5 guestfs - Library for accessing and modifying virtual machine images
11 guestfs_h *g = guestfs_create ();
12 guestfs_add_drive (g, "guest.img");
14 guestfs_mount (g, "/dev/sda1", "/");
15 guestfs_touch (g, "/hello");
16 guestfs_umount (g, "/");
20 cc prog.c -o prog -lguestfs
22 cc prog.c -o prog `pkg-config libguestfs --cflags --libs`
26 Libguestfs is a library for accessing and modifying guest disk images.
27 Amongst the things this is good for: making batch configuration
28 changes to guests, getting disk used/free statistics (see also:
29 virt-df), migrating between virtualization systems (see also:
30 virt-p2v), performing partial backups, performing partial guest
31 clones, cloning guests and changing registry/UUID/hostname info, and
34 Libguestfs uses Linux kernel and qemu code, and can access any type of
35 guest filesystem that Linux and qemu can, including but not limited
36 to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition
37 schemes, qcow, qcow2, vmdk.
39 Libguestfs provides ways to enumerate guest storage (eg. partitions,
40 LVs, what filesystem is in each LV, etc.). It can also run commands
41 in the context of the guest. Also you can access filesystems over
44 Libguestfs is a library that can be linked with C and C++ management
45 programs (or management programs written in OCaml, Perl, Python, Ruby,
46 Java, PHP, Haskell or C#). You can also use it from shell scripts or the
49 You don't need to be root to use libguestfs, although obviously you do
50 need enough permissions to access the disk images.
52 Libguestfs is a large API because it can do many things. For a gentle
53 introduction, please read the L</API OVERVIEW> section next.
55 There are also some example programs in the L<guestfs-examples(3)>
60 This section provides a gentler overview of the libguestfs API. We
61 also try to group API calls together, where that may not be obvious
62 from reading about the individual calls in the main section of this
67 Before you can use libguestfs calls, you have to create a handle.
68 Then you must add at least one disk image to the handle, followed by
69 launching the handle, then performing whatever operations you want,
70 and finally closing the handle. By convention we use the single
71 letter C<g> for the name of the handle variable, although of course
72 you can use any name you want.
74 The general structure of all libguestfs-using programs looks like
77 guestfs_h *g = guestfs_create ();
79 /* Call guestfs_add_drive additional times if there are
80 * multiple disk images.
82 guestfs_add_drive (g, "guest.img");
84 /* Most manipulation calls won't work until you've launched
85 * the handle 'g'. You have to do this _after_ adding drives
86 * and _before_ other commands.
90 /* Now you can examine what partitions, LVs etc are available.
92 char **partitions = guestfs_list_partitions (g);
93 char **logvols = guestfs_lvs (g);
95 /* To access a filesystem in the image, you must mount it.
97 guestfs_mount (g, "/dev/sda1", "/");
99 /* Now you can perform filesystem actions on the guest
102 guestfs_touch (g, "/hello");
104 /* You only need to call guestfs_sync if you have made
105 * changes to the guest image. (But if you've made changes
106 * then you *must* sync). See also: guestfs_umount and
107 * guestfs_umount_all calls.
111 /* Close the handle 'g'. */
114 The code above doesn't include any error checking. In real code you
115 should check return values carefully for errors. In general all
116 functions that return integers return C<-1> on error, and all
117 functions that return pointers return C<NULL> on error. See section
118 L</ERROR HANDLING> below for how to handle errors, and consult the
119 documentation for each function call below to see precisely how they
120 return error indications.
124 The image filename (C<"guest.img"> in the example above) could be a
125 disk image from a virtual machine, a L<dd(1)> copy of a physical hard
126 disk, an actual block device, or simply an empty file of zeroes that
127 you have created through L<posix_fallocate(3)>. Libguestfs lets you
128 do useful things to all of these.
130 The call you should use in modern code for adding drives is
131 L</guestfs_add_drive_opts>. To add a disk image, allowing writes, and
132 specifying that the format is raw, do:
134 guestfs_add_drive_opts (g, filename,
135 GUESTFS_ADD_DRIVE_OPTS_FORMAT, "raw",
138 You can add a disk read-only using:
140 guestfs_add_drive_opts (g, filename,
141 GUESTFS_ADD_DRIVE_OPTS_FORMAT, "raw",
142 GUESTFS_ADD_DRIVE_OPTS_READONLY, 1,
145 or by calling the older function L</guestfs_add_drive_ro>. In either
146 case libguestfs won't modify the file.
148 Be extremely cautious if the disk image is in use, eg. if it is being
149 used by a virtual machine. Adding it read-write will almost certainly
150 cause disk corruption, but adding it read-only is safe.
152 You must add at least one disk image, and you may add multiple disk
153 images. In the API, the disk images are usually referred to as
154 C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
157 Once L</guestfs_launch> has been called you cannot add any more images.
158 You can call L</guestfs_list_devices> to get a list of the device
159 names, in the order that you added them. See also L</BLOCK DEVICE
164 Before you can read or write files, create directories and so on in a
165 disk image that contains filesystems, you have to mount those
166 filesystems using L</guestfs_mount>. If you already know that a disk
167 image contains (for example) one partition with a filesystem on that
168 partition, then you can mount it directly:
170 guestfs_mount (g, "/dev/sda1", "/");
172 where C</dev/sda1> means literally the first partition (C<1>) of the
173 first disk image that we added (C</dev/sda>). If the disk contains
174 Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).
176 If you are given a disk image and you don't know what it contains then
177 you have to find out. Libguestfs can do that too: use
178 L</guestfs_list_partitions> and L</guestfs_lvs> to list possible
179 partitions and LVs, and either try mounting each to see what is
180 mountable, or else examine them with L</guestfs_vfs_type> or
181 L</guestfs_file>. Libguestfs also has a set of APIs for inspection of
182 disk images (see L</INSPECTION> below). But you might find it easier
183 to look at higher level programs built on top of libguestfs, in
184 particular L<virt-inspector(1)>.
186 To mount a disk image read-only, use L</guestfs_mount_ro>. There are
187 several other variations of the C<guestfs_mount_*> call.
189 =head2 FILESYSTEM ACCESS AND MODIFICATION
191 The majority of the libguestfs API consists of fairly low-level calls
192 for accessing and modifying the files, directories, symlinks etc on
193 mounted filesystems. There are over a hundred such calls which you
194 can find listed in detail below in this man page, and we don't even
195 pretend to cover them all in this overview.
197 Specify filenames as full paths, starting with C<"/"> and including
200 For example, if you mounted a filesystem at C<"/"> and you want to
201 read the file called C<"etc/passwd"> then you could do:
203 char *data = guestfs_cat (g, "/etc/passwd");
205 This would return C<data> as a newly allocated buffer containing the
206 full content of that file (with some conditions: see also
207 L</DOWNLOADING> below), or C<NULL> if there was an error.
209 As another example, to create a top-level directory on that filesystem
210 called C<"var"> you would do:
212 guestfs_mkdir (g, "/var");
214 To create a symlink you could do:
216 guestfs_ln_s (g, "/etc/init.d/portmap",
217 "/etc/rc3.d/S30portmap");
219 Libguestfs will reject attempts to use relative paths and there is no
220 concept of a current working directory.
222 Libguestfs can return errors in many situations: for example if the
223 filesystem isn't writable, or if a file or directory that you
224 requested doesn't exist. If you are using the C API (documented here)
225 you have to check for those error conditions after each call. (Other
226 language bindings turn these errors into exceptions).
228 File writes are affected by the per-handle umask, set by calling
229 L</guestfs_umask> and defaulting to 022. See L</UMASK>.
233 Libguestfs contains API calls to read, create and modify partition
234 tables on disk images.
236 In the common case where you want to create a single partition
237 covering the whole disk, you should use the L</guestfs_part_disk>
240 const char *parttype = "mbr";
241 if (disk_is_larger_than_2TB)
243 guestfs_part_disk (g, "/dev/sda", parttype);
245 Obviously this effectively wipes anything that was on that disk image
250 Libguestfs provides access to a large part of the LVM2 API, such as
251 L</guestfs_lvcreate> and L</guestfs_vgremove>. It won't make much sense
252 unless you familiarize yourself with the concepts of physical volumes,
253 volume groups and logical volumes.
255 This author strongly recommends reading the LVM HOWTO, online at
256 L<http://tldp.org/HOWTO/LVM-HOWTO/>.
260 Use L</guestfs_cat> to download small, text only files. This call
261 is limited to files which are less than 2 MB and which cannot contain
262 any ASCII NUL (C<\0>) characters. However it has a very simple
265 L</guestfs_read_file> can be used to read files which contain
266 arbitrary 8 bit data, since it returns a (pointer, size) pair.
267 However it is still limited to "small" files, less than 2 MB.
269 L</guestfs_download> can be used to download any file, with no
270 limits on content or size (even files larger than 4 GB).
272 To download multiple files, see L</guestfs_tar_out> and
277 It's often the case that you want to write a file or files to the disk
280 To write a small file with fixed content, use L</guestfs_write>. To
281 create a file of all zeroes, use L</guestfs_truncate_size> (sparse) or
282 L</guestfs_fallocate64> (with all disk blocks allocated). There are a
283 variety of other functions for creating test files, for example
284 L</guestfs_fill> and L</guestfs_fill_pattern>.
286 To upload a single file, use L</guestfs_upload>. This call has no
287 limits on file content or size (even files larger than 4 GB).
289 To upload multiple files, see L</guestfs_tar_in> and L</guestfs_tgz_in>.
291 However the fastest way to upload I<large numbers of arbitrary files>
292 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
293 L<mkisofs(8)>), then attach this using L</guestfs_add_drive_ro>. If
294 you add the drive in a predictable way (eg. adding it last after all
295 other drives) then you can get the device name from
296 L</guestfs_list_devices> and mount it directly using
297 L</guestfs_mount_ro>. Note that squashfs images are sometimes
298 non-portable between kernel versions, and they don't support labels or
299 UUIDs. If you want to pre-build an image or you need to mount it
300 using a label or UUID, use an ISO image instead.
304 There are various different commands for copying between files and
305 devices and in and out of the guest filesystem. These are summarised
310 =item B<file> to B<file>
312 Use L</guestfs_cp> to copy a single file, or
313 L</guestfs_cp_a> to copy directories recursively.
315 =item B<file or device> to B<file or device>
317 Use L</guestfs_dd> which efficiently uses L<dd(1)>
318 to copy between files and devices in the guest.
320 Example: duplicate the contents of an LV:
322 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
324 The destination (C</dev/VG/Copy>) must be at least as large as the
325 source (C</dev/VG/Original>). To copy less than the whole
326 source device, use L</guestfs_copy_size>.
328 =item B<file on the host> to B<file or device>
330 Use L</guestfs_upload>. See L</UPLOADING> above.
332 =item B<file or device> to B<file on the host>
334 Use L</guestfs_download>. See L</DOWNLOADING> above.
340 L</guestfs_ll> is just designed for humans to read (mainly when using
341 the L<guestfish(1)>-equivalent command C<ll>).
343 L</guestfs_ls> is a quick way to get a list of files in a directory
344 from programs, as a flat list of strings.
346 L</guestfs_readdir> is a programmatic way to get a list of files in a
347 directory, plus additional information about each one. It is more
348 equivalent to using the L<readdir(3)> call on a local filesystem.
350 L</guestfs_find> and L</guestfs_find0> can be used to recursively list
353 =head2 RUNNING COMMANDS
355 Although libguestfs is primarily an API for manipulating files
356 inside guest images, we also provide some limited facilities for
357 running commands inside guests.
359 There are many limitations to this:
365 The kernel version that the command runs under will be different
366 from what it expects.
370 If the command needs to communicate with daemons, then most likely
371 they won't be running.
375 The command will be running in limited memory.
379 The network may not be available unless you enable it
380 (see L</guestfs_set_network>).
384 Only supports Linux guests (not Windows, BSD, etc).
388 Architecture limitations (eg. won't work for a PPC guest on
393 For SELinux guests, you may need to enable SELinux and load policy
394 first. See L</SELINUX> in this manpage.
398 I<Security:> It is not safe to run commands from untrusted, possibly
399 malicious guests. These commands may attempt to exploit your program
400 by sending unexpected output. They could also try to exploit the
401 Linux kernel or qemu provided by the libguestfs appliance. They could
402 use the network provided by the libguestfs appliance to bypass
403 ordinary network partitions and firewalls. They could use the
404 elevated privileges or different SELinux context of your program
407 A secure alternative is to use libguestfs to install a "firstboot"
408 script (a script which runs when the guest next boots normally), and
409 to have this script run the commands you want in the normal context of
410 the running guest, network security and so on. For information about
411 other security issues, see L</SECURITY>.
415 The two main API calls to run commands are L</guestfs_command> and
416 L</guestfs_sh> (there are also variations).
418 The difference is that L</guestfs_sh> runs commands using the shell, so
419 any shell globs, redirections, etc will work.
421 =head2 CONFIGURATION FILES
423 To read and write configuration files in Linux guest filesystems, we
424 strongly recommend using Augeas. For example, Augeas understands how
425 to read and write, say, a Linux shadow password file or X.org
426 configuration file, and so avoids you having to write that code.
428 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
429 don't document Augeas itself here because there is excellent
430 documentation on the L<http://augeas.net/> website.
432 If you don't want to use Augeas (you fool!) then try calling
433 L</guestfs_read_lines> to get the file as a list of lines which
434 you can iterate over.
438 We support SELinux guests. To ensure that labeling happens correctly
439 in SELinux guests, you need to enable SELinux and load the guest's
446 Before launching, do:
448 guestfs_set_selinux (g, 1);
452 After mounting the guest's filesystem(s), load the policy. This
453 is best done by running the L<load_policy(8)> command in the
456 guestfs_sh (g, "/usr/sbin/load_policy");
458 (Older versions of C<load_policy> require you to specify the
459 name of the policy file).
463 Optionally, set the security context for the API. The correct
464 security context to use can only be known by inspecting the
465 guest. As an example:
467 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
471 This will work for running commands and editing existing files.
473 When new files are created, you may need to label them explicitly,
474 for example by running the external command
475 C<restorecon pathname>.
479 Certain calls are affected by the current file mode creation mask (the
480 "umask"). In particular ones which create files or directories, such
481 as L</guestfs_touch>, L</guestfs_mknod> or L</guestfs_mkdir>. This
482 affects either the default mode that the file is created with or
483 modifies the mode that you supply.
485 The default umask is C<022>, so files are created with modes such as
486 C<0644> and directories with C<0755>.
488 There are two ways to avoid being affected by umask. Either set umask
489 to 0 (call C<guestfs_umask (g, 0)> early after launching). Or call
490 L</guestfs_chmod> after creating each file or directory.
492 For more information about umask, see L<umask(2)>.
494 =head2 ENCRYPTED DISKS
496 Libguestfs allows you to access Linux guests which have been
497 encrypted using whole disk encryption that conforms to the
498 Linux Unified Key Setup (LUKS) standard. This includes
499 nearly all whole disk encryption systems used by modern
502 Use L</guestfs_vfs_type> to identify LUKS-encrypted block
503 devices (it returns the string C<crypto_LUKS>).
505 Then open these devices by calling L</guestfs_luks_open>.
506 Obviously you will require the passphrase!
508 Opening a LUKS device creates a new device mapper device
509 called C</dev/mapper/mapname> (where C<mapname> is the
510 string you supply to L</guestfs_luks_open>).
511 Reads and writes to this mapper device are decrypted from and
512 encrypted to the underlying block device respectively.
514 LVM volume groups on the device can be made visible by calling
515 L</guestfs_vgscan> followed by L</guestfs_vg_activate_all>.
516 The logical volume(s) can now be mounted in the usual way.
518 Use the reverse process to close a LUKS device. Unmount
519 any logical volumes on it, deactivate the volume groups
520 by caling C<guestfs_vg_activate (g, 0, ["/dev/VG"])>.
521 Then close the mapper device by calling
522 L</guestfs_luks_close> on the C</dev/mapper/mapname>
523 device (I<not> the underlying encrypted block device).
527 Libguestfs has APIs for inspecting an unknown disk image to find out
528 if it contains operating systems. (These APIs used to be in a
529 separate Perl-only library called L<Sys::Guestfs::Lib(3)> but since
530 version 1.5.3 the most frequently used part of this library has been
531 rewritten in C and moved into the core code).
533 Add all disks belonging to the unknown virtual machine and call
534 L</guestfs_launch> in the usual way.
536 Then call L</guestfs_inspect_os>. This function uses other libguestfs
537 calls and certain heuristics, and returns a list of operating systems
538 that were found. An empty list means none were found. A single
539 element is the root filesystem of the operating system. For dual- or
540 multi-boot guests, multiple roots can be returned, each one
541 corresponding to a separate operating system. (Multi-boot virtual
542 machines are extremely rare in the world of virtualization, but since
543 this scenario can happen, we have built libguestfs to deal with it.)
545 For each root, you can then call various C<guestfs_inspect_get_*>
546 functions to get additional details about that operating system. For
547 example, call L</guestfs_inspect_get_type> to return the string
548 C<windows> or C<linux> for Windows and Linux-based operating systems
551 Un*x-like and Linux-based operating systems usually consist of several
552 filesystems which are mounted at boot time (for example, a separate
553 boot partition mounted on C</boot>). The inspection rules are able to
554 detect how filesystems correspond to mount points. Call
555 C<guestfs_inspect_get_mountpoints> to get this mapping. It might
556 return a hash table like this example:
559 / => /dev/vg_guest/lv_root
560 /usr => /dev/vg_guest/lv_usr
562 The caller can then make calls to L</guestfs_mount_options> to
563 mount the filesystems as suggested.
565 Be careful to mount filesystems in the right order (eg. C</> before
566 C</usr>). Sorting the keys of the hash by length, shortest first,
569 Inspection currently only works for some common operating systems.
570 Contributors are welcome to send patches for other operating systems
571 that we currently cannot detect.
573 Encrypted disks must be opened before inspection. See
574 L</ENCRYPTED DISKS> for more details. The L</guestfs_inspect_os>
575 function just ignores any encrypted devices.
577 A note on the implementation: The call L</guestfs_inspect_os> performs
578 inspection and caches the results in the guest handle. Subsequent
579 calls to C<guestfs_inspect_get_*> return this cached information, but
580 I<do not> re-read the disks. If you change the content of the guest
581 disks, you can redo inspection by calling L</guestfs_inspect_os>
582 again. (L</guestfs_inspect_list_applications> works a little
583 differently from the other calls and does read the disks. See
584 documentation for that function for details).
586 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
588 Libguestfs can mount NTFS partitions. It does this using the
589 L<http://www.ntfs-3g.org/> driver.
591 DOS and Windows still use drive letters, and the filesystems are
592 always treated as case insensitive by Windows itself, and therefore
593 you might find a Windows configuration file referring to a path like
594 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
595 that directory might be referred to as C</WINDOWS/System32>.
597 Drive letter mappings are outside the scope of libguestfs. You have
598 to use libguestfs to read the appropriate Windows Registry and
599 configuration files, to determine yourself how drives are mapped (see
600 also L<hivex(3)> and L<virt-inspector(1)>).
602 Replacing backslash characters with forward slash characters is also
603 outside the scope of libguestfs, but something that you can easily do.
605 Where we can help is in resolving the case insensitivity of paths.
606 For this, call L</guestfs_case_sensitive_path>.
608 Libguestfs also provides some help for decoding Windows Registry
609 "hive" files, through the library C<hivex> which is part of the
610 libguestfs project although ships as a separate tarball. You have to
611 locate and download the hive file(s) yourself, and then pass them to
612 C<hivex> functions. See also the programs L<hivexml(1)>,
613 L<hivexsh(1)>, L<hivexregedit(1)> and L<virt-win-reg(1)> for more help
616 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
618 Although we don't want to discourage you from using the C API, we will
619 mention here that the same API is also available in other languages.
621 The API is broadly identical in all supported languages. This means
622 that the C call C<guestfs_mount(g,path)> is
623 C<$g-E<gt>mount($path)> in Perl, C<g.mount(path)> in Python,
624 and C<Guestfs.mount g path> in OCaml. In other words, a
625 straightforward, predictable isomorphism between each language.
627 Error messages are automatically transformed
628 into exceptions if the language supports it.
630 We don't try to "object orientify" parts of the API in OO languages,
631 although contributors are welcome to write higher level APIs above
632 what we provide in their favourite languages if they wish.
638 You can use the I<guestfs.h> header file from C++ programs. The C++
639 API is identical to the C API. C++ classes and exceptions are not
644 The C# bindings are highly experimental. Please read the warnings
645 at the top of C<csharp/Libguestfs.cs>.
649 This is the only language binding that is working but incomplete.
650 Only calls which return simple integers have been bound in Haskell,
651 and we are looking for help to complete this binding.
655 Full documentation is contained in the Javadoc which is distributed
660 For documentation see L<guestfs-ocaml(3)>.
664 For documentation see L<Sys::Guestfs(3)>.
668 For documentation see C<README-PHP> supplied with libguestfs
669 sources or in the php-libguestfs package for your distribution.
671 The PHP binding only works correctly on 64 bit machines.
675 For documentation see L<guestfs-python(3)>.
679 For documentation see L<guestfs-ruby(3)>.
681 =item B<shell scripts>
683 For documentation see L<guestfish(1)>.
687 =head2 LIBGUESTFS GOTCHAS
689 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
690 system [...] that works in the way it is documented but is
691 counterintuitive and almost invites mistakes."
693 Since we developed libguestfs and the associated tools, there are
694 several things we would have designed differently, but are now stuck
695 with for backwards compatibility or other reasons. If there is ever a
696 libguestfs 2.0 release, you can expect these to change. Beware of
701 =item Autosync / forgetting to sync.
703 When modifying a filesystem from C or another language, you B<must>
704 unmount all filesystems and call L</guestfs_sync> explicitly before
705 you close the libguestfs handle. You can also call:
707 guestfs_set_autosync (g, 1);
709 to have the unmount/sync done automatically for you when the handle 'g'
710 is closed. (This feature is called "autosync", L</guestfs_set_autosync>
713 If you forget to do this, then it is entirely possible that your
714 changes won't be written out, or will be partially written, or (very
715 rarely) that you'll get disk corruption.
717 Note that in L<guestfish(3)> autosync is the default. So quick and
718 dirty guestfish scripts that forget to sync will work just fine, which
719 can make this very puzzling if you are trying to debug a problem.
721 Update: Autosync is enabled by default for all API users starting from
724 =item Mount option C<-o sync> should not be the default.
726 If you use L</guestfs_mount>, then C<-o sync,noatime> are added
727 implicitly. However C<-o sync> does not add any reliability benefit,
728 but does have a very large performance impact.
730 The work around is to use L</guestfs_mount_options> and set the mount
731 options that you actually want to use.
733 =item Read-only should be the default.
735 In L<guestfish(3)>, I<--ro> should be the default, and you should
736 have to specify I<--rw> if you want to make changes to the image.
738 This would reduce the potential to corrupt live VM images.
740 Note that many filesystems change the disk when you just mount and
741 unmount, even if you didn't perform any writes. You need to use
742 L</guestfs_add_drive_ro> to guarantee that the disk is not changed.
744 =item guestfish command line is hard to use.
746 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
747 for examination). It tries to run a guestfish command C<disk.img>
748 which doesn't exist, so it fails. In earlier versions of guestfish
749 the error message was also unintuitive, but we have corrected this
750 since. Like the Bourne shell, we should have used C<guestfish -c
751 command> to run commands.
753 =item guestfish megabyte modifiers don't work right on all commands
755 In recent guestfish you can use C<1M> to mean 1 megabyte (and
756 similarly for other modifiers). What guestfish actually does is to
757 multiply the number part by the modifier part and pass the result to
758 the C API. However this doesn't work for a few APIs which aren't
759 expecting bytes, but are already expecting some other unit
762 The most common is L</guestfs_lvcreate>. The guestfish command:
766 does not do what you might expect. Instead because
767 L</guestfs_lvcreate> is already expecting megabytes, this tries to
768 create a 100 I<terabyte> (100 megabytes * megabytes) logical volume.
769 The error message you get from this is also a little obscure.
771 This could be fixed in the generator by specially marking parameters
772 and return values which take bytes or other units.
774 =item Ambiguity between devices and paths
776 There is a subtle ambiguity in the API between a device name
777 (eg. C</dev/sdb2>) and a similar pathname. A file might just happen
778 to be called C<sdb2> in the directory C</dev> (consider some non-Unix
781 In the current API we usually resolve this ambiguity by having two
782 separate calls, for example L</guestfs_checksum> and
783 L</guestfs_checksum_device>. Some API calls are ambiguous and
784 (incorrectly) resolve the problem by detecting if the path supplied
785 begins with C</dev/>.
787 To avoid both the ambiguity and the need to duplicate some calls, we
788 could make paths/devices into structured names. One way to do this
789 would be to use a notation like grub (C<hd(0,0)>), although nobody
790 really likes this aspect of grub. Another way would be to use a
791 structured type, equivalent to this OCaml type:
793 type path = Path of string | Device of int | Partition of int * int
795 which would allow you to pass arguments like:
798 Device 1 (* /dev/sdb, or perhaps /dev/sda *)
799 Partition (1, 2) (* /dev/sdb2 (or is it /dev/sda2 or /dev/sdb3?) *)
800 Path "/dev/sdb2" (* not a device *)
802 As you can see there are still problems to resolve even with this
803 representation. Also consider how it might work in guestfish.
807 =head2 PROTOCOL LIMITS
809 Internally libguestfs uses a message-based protocol to pass API calls
810 and their responses to and from a small "appliance" (see L</INTERNALS>
811 for plenty more detail about this). The maximum message size used by
812 the protocol is slightly less than 4 MB. For some API calls you may
813 need to be aware of this limit. The API calls which may be affected
814 are individually documented, with a link back to this section of the
817 A simple call such as L</guestfs_cat> returns its result (the file
818 data) in a simple string. Because this string is at some point
819 internally encoded as a message, the maximum size that it can return
820 is slightly under 4 MB. If the requested file is larger than this
821 then you will get an error.
823 In order to transfer large files into and out of the guest filesystem,
824 you need to use particular calls that support this. The sections
825 L</UPLOADING> and L</DOWNLOADING> document how to do this.
827 You might also consider mounting the disk image using our FUSE
828 filesystem support (L<guestmount(1)>).
830 =head2 KEYS AND PASSPHRASES
832 Certain libguestfs calls take a parameter that contains sensitive key
833 material, passed in as a C string.
835 In the future we would hope to change the libguestfs implementation so
836 that keys are L<mlock(2)>-ed into physical RAM, and thus can never end
837 up in swap. However this is I<not> done at the moment, because of the
838 complexity of such an implementation.
840 Therefore you should be aware that any key parameter you pass to
841 libguestfs might end up being written out to the swap partition. If
842 this is a concern, scrub the swap partition or don't use libguestfs on
845 =head2 MULTIPLE HANDLES AND MULTIPLE THREADS
847 All high-level libguestfs actions are synchronous. If you want
848 to use libguestfs asynchronously then you must create a thread.
850 Only use the handle from a single thread. Either use the handle
851 exclusively from one thread, or provide your own mutex so that two
852 threads cannot issue calls on the same handle at the same time.
854 See the graphical program guestfs-browser for one possible
855 architecture for multithreaded programs using libvirt and libguestfs.
859 Libguestfs needs a kernel and initrd.img, which it finds by looking
860 along an internal path.
862 By default it looks for these in the directory C<$libdir/guestfs>
863 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
865 Use L</guestfs_set_path> or set the environment variable
866 L</LIBGUESTFS_PATH> to change the directories that libguestfs will
867 search in. The value is a colon-separated list of paths. The current
868 directory is I<not> searched unless the path contains an empty element
869 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
870 search the current directory and then C</usr/lib/guestfs>.
874 If you want to compile your own qemu, run qemu from a non-standard
875 location, or pass extra arguments to qemu, then you can write a
876 shell-script wrapper around qemu.
878 There is one important rule to remember: you I<must C<exec qemu>> as
879 the last command in the shell script (so that qemu replaces the shell
880 and becomes the direct child of the libguestfs-using program). If you
881 don't do this, then the qemu process won't be cleaned up correctly.
883 Here is an example of a wrapper, where I have built my own copy of
887 qemudir=/home/rjones/d/qemu
888 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
890 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
891 and then use it by setting the LIBGUESTFS_QEMU environment variable.
894 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
896 Note that libguestfs also calls qemu with the -help and -version
897 options in order to determine features.
901 We guarantee the libguestfs ABI (binary interface), for public,
902 high-level actions as outlined in this section. Although we will
903 deprecate some actions, for example if they get replaced by newer
904 calls, we will keep the old actions forever. This allows you the
905 developer to program in confidence against the libguestfs API.
907 =head2 BLOCK DEVICE NAMING
909 In the kernel there is now quite a profusion of schemata for naming
910 block devices (in this context, by I<block device> I mean a physical
911 or virtual hard drive). The original Linux IDE driver used names
912 starting with C</dev/hd*>. SCSI devices have historically used a
913 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
914 driver became a popular replacement for the old IDE driver
915 (particularly for SATA devices) those devices also used the
916 C</dev/sd*> scheme. Additionally we now have virtual machines with
917 paravirtualized drivers. This has created several different naming
918 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
921 As discussed above, libguestfs uses a qemu appliance running an
922 embedded Linux kernel to access block devices. We can run a variety
923 of appliances based on a variety of Linux kernels.
925 This causes a problem for libguestfs because many API calls use device
926 or partition names. Working scripts and the recipe (example) scripts
927 that we make available over the internet could fail if the naming
930 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
931 scheme>. Internally C</dev/sd*> names are translated, if necessary,
932 to other names as required. For example, under RHEL 5 which uses the
933 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
934 C</dev/hda2> transparently.
936 Note that this I<only> applies to parameters. The
937 L</guestfs_list_devices>, L</guestfs_list_partitions> and similar calls
938 return the true names of the devices and partitions as known to the
941 =head3 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
943 Usually this translation is transparent. However in some (very rare)
944 cases you may need to know the exact algorithm. Such cases include
945 where you use L</guestfs_config> to add a mixture of virtio and IDE
946 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
947 and C</dev/vd*> devices.
949 The algorithm is applied only to I<parameters> which are known to be
950 either device or partition names. Return values from functions such
951 as L</guestfs_list_devices> are never changed.
957 Is the string a parameter which is a device or partition name?
961 Does the string begin with C</dev/sd>?
965 Does the named device exist? If so, we use that device.
966 However if I<not> then we continue with this algorithm.
970 Replace initial C</dev/sd> string with C</dev/hd>.
972 For example, change C</dev/sda2> to C</dev/hda2>.
974 If that named device exists, use it. If not, continue.
978 Replace initial C</dev/sd> string with C</dev/vd>.
980 If that named device exists, use it. If not, return an error.
984 =head3 PORTABILITY CONCERNS WITH BLOCK DEVICE NAMING
986 Although the standard naming scheme and automatic translation is
987 useful for simple programs and guestfish scripts, for larger programs
988 it is best not to rely on this mechanism.
990 Where possible for maximum future portability programs using
991 libguestfs should use these future-proof techniques:
997 Use L</guestfs_list_devices> or L</guestfs_list_partitions> to list
998 actual device names, and then use those names directly.
1000 Since those device names exist by definition, they will never be
1005 Use higher level ways to identify filesystems, such as LVM names,
1006 UUIDs and filesystem labels.
1012 This section discusses security implications of using libguestfs,
1013 particularly with untrusted or malicious guests or disk images.
1015 =head2 GENERAL SECURITY CONSIDERATIONS
1017 Be careful with any files or data that you download from a guest (by
1018 "download" we mean not just the L</guestfs_download> command but any
1019 command that reads files, filenames, directories or anything else from
1020 a disk image). An attacker could manipulate the data to fool your
1021 program into doing the wrong thing. Consider cases such as:
1027 the data (file etc) not being present
1031 being present but empty
1035 being much larger than normal
1039 containing arbitrary 8 bit data
1043 being in an unexpected character encoding
1047 containing homoglyphs.
1051 =head2 SECURITY OF MOUNTING FILESYSTEMS
1053 When you mount a filesystem under Linux, mistakes in the kernel
1054 filesystem (VFS) module can sometimes be escalated into exploits by
1055 deliberately creating a malicious, malformed filesystem. These
1056 exploits are very severe for two reasons. Firstly there are very many
1057 filesystem drivers in the kernel, and many of them are infrequently
1058 used and not much developer attention has been paid to the code.
1059 Linux userspace helps potential crackers by detecting the filesystem
1060 type and automatically choosing the right VFS driver, even if that
1061 filesystem type is obscure or unexpected for the administrator.
1062 Secondly, a kernel-level exploit is like a local root exploit (worse
1063 in some ways), giving immediate and total access to the system right
1064 down to the hardware level.
1066 That explains why you should never mount a filesystem from an
1067 untrusted guest on your host kernel. How about libguestfs? We run a
1068 Linux kernel inside a qemu virtual machine, usually running as a
1069 non-root user. The attacker would need to write a filesystem which
1070 first exploited the kernel, and then exploited either qemu
1071 virtualization (eg. a faulty qemu driver) or the libguestfs protocol,
1072 and finally to be as serious as the host kernel exploit it would need
1073 to escalate its privileges to root. This multi-step escalation,
1074 performed by a static piece of data, is thought to be extremely hard
1075 to do, although we never say 'never' about security issues.
1077 In any case callers can reduce the attack surface by forcing the
1078 filesystem type when mounting (use L</guestfs_mount_vfs>).
1080 =head2 PROTOCOL SECURITY
1082 The protocol is designed to be secure, being based on RFC 4506 (XDR)
1083 with a defined upper message size. However a program that uses
1084 libguestfs must also take care - for example you can write a program
1085 that downloads a binary from a disk image and executes it locally, and
1086 no amount of protocol security will save you from the consequences.
1088 =head2 INSPECTION SECURITY
1090 Parts of the inspection API (see L</INSPECTION>) return untrusted
1091 strings directly from the guest, and these could contain any 8 bit
1092 data. Callers should be careful to escape these before printing them
1093 to a structured file (for example, use HTML escaping if creating a web
1096 Guest configuration may be altered in unusual ways by the
1097 administrator of the virtual machine, and may not reflect reality
1098 (particularly for untrusted or actively malicious guests). For
1099 example we parse the hostname from configuration files like
1100 C</etc/sysconfig/network> that we find in the guest, but the guest
1101 administrator can easily manipulate these files to provide the wrong
1104 The inspection API parses guest configuration using two external
1105 libraries: Augeas (Linux configuration) and hivex (Windows Registry).
1106 Both are designed to be robust in the face of malicious data, although
1107 denial of service attacks are still possible, for example with
1108 oversized configuration files.
1110 =head2 RUNNING UNTRUSTED GUEST COMMANDS
1112 Be very cautious about running commands from the guest. By running a
1113 command in the guest, you are giving CPU time to a binary that you do
1114 not control, under the same user account as the library, albeit
1115 wrapped in qemu virtualization. More information and alternatives can
1116 be found in the section L</RUNNING COMMANDS>.
1118 =head2 CVE-2010-3851
1120 https://bugzilla.redhat.com/642934
1122 This security bug concerns the automatic disk format detection that
1123 qemu does on disk images.
1125 A raw disk image is just the raw bytes, there is no header. Other
1126 disk images like qcow2 contain a special header. Qemu deals with this
1127 by looking for one of the known headers, and if none is found then
1128 assuming the disk image must be raw.
1130 This allows a guest which has been given a raw disk image to write
1131 some other header. At next boot (or when the disk image is accessed
1132 by libguestfs) qemu would do autodetection and think the disk image
1133 format was, say, qcow2 based on the header written by the guest.
1135 This in itself would not be a problem, but qcow2 offers many features,
1136 one of which is to allow a disk image to refer to another image
1137 (called the "backing disk"). It does this by placing the path to the
1138 backing disk into the qcow2 header. This path is not validated and
1139 could point to any host file (eg. "/etc/passwd"). The backing disk is
1140 then exposed through "holes" in the qcow2 disk image, which of course
1141 is completely under the control of the attacker.
1143 In libguestfs this is rather hard to exploit except under two
1150 You have enabled the network or have opened the disk in write mode.
1154 You are also running untrusted code from the guest (see
1155 L</RUNNING COMMANDS>).
1159 The way to avoid this is to specify the expected disk format when
1160 adding disks (the optional C<format> option to
1161 L</guestfs_add_drive_opts>). You should always do this if the disk is
1162 raw format, and it's a good idea for other cases too.
1164 For disks added from libvirt using calls like L</guestfs_add_domain>,
1165 the format is fetched from libvirt and passed through.
1167 For libguestfs tools, use the I<--format> command line parameter as
1170 =head1 CONNECTION MANAGEMENT
1174 C<guestfs_h> is the opaque type representing a connection handle.
1175 Create a handle by calling L</guestfs_create>. Call L</guestfs_close>
1176 to free the handle and release all resources used.
1178 For information on using multiple handles and threads, see the section
1179 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
1181 =head2 guestfs_create
1183 guestfs_h *guestfs_create (void);
1185 Create a connection handle.
1187 You have to call L</guestfs_add_drive_opts> (or one of the equivalent
1188 calls) on the handle at least once.
1190 This function returns a non-NULL pointer to a handle on success or
1193 After configuring the handle, you have to call L</guestfs_launch>.
1195 You may also want to configure error handling for the handle. See
1196 L</ERROR HANDLING> section below.
1198 =head2 guestfs_close
1200 void guestfs_close (guestfs_h *g);
1202 This closes the connection handle and frees up all resources used.
1204 =head1 ERROR HANDLING
1206 API functions can return errors. For example, almost all functions
1207 that return C<int> will return C<-1> to indicate an error.
1209 Additional information is available for errors: an error message
1210 string and optionally an error number (errno) if the thing that failed
1213 You can get at the additional information about the last error on the
1214 handle by calling L</guestfs_last_error>, L</guestfs_last_errno>,
1215 and/or by setting up an error handler with
1216 L</guestfs_set_error_handler>.
1218 When the handle is created, a default error handler is installed which
1219 prints the error message string to C<stderr>. For small short-running
1220 command line programs it is sufficient to do:
1222 if (guestfs_launch (g) == -1)
1223 exit (EXIT_FAILURE);
1225 since the default error handler will ensure that an error message has
1226 been printed to C<stderr> before the program exits.
1228 For other programs the caller will almost certainly want to install an
1229 alternate error handler or do error handling in-line like this:
1231 g = guestfs_create ();
1233 /* This disables the default behaviour of printing errors
1235 guestfs_set_error_handler (g, NULL, NULL);
1237 if (guestfs_launch (g) == -1) {
1238 /* Examine the error message and print it etc. */
1239 char *msg = guestfs_last_error (g);
1240 int errnum = guestfs_last_errno (g);
1241 fprintf (stderr, "%s\n", msg);
1245 Out of memory errors are handled differently. The default action is
1246 to call L<abort(3)>. If this is undesirable, then you can set a
1247 handler using L</guestfs_set_out_of_memory_handler>.
1249 L</guestfs_create> returns C<NULL> if the handle cannot be created,
1250 and because there is no handle if this happens there is no way to get
1251 additional error information. However L</guestfs_create> is supposed
1252 to be a lightweight operation which can only fail because of
1253 insufficient memory (it returns NULL in this case).
1255 =head2 guestfs_last_error
1257 const char *guestfs_last_error (guestfs_h *g);
1259 This returns the last error message that happened on C<g>. If
1260 there has not been an error since the handle was created, then this
1263 The lifetime of the returned string is until the next error occurs, or
1264 L</guestfs_close> is called.
1266 =head2 guestfs_last_errno
1268 int guestfs_last_errno (guestfs_h *g);
1270 This returns the last error number (errno) that happened on C<g>.
1272 If successful, an errno integer not equal to zero is returned.
1274 If no error, this returns 0. This call can return 0 in three
1281 There has not been any error on the handle.
1285 There has been an error but the errno was meaningless. This
1286 corresponds to the case where the error did not come from a
1287 failed system call, but for some other reason.
1291 There was an error from a failed system call, but for some
1292 reason the errno was not captured and returned. This usually
1293 indicates a bug in libguestfs.
1297 Libguestfs tries to convert the errno from inside the applicance into
1298 a corresponding errno for the caller (not entirely trivial: the
1299 appliance might be running a completely different operating system
1300 from the library and error numbers are not standardized across
1301 Un*xen). If this could not be done, then the error is translated to
1302 C<EINVAL>. In practice this should only happen in very rare
1305 =head2 guestfs_set_error_handler
1307 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
1310 void guestfs_set_error_handler (guestfs_h *g,
1311 guestfs_error_handler_cb cb,
1314 The callback C<cb> will be called if there is an error. The
1315 parameters passed to the callback are an opaque data pointer and the
1316 error message string.
1318 C<errno> is not passed to the callback. To get that the callback must
1319 call L</guestfs_last_errno>.
1321 Note that the message string C<msg> is freed as soon as the callback
1322 function returns, so if you want to stash it somewhere you must make
1325 The default handler prints messages on C<stderr>.
1327 If you set C<cb> to C<NULL> then I<no> handler is called.
1329 =head2 guestfs_get_error_handler
1331 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g,
1334 Returns the current error handler callback.
1336 =head2 guestfs_set_out_of_memory_handler
1338 typedef void (*guestfs_abort_cb) (void);
1339 int guestfs_set_out_of_memory_handler (guestfs_h *g,
1342 The callback C<cb> will be called if there is an out of memory
1343 situation. I<Note this callback must not return>.
1345 The default is to call L<abort(3)>.
1347 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
1350 =head2 guestfs_get_out_of_memory_handler
1352 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *g);
1354 This returns the current out of memory handler.
1366 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
1368 Using L</guestfs_available> you can test availability of
1369 the following groups of functions. This test queries the
1370 appliance to see if the appliance you are currently using
1371 supports the functionality.
1375 =head2 GUESTFISH supported COMMAND
1377 In L<guestfish(3)> there is a handy interactive command
1378 C<supported> which prints out the available groups and
1379 whether they are supported by this build of libguestfs.
1380 Note however that you have to do C<run> first.
1382 =head2 SINGLE CALLS AT COMPILE TIME
1384 Since version 1.5.8, C<E<lt>guestfs.hE<gt>> defines symbols
1385 for each C API function, such as:
1387 #define LIBGUESTFS_HAVE_DD 1
1389 if L</guestfs_dd> is available.
1391 Before version 1.5.8, if you needed to test whether a single
1392 libguestfs function is available at compile time, we recommended using
1393 build tools such as autoconf or cmake. For example in autotools you
1396 AC_CHECK_LIB([guestfs],[guestfs_create])
1397 AC_CHECK_FUNCS([guestfs_dd])
1399 which would result in C<HAVE_GUESTFS_DD> being either defined
1400 or not defined in your program.
1402 =head2 SINGLE CALLS AT RUN TIME
1404 Testing at compile time doesn't guarantee that a function really
1405 exists in the library. The reason is that you might be dynamically
1406 linked against a previous I<libguestfs.so> (dynamic library)
1407 which doesn't have the call. This situation unfortunately results
1408 in a segmentation fault, which is a shortcoming of the C dynamic
1409 linking system itself.
1411 You can use L<dlopen(3)> to test if a function is available
1412 at run time, as in this example program (note that you still
1413 need the compile time check as well):
1419 #include <guestfs.h>
1423 #ifdef LIBGUESTFS_HAVE_DD
1427 /* Test if the function guestfs_dd is really available. */
1428 dl = dlopen (NULL, RTLD_LAZY);
1430 fprintf (stderr, "dlopen: %s\n", dlerror ());
1431 exit (EXIT_FAILURE);
1433 has_function = dlsym (dl, "guestfs_dd") != NULL;
1437 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
1439 printf ("this libguestfs.so has guestfs_dd function\n");
1440 /* Now it's safe to call
1441 guestfs_dd (g, "foo", "bar");
1445 printf ("guestfs_dd function was not found at compile time\n");
1449 You may think the above is an awful lot of hassle, and it is.
1450 There are other ways outside of the C linking system to ensure
1451 that this kind of incompatibility never arises, such as using
1454 Requires: libguestfs >= 1.0.80
1456 =head1 CALLS WITH OPTIONAL ARGUMENTS
1458 A recent feature of the API is the introduction of calls which take
1459 optional arguments. In C these are declared 3 ways. The main way is
1460 as a call which takes variable arguments (ie. C<...>), as in this
1463 int guestfs_add_drive_opts (guestfs_h *g, const char *filename, ...);
1465 Call this with a list of optional arguments, terminated by C<-1>.
1466 So to call with no optional arguments specified:
1468 guestfs_add_drive_opts (g, filename, -1);
1470 With a single optional argument:
1472 guestfs_add_drive_opts (g, filename,
1473 GUESTFS_ADD_DRIVE_OPTS_FORMAT, "qcow2",
1478 guestfs_add_drive_opts (g, filename,
1479 GUESTFS_ADD_DRIVE_OPTS_FORMAT, "qcow2",
1480 GUESTFS_ADD_DRIVE_OPTS_READONLY, 1,
1483 and so forth. Don't forget the terminating C<-1> otherwise
1484 Bad Things will happen!
1486 =head2 USING va_list FOR OPTIONAL ARGUMENTS
1488 The second variant has the same name with the suffix C<_va>, which
1489 works the same way but takes a C<va_list>. See the C manual for
1490 details. For the example function, this is declared:
1492 int guestfs_add_drive_opts_va (guestfs_h *g, const char *filename,
1495 =head2 CONSTRUCTING OPTIONAL ARGUMENTS
1497 The third variant is useful where you need to construct these
1498 calls. You pass in a structure where you fill in the optional
1499 fields. The structure has a bitmask as the first element which
1500 you must set to indicate which fields you have filled in. For
1501 our example function the structure and call are declared:
1503 struct guestfs_add_drive_opts_argv {
1509 int guestfs_add_drive_opts_argv (guestfs_h *g, const char *filename,
1510 const struct guestfs_add_drive_opts_argv *optargs);
1512 You could call it like this:
1514 struct guestfs_add_drive_opts_argv optargs = {
1515 .bitmask = GUESTFS_ADD_DRIVE_OPTS_READONLY_BITMASK |
1516 GUESTFS_ADD_DRIVE_OPTS_FORMAT_BITMASK,
1521 guestfs_add_drive_opts_argv (g, filename, &optargs);
1529 The C<_BITMASK> suffix on each option name when specifying the
1534 You do not need to fill in all fields of the structure.
1538 There must be a one-to-one correspondence between fields of the
1539 structure that are filled in, and bits set in the bitmask.
1543 =head2 OPTIONAL ARGUMENTS IN OTHER LANGUAGES
1545 In other languages, optional arguments are expressed in the
1546 way that is natural for that language. We refer you to the
1547 language-specific documentation for more details on that.
1549 For guestfish, see L<guestfish(1)/OPTIONAL ARGUMENTS>.
1551 =head2 SETTING CALLBACKS TO HANDLE EVENTS
1553 The child process generates events in some situations. Current events
1554 include: receiving a log message, the child process exits.
1556 Use the C<guestfs_set_*_callback> functions to set a callback for
1557 different types of events.
1559 Only I<one callback of each type> can be registered for each handle.
1560 Calling C<guestfs_set_*_callback> again overwrites the previous
1561 callback of that type. Cancel all callbacks of this type by calling
1562 this function with C<cb> set to C<NULL>.
1564 =head2 guestfs_set_log_message_callback
1566 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
1567 char *buf, int len);
1568 void guestfs_set_log_message_callback (guestfs_h *g,
1569 guestfs_log_message_cb cb,
1572 The callback function C<cb> will be called whenever qemu or the guest
1573 writes anything to the console.
1575 Use this function to capture kernel messages and similar.
1577 Normally there is no log message handler, and log messages are just
1580 =head2 guestfs_set_subprocess_quit_callback
1582 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
1583 void guestfs_set_subprocess_quit_callback (guestfs_h *g,
1584 guestfs_subprocess_quit_cb cb,
1587 The callback function C<cb> will be called when the child process
1588 quits, either asynchronously or if killed by
1589 L</guestfs_kill_subprocess>. (This corresponds to a transition from
1590 any state to the CONFIG state).
1592 =head2 guestfs_set_launch_done_callback
1594 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
1595 void guestfs_set_launch_done_callback (guestfs_h *g,
1596 guestfs_launch_done_cb cb,
1599 The callback function C<cb> will be called when the child process
1600 becomes ready first time after it has been launched. (This
1601 corresponds to a transition from LAUNCHING to the READY state).
1603 =head2 guestfs_set_close_callback
1605 typedef void (*guestfs_close_cb) (guestfs_h *g, void *opaque);
1606 void guestfs_set_close_callback (guestfs_h *g,
1607 guestfs_close_cb cb,
1610 The callback function C<cb> will be called while the handle
1611 is being closed (synchronously from L</guestfs_close>).
1613 Note that libguestfs installs an L<atexit(3)> handler to try to
1614 clean up handles that are open when the program exits. This
1615 means that this callback might be called indirectly from
1616 L<exit(3)>, which can cause unexpected problems in higher-level
1617 languages (eg. if your HLL interpreter has already been cleaned
1618 up by the time this is called, and if your callback then jumps
1619 into some HLL function).
1621 =head2 guestfs_set_progress_callback
1623 typedef void (*guestfs_progress_cb) (guestfs_h *g, void *opaque,
1624 int proc_nr, int serial,
1625 uint64_t position, uint64_t total);
1626 void guestfs_set_progress_callback (guestfs_h *g,
1627 guestfs_progress_cb cb,
1630 Some long-running operations can generate progress messages. If
1631 this callback is registered, then it will be called each time a
1632 progress message is generated (usually two seconds after the
1633 operation started, and three times per second thereafter until
1634 it completes, although the frequency may change in future versions).
1636 The callback receives two numbers: C<position> and C<total>.
1637 The units of C<total> are not defined, although for some
1638 operations C<total> may relate in some way to the amount of
1639 data to be transferred (eg. in bytes or megabytes), and
1640 C<position> may be the portion which has been transferred.
1642 The only defined and stable parts of the API are:
1648 The callback can display to the user some type of progress bar or
1649 indicator which shows the ratio of C<position>:C<total>.
1653 0 E<lt>= C<position> E<lt>= C<total>
1657 If any progress notification is sent during a call, then a final
1658 progress notification is always sent when C<position> = C<total>.
1660 This is to simplify caller code, so callers can easily set the
1661 progress indicator to "100%" at the end of the operation, without
1662 requiring special code to detect this case.
1666 The callback also receives the procedure number and serial number of
1667 the call. These are only useful for debugging protocol issues, and
1668 the callback can normally ignore them. The callback may want to
1669 print these numbers in error messages or debugging messages.
1671 =head1 PRIVATE DATA AREA
1673 You can attach named pieces of private data to the libguestfs handle,
1674 and fetch them by name for the lifetime of the handle. This is called
1675 the private data area and is only available from the C API.
1677 To attach a named piece of data, use the following call:
1679 void guestfs_set_private (guestfs_h *g, const char *key, void *data);
1681 C<key> is the name to associate with this data, and C<data> is an
1682 arbitrary pointer (which can be C<NULL>). Any previous item with the
1683 same name is overwritten.
1685 You can use any C<key> you want, but names beginning with an
1686 underscore character are reserved for internal libguestfs purposes
1687 (for implementing language bindings). It is recommended to prefix the
1688 name with some unique string to avoid collisions with other users.
1690 To retrieve the pointer, use:
1692 void *guestfs_get_private (guestfs_h *g, const char *key);
1694 This function returns C<NULL> if either no data is found associated
1695 with C<key>, or if the user previously set the C<key>'s C<data>
1698 Libguestfs does not try to look at or interpret the C<data> pointer in
1699 any way. As far as libguestfs is concerned, it need not be a valid
1700 pointer at all. In particular, libguestfs does I<not> try to free the
1701 data when the handle is closed. If the data must be freed, then the
1702 caller must either free it before calling L</guestfs_close> or must
1703 set up a close callback to do it (see L</guestfs_set_close_callback>,
1704 and note that only one callback can be registered for a handle).
1706 The private data area is implemented using a hash table, and should be
1707 reasonably efficient for moderate numbers of keys.
1711 <!-- old anchor for the next section -->
1712 <a name="state_machine_and_low_level_event_api"/>
1718 Internally, libguestfs is implemented by running an appliance (a
1719 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
1720 a child process of the main program.
1726 | | child process / appliance
1727 | | __________________________
1729 +-------------------+ RPC | +-----------------+ |
1730 | libguestfs <--------------------> guestfsd | |
1731 | | | +-----------------+ |
1732 \___________________/ | | Linux kernel | |
1733 | +--^--------------+ |
1734 \_________|________________/
1742 The library, linked to the main program, creates the child process and
1743 hence the appliance in the L</guestfs_launch> function.
1745 Inside the appliance is a Linux kernel and a complete stack of
1746 userspace tools (such as LVM and ext2 programs) and a small
1747 controlling daemon called L</guestfsd>. The library talks to
1748 L</guestfsd> using remote procedure calls (RPC). There is a mostly
1749 one-to-one correspondence between libguestfs API calls and RPC calls
1750 to the daemon. Lastly the disk image(s) are attached to the qemu
1751 process which translates device access by the appliance's Linux kernel
1752 into accesses to the image.
1754 A common misunderstanding is that the appliance "is" the virtual
1755 machine. Although the disk image you are attached to might also be
1756 used by some virtual machine, libguestfs doesn't know or care about
1757 this. (But you will care if both libguestfs's qemu process and your
1758 virtual machine are trying to update the disk image at the same time,
1759 since these usually results in massive disk corruption).
1761 =head1 STATE MACHINE
1763 libguestfs uses a state machine to model the child process:
1774 / | \ \ guestfs_launch
1785 \______/ <------ \________/
1787 The normal transitions are (1) CONFIG (when the handle is created, but
1788 there is no child process), (2) LAUNCHING (when the child process is
1789 booting up), (3) alternating between READY and BUSY as commands are
1790 issued to, and carried out by, the child process.
1792 The guest may be killed by L</guestfs_kill_subprocess>, or may die
1793 asynchronously at any time (eg. due to some internal error), and that
1794 causes the state to transition back to CONFIG.
1796 Configuration commands for qemu such as L</guestfs_add_drive> can only
1797 be issued when in the CONFIG state.
1799 The API offers one call that goes from CONFIG through LAUNCHING to
1800 READY. L</guestfs_launch> blocks until the child process is READY to
1801 accept commands (or until some failure or timeout).
1802 L</guestfs_launch> internally moves the state from CONFIG to LAUNCHING
1803 while it is running.
1805 API actions such as L</guestfs_mount> can only be issued when in the
1806 READY state. These API calls block waiting for the command to be
1807 carried out (ie. the state to transition to BUSY and then back to
1808 READY). There are no non-blocking versions, and no way to issue more
1809 than one command per handle at the same time.
1811 Finally, the child process sends asynchronous messages back to the
1812 main program, such as kernel log messages. You can register a
1813 callback to receive these messages.
1817 =head2 COMMUNICATION PROTOCOL
1819 Don't rely on using this protocol directly. This section documents
1820 how it currently works, but it may change at any time.
1822 The protocol used to talk between the library and the daemon running
1823 inside the qemu virtual machine is a simple RPC mechanism built on top
1824 of XDR (RFC 1014, RFC 1832, RFC 4506).
1826 The detailed format of structures is in C<src/guestfs_protocol.x>
1827 (note: this file is automatically generated).
1829 There are two broad cases, ordinary functions that don't have any
1830 C<FileIn> and C<FileOut> parameters, which are handled with very
1831 simple request/reply messages. Then there are functions that have any
1832 C<FileIn> or C<FileOut> parameters, which use the same request and
1833 reply messages, but they may also be followed by files sent using a
1836 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1838 For ordinary functions, the request message is:
1840 total length (header + arguments,
1841 but not including the length word itself)
1842 struct guestfs_message_header (encoded as XDR)
1843 struct guestfs_<foo>_args (encoded as XDR)
1845 The total length field allows the daemon to allocate a fixed size
1846 buffer into which it slurps the rest of the message. As a result, the
1847 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1848 4MB), which means the effective size of any request is limited to
1849 somewhere under this size.
1851 Note also that many functions don't take any arguments, in which case
1852 the C<guestfs_I<foo>_args> is completely omitted.
1854 The header contains the procedure number (C<guestfs_proc>) which is
1855 how the receiver knows what type of args structure to expect, or none
1858 For functions that take optional arguments, the optional arguments are
1859 encoded in the C<guestfs_I<foo>_args> structure in the same way as
1860 ordinary arguments. A bitmask in the header indicates which optional
1861 arguments are meaningful. The bitmask is also checked to see if it
1862 contains bits set which the daemon does not know about (eg. if more
1863 optional arguments were added in a later version of the library), and
1864 this causes the call to be rejected.
1866 The reply message for ordinary functions is:
1868 total length (header + ret,
1869 but not including the length word itself)
1870 struct guestfs_message_header (encoded as XDR)
1871 struct guestfs_<foo>_ret (encoded as XDR)
1873 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1874 for functions that return no formal return values.
1876 As above the total length of the reply is limited to
1877 C<GUESTFS_MESSAGE_MAX>.
1879 In the case of an error, a flag is set in the header, and the reply
1880 message is slightly changed:
1882 total length (header + error,
1883 but not including the length word itself)
1884 struct guestfs_message_header (encoded as XDR)
1885 struct guestfs_message_error (encoded as XDR)
1887 The C<guestfs_message_error> structure contains the error message as a
1890 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1892 A C<FileIn> parameter indicates that we transfer a file I<into> the
1893 guest. The normal request message is sent (see above). However this
1894 is followed by a sequence of file chunks.
1896 total length (header + arguments,
1897 but not including the length word itself,
1898 and not including the chunks)
1899 struct guestfs_message_header (encoded as XDR)
1900 struct guestfs_<foo>_args (encoded as XDR)
1901 sequence of chunks for FileIn param #0
1902 sequence of chunks for FileIn param #1 etc.
1904 The "sequence of chunks" is:
1906 length of chunk (not including length word itself)
1907 struct guestfs_chunk (encoded as XDR)
1909 struct guestfs_chunk (encoded as XDR)
1912 struct guestfs_chunk (with data.data_len == 0)
1914 The final chunk has the C<data_len> field set to zero. Additionally a
1915 flag is set in the final chunk to indicate either successful
1916 completion or early cancellation.
1918 At time of writing there are no functions that have more than one
1919 FileIn parameter. However this is (theoretically) supported, by
1920 sending the sequence of chunks for each FileIn parameter one after
1921 another (from left to right).
1923 Both the library (sender) I<and> the daemon (receiver) may cancel the
1924 transfer. The library does this by sending a chunk with a special
1925 flag set to indicate cancellation. When the daemon sees this, it
1926 cancels the whole RPC, does I<not> send any reply, and goes back to
1927 reading the next request.
1929 The daemon may also cancel. It does this by writing a special word
1930 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1931 during the transfer, and if it gets it, it will cancel the transfer
1932 (it sends a cancel chunk). The special word is chosen so that even if
1933 cancellation happens right at the end of the transfer (after the
1934 library has finished writing and has started listening for the reply),
1935 the "spurious" cancel flag will not be confused with the reply
1938 This protocol allows the transfer of arbitrary sized files (no 32 bit
1939 limit), and also files where the size is not known in advance
1940 (eg. from pipes or sockets). However the chunks are rather small
1941 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1942 daemon need to keep much in memory.
1944 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1946 The protocol for FileOut parameters is exactly the same as for FileIn
1947 parameters, but with the roles of daemon and library reversed.
1949 total length (header + ret,
1950 but not including the length word itself,
1951 and not including the chunks)
1952 struct guestfs_message_header (encoded as XDR)
1953 struct guestfs_<foo>_ret (encoded as XDR)
1954 sequence of chunks for FileOut param #0
1955 sequence of chunks for FileOut param #1 etc.
1957 =head3 INITIAL MESSAGE
1959 When the daemon launches it sends an initial word
1960 (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest and daemon is
1961 alive. This is what L</guestfs_launch> waits for.
1963 =head3 PROGRESS NOTIFICATION MESSAGES
1965 The daemon may send progress notification messages at any time. These
1966 are distinguished by the normal length word being replaced by
1967 C<GUESTFS_PROGRESS_FLAG>, followed by a fixed size progress message.
1969 The library turns them into progress callbacks (see
1970 C<guestfs_set_progress_callback>) if there is a callback registered,
1971 or discards them if not.
1973 The daemon self-limits the frequency of progress messages it sends
1974 (see C<daemon/proto.c:notify_progress>). Not all calls generate
1977 =head1 LIBGUESTFS VERSION NUMBERS
1979 Since April 2010, libguestfs has started to make separate development
1980 and stable releases, along with corresponding branches in our git
1981 repository. These separate releases can be identified by version
1984 even numbers for stable: 1.2.x, 1.4.x, ...
1985 .-------- odd numbers for development: 1.3.x, 1.5.x, ...
1991 | `-------- sub-version
1993 `------ always '1' because we don't change the ABI
1995 Thus "1.3.5" is the 5th update to the development branch "1.3".
1997 As time passes we cherry pick fixes from the development branch and
1998 backport those into the stable branch, the effect being that the
1999 stable branch should get more stable and less buggy over time. So the
2000 stable releases are ideal for people who don't need new features but
2001 would just like the software to work.
2003 Our criteria for backporting changes are:
2009 Documentation changes which don't affect any code are
2010 backported unless the documentation refers to a future feature
2011 which is not in stable.
2015 Bug fixes which are not controversial, fix obvious problems, and
2016 have been well tested are backported.
2020 Simple rearrangements of code which shouldn't affect how it works get
2021 backported. This is so that the code in the two branches doesn't get
2022 too far out of step, allowing us to backport future fixes more easily.
2026 We I<don't> backport new features, new APIs, new tools etc, except in
2027 one exceptional case: the new feature is required in order to
2028 implement an important bug fix.
2032 A new stable branch starts when we think the new features in
2033 development are substantial and compelling enough over the current
2034 stable branch to warrant it. When that happens we create new stable
2035 and development versions 1.N.0 and 1.(N+1).0 [N is even]. The new
2036 dot-oh release won't necessarily be so stable at this point, but by
2037 backporting fixes from development, that branch will stabilize over
2040 =head1 ENVIRONMENT VARIABLES
2044 =item LIBGUESTFS_APPEND
2046 Pass additional options to the guest kernel.
2048 =item LIBGUESTFS_DEBUG
2050 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
2051 has the same effect as calling C<guestfs_set_verbose (g, 1)>.
2053 =item LIBGUESTFS_MEMSIZE
2055 Set the memory allocated to the qemu process, in megabytes. For
2058 LIBGUESTFS_MEMSIZE=700
2060 =item LIBGUESTFS_PATH
2062 Set the path that libguestfs uses to search for kernel and initrd.img.
2063 See the discussion of paths in section PATH above.
2065 =item LIBGUESTFS_QEMU
2067 Set the default qemu binary that libguestfs uses. If not set, then
2068 the qemu which was found at compile time by the configure script is
2071 See also L</QEMU WRAPPERS> above.
2073 =item LIBGUESTFS_TRACE
2075 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
2076 has the same effect as calling C<guestfs_set_trace (g, 1)>.
2080 Location of temporary directory, defaults to C</tmp>.
2082 If libguestfs was compiled to use the supermin appliance then the
2083 real appliance is cached in this directory, shared between all
2084 handles belonging to the same EUID. You can use C<$TMPDIR> to
2085 configure another directory to use in case C</tmp> is not large
2092 L<guestfs-examples(3)>,
2093 L<guestfs-ocaml(3)>,
2094 L<guestfs-python(3)>,
2101 L<virt-filesystems(1)>,
2102 L<virt-inspector(1)>,
2103 L<virt-list-filesystems(1)>,
2104 L<virt-list-partitions(1)>,
2113 L<http://libguestfs.org/>.
2115 Tools with a similar purpose:
2124 To get a list of bugs against libguestfs use this link:
2126 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
2128 To report a new bug against libguestfs use this link:
2130 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
2132 When reporting a bug, please check:
2138 That the bug hasn't been reported already.
2142 That you are testing a recent version.
2146 Describe the bug accurately, and give a way to reproduce it.
2150 Run libguestfs-test-tool and paste the B<complete, unedited>
2151 output into the bug report.
2157 Richard W.M. Jones (C<rjones at redhat dot com>)
2161 Copyright (C) 2009-2010 Red Hat Inc.
2162 L<http://libguestfs.org/>
2164 This library is free software; you can redistribute it and/or
2165 modify it under the terms of the GNU Lesser General Public
2166 License as published by the Free Software Foundation; either
2167 version 2 of the License, or (at your option) any later version.
2169 This library is distributed in the hope that it will be useful,
2170 but WITHOUT ANY WARRANTY; without even the implied warranty of
2171 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
2172 Lesser General Public License for more details.
2174 You should have received a copy of the GNU Lesser General Public
2175 License along with this library; if not, write to the Free Software
2176 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA