=head1 NAME

guestfs-hacking - extending and contributing to libguestfs

=head1 DESCRIPTION

This manual page is for hackers who want to extend libguestfs itself.

=head1 THE SOURCE CODE

Libguestfs source is located in the github repository
L<https://github.com/libguestfs/libguestfs>

Large amounts of boilerplate code in libguestfs (RPC, bindings,
documentation) are generated.  This means that many source files will
appear to be missing from a straightforward git checkout.  You have to
run the generator (C<./configure && make -C generator>) in order to
create those files.

Libguestfs uses an autotools-based build system, with the main files
being F<configure.ac> and F<Makefile.am>.  See L</THE BUILD SYSTEM>.

The F<generator> subdirectory contains the generator, plus files
describing the API.  The F<lib> subdirectory contains source for the
library.  The F<appliance> and F<daemon> subdirectories contain the
source for the code that builds the appliance, and the code that runs
in the appliance respectively.  Other directories are covered in the
section L<SOURCE CODE SUBDIRECTORIES> below.

Apart from the fact that all API entry points go via some generated
code, the library is straightforward.  (In fact, even the generated
code is designed to be readable, and should be read as ordinary code).
Some actions run entirely in the library, and are written as C
functions in files under F<lib>.  Others are forwarded to the daemon
where (after some generated RPC marshalling) they appear as C
functions in files under F<daemon>.

To build from source, first read the L<guestfs-building(1)>.

=head2 SOURCE CODE SUBDIRECTORIES

There are a lot of subdirectories in the source tree!  Which ones
should you concentrate on first?  F<lib> and F<daemon> which contain
the source code of the core library.  F<generator> is the code
generator described above, so that is important.  The F<Makefile.am>
in the root directory will tell you in which order the subdirectories
get built.  And then if you are looking at a particular tool
(eg. F<customize>) or language binding (eg. F<python>), go straight to that
subdirectory, but remember that if you didn't run the generator yet,
then you may find files which appear to be missing.

=over 4

=item F<align>

L<virt-alignment-scan(1)> command and documentation.

=item F<appliance>

The libguestfs appliance, build scripts and so on.

=item F<bash>

Bash tab-completion scripts.

=item F<build-aux>

Various build scripts used by autotools.

=item F<builder>

L<virt-builder(1)> command and documentation.

=item F<cat>

The L<virt-cat(1)>, L<virt-filesystems(1)>, L<virt-log(1)>,
L<virt-ls(1)> and L<virt-tail(1)> commands and documentation.

=item F<common>

Various libraries of internal code can be found in the F<common>
subdirectory:

=over 4

=item F<common/edit>

Common code for interactively and non-interactively editing files
within a libguestfs filesystem.

=item F<common/errnostring>

The communication protocol used between the library and the daemon
running inside the appliance has to encode errnos as strings, which is
handled by this library.

=item F<common/mlcustomize>

Library code associated with C<virt-customize> but also used in other
tools.

=item F<common/mlgettext>

Small, generated wrapper which allows libguestfs to be compiled with
or without ocaml-gettext.  This is generated by F<./configure>.

=item F<common/mlpcre>

Lightweight OCaml bindings for Perl Compatible Regular Expressions
(PCRE).  Note this is not related in any way to Markus Mottl's
ocaml-pcre library.

=item F<common/mlprogress>

OCaml bindings for the progress bar functions (see F<common/progress>).

=item F<common/mlstdutils>

A library of pure OCaml utility functions used in many places.

=item F<common/mltools>

OCaml utility functions only used by the OCaml virt tools (like
C<virt-sysprep>, C<virt-customize> etc.)

=item F<common/mlutils>

OCaml bindings for C functions in C<common/utils>, and some POSIX
bindings which are missing from the OCaml stdlib.

=item F<common/mlvisit>

OCaml bindings for the visit functions (see F<common/visit>).

=item F<common/mlxml>

OCaml bindings for the libxml2 library.

=item F<common/options>

Common options parsing for guestfish, guestmount and some virt tools.

=item F<common/parallel>

A framework used for processing multiple libvirt domains in parallel.

=item F<common/progress>

Common code for printing progress bars.

=item F<common/protocol>

The XDR-based communication protocol used between the library
and the daemon running inside the appliance is defined here.

=item F<common/qemuopts>

Mini-library for writing qemu command lines and qemu config files.

=item F<common/structs>

Common code for printing and freeing libguestfs structs, used by the
library and some tools.

=item F<common/utils>

Various utility functions used throughout the library and tools.

=item F<common/visit>

Recursively visit a guestfs filesystem hierarchy.

=item F<common/windows>

Utility functions for handling Windows drive letters.

=back

=item F<contrib>

Outside contributions, experimental parts.

=item F<customize>

L<virt-customize(1)> command and documentation.

=item F<daemon>

The daemon that runs inside the libguestfs appliance and carries out
actions.

=item F<df>

L<virt-df(1)> command and documentation.

=item F<dib>

L<virt-dib(1)> command and documentation.

=item F<diff>

L<virt-diff(1)> command and documentation.

=item F<docs>

Miscellaneous manual pages.

=item F<edit>

L<virt-edit(1)> command and documentation.

=item F<examples>

C API example code.

=item F<fish>

L<guestfish(1)>, the command-line shell, and various shell scripts
built on top such as L<virt-copy-in(1)>, L<virt-copy-out(1)>,
L<virt-tar-in(1)>, L<virt-tar-out(1)>.

=item F<format>

L<virt-format(1)> command and documentation.

=item F<fuse>

L<guestmount(1)>, FUSE (userspace filesystem) built on top of libguestfs.

=item F<generator>

The crucially important generator, used to automatically generate
large amounts of boilerplate C code for things like RPC and bindings.

=item F<get-kernel>

L<virt-get-kernel(1)> command and documentation.

=item F<inspector>

L<virt-inspector(1)>, the virtual machine image inspector.

=item F<lib>

Source code to the C library.

=item F<logo>

Logo used on the website.  The fish is called Arthur by the way.

=item F<m4>

M4 macros used by autoconf.  See L</THE BUILD SYSTEM>.

=item F<make-fs>

L<virt-make-fs(1)> command and documentation.

=item F<po>

Translations of simple gettext strings.

=item F<po-docs>

The build infrastructure and PO files for translations of manpages and
POD files.  Eventually this will be combined with the F<po> directory,
but that is rather complicated.

=item F<rescue>

L<virt-rescue(1)> command and documentation.

=item F<resize>

L<virt-resize(1)> command and documentation.

=item F<sparsify>

L<virt-sparsify(1)> command and documentation.

=item F<sysprep>

L<virt-sysprep(1)> command and documentation.

=item F<tests>

Tests.

=item F<test-data>

Files and other test data used by the tests.

=item F<test-tool>

Test tool for end users to test if their qemu/kernel combination
will work with libguestfs.

=item F<tmp>

Used for temporary files when running the tests (instead of F</tmp>
etc).  The reason is so that you can run multiple parallel tests of
libguestfs without having one set of tests overwriting the appliance
created by another.

=item F<tools>

Command line tools written in Perl (L<virt-win-reg(1)> and many others).

=item F<utils>

Miscellaneous utilities, such as C<boot-benchmark>.

=item F<v2v>

Up to libguestfs E<gt> 1.42 this contained the L<virt-v2v(1)> tool,
but this has now moved into a separate repository:
L<https://github.com/libguestfs/virt-v2v>

=item F<website>

The L<http://libguestfs.org> website files.

=item F<csharp>

=item F<erlang>

=item F<gobject>

=item F<golang>

=item F<haskell>

=item F<java>

=item F<lua>

=item F<ocaml>

=item F<php>

=item F<perl>

=item F<python>

=item F<ruby>

Language bindings.

=back

=head2 THE BUILD SYSTEM

Libguestfs uses the GNU autotools build system (autoconf, automake,
libtool).

The F<./configure> script is generated from F<configure.ac> and
F<m4/guestfs-*.m4>.  Most of the configure script is split over many
m4 macro files by topic, for example F<m4/guestfs-daemon.m4> deals
with the dependencies of the daemon.

The job of the top level F<Makefile.am> is mainly to list the
subdirectories (C<SUBDIRS>) in the order they should be compiled.

F<common-rules.mk> is included in every F<Makefile.am> (top level and
subdirectories).  F<subdir-rules.mk> is included only in subdirectory
F<Makefile.am> files.

There are many make targets.  Use this command to list them all:

 make help

=head1 EXTENDING LIBGUESTFS

=head2 ADDING A NEW API

Because large amounts of boilerplate code in libguestfs are generated,
this makes it easy to extend the libguestfs API.

To add a new API action there are two changes:

=over 4

=item 1.

You need to add a description of the call (name, parameters, return
type, tests, documentation) to F<generator/actions_*.ml> and
possibly F<generator/proc_nr.ml>.

There are two sorts of API action, depending on whether the call goes
through to the daemon in the appliance, or is serviced entirely by the
library (see L<guestfs-internals(1)/ARCHITECTURE>).  L<guestfs(3)/guestfs_sync> is an example
of the former, since the sync is done in the appliance.
L<guestfs(3)/guestfs_set_trace> is an example of the latter, since a trace flag
is maintained in the handle and all tracing is done on the library
side.

Most new actions are of the first type, and get added to the
C<daemon_functions> list.  Each function has a unique procedure number
used in the RPC protocol which is assigned to that action when we
publish libguestfs and cannot be reused.  Take the latest procedure
number and increment it.

For library-only actions of the second type, add to the
C<non_daemon_functions> list.  Since these functions are serviced by
the library and do not travel over the RPC mechanism to the daemon,
these functions do not need a procedure number, and so the procedure
number is set to C<-1>.

=item 2.

Implement the action (in C):

For daemon actions, implement the function C<do_E<lt>nameE<gt>> in the
C<daemon/> directory.

For library actions, implement the function C<guestfs_impl_E<lt>nameE<gt>>
in the C<lib/> directory.

In either case, use another function as an example of what to do.

=item 3.

As an alternative to step 2: Since libguestfs 1.38, daemon actions
can be implemented in OCaml.  You have to set the C<impl = OCaml ...>
flag in the generator.  Take a look at F<daemon/file.ml> for an
example.

=back

After making these changes, use C<make> to compile.

Note that you don’t need to implement the RPC, language bindings,
manual pages or anything else.  It’s all automatically generated from
the OCaml description.

=head3 Adding tests for an API

You can supply zero or as many tests as you want per API call.  The
tests can either be added as part of the API description
(F<generator/actions_*.ml>), or in some rarer cases you may want to drop
a script into C<tests/*/>.  Note that adding a script to C<tests/*/>
is slower, so if possible use the first method.

The following describes the test environment used when you add an API
test in F<actions_*.ml>.

The test environment has 4 block devices:

=over 4

=item F</dev/sda> 2 GB

General block device for testing.

=item F</dev/sdb> 2 GB

F</dev/sdb1> is an ext2 filesystem used for testing
filesystem write operations.

=item F</dev/sdc> 10 MB

Used in a few tests where two block devices are needed.

=item F</dev/sdd>

ISO with fixed content (see F<images/test.iso>).

=back

To be able to run the tests in a reasonable amount of time, the
libguestfs appliance and block devices are reused between tests.  So
don't try testing L<guestfs(3)/guestfs_kill_subprocess> :-x

Each test starts with an initial scenario, selected using one of the
C<Init*> expressions, described in F<generator/types.ml>.  These
initialize the disks mentioned above in a particular way as documented
in F<types.ml>.  You should not assume anything about the previous
contents of other disks that are not initialized.

You can add a prerequisite clause to any individual test.  This is a
run-time check, which, if it fails, causes the test to be skipped.
Useful if testing a command which might not work on all variations of
libguestfs builds.  A test that has prerequisite of C<Always> means to
run unconditionally.

In addition, packagers can skip individual tests by setting
environment variables before running C<make check>.

 SKIP_TEST_<CMD>_<NUM>=1

eg: C<SKIP_TEST_COMMAND_3=1> skips test #3 of L<guestfs(3)/guestfs_command>.

or:

 SKIP_TEST_<CMD>=1

eg: C<SKIP_TEST_ZEROFREE=1> skips all L<guestfs(3)/guestfs_zerofree> tests.

Packagers can run only certain tests by setting for example:

 TEST_ONLY="vfs_type zerofree"

See F<tests/c-api/tests.c> for more details of how these environment
variables work.

=head3 Debugging new APIs

Test new actions work before submitting them.

You can use guestfish to try out new commands.

Debugging the daemon is a problem because it runs inside a minimal
environment.  However you can fprintf messages in the daemon to
stderr, and they will show up if you use C<guestfish -v>.

=head2 ADDING A NEW LANGUAGE BINDING

All language bindings must be generated by the generator
(see the F<generator> subdirectory).

There is no documentation for this yet.  We suggest you look
at an existing binding, eg. F<generator/ocaml.ml> or
F<generator/perl.ml>.

=head3 Adding tests for language bindings

Language bindings should come with tests.  Previously testing of
language bindings was rather ad-hoc, but we have been trying to
formalize the set of tests that every language binding should use.

Currently only the OCaml and Perl bindings actually implement the full
set of tests, and the OCaml bindings are canonical, so you should
emulate what the OCaml tests do.

This is the numbering scheme used by the tests:

 - 000+ basic tests:
 
   010  load the library
   020  create
   030  create-flags
   040  create multiple handles
   050  test setting and getting config properties
   060  explicit close
   065  implicit close (in GC'd languages)
   070  optargs
   080  version
   090  retvalues
 
 - 100  launch, create partitions and LVs and filesystems
 
 - 400+ events:
 
   410  close event
   420  log messages
   430  progress messages
 
 - 800+ regression tests (specific to the language)
 
 - 900+ any other custom tests for the language

To save time when running the tests, only 100, 430, 800+, 900+ should
launch the handle.

=head2 FORMATTING CODE

Our C source code generally adheres to some basic code-formatting
conventions.  The existing code base is not totally consistent on this
front, but we do prefer that contributed code be formatted similarly.
In short, use spaces-not-TABs for indentation, use 2 spaces for each
indentation level, and other than that, follow the K&R style.

If you use Emacs, add the following to one of your start-up files
(e.g., ~/.emacs), to help ensure that you get indentation right:

 ;;; In libguestfs, indent with spaces everywhere (not TABs).
 ;;; Exceptions: Makefile and ChangeLog modes.
 (add-hook 'find-file-hook
     '(lambda () (if (and buffer-file-name
                          (string-match "/libguestfs\\>"
                              (buffer-file-name))
                          (not (string-equal mode-name "Change Log"))
                          (not (string-equal mode-name "Makefile")))
                     (setq indent-tabs-mode nil))))
 
 ;;; When editing C sources in libguestfs, use this style.
 (defun libguestfs-c-mode ()
   "C mode with adjusted defaults for use with libguestfs."
   (interactive)
   (c-set-style "K&R")
   (setq c-indent-level 2)
   (setq c-basic-offset 2))
 (add-hook 'c-mode-hook
           '(lambda () (if (string-match "/libguestfs\\>"
                               (buffer-file-name))
                           (libguestfs-c-mode))))

=head2 TESTING YOUR CHANGES

Turn warnings into errors when developing to make warnings hard to
ignore:

 ./configure --enable-werror

Useful targets are:

=over 4

=item C<make check>

Runs the regular test suite.

This is implemented using the regular automake C<TESTS> target.  See
the automake documentation for details.

=item C<make check-valgrind>

Runs a subset of the test suite under valgrind.

See L</VALGRIND> below.

=item C<make check-valgrind-local-guests>

Runs a subset of the test suite under valgrind
using locally installed libvirt guests (read-only).

=item C<make check-direct>

Runs all tests using default appliance back-end.  This only
has any effect if a non-default backend was selected
using C<./configure --with-default-backend=...>

=item C<make check-valgrind-direct>

Run a subset of the test suite under valgrind using the
default appliance back-end.

=item C<make check-with-upstream-qemu>

Runs all tests using a local qemu binary.  It looks for the qemu
binary in QEMUDIR (defaults to F<$HOME/d/qemu>), but you can set this
to another directory on the command line, eg:

 make check-with-upstream-qemu QEMUDIR=/usr/src/qemu

=item C<make check-with-upstream-libvirt>

Runs all tests using a local libvirt.  This only has any effect if the
libvirt backend was selected using
C<./configure --with-default-backend=libvirt>

It looks for libvirt in LIBVIRTDIR (defaults to F<$HOME/d/libvirt>),
but you can set this to another directory on the command line, eg:

 make check-with-upstream-libvirt LIBVIRTDIR=/usr/src/libvirt

=item C<make check-slow>

Runs some slow/long-running tests which are not run by default.

To mark a test as slow/long-running:

=over 4

=item *

Add it to the list of C<TESTS> in the F<Makefile.am>, just like a
normal test.

=item *

Modify the test so it checks if the C<SLOW=1> environment variable is
set, and if I<not> set it skips (ie. returns with exit code 77).  If
using F<tests/functions.sh>, you can call the function C<slow_test>
for this.

=item *

Add a variable C<SLOW_TESTS> to the F<Makefile.am> listing the slow
tests.

=item *

Add a rule to the F<Makefile.am>:

 check-slow:
   $(MAKE) check TESTS="$(SLOW_TESTS)" SLOW=1

=back

=item C<sudo make check-root>

Runs some tests which require root privileges.  These are supposed to
be safe, but take care.  You have to run this as root (eg. using
L<sudo(8)> explicitly).

To mark a test as requiring root:

=over 4

=item *

Add it to the list of C<TESTS> in the F<Makefile.am>, just like a
normal test.

=item *

Modify the test so it checks if euid == 0, and if I<not> set it skips
(ie. returns with exit code 77).  If using F<tests/functions.sh>, you
can call the function C<root_test> for this.

=item *

Add a variable C<ROOT_TESTS> to the F<Makefile.am> listing the root
tests.

=item *

Add a rule to the F<Makefile.am>:

 check-root:
   $(MAKE) check TESTS="$(ROOT_TESTS)"

=back

=item C<make check-all>

Equivalent to running all C<make check*> rules except C<check-root>.

=item C<make check-release>

Runs a subset of C<make check*> rules that are required to pass
before a tarball can be released.  Currently this is:

=over 4

=item *

check

=item *

check-valgrind

=item *

check-direct

=item *

check-valgrind-direct

=item *

check-slow

=back

=item C<make installcheck>

Run C<make check> on the installed copy of libguestfs.

The version of installed libguestfs being tested, and the version of
the libguestfs source tree must be the same.

Do:

 ./configure
 make clean ||:
 make
 make installcheck

=back

=head2 VALGRIND

When you do C<make check-valgrind>, it searches for any F<Makefile.am>
in the tree that has a C<check-valgrind:> target and runs it.

Writing the F<Makefile.am> and tests correctly to use valgrind and
working with automake parallel tests is subtle.

If your tests are run via a shell script wrapper, then in the wrapper
use:

 $VG virt-foo

and in the F<Makefile.am> use:

 check-valgrind:
     make VG="@VG@" check

However, if your binaries run directly from the C<TESTS> rule, you
have to modify the F<Makefile.am> like this:

 LOG_COMPILER = $(VG)
 
 check-valgrind:
     make VG="@VG@" check

In either case, check that the right program is being tested by
examining the F<tmp/valgrind*> log files carefully.

=head2 SUBMITTING PATCHES

Submit patches to the mailing list:
L<https://lists.libguestfs.org>
and CC to L<rjones@redhat.com>.

You do not need to subscribe to the mailing list if you don’t want to.
There may be a short delay while your message is moderated.

=head2 INTERNATIONALIZATION (I18N) SUPPORT

We support i18n (gettext anyhow) in the library.

However many messages come from the daemon, and we don’t translate
those at the moment.  One reason is that the appliance generally has
all locale files removed from it, because they take up a lot of space.
So we'd have to readd some of those, as well as copying our PO files
into the appliance.

Debugging messages are never translated, since they are intended for
the programmers.

=head1 MISCELLANEOUS TOPICS

=head2 HOW OCAML PROGRAMS ARE COMPILED AND LINKED

Mostly this section is "how we make automake & ocamlopt work together"
since OCaml programs themselves are easy to compile.

Automake has no native support for OCaml programs, ocamlc nor
ocamlopt.  What we do instead is to treat OCaml programs as C programs
which happen to contain these "other objects" (C<"DEPENDENCIES"> in
automake-speak) that happen to be the OCaml objects.  This works
because OCaml programs usually have C files for native bindings etc.

So a typical program is described as just its C sources:

 virt_customize_SOURCES = ... crypt-c.c perl_edit-c.c

For programs that have no explicit C sources, we create an empty
F<dummy.c> file, and list that instead:

 virt_resize_SOURCES = dummy.c

The OCaml objects which contain most of the code are listed as
automake dependencies (other dependencies may also be listed):

 virt_customize_DEPENDENCIES = ... customize_main.cmx

The only other special thing we need to do is to provide a custom link
command.  This is needed because automake won't assemble the ocamlopt
command, the list of objects and the C<-cclib> libraries in the
correct order otherwise.

 virt_customize_LINK = \
     $(top_builddir)/ocaml-link.sh -cclib '-lutils' -- ...

The actual rules, which you can examine in F<customize/Makefile.am>, are a
little bit more complicated than this because they have to handle:

=over 4

=item *

Compiling for byte code or native code.

=item *

The pattern rules needed to compile the OCaml sources to objects.

These are now kept in F<subdir-rules.mk> at the top level, which is
included in every subdirectory F<Makefile.am>.

=item *

Adding OCaml sources files to C<EXTRA_DIST>.

Automake isn't aware of the complete list of sources for a binary, so
it will not add them all automatically.

=back

=head1 MAINTAINER TASKS

=head2 MAINTAINER MAKEFILE TARGETS

These C<make> targets probably won’t work and aren't useful unless you
are a libguestfs maintainer.

=head3 make maintainer-commit

This commits everything in the working directory with the commit
message C<Version $(VERSION).>.  You must update F<configure.ac>,
clean and rebuild first.

=head3 make maintainer-tag

This tags the current HEAD commit with the tag C<v$(VERSION)> and one
of the messages:

 Version $(VERSION) stable

 Version $(VERSION) development

(See L<guestfs(3)/LIBGUESTFS VERSION NUMBERS> for the difference
between a stable and development release.)

=head3 make maintainer-check-authors

Check that all authors (found in git commit messages) are included in
the F<generator/authors.ml> file.

=head3 make maintainer-check-extra-dist

This rule must be run after C<make dist> (so there is a tarball in the
working directory).  It compares the contents of the tarball with the
contents of git to ensure that no files have been missed from
F<Makefile.am> C<EXTRA_DIST> rules.

=head3 make maintainer-upload-website

This is used by the software used to automate libguestfs releases to
copy the libguestfs website to another git repository before it is
uploaded to the web server.

=head2 MAKING A STABLE RELEASE

When we make a stable release, there are several steps documented
here.  See L<guestfs(3)/LIBGUESTFS VERSION NUMBERS> for general information
about the stable branch policy.

=over 4

=item *

Check C<make && make check> works on at least:

=over 4

=item Fedora (x86-64)

=item Debian (x86-64)

=item Ubuntu (x86-64)

=item Fedora (aarch64)

=item Fedora (ppc64)

=item Fedora (ppc64le)

=back

=item *

Check C<./configure --without-libvirt> works.

=item *

Finalize F<guestfs-release-notes.pod>

=item *

Create new stable and development directories under
L<http://libguestfs.org/download>.

=item *

Edit F<website/index.html.in>.

=item *

Set the version (in F<configure.ac>) to the new I<stable> version,
ie. 1.XX.0, and commit it:

 ./localconfigure
 make distclean -k
 ./localconfigure
 make && make dist
 make maintainer-commit
 make maintainer-tag

=item *

Create the stable branch in git:

 git branch stable-1.XX
 git push origin stable-1.XX

=item *

Do a full release of the stable branch.

=item *

Set the version to the next development version and commit that.
Optionally do a full release of the development branch.

=back

=head1 INTERNAL DOCUMENTATION

This section documents internal functions inside libguestfs and
various utilities.  It is intended for libguestfs developers only.

This section is autogenerated from C</**> comments in source files,
which are marked up in POD format.

B<These functions are not publicly exported, and may change or be
removed at any time.>

__INTERNAL_DOCUMENTATION__

=head1 SEE ALSO

L<guestfs(3)>,
L<guestfs-building(1)>,
L<guestfs-examples(3)>,
L<guestfs-internals(1)>,
L<guestfs-performance(1)>,
L<guestfs-release-notes(1)>,
L<guestfs-testing(1)>,
L<libguestfs-test-tool(1)>,
L<libguestfs-make-fixed-appliance(1)>,
L<http://libguestfs.org/>.

=head1 AUTHORS

Richard W.M. Jones (C<rjones at redhat dot com>)

=head1 COPYRIGHT

Copyright (C) 2009-2025 Red Hat Inc.
