64-multilib-linux-gnu-gcc -print-sysroot` example
produces an output like this:
libc.so.6 => /lib64/libc.so.6 (0x00000000deadbeef)
ld64.so.1 => /lib64/ld64.so.1 (0x00000000deadbeef)
The load addresses are obviously bogus, as this script does not actually load the libraries.