ntfs-3g: Document problems with symlinks and alternatives (RHBZ#663407).

[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 *g = guestfs_create ();
 guestfs_add_drive (g, "guest.img");
 guestfs_launch (g);
 guestfs_mount (g, "/dev/sda1", "/");
 guestfs_touch (g, "/hello");
 guestfs_umount (g, "/");
 guestfs_close (g);

 cc prog.c -o prog -lguestfs
or:
 cc prog.c -o prog `pkg-config libguestfs --cflags --libs`

=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
FUSE.

Libguestfs is a library that can be linked with C and C++ management
programs (or management programs written in OCaml, Perl, Python, Ruby,
Java, PHP, 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.

There are also some example programs in the L<guestfs-examples(3)>
manual page.

=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 in the main section of this
manual.

=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.  By convention we use the single
letter C<g> for the name of the handle variable, although of course
you can use any name you want.

The general structure of all libguestfs-using programs looks like
this:

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

 /* This is only needed for libguestfs < 1.5.24.  Since then
  * it is done automatically when you close the handle.  See
  * discussion of autosync in this page.
  */
 guestfs_sync (g);
 
 /* Close the handle 'g'. */
 guestfs_close (g);

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.  See L<guestfs-examples(3)> for fully worked
examples.

=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.

The call you should use in modern code for adding drives is
L</guestfs_add_drive_opts>.  To add a disk image, allowing writes, and
specifying that the format is raw, do:

 guestfs_add_drive_opts (g, filename,
                         GUESTFS_ADD_DRIVE_OPTS_FORMAT, "raw",
                         -1);

You can add a disk read-only using:

 guestfs_add_drive_opts (g, filename,
                         GUESTFS_ADD_DRIVE_OPTS_FORMAT, "raw",
                         GUESTFS_ADD_DRIVE_OPTS_READONLY, 1,
                         -1);

or by calling the older function L</guestfs_add_drive_ro>.  In either
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 L</guestfs_launch> has been called you cannot add any more images.
You can call L</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 L</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 (g, "/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
L</guestfs_list_partitions> and L</guestfs_lvs> to list possible
partitions and LVs, and either try mounting each to see what is
mountable, or else examine them with L</guestfs_vfs_type> or
L</guestfs_file>.  Libguestfs also has a set of APIs for inspection of
disk images (see L</INSPECTION> below).  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 L</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, starting with C<"/"> and 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 (g, "/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 (g, "/var");

To create a symlink you could do:

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

Libguestfs will reject attempts to use relative paths and 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
L</guestfs_umask> and defaulting to 022.  See L</UMASK>.

=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 L</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.

=head2 LVM2

Libguestfs provides access to a large part of the LVM2 API, such as
L</guestfs_lvcreate> and L</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 L</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.

L</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.

L</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 L</guestfs_tar_out> and
L</guestfs_tgz_out>.

=head2 UPLOADING

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

To write a small file with fixed content, use L</guestfs_write>.  To
create a file of all zeroes, use L</guestfs_truncate_size> (sparse) or
L</guestfs_fallocate64> (with all disk blocks allocated).  There are a
variety of other functions for creating test files, for example
L</guestfs_fill> and L</guestfs_fill_pattern>.

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

To upload multiple files, see L</guestfs_tar_in> and L</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 L</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
L</guestfs_list_devices> and mount it directly using
L</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>).  To copy less than the whole
source device, use L</guestfs_copy_size>.

=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

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

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

L</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.

L</guestfs_find> and L</guestfs_find0> can be used to recursively list
files.

=head2 RUNNING COMMANDS

Although libguestfs is 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 *

The network may not be available unless you enable it
(see L</guestfs_set_network>).

=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.

=item *

I<Security:> It is not safe to run commands from untrusted, possibly
malicious guests.  These commands may attempt to exploit your program
by sending unexpected output.  They could also try to exploit the
Linux kernel or qemu provided by the libguestfs appliance.  They could
use the network provided by the libguestfs appliance to bypass
ordinary network partitions and firewalls.  They could use the
elevated privileges or different SELinux context of your program
to their advantage.

A secure alternative is to use libguestfs to install a "firstboot"
script (a script which runs when the guest next boots normally), and
to have this script run the commands you want in the normal context of
the running guest, network security and so on.  For information about
other security issues, see L</SECURITY>.

=back

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

The difference is that L</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
L</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 UMASK

Certain calls are affected by the current file mode creation mask (the
"umask").  In particular ones which create files or directories, such
as L</guestfs_touch>, L</guestfs_mknod> or L</guestfs_mkdir>.  This
affects either the default mode that the file is created with or
modifies the mode that you supply.

The default umask is C<022>, so files are created with modes such as
C<0644> and directories with C<0755>.

There are two ways to avoid being affected by umask.  Either set umask
to 0 (call C<guestfs_umask (g, 0)> early after launching).  Or call
L</guestfs_chmod> after creating each file or directory.

For more information about umask, see L<umask(2)>.

=head2 ENCRYPTED DISKS

Libguestfs allows you to access Linux guests which have been
encrypted using whole disk encryption that conforms to the
Linux Unified Key Setup (LUKS) standard.  This includes
nearly all whole disk encryption systems used by modern
Linux guests.

Use L</guestfs_vfs_type> to identify LUKS-encrypted block
devices (it returns the string C<crypto_LUKS>).

Then open these devices by calling L</guestfs_luks_open>.
Obviously you will require the passphrase!

Opening a LUKS device creates a new device mapper device
called C</dev/mapper/mapname> (where C<mapname> is the
string you supply to L</guestfs_luks_open>).
Reads and writes to this mapper device are decrypted from and
encrypted to the underlying block device respectively.

LVM volume groups on the device can be made visible by calling
L</guestfs_vgscan> followed by L</guestfs_vg_activate_all>.
The logical volume(s) can now be mounted in the usual way.

Use the reverse process to close a LUKS device.  Unmount
any logical volumes on it, deactivate the volume groups
by caling C<guestfs_vg_activate (g, 0, ["/dev/VG"])>.
Then close the mapper device by calling
L</guestfs_luks_close> on the C</dev/mapper/mapname>
device (I<not> the underlying encrypted block device).

=head2 INSPECTION

Libguestfs has APIs for inspecting an unknown disk image to find out
if it contains operating systems.  (These APIs used to be in a
separate Perl-only library called L<Sys::Guestfs::Lib(3)> but since
version 1.5.3 the most frequently used part of this library has been
rewritten in C and moved into the core code).

Add all disks belonging to the unknown virtual machine and call
L</guestfs_launch> in the usual way.

Then call L</guestfs_inspect_os>.  This function uses other libguestfs
calls and certain heuristics, and returns a list of operating systems
that were found.  An empty list means none were found.  A single
element is the root filesystem of the operating system.  For dual- or
multi-boot guests, multiple roots can be returned, each one
corresponding to a separate operating system.  (Multi-boot virtual
machines are extremely rare in the world of virtualization, but since
this scenario can happen, we have built libguestfs to deal with it.)

For each root, you can then call various C<guestfs_inspect_get_*>
functions to get additional details about that operating system.  For
example, call L</guestfs_inspect_get_type> to return the string
C<windows> or C<linux> for Windows and Linux-based operating systems
respectively.

Un*x-like and Linux-based operating systems usually consist of several
filesystems which are mounted at boot time (for example, a separate
boot partition mounted on C</boot>).  The inspection rules are able to
detect how filesystems correspond to mount points.  Call
C<guestfs_inspect_get_mountpoints> to get this mapping.  It might
return a hash table like this example:

 /boot => /dev/sda1
 /     => /dev/vg_guest/lv_root
 /usr  => /dev/vg_guest/lv_usr

The caller can then make calls to L</guestfs_mount_options> to
mount the filesystems as suggested.

Be careful to mount filesystems in the right order (eg. C</> before
C</usr>).  Sorting the keys of the hash by length, shortest first,
should work.

Inspection currently only works for some common operating systems.
Contributors are welcome to send patches for other operating systems
that we currently cannot detect.

Encrypted disks must be opened before inspection.  See
L</ENCRYPTED DISKS> for more details.  The L</guestfs_inspect_os>
function just ignores any encrypted devices.

A note on the implementation: The call L</guestfs_inspect_os> performs
inspection and caches the results in the guest handle.  Subsequent
calls to C<guestfs_inspect_get_*> return this cached information, but
I<do not> re-read the disks.  If you change the content of the guest
disks, you can redo inspection by calling L</guestfs_inspect_os>
again.  (L</guestfs_inspect_list_applications> works a little
differently from the other calls and does read the disks.  See
documentation for that function for details).

=head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS

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

=head3 DRIVE LETTERS AND PATHS

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<hivex(3)> and 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 L</guestfs_case_sensitive_path>.

=head3 ACCESSING THE WINDOWS REGISTRY

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

=head3 SYMLINKS ON NTFS-3G FILESYSTEMS

Ntfs-3g tries to rewrite "Junction Points" and NTFS "symbolic links"
to provide something which looks like a Linux symlink.  The way it
tries to do the rewriting is described here:

L<http://www.tuxera.com/community/ntfs-3g-advanced/junction-points-and-symbolic-links/>

The essential problem is that ntfs-3g simply does not have enough
information to do a correct job.  NTFS links can contain drive letters
and references to external device GUIDs that ntfs-3g has no way of
resolving.  It is almost certainly the case that libguestfs callers
should ignore what ntfs-3g does (ie. don't use L</guestfs_readlink> on
NTFS volumes).

Instead if you encounter a symbolic link on an ntfs-3g filesystem, use
L</guestfs_lgetxattr> to read the C<system.ntfs_reparse_data> extended
attribute, and read the raw reparse data from that (you can find the
format documented in various places around the web).

=head3 EXTENDED ATTRIBUTES ON NTFS-3G FILESYSTEMS

There are other useful extended attributes that can be read from
ntfs-3g filesystems (using L</guestfs_getxattr>).  See:

L<http://www.tuxera.com/community/ntfs-3g-advanced/extended-attributes/>

=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(g,path)> is
C<$g-E<gt>mount($path)> in Perl, C<g.mount(path)> in Python,
and C<Guestfs.mount g 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
used.

=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 is 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 L<guestfs-ocaml(3)>.

=item B<Perl>

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

=item B<PHP>

For documentation see C<README-PHP> supplied with libguestfs
sources or in the php-libguestfs package for your distribution.

The PHP binding only works correctly on 64 bit machines.

=item B<Python>

For documentation see L<guestfs-python(3)>.

=item B<Ruby>

For documentation see L<guestfs-ruby(3)>.

=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 (g, 1);

to have the unmount/sync done automatically for you when the handle 'g'
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)> autosync is the default.  So quick and
dirty guestfish scripts that forget to sync will work just fine, which
can make this very puzzling if you are trying to debug a problem.

Update: Autosync is enabled by default for all API users starting from
libguestfs 1.5.24.

=item Mount option C<-o sync> should not be the default.

If you use L</guestfs_mount>, then C<-o sync,noatime> are added
implicitly.  However C<-o sync> does not add any reliability benefit,
but does have a very large performance impact.

The work around is to use L</guestfs_mount_options> and set the mount
options that you actually want to use.

=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
L</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.  In earlier versions of guestfish
the error message was also unintuitive, but we have corrected this
since.  Like the Bourne shell, we should have used C<guestfish -c
command> to run commands.

=item guestfish megabyte modifiers don't work right on all commands

In recent guestfish you can use C<1M> to mean 1 megabyte (and
similarly for other modifiers).  What guestfish actually does is to
multiply the number part by the modifier part and pass the result to
the C API.  However this doesn't work for a few APIs which aren't
expecting bytes, but are already expecting some other unit
(eg. megabytes).

The most common is L</guestfs_lvcreate>.  The guestfish command:

 lvcreate LV VG 100M

does not do what you might expect.  Instead because
L</guestfs_lvcreate> is already expecting megabytes, this tries to
create a 100 I<terabyte> (100 megabytes * megabytes) logical volume.
The error message you get from this is also a little obscure.

This could be fixed in the generator by specially marking parameters
and return values which take bytes or other units.

=item Ambiguity between devices and paths

There is a subtle ambiguity in the API between a device name
(eg. C</dev/sdb2>) and a similar pathname.  A file might just happen
to be called C<sdb2> in the directory C</dev> (consider some non-Unix
VM image).

In the current API we usually resolve this ambiguity by having two
separate calls, for example L</guestfs_checksum> and
L</guestfs_checksum_device>.  Some API calls are ambiguous and
(incorrectly) resolve the problem by detecting if the path supplied
begins with C</dev/>.

To avoid both the ambiguity and the need to duplicate some calls, we
could make paths/devices into structured names.  One way to do this
would be to use a notation like grub (C<hd(0,0)>), although nobody
really likes this aspect of grub.  Another way would be to use a
structured type, equivalent to this OCaml type:

 type path = Path of string | Device of int | Partition of int * int

which would allow you to pass arguments like:

 Path "/foo/bar"
 Device 1            (* /dev/sdb, or perhaps /dev/sda *)
 Partition (1, 2)    (* /dev/sdb2 (or is it /dev/sda2 or /dev/sdb3?) *)
 Path "/dev/sdb2"    (* not a device *)

As you can see there are still problems to resolve even with this
representation.  Also consider how it might work in guestfish.

=back

=head2 PROTOCOL LIMITS

Internally libguestfs uses a message-based protocol to pass API calls
and their responses to and from a small "appliance" (see L</INTERNALS>
for plenty more detail about this).  The maximum message size used by
the protocol is slightly less than 4 MB.  For some API calls you may
need to be aware of this limit.  The API calls which may be affected
are individually documented, with a link back to this section of the
documentation.

A simple call such as L</guestfs_cat> returns its result (the file
data) in a simple string.  Because this string is at some point
internally encoded as a message, the maximum size that it can return
is slightly under 4 MB.  If the requested file is larger than this
then you will get an error.

In order to transfer large files into and out of the guest filesystem,
you need to use particular calls that support this.  The sections
L</UPLOADING> and L</DOWNLOADING> document how to do this.

You might also consider mounting the disk image using our FUSE
filesystem support (L<guestmount(1)>).

=head2 KEYS AND PASSPHRASES

Certain libguestfs calls take a parameter that contains sensitive key
material, passed in as a C string.

In the future we would hope to change the libguestfs implementation so
that keys are L<mlock(2)>-ed into physical RAM, and thus can never end
up in swap.  However this is I<not> done at the moment, because of the
complexity of such an implementation.

Therefore you should be aware that any key parameter you pass to
libguestfs might end up being written out to the swap partition.  If
this is a concern, scrub the swap partition or don't use libguestfs on
encrypted devices.

=head2 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.

See the graphical program guestfs-browser for one possible
architecture for multithreaded programs using libvirt and libguestfs.

=head2 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 L</guestfs_set_path> or set the environment variable
L</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>.

=head2 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.

=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 the libguestfs API.

=head2 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
L</guestfs_list_devices>, L</guestfs_list_partitions> and similar calls
return the true names of the devices and partitions as known to the
appliance.

=head3 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 L</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 L</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

=head3 PORTABILITY CONCERNS WITH BLOCK DEVICE NAMING

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 L</guestfs_list_devices> or L</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 SECURITY

This section discusses security implications of using libguestfs,
particularly with untrusted or malicious guests or disk images.

=head2 GENERAL SECURITY CONSIDERATIONS

Be careful with any files or data that you download from a guest (by
"download" we mean not just the L</guestfs_download> command but any
command that reads files, filenames, directories or anything else from
a disk image).  An attacker could manipulate the data to fool your
program into doing the wrong thing.  Consider cases such as:

=over 4

=item *

the data (file etc) not being present

=item *

being present but empty

=item *

being much larger than normal

=item *

containing arbitrary 8 bit data

=item *

being in an unexpected character encoding

=item *

containing homoglyphs.

=back

=head2 SECURITY OF MOUNTING FILESYSTEMS

When you mount a filesystem under Linux, mistakes in the kernel
filesystem (VFS) module can sometimes be escalated into exploits by
deliberately creating a malicious, malformed filesystem.  These
exploits are very severe for two reasons.  Firstly there are very many
filesystem drivers in the kernel, and many of them are infrequently
used and not much developer attention has been paid to the code.
Linux userspace helps potential crackers by detecting the filesystem
type and automatically choosing the right VFS driver, even if that
filesystem type is obscure or unexpected for the administrator.
Secondly, a kernel-level exploit is like a local root exploit (worse
in some ways), giving immediate and total access to the system right
down to the hardware level.

That explains why you should never mount a filesystem from an
untrusted guest on your host kernel.  How about libguestfs?  We run a
Linux kernel inside a qemu virtual machine, usually running as a
non-root user.  The attacker would need to write a filesystem which
first exploited the kernel, and then exploited either qemu
virtualization (eg. a faulty qemu driver) or the libguestfs protocol,
and finally to be as serious as the host kernel exploit it would need
to escalate its privileges to root.  This multi-step escalation,
performed by a static piece of data, is thought to be extremely hard
to do, although we never say 'never' about security issues.

In any case callers can reduce the attack surface by forcing the
filesystem type when mounting (use L</guestfs_mount_vfs>).

=head2 PROTOCOL SECURITY

The protocol is designed to be secure, being based on RFC 4506 (XDR)
with a defined upper message size.  However a program that uses
libguestfs must also take care - for example you can write a program
that downloads a binary from a disk image and executes it locally, and
no amount of protocol security will save you from the consequences.

=head2 INSPECTION SECURITY

Parts of the inspection API (see L</INSPECTION>) return untrusted
strings directly from the guest, and these could contain any 8 bit
data.  Callers should be careful to escape these before printing them
to a structured file (for example, use HTML escaping if creating a web
page).

Guest configuration may be altered in unusual ways by the
administrator of the virtual machine, and may not reflect reality
(particularly for untrusted or actively malicious guests).  For
example we parse the hostname from configuration files like
C</etc/sysconfig/network> that we find in the guest, but the guest
administrator can easily manipulate these files to provide the wrong
hostname.

The inspection API parses guest configuration using two external
libraries: Augeas (Linux configuration) and hivex (Windows Registry).
Both are designed to be robust in the face of malicious data, although
denial of service attacks are still possible, for example with
oversized configuration files.

=head2 RUNNING UNTRUSTED GUEST COMMANDS

Be very cautious about running commands from the guest.  By running a
command in the guest, you are giving CPU time to a binary that you do
not control, under the same user account as the library, albeit
wrapped in qemu virtualization.  More information and alternatives can
be found in the section L</RUNNING COMMANDS>.

=head2 CVE-2010-3851

https://bugzilla.redhat.com/642934

This security bug concerns the automatic disk format detection that
qemu does on disk images.

A raw disk image is just the raw bytes, there is no header.  Other
disk images like qcow2 contain a special header.  Qemu deals with this
by looking for one of the known headers, and if none is found then
assuming the disk image must be raw.

This allows a guest which has been given a raw disk image to write
some other header.  At next boot (or when the disk image is accessed
by libguestfs) qemu would do autodetection and think the disk image
format was, say, qcow2 based on the header written by the guest.

This in itself would not be a problem, but qcow2 offers many features,
one of which is to allow a disk image to refer to another image
(called the "backing disk").  It does this by placing the path to the
backing disk into the qcow2 header.  This path is not validated and
could point to any host file (eg. "/etc/passwd").  The backing disk is
then exposed through "holes" in the qcow2 disk image, which of course
is completely under the control of the attacker.

In libguestfs this is rather hard to exploit except under two
circumstances:

=over 4

=item 1.

You have enabled the network or have opened the disk in write mode.

=item 2.

You are also running untrusted code from the guest (see
L</RUNNING COMMANDS>).

=back

The way to avoid this is to specify the expected disk format when
adding disks (the optional C<format> option to
L</guestfs_add_drive_opts>).  You should always do this if the disk is
raw format, and it's a good idea for other cases too.

For disks added from libvirt using calls like L</guestfs_add_domain>,
the format is fetched from libvirt and passed through.

For libguestfs tools, use the I<--format> command line parameter as
appropriate.

=head1 CONNECTION MANAGEMENT

=head2 guestfs_h *

C<guestfs_h> is the opaque type representing a connection handle.
Create a handle by calling L</guestfs_create>.  Call L</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 L</guestfs_add_drive_opts> (or one of the equivalent
calls) 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 L</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 *g);

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

=head1 ERROR HANDLING

API functions can return errors.  For example, almost all functions
that return C<int> will return C<-1> to indicate an error.

Additional information is available for errors: an error message
string and optionally an error number (errno) if the thing that failed
was a system call.

You can get at the additional information about the last error on the
handle by calling L</guestfs_last_error>, L</guestfs_last_errno>,
and/or by setting up an error handler with
L</guestfs_set_error_handler>.

When the handle is created, a default error handler is installed which
prints the error message string to C<stderr>.  For small short-running
command line programs it is sufficient to do:

 if (guestfs_launch (g) == -1)
   exit (EXIT_FAILURE);

since the default error handler will ensure that an error message has
been printed to C<stderr> before the program exits.

For other programs the caller will almost certainly want to install an
alternate error handler or do error handling in-line like this:

 g = guestfs_create ();
 
 /* This disables the default behaviour of printing errors
    on stderr. */
 guestfs_set_error_handler (g, NULL, NULL);
 
 if (guestfs_launch (g) == -1) {
   /* Examine the error message and print it etc. */
   char *msg = guestfs_last_error (g);
   int errnum = guestfs_last_errno (g);
   fprintf (stderr, "%s\n", msg);
   /* ... */
  }

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 L</guestfs_set_out_of_memory_handler>.

L</guestfs_create> returns C<NULL> if the handle cannot be created,
and because there is no handle if this happens there is no way to get
additional error information.  However L</guestfs_create> is supposed
to be a lightweight operation which can only fail because of
insufficient memory (it returns NULL in this case).

=head2 guestfs_last_error

 const char *guestfs_last_error (guestfs_h *g);

This returns the last error message that happened on C<g>.  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
L</guestfs_close> is called.

=head2 guestfs_last_errno

 int guestfs_last_errno (guestfs_h *g);

This returns the last error number (errno) that happened on C<g>.

If successful, an errno integer not equal to zero is returned.

If no error, this returns 0.  This call can return 0 in three
situations:

=over 4

=item 1.

There has not been any error on the handle.

=item 2.

There has been an error but the errno was meaningless.  This
corresponds to the case where the error did not come from a
failed system call, but for some other reason.

=item 3.

There was an error from a failed system call, but for some
reason the errno was not captured and returned.  This usually
indicates a bug in libguestfs.

=back

Libguestfs tries to convert the errno from inside the applicance into
a corresponding errno for the caller (not entirely trivial: the
appliance might be running a completely different operating system
from the library and error numbers are not standardized across
Un*xen).  If this could not be done, then the error is translated to
C<EINVAL>.  In practice this should only happen in very rare
circumstances.

=head2 guestfs_set_error_handler

 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
                                           void *opaque,
                                           const char *msg);
 void guestfs_set_error_handler (guestfs_h *g,
                                 guestfs_error_handler_cb cb,
                                 void *opaque);

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.

C<errno> is not passed to the callback.  To get that the callback must
call L</guestfs_last_errno>.

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 *g,
                                                     void **opaque_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 *g,
                                        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 *g);

This returns the current out of memory handler.

=head1 API CALLS

@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 GUESTFISH supported COMMAND

In L<guestfish(3)> there is a handy interactive command
C<supported> which prints out the available groups and
whether they are supported by this build of libguestfs.
Note however that you have to do C<run> first.

=head2 SINGLE CALLS AT COMPILE TIME

Since version 1.5.8, C<E<lt>guestfs.hE<gt>> defines symbols
for each C API function, such as:

 #define LIBGUESTFS_HAVE_DD 1

if L</guestfs_dd> is available.

Before version 1.5.8, if you needed to test whether a single
libguestfs function is available at compile time, we recommended 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 <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <dlfcn.h>
 #include <guestfs.h>
 
 main ()
 {
 #ifdef LIBGUESTFS_HAVE_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 (EXIT_FAILURE);
   }
   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

=head1 CALLS WITH OPTIONAL ARGUMENTS

A recent feature of the API is the introduction of calls which take
optional arguments.  In C these are declared 3 ways.  The main way is
as a call which takes variable arguments (ie. C<...>), as in this
example:

 int guestfs_add_drive_opts (guestfs_h *g, const char *filename, ...);

Call this with a list of optional arguments, terminated by C<-1>.
So to call with no optional arguments specified:

 guestfs_add_drive_opts (g, filename, -1);

With a single optional argument:

 guestfs_add_drive_opts (g, filename,
                         GUESTFS_ADD_DRIVE_OPTS_FORMAT, "qcow2",
                         -1);

With two:

 guestfs_add_drive_opts (g, filename,
                         GUESTFS_ADD_DRIVE_OPTS_FORMAT, "qcow2",
                         GUESTFS_ADD_DRIVE_OPTS_READONLY, 1,
                         -1);

and so forth.  Don't forget the terminating C<-1> otherwise
Bad Things will happen!

=head2 USING va_list FOR OPTIONAL ARGUMENTS

The second variant has the same name with the suffix C<_va>, which
works the same way but takes a C<va_list>.  See the C manual for
details.  For the example function, this is declared:

 int guestfs_add_drive_opts_va (guestfs_h *g, const char *filename,
                                va_list args);

=head2 CONSTRUCTING OPTIONAL ARGUMENTS

The third variant is useful where you need to construct these
calls.  You pass in a structure where you fill in the optional
fields.  The structure has a bitmask as the first element which
you must set to indicate which fields you have filled in.  For
our example function the structure and call are declared:

 struct guestfs_add_drive_opts_argv {
   uint64_t bitmask;
   int readonly;
   const char *format;
   /* ... */
 };
 int guestfs_add_drive_opts_argv (guestfs_h *g, const char *filename,
              const struct guestfs_add_drive_opts_argv *optargs);

You could call it like this:

 struct guestfs_add_drive_opts_argv optargs = {
   .bitmask = GUESTFS_ADD_DRIVE_OPTS_READONLY_BITMASK |
              GUESTFS_ADD_DRIVE_OPTS_FORMAT_BITMASK,
   .readonly = 1,
   .format = "qcow2"
 };
 
 guestfs_add_drive_opts_argv (g, filename, &optargs);

Notes:

=over 4

=item *

The C<_BITMASK> suffix on each option name when specifying the
bitmask.

=item *

You do not need to fill in all fields of the structure.

=item *

There must be a one-to-one correspondence between fields of the
structure that are filled in, and bits set in the bitmask.

=back

=head2 OPTIONAL ARGUMENTS IN OTHER LANGUAGES

In other languages, optional arguments are expressed in the
way that is natural for that language.  We refer you to the
language-specific documentation for more details on that.

For guestfish, see L<guestfish(1)/OPTIONAL ARGUMENTS>.

=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 *g,
                                        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 *g,
                                            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
L</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 *g,
                                        guestfs_launch_done_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).

=head2 guestfs_set_close_callback

 typedef void (*guestfs_close_cb) (guestfs_h *g, void *opaque);
 void guestfs_set_close_callback (guestfs_h *g,
                                  guestfs_close_cb cb,
                                  void *opaque);

The callback function C<cb> will be called while the handle
is being closed (synchronously from L</guestfs_close>).

Note that libguestfs installs an L<atexit(3)> handler to try to
clean up handles that are open when the program exits.  This
means that this callback might be called indirectly from
L<exit(3)>, which can cause unexpected problems in higher-level
languages (eg. if your HLL interpreter has already been cleaned
up by the time this is called, and if your callback then jumps
into some HLL function).

=head2 guestfs_set_progress_callback

 typedef void (*guestfs_progress_cb) (guestfs_h *g, void *opaque,
                                      int proc_nr, int serial,
                                      uint64_t position, uint64_t total);
 void guestfs_set_progress_callback (guestfs_h *g,
                                     guestfs_progress_cb cb,
                                     void *opaque);

Some long-running operations can generate progress messages.  If
this callback is registered, then it will be called each time a
progress message is generated (usually two seconds after the
operation started, and three times per second thereafter until
it completes, although the frequency may change in future versions).

The callback receives two numbers: C<position> and C<total>.
The units of C<total> are not defined, although for some
operations C<total> may relate in some way to the amount of
data to be transferred (eg. in bytes or megabytes), and
C<position> may be the portion which has been transferred.

The only defined and stable parts of the API are:

=over 4

=item *

The callback can display to the user some type of progress bar or
indicator which shows the ratio of C<position>:C<total>.

=item *

0 E<lt>= C<position> E<lt>= C<total>

=item *

If any progress notification is sent during a call, then a final
progress notification is always sent when C<position> = C<total>.

This is to simplify caller code, so callers can easily set the
progress indicator to "100%" at the end of the operation, without
requiring special code to detect this case.

=back

The callback also receives the procedure number and serial number of
the call.  These are only useful for debugging protocol issues, and
the callback can normally ignore them.  The callback may want to
print these numbers in error messages or debugging messages.

=head1 PRIVATE DATA AREA

You can attach named pieces of private data to the libguestfs handle,
and fetch them by name for the lifetime of the handle.  This is called
the private data area and is only available from the C API.

To attach a named piece of data, use the following call:

 void guestfs_set_private (guestfs_h *g, const char *key, void *data);

C<key> is the name to associate with this data, and C<data> is an
arbitrary pointer (which can be C<NULL>).  Any previous item with the
same name is overwritten.

You can use any C<key> you want, but names beginning with an
underscore character are reserved for internal libguestfs purposes
(for implementing language bindings).  It is recommended to prefix the
name with some unique string to avoid collisions with other users.

To retrieve the pointer, use:

 void *guestfs_get_private (guestfs_h *g, const char *key);

This function returns C<NULL> if either no data is found associated
with C<key>, or if the user previously set the C<key>'s C<data>
pointer to C<NULL>.

Libguestfs does not try to look at or interpret the C<data> pointer in
any way.  As far as libguestfs is concerned, it need not be a valid
pointer at all.  In particular, libguestfs does I<not> try to free the
data when the handle is closed.  If the data must be freed, then the
caller must either free it before calling L</guestfs_close> or must
set up a close callback to do it (see L</guestfs_set_close_callback>,
and note that only one callback can be registered for a handle).

The private data area is implemented using a hash table, and should be
reasonably efficient for moderate numbers of keys.

=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 L</guestfsd>.  The library talks to
L</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 L</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 L</guestfs_add_drive> can only
be issued when in the CONFIG state.

The API offers one call that goes from CONFIG through LAUNCHING to
READY.  L</guestfs_launch> blocks until the child process is READY to
accept commands (or until some failure or timeout).
L</guestfs_launch> internally moves the state from CONFIG to LAUNCHING
while it is running.

API actions such as L</guestfs_mount> can only be issued when in the
READY state.  These API calls block waiting for the command to be
carried out (ie. the state to transition to BUSY and then back to
READY).  There are no non-blocking versions, and no way to issue more
than one command per handle at the same time.

Finally, the child process sends asynchronous messages back to the
main program, such as kernel log messages.  You can register a
callback to receive these messages.

=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.

For functions that take optional arguments, the optional arguments are
encoded in the C<guestfs_I<foo>_args> structure in the same way as
ordinary arguments.  A bitmask in the header indicates which optional
arguments are meaningful.  The bitmask is also checked to see if it
contains bits set which the daemon does not know about (eg. if more
optional arguments were added in a later version of the library), and
this causes the call to be rejected.

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

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 L</guestfs_launch> waits for.

=head3 PROGRESS NOTIFICATION MESSAGES

The daemon may send progress notification messages at any time.  These
are distinguished by the normal length word being replaced by
C<GUESTFS_PROGRESS_FLAG>, followed by a fixed size progress message.

The library turns them into progress callbacks (see
C<guestfs_set_progress_callback>) if there is a callback registered,
or discards them if not.

The daemon self-limits the frequency of progress messages it sends
(see C<daemon/proto.c:notify_progress>).  Not all calls generate
progress messages.

=head1 LIBGUESTFS VERSION NUMBERS

Since April 2010, libguestfs has started to make separate development
and stable releases, along with corresponding branches in our git
repository.  These separate releases can be identified by version
number:

                 even numbers for stable: 1.2.x, 1.4.x, ...
       .-------- odd numbers for development: 1.3.x, 1.5.x, ...
       |
       v
 1  .  3  .  5
 ^           ^
 |           |
 |           `-------- sub-version
 |
 `------ always '1' because we don't change the ABI

Thus "1.3.5" is the 5th update to the development branch "1.3".

As time passes we cherry pick fixes from the development branch and
backport those into the stable branch, the effect being that the
stable branch should get more stable and less buggy over time.  So the
stable releases are ideal for people who don't need new features but
would just like the software to work.

Our criteria for backporting changes are:

=over 4

=item *

Documentation changes which don't affect any code are
backported unless the documentation refers to a future feature
which is not in stable.

=item *

Bug fixes which are not controversial, fix obvious problems, and
have been well tested are backported.

=item *

Simple rearrangements of code which shouldn't affect how it works get
backported.  This is so that the code in the two branches doesn't get
too far out of step, allowing us to backport future fixes more easily.

=item *

We I<don't> backport new features, new APIs, new tools etc, except in
one exceptional case: the new feature is required in order to
implement an important bug fix.

=back

A new stable branch starts when we think the new features in
development are substantial and compelling enough over the current
stable branch to warrant it.  When that happens we create new stable
and development versions 1.N.0 and 1.(N+1).0 [N is even].  The new
dot-oh release won't necessarily be so stable at this point, but by
backporting fixes from development, that branch will stabilize over
time.

=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 (g, 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 (g, 1)>.

=item TMPDIR

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

If libguestfs was compiled to use the supermin appliance then the
real appliance is cached in this directory, shared between all
handles belonging to the same EUID.  You can use C<$TMPDIR> to
configure another directory to use in case C</tmp> is not large
enough.

=back

=head1 SEE ALSO

L<guestfs-examples(3)>,
L<guestfs-ocaml(3)>,
L<guestfs-python(3)>,
L<guestfs-ruby(3)>,
L<guestfish(1)>,
L<guestmount(1)>,
L<virt-cat(1)>,
L<virt-df(1)>,
L<virt-edit(1)>,
L<virt-filesystems(1)>,
L<virt-inspector(1)>,
L<virt-list-filesystems(1)>,
L<virt-list-partitions(1)>,
L<virt-ls(1)>,
L<virt-make-fs(1)>,
L<virt-rescue(1)>,
L<virt-tar(1)>,
L<virt-win-reg(1)>,
L<qemu(1)>,
L<febootstrap(1)>,
L<hivex(3)>,
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) 2009-2010 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