Documentation: Added a section on libguestfs gotchas.

[libguestfs.git] / src / guestfs.pod

=encoding utf8

=head1 NAME

guestfs - Library for accessing and modifying virtual machine images

=head1 SYNOPSIS

 #include <guestfs.h>
 
 guestfs_h *handle = guestfs_create ();
 guestfs_add_drive (handle, "guest.img");
 guestfs_launch (handle);
 guestfs_mount (handle, "/dev/sda1", "/");
 guestfs_touch (handle, "/hello");
 guestfs_sync (handle);
 guestfs_close (handle);

=head1 DESCRIPTION

Libguestfs is a library for accessing and modifying guest disk images.
Amongst the things this is good for: making batch configuration
changes to guests, getting disk used/free statistics (see also:
virt-df), migrating between virtualization systems (see also:
virt-p2v), performing partial backups, performing partial guest
clones, cloning guests and changing registry/UUID/hostname info, and
much else besides.

Libguestfs uses Linux kernel and qemu code, and can access any type of
guest filesystem that Linux and qemu can, including but not limited
to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition
schemes, qcow, qcow2, vmdk.

Libguestfs provides ways to enumerate guest storage (eg. partitions,
LVs, what filesystem is in each LV, etc.).  It can also run commands
in the context of the guest.  Also you can access filesystems over FTP.

Libguestfs is a library that can be linked with C and C++ management
programs (or management programs written in OCaml, Perl, Python, Ruby,
Java, Haskell or C#).  You can also use it from shell scripts or the
command line.

You don't need to be root to use libguestfs, although obviously you do
need enough permissions to access the disk images.

Libguestfs is a large API because it can do many things.  For a gentle
introduction, please read the L</API OVERVIEW> section next.

=head1 API OVERVIEW

This section provides a gentler overview of the libguestfs API.  We
also try to group API calls together, where that may not be obvious
from reading about the individual calls below.

=head2 HANDLES

Before you can use libguestfs calls, you have to create a handle.
Then you must add at least one disk image to the handle, followed by
launching the handle, then performing whatever operations you want,
and finally closing the handle.  So the general structure of all
libguestfs-using programs looks like this:

 guestfs_h *handle = guestfs_create ();
 
 /* Call guestfs_add_drive additional times if there are
  * multiple disk images.
  */
 guestfs_add_drive (handle, "guest.img");
 
 /* Most manipulation calls won't work until you've launched
  * the handle.  You have to do this _after_ adding drives
  * and _before_ other commands.
  */
 guestfs_launch (handle);
 
 /* Now you can examine what partitions, LVs etc are available.
  */
 char **partitions = guestfs_list_partitions (handle);
 char **logvols = guestfs_lvs (handle);
 
 /* To access a filesystem in the image, you must mount it.
  */
 guestfs_mount (handle, "/dev/sda1", "/");
 
 /* Now you can perform filesystem actions on the guest
  * disk image.
  */
 guestfs_touch (handle, "/hello");
 
 /* You only need to call guestfs_sync if you have made
  * changes to the guest image.
  */
 guestfs_sync (handle);
 
 /* Close the handle. */
 guestfs_close (handle);

The code above doesn't include any error checking.  In real code you
should check return values carefully for errors.  In general all
functions that return integers return C<-1> on error, and all
functions that return pointers return C<NULL> on error.  See section
L</ERROR HANDLING> below for how to handle errors, and consult the
documentation for each function call below to see precisely how they
return error indications.

=head2 DISK IMAGES

The image filename (C<"guest.img"> in the example above) could be a
disk image from a virtual machine, a L<dd(1)> copy of a physical hard
disk, an actual block device, or simply an empty file of zeroes that
you have created through L<posix_fallocate(3)>.  Libguestfs lets you
do useful things to all of these.

You can add a disk read-only using C<guestfs_add_drive_ro>, in which
case libguestfs won't modify the file.

Be extremely cautious if the disk image is in use, eg. if it is being
used by a virtual machine.  Adding it read-write will almost certainly
cause disk corruption, but adding it read-only is safe.

You must add at least one disk image, and you may add multiple disk
images.  In the API, the disk images are usually referred to as
C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
one you added), etc.

Once C<guestfs_launch> has been called you cannot add any more images.
You can call C<guestfs_list_devices> to get a list of the device
names, in the order that you added them.  See also L</BLOCK DEVICE
NAMING> below.

=head2 MOUNTING

Before you can read or write files, create directories and so on in a
disk image that contains filesystems, you have to mount those
filesystems using C<guestfs_mount>.  If you already know that a disk
image contains (for example) one partition with a filesystem on that
partition, then you can mount it directly:

 guestfs_mount (handle, "/dev/sda1", "/");

where C</dev/sda1> means literally the first partition (C<1>) of the
first disk image that we added (C</dev/sda>).  If the disk contains
Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).

If you are given a disk image and you don't know what it contains then
you have to find out.  Libguestfs can do that too: use
C<guestfs_list_partitions> and C<guestfs_lvs> to list possible
partitions and LVs, and either try mounting each to see what is
mountable, or else examine them with C<guestfs_file>.  But you might
find it easier to look at higher level programs built on top of
libguestfs, in particular L<virt-inspector(1)>.

To mount a disk image read-only, use C<guestfs_mount_ro>.  There are
several other variations of the C<guestfs_mount_*> call.

=head2 FILESYSTEM ACCESS AND MODIFICATION

The majority of the libguestfs API consists of fairly low-level calls
for accessing and modifying the files, directories, symlinks etc on
mounted filesystems.  There are over a hundred such calls which you
can find listed in detail below in this man page, and we don't even
pretend to cover them all in this overview.

Specify filenames as full paths including the mount point.

For example, if you mounted a filesystem at C<"/"> and you want to
read the file called C<"etc/passwd"> then you could do:

 char *data = guestfs_cat (handle, "/etc/passwd");

This would return C<data> as a newly allocated buffer containing the
full content of that file (with some conditions: see also
L</DOWNLOADING> below), or C<NULL> if there was an error.

As another example, to create a top-level directory on that filesystem
called C<"var"> you would do:

 guestfs_mkdir (handle, "/var");

To create a symlink you could do:

 guestfs_ln_s (handle, "/etc/init.d/portmap",
               "/etc/rc3.d/S30portmap");

Libguestfs will reject attempts to use relative paths.  There is no
concept of a current working directory.  Libguestfs can return errors
in many situations: for example if the filesystem isn't writable, or
if a file or directory that you requested doesn't exist.  If you are
using the C API (documented here) you have to check for those error
conditions after each call.  (Other language bindings turn these
errors into exceptions).

File writes are affected by the per-handle umask, set by calling
C<guestfs_umask> and defaulting to 022.

=head2 PARTITIONING

Libguestfs contains API calls to read, create and modify partition
tables on disk images.

In the common case where you want to create a single partition
covering the whole disk, you should use the C<guestfs_part_disk>
call:

 const char *parttype = "mbr";
 if (disk_is_larger_than_2TB)
   parttype = "gpt";
 guestfs_part_disk (g, "/dev/sda", parttype);

Obviously this effectively wipes anything that was on that disk image
before.

In general MBR partitions are both unnecessarily complicated and
depend on archaic details, namely the Cylinder-Head-Sector (CHS)
geometry of the disk.  C<guestfs_sfdiskM> can be used to
create more complex arrangements where the relative sizes are
expressed in megabytes instead of cylinders, which is a small win.
C<guestfs_sfdiskM> will choose the nearest cylinder to approximate the
requested size.  There's a lot of crazy stuff to do with IDE and
virtio disks having different, incompatible CHS geometries, that you
probably don't want to know about.

My advice: make a single partition to cover the whole disk, then use
LVM on top.

=head2 LVM2

Libguestfs provides access to a large part of the LVM2 API, such as
C<guestfs_lvcreate> and C<guestfs_vgremove>.  It won't make much sense
unless you familiarize yourself with the concepts of physical volumes,
volume groups and logical volumes.

This author strongly recommends reading the LVM HOWTO, online at
L<http://tldp.org/HOWTO/LVM-HOWTO/>.

=head2 DOWNLOADING

Use C<guestfs_cat> to download small, text only files.  This call
is limited to files which are less than 2 MB and which cannot contain
any ASCII NUL (C<\0>) characters.  However it has a very simple
to use API.

C<guestfs_read_file> can be used to read files which contain
arbitrary 8 bit data, since it returns a (pointer, size) pair.
However it is still limited to "small" files, less than 2 MB.

C<guestfs_download> can be used to download any file, with no
limits on content or size (even files larger than 4 GB).

To download multiple files, see C<guestfs_tar_out> and
C<guestfs_tgz_out>.

=head2 UPLOADING

It's often the case that you want to write a file or files to the disk
image.

For small, single files, use C<guestfs_write_file>.  This call
currently contains a bug which limits the call to plain text files
(not containing ASCII NUL characters).

To upload a single file, use C<guestfs_upload>.  This call has no
limits on file content or size (even files larger than 4 GB).

To upload multiple files, see C<guestfs_tar_in> and C<guestfs_tgz_in>.

However the fastest way to upload I<large numbers of arbitrary files>
is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
L<mkisofs(8)>), then attach this using C<guestfs_add_drive_ro>.  If
you add the drive in a predictable way (eg. adding it last after all
other drives) then you can get the device name from
C<guestfs_list_devices> and mount it directly using
C<guestfs_mount_ro>.  Note that squashfs images are sometimes
non-portable between kernel versions, and they don't support labels or
UUIDs.  If you want to pre-build an image or you need to mount it
using a label or UUID, use an ISO image instead.

=head2 COPYING

There are various different commands for copying between files and
devices and in and out of the guest filesystem.  These are summarised
in the table below.

=over 4

=item B<file> to B<file>

Use L</guestfs_cp> to copy a single file, or
L</guestfs_cp_a> to copy directories recursively.

=item B<file or device> to B<file or device>

Use L</guestfs_dd> which efficiently uses L<dd(1)>
to copy between files and devices in the guest.

Example: duplicate the contents of an LV:

 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");

The destination (C</dev/VG/Copy>) must be at least as large as the
source (C</dev/VG/Original>).

=item B<file on the host> to B<file or device>

Use L</guestfs_upload>.  See L</UPLOADING> above.

=item B<file or device> to B<file on the host>

Use L</guestfs_download>.  See L</DOWNLOADING> above.

=back

=head2 LISTING FILES

C<guestfs_ll> is just designed for humans to read (mainly when using
the L<guestfish(1)>-equivalent command C<ll>).

C<guestfs_ls> is a quick way to get a list of files in a directory
from programs, as a flat list of strings.

C<guestfs_readdir> is a programmatic way to get a list of files in a
directory, plus additional information about each one.  It is more
equivalent to using the L<readdir(3)> call on a local filesystem.

C<guestfs_find> can be used to recursively list files.

=head2 RUNNING COMMANDS

Although libguestfs is a primarily an API for manipulating files
inside guest images, we also provide some limited facilities for
running commands inside guests.

There are many limitations to this:

=over 4

=item *

The kernel version that the command runs under will be different
from what it expects.

=item *

If the command needs to communicate with daemons, then most likely
they won't be running.

=item *

The command will be running in limited memory.

=item *

Only supports Linux guests (not Windows, BSD, etc).

=item *

Architecture limitations (eg. won't work for a PPC guest on
an X86 host).

=item *

For SELinux guests, you may need to enable SELinux and load policy
first.  See L</SELINUX> in this manpage.

=back

The two main API calls to run commands are C<guestfs_command> and
C<guestfs_sh> (there are also variations).

The difference is that C<guestfs_sh> runs commands using the shell, so
any shell globs, redirections, etc will work.

=head2 CONFIGURATION FILES

To read and write configuration files in Linux guest filesystems, we
strongly recommend using Augeas.  For example, Augeas understands how
to read and write, say, a Linux shadow password file or X.org
configuration file, and so avoids you having to write that code.

The main Augeas calls are bound through the C<guestfs_aug_*> APIs.  We
don't document Augeas itself here because there is excellent
documentation on the L<http://augeas.net/> website.

If you don't want to use Augeas (you fool!) then try calling
C<guestfs_read_lines> to get the file as a list of lines which
you can iterate over.

=head2 SELINUX

We support SELinux guests.  To ensure that labeling happens correctly
in SELinux guests, you need to enable SELinux and load the guest's
policy:

=over 4

=item 1.

Before launching, do:

 guestfs_set_selinux (g, 1);

=item 2.

After mounting the guest's filesystem(s), load the policy.  This
is best done by running the L<load_policy(8)> command in the
guest itself:

 guestfs_sh (g, "/usr/sbin/load_policy");

(Older versions of C<load_policy> require you to specify the
name of the policy file).

=item 3.

Optionally, set the security context for the API.  The correct
security context to use can only be known by inspecting the
guest.  As an example:

 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");

=back

This will work for running commands and editing existing files.

When new files are created, you may need to label them explicitly,
for example by running the external command
C<restorecon pathname>.

=head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS

Libguestfs can mount NTFS partitions.  It does this using the
L<http://www.ntfs-3g.org/> driver.

DOS and Windows still use drive letters, and the filesystems are
always treated as case insensitive by Windows itself, and therefore
you might find a Windows configuration file referring to a path like
C<c:\windows\system32>.  When the filesystem is mounted in libguestfs,
that directory might be referred to as C</WINDOWS/System32>.

Drive letter mappings are outside the scope of libguestfs.  You have
to use libguestfs to read the appropriate Windows Registry and
configuration files, to determine yourself how drives are mapped (see
also L<virt-inspector(1)>).

Replacing backslash characters with forward slash characters is also
outside the scope of libguestfs, but something that you can easily do.

Where we can help is in resolving the case insensitivity of paths.
For this, call C<guestfs_case_sensitive_path>.

Libguestfs also provides some help for decoding Windows Registry
"hive" files, through the library C<libhivex> which is part of
libguestfs.  You have to locate and download the hive file(s)
yourself, and then pass them to C<libhivex> functions.  See also the
programs L<hivexml(1)>, L<hivexget(1)> and L<virt-win-reg(1)> for more
help on this issue.

=head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES

Although we don't want to discourage you from using the C API, we will
mention here that the same API is also available in other languages.

The API is broadly identical in all supported languages.  This means
that the C call C<guestfs_mount(handle,path)> is
C<$handle-E<gt>mount($path)> in Perl, C<handle.mount(path)> in Python,
and C<Guestfs.mount handle path> in OCaml.  In other words, a
straightforward, predictable isomorphism between each language.

Error messages are automatically transformed
into exceptions if the language supports it.

We don't try to "object orientify" parts of the API in OO languages,
although contributors are welcome to write higher level APIs above
what we provide in their favourite languages if they wish.

=over 4

=item B<C++>

You can use the I<guestfs.h> header file from C++ programs.  The C++
API is identical to the C API.  C++ classes and exceptions are
not implemented.

=item B<C#>

The C# bindings are highly experimental.  Please read the warnings
at the top of C<csharp/Libguestfs.cs>.

=item B<Haskell>

This is the only language binding that working but incomplete.  Only
calls which return simple integers have been bound in Haskell, and we
are looking for help to complete this binding.

=item B<Java>

Full documentation is contained in the Javadoc which is distributed
with libguestfs.

=item B<OCaml>

For documentation see the file C<guestfs.mli>.

=item B<Perl>

For documentation see L<Sys::Guestfs(3)>.

=item B<Python>

For documentation do:

 $ python
 >>> import guestfs
 >>> help (guestfs)

=item B<Ruby>

Use the Guestfs module.  There is no Ruby-specific documentation, but
you can find examples written in Ruby in the libguestfs source.

=item B<shell scripts>

For documentation see L<guestfish(1)>.

=back

=head2 LIBGUESTFS GOTCHAS

L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
system [...] that works in the way it is documented but is
counterintuitive and almost invites mistakes."

Since we developed libguestfs and the associated tools, there are
several things we would have designed differently, but are now stuck
with for backwards compatibility or other reasons.  If there is ever a
libguestfs 2.0 release, you can expect these to change.  Beware of
them.

=over 4

=item Autosync / forgetting to sync.

When modifying a filesystem from C or another language, you B<must>
unmount all filesystems and call L</guestfs_sync> explicitly before
you close the libguestfs handle.  You can also call:

 guestfs_set_autosync (handle, 1);

to have the unmount/sync done automatically for you when the handle is
closed.  (This feature is called "autosync", L</guestfs_set_autosync>
q.v.)

If you forget to do this, then it is entirely possible that your
changes won't be written out, or will be partially written, or (very
rarely) that you'll get disk corruption.

Note that in L<guestfish(3)> I<autosync is the default>.  So quick and
dirty guestfish scripts that forget to sync will work just fine, which
can make this extra-puzzling if you are trying to debug a problem.

=item Read-only should be the default.

In L<guestfish(3)>, I<--ro> should be the default, and you should
have to specify I<--rw> if you want to make changes to the image.

This would reduce the potential to corrupt live VM images.

Note that many filesystems change the disk when you just mount and
unmount, even if you didn't perform any writes.  You need to use
C<guestfs_add_drive_ro> to guarantee that the disk is not changed.

=item guestfish command line is hard to use.

C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
for examination).  It tries to run a guestfish command C<disk.img>
which doesn't exist, so it fails, and it fails with a strange and
unintuitive error message.  Like the Bourne shell, we should have used
C<guestfish -c command> to run commands.

=back

=head1 CONNECTION MANAGEMENT

=head2 guestfs_h *

C<guestfs_h> is the opaque type representing a connection handle.
Create a handle by calling C<guestfs_create>.  Call C<guestfs_close>
to free the handle and release all resources used.

For information on using multiple handles and threads, see the section
L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.

=head2 guestfs_create

 guestfs_h *guestfs_create (void);

Create a connection handle.

You have to call C<guestfs_add_drive> on the handle at least once.

This function returns a non-NULL pointer to a handle on success or
NULL on error.

After configuring the handle, you have to call C<guestfs_launch>.

You may also want to configure error handling for the handle.  See
L</ERROR HANDLING> section below.

=head2 guestfs_close

 void guestfs_close (guestfs_h *handle);

This closes the connection handle and frees up all resources used.

=head1 ERROR HANDLING

The convention in all functions that return C<int> is that they return
C<-1> to indicate an error.  You can get additional information on
errors by calling C<guestfs_last_error> and/or by setting up an error
handler with C<guestfs_set_error_handler>.

The default error handler prints the information string to C<stderr>.

Out of memory errors are handled differently.  The default action is
to call L<abort(3)>.  If this is undesirable, then you can set a
handler using C<guestfs_set_out_of_memory_handler>.

=head2 guestfs_last_error

 const char *guestfs_last_error (guestfs_h *handle);

This returns the last error message that happened on C<handle>.  If
there has not been an error since the handle was created, then this
returns C<NULL>.

The lifetime of the returned string is until the next error occurs, or
C<guestfs_close> is called.

The error string is not localized (ie. is always in English), because
this makes searching for error messages in search engines give the
largest number of results.

=head2 guestfs_set_error_handler

 typedef void (*guestfs_error_handler_cb) (guestfs_h *handle,
                                           void *data,
                                           const char *msg);
 void guestfs_set_error_handler (guestfs_h *handle,
                                 guestfs_error_handler_cb cb,
                                 void *data);

The callback C<cb> will be called if there is an error.  The
parameters passed to the callback are an opaque data pointer and the
error message string.

Note that the message string C<msg> is freed as soon as the callback
function returns, so if you want to stash it somewhere you must make
your own copy.

The default handler prints messages on C<stderr>.

If you set C<cb> to C<NULL> then I<no> handler is called.

=head2 guestfs_get_error_handler

 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *handle,
                                                     void **data_rtn);

Returns the current error handler callback.

=head2 guestfs_set_out_of_memory_handler

 typedef void (*guestfs_abort_cb) (void);
 int guestfs_set_out_of_memory_handler (guestfs_h *handle,
                                        guestfs_abort_cb);

The callback C<cb> will be called if there is an out of memory
situation.  I<Note this callback must not return>.

The default is to call L<abort(3)>.

You cannot set C<cb> to C<NULL>.  You can't ignore out of memory
situations.

=head2 guestfs_get_out_of_memory_handler

 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *handle);

This returns the current out of memory handler.

=head1 PATH

Libguestfs needs a kernel and initrd.img, which it finds by looking
along an internal path.

By default it looks for these in the directory C<$libdir/guestfs>
(eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).

Use C<guestfs_set_path> or set the environment variable
C<LIBGUESTFS_PATH> to change the directories that libguestfs will
search in.  The value is a colon-separated list of paths.  The current
directory is I<not> searched unless the path contains an empty element
or C<.>.  For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
search the current directory and then C</usr/lib/guestfs>.

=head1 HIGH-LEVEL API ACTIONS

=head2 ABI GUARANTEE

We guarantee the libguestfs ABI (binary interface), for public,
high-level actions as outlined in this section.  Although we will
deprecate some actions, for example if they get replaced by newer
calls, we will keep the old actions forever.  This allows you the
developer to program in confidence against libguestfs.

@ACTIONS@

=head1 STRUCTURES

@STRUCTS@

=head1 AVAILABILITY

=head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE

Using L</guestfs_available> you can test availability of
the following groups of functions.  This test queries the
appliance to see if the appliance you are currently using
supports the functionality.

@AVAILABILITY@

=head2 SINGLE CALLS AT COMPILE TIME

If you need to test whether a single libguestfs function is
available at compile time, we recommend using build tools
such as autoconf or cmake.  For example in autotools you could
use:

 AC_CHECK_LIB([guestfs],[guestfs_create])
 AC_CHECK_FUNCS([guestfs_dd])

which would result in C<HAVE_GUESTFS_DD> being either defined
or not defined in your program.

=head2 SINGLE CALLS AT RUN TIME

Testing at compile time doesn't guarantee that a function really
exists in the library.  The reason is that you might be dynamically
linked against a previous I<libguestfs.so> (dynamic library)
which doesn't have the call.  This situation unfortunately results
in a segmentation fault, which is a shortcoming of the C dynamic
linking system itself.

You can use L<dlopen(3)> to test if a function is available
at run time, as in this example program (note that you still
need the compile time check as well):

 #include <config.h>
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <dlfcn.h>
 #include <guestfs.h>
 
 main ()
 {
 #ifdef HAVE_GUESTFS_DD
   void *dl;
   int has_function;
 
   /* Test if the function guestfs_dd is really available. */
   dl = dlopen (NULL, RTLD_LAZY);
   if (!dl) {
     fprintf (stderr, "dlopen: %s\n", dlerror ());
     exit (1);
   }
   has_function = dlsym (dl, "guestfs_dd") != NULL;
   dlclose (dl);
 
   if (!has_function)
     printf ("this libguestfs.so does NOT have guestfs_dd function\n");
   else {
     printf ("this libguestfs.so has guestfs_dd function\n");
     /* Now it's safe to call
     guestfs_dd (g, "foo", "bar");
     */
   }
 #else
   printf ("guestfs_dd function was not found at compile time\n");
 #endif
  }

You may think the above is an awful lot of hassle, and it is.
There are other ways outside of the C linking system to ensure
that this kind of incompatibility never arises, such as using
package versioning:

 Requires: libguestfs >= 1.0.80

=begin html

<!-- old anchor for the next section -->
<a name="state_machine_and_low_level_event_api"/>

=end html

=head1 ARCHITECTURE

Internally, libguestfs is implemented by running an appliance (a
special type of small virtual machine) using L<qemu(1)>.  Qemu runs as
a child process of the main program.

  ___________________
 /                   \
 | main program      |
 |                   |
 |                   |           child process / appliance
 |                   |           __________________________
 |                   |          / qemu                     \
 +-------------------+   RPC    |      +-----------------+ |
 | libguestfs     <--------------------> guestfsd        | |
 |                   |          |      +-----------------+ |
 \___________________/          |      | Linux kernel    | |
                                |      +--^--------------+ |
                                \_________|________________/
                                          |
                                   _______v______
                                  /              \
                                  | Device or    |
                                  | disk image   |
                                  \______________/

The library, linked to the main program, creates the child process and
hence the appliance in the L</guestfs_launch> function.

Inside the appliance is a Linux kernel and a complete stack of
userspace tools (such as LVM and ext2 programs) and a small
controlling daemon called C<guestfsd>.  The library talks to
C<guestfsd> using remote procedure calls (RPC).  There is a mostly
one-to-one correspondence between libguestfs API calls and RPC calls
to the daemon.  Lastly the disk image(s) are attached to the qemu
process which translates device access by the appliance's Linux kernel
into accesses to the image.

A common misunderstanding is that the appliance "is" the virtual
machine.  Although the disk image you are attached to might also be
used by some virtual machine, libguestfs doesn't know or care about
this.  (But you will care if both libguestfs's qemu process and your
virtual machine are trying to update the disk image at the same time,
since these usually results in massive disk corruption).

=head1 STATE MACHINE

libguestfs uses a state machine to model the child process:

                         |
                    guestfs_create
                         |
                         |
                     ____V_____
                    /          \
                    |  CONFIG  |
                    \__________/
                     ^ ^   ^  \
                    /  |    \  \ guestfs_launch
                   /   |    _\__V______
                  /    |   /           \
                 /     |   | LAUNCHING |
                /      |   \___________/
               /       |       /
              /        |  guestfs_launch
             /         |     /
    ______  /        __|____V
   /      \ ------> /        \
   | BUSY |         | READY  |
   \______/ <------ \________/

The normal transitions are (1) CONFIG (when the handle is created, but
there is no child process), (2) LAUNCHING (when the child process is
booting up), (3) alternating between READY and BUSY as commands are
issued to, and carried out by, the child process.

The guest may be killed by C<guestfs_kill_subprocess>, or may die
asynchronously at any time (eg. due to some internal error), and that
causes the state to transition back to CONFIG.

Configuration commands for qemu such as C<guestfs_add_drive> can only
be issued when in the CONFIG state.

The high-level API offers two calls that go from CONFIG through
LAUNCHING to READY.  C<guestfs_launch> blocks until the child process
is READY to accept commands (or until some failure or timeout).
C<guestfs_launch> internally moves the state from CONFIG to LAUNCHING
while it is running.

High-level API actions such as C<guestfs_mount> can only be issued
when in the READY state.  These high-level API calls block waiting for
the command to be carried out (ie. the state to transition to BUSY and
then back to READY).  But using the low-level event API, you get
non-blocking versions.  (But you can still only carry out one
operation per handle at a time - that is a limitation of the
communications protocol we use).

Finally, the child process sends asynchronous messages back to the
main program, such as kernel log messages.  Mostly these are ignored
by the high-level API, but using the low-level event API you can
register to receive these messages.

=head2 SETTING CALLBACKS TO HANDLE EVENTS

The child process generates events in some situations.  Current events
include: receiving a log message, the child process exits.

Use the C<guestfs_set_*_callback> functions to set a callback for
different types of events.

Only I<one callback of each type> can be registered for each handle.
Calling C<guestfs_set_*_callback> again overwrites the previous
callback of that type.  Cancel all callbacks of this type by calling
this function with C<cb> set to C<NULL>.

=head2 guestfs_set_log_message_callback

 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
                                         char *buf, int len);
 void guestfs_set_log_message_callback (guestfs_h *handle,
                                        guestfs_log_message_cb cb,
                                        void *opaque);

The callback function C<cb> will be called whenever qemu or the guest
writes anything to the console.

Use this function to capture kernel messages and similar.

Normally there is no log message handler, and log messages are just
discarded.

=head2 guestfs_set_subprocess_quit_callback

 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
 void guestfs_set_subprocess_quit_callback (guestfs_h *handle,
                                            guestfs_subprocess_quit_cb cb,
                                            void *opaque);

The callback function C<cb> will be called when the child process
quits, either asynchronously or if killed by
C<guestfs_kill_subprocess>.  (This corresponds to a transition from
any state to the CONFIG state).

=head2 guestfs_set_launch_done_callback

 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
 void guestfs_set_launch_done_callback (guestfs_h *handle,
                                        guestfs_ready_cb cb,
                                        void *opaque);

The callback function C<cb> will be called when the child process
becomes ready first time after it has been launched.  (This
corresponds to a transition from LAUNCHING to the READY state).

=head1 BLOCK DEVICE NAMING

In the kernel there is now quite a profusion of schemata for naming
block devices (in this context, by I<block device> I mean a physical
or virtual hard drive).  The original Linux IDE driver used names
starting with C</dev/hd*>.  SCSI devices have historically used a
different naming scheme, C</dev/sd*>.  When the Linux kernel I<libata>
driver became a popular replacement for the old IDE driver
(particularly for SATA devices) those devices also used the
C</dev/sd*> scheme.  Additionally we now have virtual machines with
paravirtualized drivers.  This has created several different naming
systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
PV disks.

As discussed above, libguestfs uses a qemu appliance running an
embedded Linux kernel to access block devices.  We can run a variety
of appliances based on a variety of Linux kernels.

This causes a problem for libguestfs because many API calls use device
or partition names.  Working scripts and the recipe (example) scripts
that we make available over the internet could fail if the naming
scheme changes.

Therefore libguestfs defines C</dev/sd*> as the I<standard naming
scheme>.  Internally C</dev/sd*> names are translated, if necessary,
to other names as required.  For example, under RHEL 5 which uses the
C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
C</dev/hda2> transparently.

Note that this I<only> applies to parameters.  The
C<guestfs_list_devices>, C<guestfs_list_partitions> and similar calls
return the true names of the devices and partitions as known to the
appliance.

=head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION

Usually this translation is transparent.  However in some (very rare)
cases you may need to know the exact algorithm.  Such cases include
where you use C<guestfs_config> to add a mixture of virtio and IDE
devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
and C</dev/vd*> devices.

The algorithm is applied only to I<parameters> which are known to be
either device or partition names.  Return values from functions such
as C<guestfs_list_devices> are never changed.

=over 4

=item *

Is the string a parameter which is a device or partition name?

=item *

Does the string begin with C</dev/sd>?

=item *

Does the named device exist?  If so, we use that device.
However if I<not> then we continue with this algorithm.

=item *

Replace initial C</dev/sd> string with C</dev/hd>.

For example, change C</dev/sda2> to C</dev/hda2>.

If that named device exists, use it.  If not, continue.

=item *

Replace initial C</dev/sd> string with C</dev/vd>.

If that named device exists, use it.  If not, return an error.

=back

=head2 PORTABILITY CONCERNS

Although the standard naming scheme and automatic translation is
useful for simple programs and guestfish scripts, for larger programs
it is best not to rely on this mechanism.

Where possible for maximum future portability programs using
libguestfs should use these future-proof techniques:

=over 4

=item *

Use C<guestfs_list_devices> or C<guestfs_list_partitions> to list
actual device names, and then use those names directly.

Since those device names exist by definition, they will never be
translated.

=item *

Use higher level ways to identify filesystems, such as LVM names,
UUIDs and filesystem labels.

=back

=head1 INTERNALS

=head2 COMMUNICATION PROTOCOL

Don't rely on using this protocol directly.  This section documents
how it currently works, but it may change at any time.

The protocol used to talk between the library and the daemon running
inside the qemu virtual machine is a simple RPC mechanism built on top
of XDR (RFC 1014, RFC 1832, RFC 4506).

The detailed format of structures is in C<src/guestfs_protocol.x>
(note: this file is automatically generated).

There are two broad cases, ordinary functions that don't have any
C<FileIn> and C<FileOut> parameters, which are handled with very
simple request/reply messages.  Then there are functions that have any
C<FileIn> or C<FileOut> parameters, which use the same request and
reply messages, but they may also be followed by files sent using a
chunked encoding.

=head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)

For ordinary functions, the request message is:

 total length (header + arguments,
      but not including the length word itself)
 struct guestfs_message_header (encoded as XDR)
 struct guestfs_<foo>_args (encoded as XDR)

The total length field allows the daemon to allocate a fixed size
buffer into which it slurps the rest of the message.  As a result, the
total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
4MB), which means the effective size of any request is limited to
somewhere under this size.

Note also that many functions don't take any arguments, in which case
the C<guestfs_I<foo>_args> is completely omitted.

The header contains the procedure number (C<guestfs_proc>) which is
how the receiver knows what type of args structure to expect, or none
at all.

The reply message for ordinary functions is:

 total length (header + ret,
      but not including the length word itself)
 struct guestfs_message_header (encoded as XDR)
 struct guestfs_<foo>_ret (encoded as XDR)

As above the C<guestfs_I<foo>_ret> structure may be completely omitted
for functions that return no formal return values.

As above the total length of the reply is limited to
C<GUESTFS_MESSAGE_MAX>.

In the case of an error, a flag is set in the header, and the reply
message is slightly changed:

 total length (header + error,
      but not including the length word itself)
 struct guestfs_message_header (encoded as XDR)
 struct guestfs_message_error (encoded as XDR)

The C<guestfs_message_error> structure contains the error message as a
string.

=head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS

A C<FileIn> parameter indicates that we transfer a file I<into> the
guest.  The normal request message is sent (see above).  However this
is followed by a sequence of file chunks.

 total length (header + arguments,
      but not including the length word itself,
      and not including the chunks)
 struct guestfs_message_header (encoded as XDR)
 struct guestfs_<foo>_args (encoded as XDR)
 sequence of chunks for FileIn param #0
 sequence of chunks for FileIn param #1 etc.

The "sequence of chunks" is:

 length of chunk (not including length word itself)
 struct guestfs_chunk (encoded as XDR)
 length of chunk
 struct guestfs_chunk (encoded as XDR)
   ...
 length of chunk
 struct guestfs_chunk (with data.data_len == 0)

The final chunk has the C<data_len> field set to zero.  Additionally a
flag is set in the final chunk to indicate either successful
completion or early cancellation.

At time of writing there are no functions that have more than one
FileIn parameter.  However this is (theoretically) supported, by
sending the sequence of chunks for each FileIn parameter one after
another (from left to right).

Both the library (sender) I<and> the daemon (receiver) may cancel the
transfer.  The library does this by sending a chunk with a special
flag set to indicate cancellation.  When the daemon sees this, it
cancels the whole RPC, does I<not> send any reply, and goes back to
reading the next request.

The daemon may also cancel.  It does this by writing a special word
C<GUESTFS_CANCEL_FLAG> to the socket.  The library listens for this
during the transfer, and if it gets it, it will cancel the transfer
(it sends a cancel chunk).  The special word is chosen so that even if
cancellation happens right at the end of the transfer (after the
library has finished writing and has started listening for the reply),
the "spurious" cancel flag will not be confused with the reply
message.

This protocol allows the transfer of arbitrary sized files (no 32 bit
limit), and also files where the size is not known in advance
(eg. from pipes or sockets).  However the chunks are rather small
(C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
daemon need to keep much in memory.

=head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS

The protocol for FileOut parameters is exactly the same as for FileIn
parameters, but with the roles of daemon and library reversed.

 total length (header + ret,
      but not including the length word itself,
      and not including the chunks)
 struct guestfs_message_header (encoded as XDR)
 struct guestfs_<foo>_ret (encoded as XDR)
 sequence of chunks for FileOut param #0
 sequence of chunks for FileOut param #1 etc.

=head3 INITIAL MESSAGE

Because the underlying channel (QEmu -net channel) doesn't have any
sort of connection control, when the daemon launches it sends an
initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
and daemon is alive.  This is what C<guestfs_launch> waits for.

=head1 MULTIPLE HANDLES AND MULTIPLE THREADS

All high-level libguestfs actions are synchronous.  If you want
to use libguestfs asynchronously then you must create a thread.

Only use the handle from a single thread.  Either use the handle
exclusively from one thread, or provide your own mutex so that two
threads cannot issue calls on the same handle at the same time.

=head1 QEMU WRAPPERS

If you want to compile your own qemu, run qemu from a non-standard
location, or pass extra arguments to qemu, then you can write a
shell-script wrapper around qemu.

There is one important rule to remember: you I<must C<exec qemu>> as
the last command in the shell script (so that qemu replaces the shell
and becomes the direct child of the libguestfs-using program).  If you
don't do this, then the qemu process won't be cleaned up correctly.

Here is an example of a wrapper, where I have built my own copy of
qemu from source:

 #!/bin/sh -
 qemudir=/home/rjones/d/qemu
 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"

Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
and then use it by setting the LIBGUESTFS_QEMU environment variable.
For example:

 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish

Note that libguestfs also calls qemu with the -help and -version
options in order to determine features.

=head1 ENVIRONMENT VARIABLES

=over 4

=item LIBGUESTFS_APPEND

Pass additional options to the guest kernel.

=item LIBGUESTFS_DEBUG

Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages.  This
has the same effect as calling C<guestfs_set_verbose (handle, 1)>.

=item LIBGUESTFS_MEMSIZE

Set the memory allocated to the qemu process, in megabytes.  For
example:

 LIBGUESTFS_MEMSIZE=700

=item LIBGUESTFS_PATH

Set the path that libguestfs uses to search for kernel and initrd.img.
See the discussion of paths in section PATH above.

=item LIBGUESTFS_QEMU

Set the default qemu binary that libguestfs uses.  If not set, then
the qemu which was found at compile time by the configure script is
used.

See also L</QEMU WRAPPERS> above.

=item LIBGUESTFS_TRACE

Set C<LIBGUESTFS_TRACE=1> to enable command traces.  This
has the same effect as calling C<guestfs_set_trace (handle, 1)>.

=item TMPDIR

Location of temporary directory, defaults to C</tmp>.

If libguestfs was compiled to use the supermin appliance then each
handle will require rather a large amount of space in this directory
for short periods of time (~ 80 MB).  You can use C<$TMPDIR> to
configure another directory to use in case C</tmp> is not large
enough.

=back

=head1 SEE ALSO

L<guestfish(1)>,
L<qemu(1)>,
L<febootstrap(1)>,
L<http://libguestfs.org/>.

Tools with a similar purpose:
L<fdisk(8)>,
L<parted(8)>,
L<kpartx(8)>,
L<lvm(8)>,
L<disktype(1)>.

=head1 BUGS

To get a list of bugs against libguestfs use this link:

L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>

To report a new bug against libguestfs use this link:

L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>

When reporting a bug, please check:

=over 4

=item *

That the bug hasn't been reported already.

=item *

That you are testing a recent version.

=item *

Describe the bug accurately, and give a way to reproduce it.

=item *

Run libguestfs-test-tool and paste the B<complete, unedited>
output into the bug report.

=back

=head1 AUTHORS

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

=head1 COPYRIGHT

Copyright (C) 2009 Red Hat Inc.
L<http://libguestfs.org/>

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA