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 the file C<guestfs.mli>.
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 do:
683 Use the Guestfs module. There is no Ruby-specific documentation, but
684 you can find examples written in Ruby in the libguestfs source.
686 =item B<shell scripts>
688 For documentation see L<guestfish(1)>.
692 =head2 LIBGUESTFS GOTCHAS
694 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
695 system [...] that works in the way it is documented but is
696 counterintuitive and almost invites mistakes."
698 Since we developed libguestfs and the associated tools, there are
699 several things we would have designed differently, but are now stuck
700 with for backwards compatibility or other reasons. If there is ever a
701 libguestfs 2.0 release, you can expect these to change. Beware of
706 =item Autosync / forgetting to sync.
708 When modifying a filesystem from C or another language, you B<must>
709 unmount all filesystems and call L</guestfs_sync> explicitly before
710 you close the libguestfs handle. You can also call:
712 guestfs_set_autosync (g, 1);
714 to have the unmount/sync done automatically for you when the handle 'g'
715 is closed. (This feature is called "autosync", L</guestfs_set_autosync>
718 If you forget to do this, then it is entirely possible that your
719 changes won't be written out, or will be partially written, or (very
720 rarely) that you'll get disk corruption.
722 Note that in L<guestfish(3)> autosync is the default. So quick and
723 dirty guestfish scripts that forget to sync will work just fine, which
724 can make this very puzzling if you are trying to debug a problem.
726 Update: Autosync is enabled by default for all API users starting from
729 =item Mount option C<-o sync> should not be the default.
731 If you use L</guestfs_mount>, then C<-o sync,noatime> are added
732 implicitly. However C<-o sync> does not add any reliability benefit,
733 but does have a very large performance impact.
735 The work around is to use L</guestfs_mount_options> and set the mount
736 options that you actually want to use.
738 =item Read-only should be the default.
740 In L<guestfish(3)>, I<--ro> should be the default, and you should
741 have to specify I<--rw> if you want to make changes to the image.
743 This would reduce the potential to corrupt live VM images.
745 Note that many filesystems change the disk when you just mount and
746 unmount, even if you didn't perform any writes. You need to use
747 L</guestfs_add_drive_ro> to guarantee that the disk is not changed.
749 =item guestfish command line is hard to use.
751 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
752 for examination). It tries to run a guestfish command C<disk.img>
753 which doesn't exist, so it fails. In earlier versions of guestfish
754 the error message was also unintuitive, but we have corrected this
755 since. Like the Bourne shell, we should have used C<guestfish -c
756 command> to run commands.
758 =item guestfish megabyte modifiers don't work right on all commands
760 In recent guestfish you can use C<1M> to mean 1 megabyte (and
761 similarly for other modifiers). What guestfish actually does is to
762 multiply the number part by the modifier part and pass the result to
763 the C API. However this doesn't work for a few APIs which aren't
764 expecting bytes, but are already expecting some other unit
767 The most common is L</guestfs_lvcreate>. The guestfish command:
771 does not do what you might expect. Instead because
772 L</guestfs_lvcreate> is already expecting megabytes, this tries to
773 create a 100 I<terabyte> (100 megabytes * megabytes) logical volume.
774 The error message you get from this is also a little obscure.
776 This could be fixed in the generator by specially marking parameters
777 and return values which take bytes or other units.
779 =item Ambiguity between devices and paths
781 There is a subtle ambiguity in the API between a device name
782 (eg. C</dev/sdb2>) and a similar pathname. A file might just happen
783 to be called C<sdb2> in the directory C</dev> (consider some non-Unix
786 In the current API we usually resolve this ambiguity by having two
787 separate calls, for example L</guestfs_checksum> and
788 L</guestfs_checksum_device>. Some API calls are ambiguous and
789 (incorrectly) resolve the problem by detecting if the path supplied
790 begins with C</dev/>.
792 To avoid both the ambiguity and the need to duplicate some calls, we
793 could make paths/devices into structured names. One way to do this
794 would be to use a notation like grub (C<hd(0,0)>), although nobody
795 really likes this aspect of grub. Another way would be to use a
796 structured type, equivalent to this OCaml type:
798 type path = Path of string | Device of int | Partition of int * int
800 which would allow you to pass arguments like:
803 Device 1 (* /dev/sdb, or perhaps /dev/sda *)
804 Partition (1, 2) (* /dev/sdb2 (or is it /dev/sda2 or /dev/sdb3?) *)
805 Path "/dev/sdb2" (* not a device *)
807 As you can see there are still problems to resolve even with this
808 representation. Also consider how it might work in guestfish.
812 =head2 PROTOCOL LIMITS
814 Internally libguestfs uses a message-based protocol to pass API calls
815 and their responses to and from a small "appliance" (see L</INTERNALS>
816 for plenty more detail about this). The maximum message size used by
817 the protocol is slightly less than 4 MB. For some API calls you may
818 need to be aware of this limit. The API calls which may be affected
819 are individually documented, with a link back to this section of the
822 A simple call such as L</guestfs_cat> returns its result (the file
823 data) in a simple string. Because this string is at some point
824 internally encoded as a message, the maximum size that it can return
825 is slightly under 4 MB. If the requested file is larger than this
826 then you will get an error.
828 In order to transfer large files into and out of the guest filesystem,
829 you need to use particular calls that support this. The sections
830 L</UPLOADING> and L</DOWNLOADING> document how to do this.
832 You might also consider mounting the disk image using our FUSE
833 filesystem support (L<guestmount(1)>).
835 =head2 KEYS AND PASSPHRASES
837 Certain libguestfs calls take a parameter that contains sensitive key
838 material, passed in as a C string.
840 In the future we would hope to change the libguestfs implementation so
841 that keys are L<mlock(2)>-ed into physical RAM, and thus can never end
842 up in swap. However this is I<not> done at the moment, because of the
843 complexity of such an implementation.
845 Therefore you should be aware that any key parameter you pass to
846 libguestfs might end up being written out to the swap partition. If
847 this is a concern, scrub the swap partition or don't use libguestfs on
850 =head2 MULTIPLE HANDLES AND MULTIPLE THREADS
852 All high-level libguestfs actions are synchronous. If you want
853 to use libguestfs asynchronously then you must create a thread.
855 Only use the handle from a single thread. Either use the handle
856 exclusively from one thread, or provide your own mutex so that two
857 threads cannot issue calls on the same handle at the same time.
859 See the graphical program guestfs-browser for one possible
860 architecture for multithreaded programs using libvirt and libguestfs.
864 Libguestfs needs a kernel and initrd.img, which it finds by looking
865 along an internal path.
867 By default it looks for these in the directory C<$libdir/guestfs>
868 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
870 Use L</guestfs_set_path> or set the environment variable
871 L</LIBGUESTFS_PATH> to change the directories that libguestfs will
872 search in. The value is a colon-separated list of paths. The current
873 directory is I<not> searched unless the path contains an empty element
874 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
875 search the current directory and then C</usr/lib/guestfs>.
879 If you want to compile your own qemu, run qemu from a non-standard
880 location, or pass extra arguments to qemu, then you can write a
881 shell-script wrapper around qemu.
883 There is one important rule to remember: you I<must C<exec qemu>> as
884 the last command in the shell script (so that qemu replaces the shell
885 and becomes the direct child of the libguestfs-using program). If you
886 don't do this, then the qemu process won't be cleaned up correctly.
888 Here is an example of a wrapper, where I have built my own copy of
892 qemudir=/home/rjones/d/qemu
893 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
895 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
896 and then use it by setting the LIBGUESTFS_QEMU environment variable.
899 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
901 Note that libguestfs also calls qemu with the -help and -version
902 options in order to determine features.
906 We guarantee the libguestfs ABI (binary interface), for public,
907 high-level actions as outlined in this section. Although we will
908 deprecate some actions, for example if they get replaced by newer
909 calls, we will keep the old actions forever. This allows you the
910 developer to program in confidence against the libguestfs API.
912 =head2 BLOCK DEVICE NAMING
914 In the kernel there is now quite a profusion of schemata for naming
915 block devices (in this context, by I<block device> I mean a physical
916 or virtual hard drive). The original Linux IDE driver used names
917 starting with C</dev/hd*>. SCSI devices have historically used a
918 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
919 driver became a popular replacement for the old IDE driver
920 (particularly for SATA devices) those devices also used the
921 C</dev/sd*> scheme. Additionally we now have virtual machines with
922 paravirtualized drivers. This has created several different naming
923 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
926 As discussed above, libguestfs uses a qemu appliance running an
927 embedded Linux kernel to access block devices. We can run a variety
928 of appliances based on a variety of Linux kernels.
930 This causes a problem for libguestfs because many API calls use device
931 or partition names. Working scripts and the recipe (example) scripts
932 that we make available over the internet could fail if the naming
935 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
936 scheme>. Internally C</dev/sd*> names are translated, if necessary,
937 to other names as required. For example, under RHEL 5 which uses the
938 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
939 C</dev/hda2> transparently.
941 Note that this I<only> applies to parameters. The
942 L</guestfs_list_devices>, L</guestfs_list_partitions> and similar calls
943 return the true names of the devices and partitions as known to the
946 =head3 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
948 Usually this translation is transparent. However in some (very rare)
949 cases you may need to know the exact algorithm. Such cases include
950 where you use L</guestfs_config> to add a mixture of virtio and IDE
951 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
952 and C</dev/vd*> devices.
954 The algorithm is applied only to I<parameters> which are known to be
955 either device or partition names. Return values from functions such
956 as L</guestfs_list_devices> are never changed.
962 Is the string a parameter which is a device or partition name?
966 Does the string begin with C</dev/sd>?
970 Does the named device exist? If so, we use that device.
971 However if I<not> then we continue with this algorithm.
975 Replace initial C</dev/sd> string with C</dev/hd>.
977 For example, change C</dev/sda2> to C</dev/hda2>.
979 If that named device exists, use it. If not, continue.
983 Replace initial C</dev/sd> string with C</dev/vd>.
985 If that named device exists, use it. If not, return an error.
989 =head3 PORTABILITY CONCERNS WITH BLOCK DEVICE NAMING
991 Although the standard naming scheme and automatic translation is
992 useful for simple programs and guestfish scripts, for larger programs
993 it is best not to rely on this mechanism.
995 Where possible for maximum future portability programs using
996 libguestfs should use these future-proof techniques:
1002 Use L</guestfs_list_devices> or L</guestfs_list_partitions> to list
1003 actual device names, and then use those names directly.
1005 Since those device names exist by definition, they will never be
1010 Use higher level ways to identify filesystems, such as LVM names,
1011 UUIDs and filesystem labels.
1017 This section discusses security implications of using libguestfs,
1018 particularly with untrusted or malicious guests or disk images.
1020 =head2 GENERAL SECURITY CONSIDERATIONS
1022 Be careful with any files or data that you download from a guest (by
1023 "download" we mean not just the L</guestfs_download> command but any
1024 command that reads files, filenames, directories or anything else from
1025 a disk image). An attacker could manipulate the data to fool your
1026 program into doing the wrong thing. Consider cases such as:
1032 the data (file etc) not being present
1036 being present but empty
1040 being much larger than normal
1044 containing arbitrary 8 bit data
1048 being in an unexpected character encoding
1052 containing homoglyphs.
1056 =head2 SECURITY OF MOUNTING FILESYSTEMS
1058 When you mount a filesystem under Linux, mistakes in the kernel
1059 filesystem (VFS) module can sometimes be escalated into exploits by
1060 deliberately creating a malicious, malformed filesystem. These
1061 exploits are very severe for two reasons. Firstly there are very many
1062 filesystem drivers in the kernel, and many of them are infrequently
1063 used and not much developer attention has been paid to the code.
1064 Linux userspace helps potential crackers by detecting the filesystem
1065 type and automatically choosing the right VFS driver, even if that
1066 filesystem type is obscure or unexpected for the administrator.
1067 Secondly, a kernel-level exploit is like a local root exploit (worse
1068 in some ways), giving immediate and total access to the system right
1069 down to the hardware level.
1071 That explains why you should never mount a filesystem from an
1072 untrusted guest on your host kernel. How about libguestfs? We run a
1073 Linux kernel inside a qemu virtual machine, usually running as a
1074 non-root user. The attacker would need to write a filesystem which
1075 first exploited the kernel, and then exploited either qemu
1076 virtualization (eg. a faulty qemu driver) or the libguestfs protocol,
1077 and finally to be as serious as the host kernel exploit it would need
1078 to escalate its privileges to root. This multi-step escalation,
1079 performed by a static piece of data, is thought to be extremely hard
1080 to do, although we never say 'never' about security issues.
1082 In any case callers can reduce the attack surface by forcing the
1083 filesystem type when mounting (use L</guestfs_mount_vfs>).
1085 =head2 PROTOCOL SECURITY
1087 The protocol is designed to be secure, being based on RFC 4506 (XDR)
1088 with a defined upper message size. However a program that uses
1089 libguestfs must also take care - for example you can write a program
1090 that downloads a binary from a disk image and executes it locally, and
1091 no amount of protocol security will save you from the consequences.
1093 =head2 INSPECTION SECURITY
1095 Parts of the inspection API (see L</INSPECTION>) return untrusted
1096 strings directly from the guest, and these could contain any 8 bit
1097 data. Callers should be careful to escape these before printing them
1098 to a structured file (for example, use HTML escaping if creating a web
1101 Guest configuration may be altered in unusual ways by the
1102 administrator of the virtual machine, and may not reflect reality
1103 (particularly for untrusted or actively malicious guests). For
1104 example we parse the hostname from configuration files like
1105 C</etc/sysconfig/network> that we find in the guest, but the guest
1106 administrator can easily manipulate these files to provide the wrong
1109 The inspection API parses guest configuration using two external
1110 libraries: Augeas (Linux configuration) and hivex (Windows Registry).
1111 Both are designed to be robust in the face of malicious data, although
1112 denial of service attacks are still possible, for example with
1113 oversized configuration files.
1115 =head2 RUNNING UNTRUSTED GUEST COMMANDS
1117 Be very cautious about running commands from the guest. By running a
1118 command in the guest, you are giving CPU time to a binary that you do
1119 not control, under the same user account as the library, albeit
1120 wrapped in qemu virtualization. More information and alternatives can
1121 be found in the section L</RUNNING COMMANDS>.
1123 =head2 CVE-2010-3851
1125 https://bugzilla.redhat.com/642934
1127 This security bug concerns the automatic disk format detection that
1128 qemu does on disk images.
1130 A raw disk image is just the raw bytes, there is no header. Other
1131 disk images like qcow2 contain a special header. Qemu deals with this
1132 by looking for one of the known headers, and if none is found then
1133 assuming the disk image must be raw.
1135 This allows a guest which has been given a raw disk image to write
1136 some other header. At next boot (or when the disk image is accessed
1137 by libguestfs) qemu would do autodetection and think the disk image
1138 format was, say, qcow2 based on the header written by the guest.
1140 This in itself would not be a problem, but qcow2 offers many features,
1141 one of which is to allow a disk image to refer to another image
1142 (called the "backing disk"). It does this by placing the path to the
1143 backing disk into the qcow2 header. This path is not validated and
1144 could point to any host file (eg. "/etc/passwd"). The backing disk is
1145 then exposed through "holes" in the qcow2 disk image, which of course
1146 is completely under the control of the attacker.
1148 In libguestfs this is rather hard to exploit except under two
1155 You have enabled the network or have opened the disk in write mode.
1159 You are also running untrusted code from the guest (see
1160 L</RUNNING COMMANDS>).
1164 The way to avoid this is to specify the expected disk format when
1165 adding disks (the optional C<format> option to
1166 L</guestfs_add_drive_opts>). You should always do this if the disk is
1167 raw format, and it's a good idea for other cases too.
1169 For disks added from libvirt using calls like L</guestfs_add_domain>,
1170 the format is fetched from libvirt and passed through.
1172 For libguestfs tools, use the I<--format> command line parameter as
1175 =head1 CONNECTION MANAGEMENT
1179 C<guestfs_h> is the opaque type representing a connection handle.
1180 Create a handle by calling L</guestfs_create>. Call L</guestfs_close>
1181 to free the handle and release all resources used.
1183 For information on using multiple handles and threads, see the section
1184 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
1186 =head2 guestfs_create
1188 guestfs_h *guestfs_create (void);
1190 Create a connection handle.
1192 You have to call L</guestfs_add_drive_opts> (or one of the equivalent
1193 calls) on the handle at least once.
1195 This function returns a non-NULL pointer to a handle on success or
1198 After configuring the handle, you have to call L</guestfs_launch>.
1200 You may also want to configure error handling for the handle. See
1201 L</ERROR HANDLING> section below.
1203 =head2 guestfs_close
1205 void guestfs_close (guestfs_h *g);
1207 This closes the connection handle and frees up all resources used.
1209 =head1 ERROR HANDLING
1211 API functions can return errors. For example, almost all functions
1212 that return C<int> will return C<-1> to indicate an error.
1214 Additional information is available for errors: an error message
1215 string and optionally an error number (errno) if the thing that failed
1218 You can get at the additional information about the last error on the
1219 handle by calling L</guestfs_last_error>, L</guestfs_last_errno>,
1220 and/or by setting up an error handler with
1221 L</guestfs_set_error_handler>.
1223 When the handle is created, a default error handler is installed which
1224 prints the error message string to C<stderr>. For small short-running
1225 command line programs it is sufficient to do:
1227 if (guestfs_launch (g) == -1)
1228 exit (EXIT_FAILURE);
1230 since the default error handler will ensure that an error message has
1231 been printed to C<stderr> before the program exits.
1233 For other programs the caller will almost certainly want to install an
1234 alternate error handler or do error handling in-line like this:
1236 g = guestfs_create ();
1238 /* This disables the default behaviour of printing errors
1240 guestfs_set_error_handler (g, NULL, NULL);
1242 if (guestfs_launch (g) == -1) {
1243 /* Examine the error message and print it etc. */
1244 char *msg = guestfs_last_error (g);
1245 int errnum = guestfs_last_errno (g);
1246 fprintf (stderr, "%s\n", msg);
1250 Out of memory errors are handled differently. The default action is
1251 to call L<abort(3)>. If this is undesirable, then you can set a
1252 handler using L</guestfs_set_out_of_memory_handler>.
1254 L</guestfs_create> returns C<NULL> if the handle cannot be created,
1255 and because there is no handle if this happens there is no way to get
1256 additional error information. However L</guestfs_create> is supposed
1257 to be a lightweight operation which can only fail because of
1258 insufficient memory (it returns NULL in this case).
1260 =head2 guestfs_last_error
1262 const char *guestfs_last_error (guestfs_h *g);
1264 This returns the last error message that happened on C<g>. If
1265 there has not been an error since the handle was created, then this
1268 The lifetime of the returned string is until the next error occurs, or
1269 L</guestfs_close> is called.
1271 =head2 guestfs_last_errno
1273 int guestfs_last_errno (guestfs_h *g);
1275 This returns the last error number (errno) that happened on C<g>.
1277 If successful, an errno integer not equal to zero is returned.
1279 If no error, this returns 0. This call can return 0 in three
1286 There has not been any error on the handle.
1290 There has been an error but the errno was meaningless. This
1291 corresponds to the case where the error did not come from a
1292 failed system call, but for some other reason.
1296 There was an error from a failed system call, but for some
1297 reason the errno was not captured and returned. This usually
1298 indicates a bug in libguestfs.
1302 Libguestfs tries to convert the errno from inside the applicance into
1303 a corresponding errno for the caller (not entirely trivial: the
1304 appliance might be running a completely different operating system
1305 from the library and error numbers are not standardized across
1306 Un*xen). If this could not be done, then the error is translated to
1307 C<EINVAL>. In practice this should only happen in very rare
1310 =head2 guestfs_set_error_handler
1312 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
1315 void guestfs_set_error_handler (guestfs_h *g,
1316 guestfs_error_handler_cb cb,
1319 The callback C<cb> will be called if there is an error. The
1320 parameters passed to the callback are an opaque data pointer and the
1321 error message string.
1323 C<errno> is not passed to the callback. To get that the callback must
1324 call L</guestfs_last_errno>.
1326 Note that the message string C<msg> is freed as soon as the callback
1327 function returns, so if you want to stash it somewhere you must make
1330 The default handler prints messages on C<stderr>.
1332 If you set C<cb> to C<NULL> then I<no> handler is called.
1334 =head2 guestfs_get_error_handler
1336 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g,
1339 Returns the current error handler callback.
1341 =head2 guestfs_set_out_of_memory_handler
1343 typedef void (*guestfs_abort_cb) (void);
1344 int guestfs_set_out_of_memory_handler (guestfs_h *g,
1347 The callback C<cb> will be called if there is an out of memory
1348 situation. I<Note this callback must not return>.
1350 The default is to call L<abort(3)>.
1352 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
1355 =head2 guestfs_get_out_of_memory_handler
1357 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *g);
1359 This returns the current out of memory handler.
1371 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
1373 Using L</guestfs_available> you can test availability of
1374 the following groups of functions. This test queries the
1375 appliance to see if the appliance you are currently using
1376 supports the functionality.
1380 =head2 GUESTFISH supported COMMAND
1382 In L<guestfish(3)> there is a handy interactive command
1383 C<supported> which prints out the available groups and
1384 whether they are supported by this build of libguestfs.
1385 Note however that you have to do C<run> first.
1387 =head2 SINGLE CALLS AT COMPILE TIME
1389 Since version 1.5.8, C<E<lt>guestfs.hE<gt>> defines symbols
1390 for each C API function, such as:
1392 #define LIBGUESTFS_HAVE_DD 1
1394 if L</guestfs_dd> is available.
1396 Before version 1.5.8, if you needed to test whether a single
1397 libguestfs function is available at compile time, we recommended using
1398 build tools such as autoconf or cmake. For example in autotools you
1401 AC_CHECK_LIB([guestfs],[guestfs_create])
1402 AC_CHECK_FUNCS([guestfs_dd])
1404 which would result in C<HAVE_GUESTFS_DD> being either defined
1405 or not defined in your program.
1407 =head2 SINGLE CALLS AT RUN TIME
1409 Testing at compile time doesn't guarantee that a function really
1410 exists in the library. The reason is that you might be dynamically
1411 linked against a previous I<libguestfs.so> (dynamic library)
1412 which doesn't have the call. This situation unfortunately results
1413 in a segmentation fault, which is a shortcoming of the C dynamic
1414 linking system itself.
1416 You can use L<dlopen(3)> to test if a function is available
1417 at run time, as in this example program (note that you still
1418 need the compile time check as well):
1424 #include <guestfs.h>
1428 #ifdef LIBGUESTFS_HAVE_DD
1432 /* Test if the function guestfs_dd is really available. */
1433 dl = dlopen (NULL, RTLD_LAZY);
1435 fprintf (stderr, "dlopen: %s\n", dlerror ());
1436 exit (EXIT_FAILURE);
1438 has_function = dlsym (dl, "guestfs_dd") != NULL;
1442 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
1444 printf ("this libguestfs.so has guestfs_dd function\n");
1445 /* Now it's safe to call
1446 guestfs_dd (g, "foo", "bar");
1450 printf ("guestfs_dd function was not found at compile time\n");
1454 You may think the above is an awful lot of hassle, and it is.
1455 There are other ways outside of the C linking system to ensure
1456 that this kind of incompatibility never arises, such as using
1459 Requires: libguestfs >= 1.0.80
1461 =head1 CALLS WITH OPTIONAL ARGUMENTS
1463 A recent feature of the API is the introduction of calls which take
1464 optional arguments. In C these are declared 3 ways. The main way is
1465 as a call which takes variable arguments (ie. C<...>), as in this
1468 int guestfs_add_drive_opts (guestfs_h *g, const char *filename, ...);
1470 Call this with a list of optional arguments, terminated by C<-1>.
1471 So to call with no optional arguments specified:
1473 guestfs_add_drive_opts (g, filename, -1);
1475 With a single optional argument:
1477 guestfs_add_drive_opts (g, filename,
1478 GUESTFS_ADD_DRIVE_OPTS_FORMAT, "qcow2",
1483 guestfs_add_drive_opts (g, filename,
1484 GUESTFS_ADD_DRIVE_OPTS_FORMAT, "qcow2",
1485 GUESTFS_ADD_DRIVE_OPTS_READONLY, 1,
1488 and so forth. Don't forget the terminating C<-1> otherwise
1489 Bad Things will happen!
1491 =head2 USING va_list FOR OPTIONAL ARGUMENTS
1493 The second variant has the same name with the suffix C<_va>, which
1494 works the same way but takes a C<va_list>. See the C manual for
1495 details. For the example function, this is declared:
1497 int guestfs_add_drive_opts_va (guestfs_h *g, const char *filename,
1500 =head2 CONSTRUCTING OPTIONAL ARGUMENTS
1502 The third variant is useful where you need to construct these
1503 calls. You pass in a structure where you fill in the optional
1504 fields. The structure has a bitmask as the first element which
1505 you must set to indicate which fields you have filled in. For
1506 our example function the structure and call are declared:
1508 struct guestfs_add_drive_opts_argv {
1514 int guestfs_add_drive_opts_argv (guestfs_h *g, const char *filename,
1515 const struct guestfs_add_drive_opts_argv *optargs);
1517 You could call it like this:
1519 struct guestfs_add_drive_opts_argv optargs = {
1520 .bitmask = GUESTFS_ADD_DRIVE_OPTS_READONLY_BITMASK |
1521 GUESTFS_ADD_DRIVE_OPTS_FORMAT_BITMASK,
1526 guestfs_add_drive_opts_argv (g, filename, &optargs);
1534 The C<_BITMASK> suffix on each option name when specifying the
1539 You do not need to fill in all fields of the structure.
1543 There must be a one-to-one correspondence between fields of the
1544 structure that are filled in, and bits set in the bitmask.
1548 =head2 OPTIONAL ARGUMENTS IN OTHER LANGUAGES
1550 In other languages, optional arguments are expressed in the
1551 way that is natural for that language. We refer you to the
1552 language-specific documentation for more details on that.
1554 For guestfish, see L<guestfish(1)/OPTIONAL ARGUMENTS>.
1556 =head2 SETTING CALLBACKS TO HANDLE EVENTS
1558 The child process generates events in some situations. Current events
1559 include: receiving a log message, the child process exits.
1561 Use the C<guestfs_set_*_callback> functions to set a callback for
1562 different types of events.
1564 Only I<one callback of each type> can be registered for each handle.
1565 Calling C<guestfs_set_*_callback> again overwrites the previous
1566 callback of that type. Cancel all callbacks of this type by calling
1567 this function with C<cb> set to C<NULL>.
1569 =head2 guestfs_set_log_message_callback
1571 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
1572 char *buf, int len);
1573 void guestfs_set_log_message_callback (guestfs_h *g,
1574 guestfs_log_message_cb cb,
1577 The callback function C<cb> will be called whenever qemu or the guest
1578 writes anything to the console.
1580 Use this function to capture kernel messages and similar.
1582 Normally there is no log message handler, and log messages are just
1585 =head2 guestfs_set_subprocess_quit_callback
1587 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
1588 void guestfs_set_subprocess_quit_callback (guestfs_h *g,
1589 guestfs_subprocess_quit_cb cb,
1592 The callback function C<cb> will be called when the child process
1593 quits, either asynchronously or if killed by
1594 L</guestfs_kill_subprocess>. (This corresponds to a transition from
1595 any state to the CONFIG state).
1597 =head2 guestfs_set_launch_done_callback
1599 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
1600 void guestfs_set_launch_done_callback (guestfs_h *g,
1601 guestfs_launch_done_cb cb,
1604 The callback function C<cb> will be called when the child process
1605 becomes ready first time after it has been launched. (This
1606 corresponds to a transition from LAUNCHING to the READY state).
1608 =head2 guestfs_set_close_callback
1610 typedef void (*guestfs_close_cb) (guestfs_h *g, void *opaque);
1611 void guestfs_set_close_callback (guestfs_h *g,
1612 guestfs_close_cb cb,
1615 The callback function C<cb> will be called while the handle
1616 is being closed (synchronously from L</guestfs_close>).
1618 Note that libguestfs installs an L<atexit(3)> handler to try to
1619 clean up handles that are open when the program exits. This
1620 means that this callback might be called indirectly from
1621 L<exit(3)>, which can cause unexpected problems in higher-level
1622 languages (eg. if your HLL interpreter has already been cleaned
1623 up by the time this is called, and if your callback then jumps
1624 into some HLL function).
1626 =head2 guestfs_set_progress_callback
1628 typedef void (*guestfs_progress_cb) (guestfs_h *g, void *opaque,
1629 int proc_nr, int serial,
1630 uint64_t position, uint64_t total);
1631 void guestfs_set_progress_callback (guestfs_h *g,
1632 guestfs_progress_cb cb,
1635 Some long-running operations can generate progress messages. If
1636 this callback is registered, then it will be called each time a
1637 progress message is generated (usually two seconds after the
1638 operation started, and three times per second thereafter until
1639 it completes, although the frequency may change in future versions).
1641 The callback receives two numbers: C<position> and C<total>.
1642 The units of C<total> are not defined, although for some
1643 operations C<total> may relate in some way to the amount of
1644 data to be transferred (eg. in bytes or megabytes), and
1645 C<position> may be the portion which has been transferred.
1647 The only defined and stable parts of the API are:
1653 The callback can display to the user some type of progress bar or
1654 indicator which shows the ratio of C<position>:C<total>.
1658 0 E<lt>= C<position> E<lt>= C<total>
1662 If any progress notification is sent during a call, then a final
1663 progress notification is always sent when C<position> = C<total>.
1665 This is to simplify caller code, so callers can easily set the
1666 progress indicator to "100%" at the end of the operation, without
1667 requiring special code to detect this case.
1671 The callback also receives the procedure number and serial number of
1672 the call. These are only useful for debugging protocol issues, and
1673 the callback can normally ignore them. The callback may want to
1674 print these numbers in error messages or debugging messages.
1676 =head1 PRIVATE DATA AREA
1678 You can attach named pieces of private data to the libguestfs handle,
1679 and fetch them by name for the lifetime of the handle. This is called
1680 the private data area and is only available from the C API.
1682 To attach a named piece of data, use the following call:
1684 void guestfs_set_private (guestfs_h *g, const char *key, void *data);
1686 C<key> is the name to associate with this data, and C<data> is an
1687 arbitrary pointer (which can be C<NULL>). Any previous item with the
1688 same name is overwritten.
1690 You can use any C<key> you want, but names beginning with an
1691 underscore character are reserved for internal libguestfs purposes
1692 (for implementing language bindings). It is recommended to prefix the
1693 name with some unique string to avoid collisions with other users.
1695 To retrieve the pointer, use:
1697 void *guestfs_get_private (guestfs_h *g, const char *key);
1699 This function returns C<NULL> if either no data is found associated
1700 with C<key>, or if the user previously set the C<key>'s C<data>
1703 Libguestfs does not try to look at or interpret the C<data> pointer in
1704 any way. As far as libguestfs is concerned, it need not be a valid
1705 pointer at all. In particular, libguestfs does I<not> try to free the
1706 data when the handle is closed. If the data must be freed, then the
1707 caller must either free it before calling L</guestfs_close> or must
1708 set up a close callback to do it (see L</guestfs_set_close_callback>,
1709 and note that only one callback can be registered for a handle).
1711 The private data area is implemented using a hash table, and should be
1712 reasonably efficient for moderate numbers of keys.
1716 <!-- old anchor for the next section -->
1717 <a name="state_machine_and_low_level_event_api"/>
1723 Internally, libguestfs is implemented by running an appliance (a
1724 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
1725 a child process of the main program.
1731 | | child process / appliance
1732 | | __________________________
1734 +-------------------+ RPC | +-----------------+ |
1735 | libguestfs <--------------------> guestfsd | |
1736 | | | +-----------------+ |
1737 \___________________/ | | Linux kernel | |
1738 | +--^--------------+ |
1739 \_________|________________/
1747 The library, linked to the main program, creates the child process and
1748 hence the appliance in the L</guestfs_launch> function.
1750 Inside the appliance is a Linux kernel and a complete stack of
1751 userspace tools (such as LVM and ext2 programs) and a small
1752 controlling daemon called L</guestfsd>. The library talks to
1753 L</guestfsd> using remote procedure calls (RPC). There is a mostly
1754 one-to-one correspondence between libguestfs API calls and RPC calls
1755 to the daemon. Lastly the disk image(s) are attached to the qemu
1756 process which translates device access by the appliance's Linux kernel
1757 into accesses to the image.
1759 A common misunderstanding is that the appliance "is" the virtual
1760 machine. Although the disk image you are attached to might also be
1761 used by some virtual machine, libguestfs doesn't know or care about
1762 this. (But you will care if both libguestfs's qemu process and your
1763 virtual machine are trying to update the disk image at the same time,
1764 since these usually results in massive disk corruption).
1766 =head1 STATE MACHINE
1768 libguestfs uses a state machine to model the child process:
1779 / | \ \ guestfs_launch
1790 \______/ <------ \________/
1792 The normal transitions are (1) CONFIG (when the handle is created, but
1793 there is no child process), (2) LAUNCHING (when the child process is
1794 booting up), (3) alternating between READY and BUSY as commands are
1795 issued to, and carried out by, the child process.
1797 The guest may be killed by L</guestfs_kill_subprocess>, or may die
1798 asynchronously at any time (eg. due to some internal error), and that
1799 causes the state to transition back to CONFIG.
1801 Configuration commands for qemu such as L</guestfs_add_drive> can only
1802 be issued when in the CONFIG state.
1804 The API offers one call that goes from CONFIG through LAUNCHING to
1805 READY. L</guestfs_launch> blocks until the child process is READY to
1806 accept commands (or until some failure or timeout).
1807 L</guestfs_launch> internally moves the state from CONFIG to LAUNCHING
1808 while it is running.
1810 API actions such as L</guestfs_mount> can only be issued when in the
1811 READY state. These API calls block waiting for the command to be
1812 carried out (ie. the state to transition to BUSY and then back to
1813 READY). There are no non-blocking versions, and no way to issue more
1814 than one command per handle at the same time.
1816 Finally, the child process sends asynchronous messages back to the
1817 main program, such as kernel log messages. You can register a
1818 callback to receive these messages.
1822 =head2 COMMUNICATION PROTOCOL
1824 Don't rely on using this protocol directly. This section documents
1825 how it currently works, but it may change at any time.
1827 The protocol used to talk between the library and the daemon running
1828 inside the qemu virtual machine is a simple RPC mechanism built on top
1829 of XDR (RFC 1014, RFC 1832, RFC 4506).
1831 The detailed format of structures is in C<src/guestfs_protocol.x>
1832 (note: this file is automatically generated).
1834 There are two broad cases, ordinary functions that don't have any
1835 C<FileIn> and C<FileOut> parameters, which are handled with very
1836 simple request/reply messages. Then there are functions that have any
1837 C<FileIn> or C<FileOut> parameters, which use the same request and
1838 reply messages, but they may also be followed by files sent using a
1841 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1843 For ordinary functions, the request message is:
1845 total length (header + arguments,
1846 but not including the length word itself)
1847 struct guestfs_message_header (encoded as XDR)
1848 struct guestfs_<foo>_args (encoded as XDR)
1850 The total length field allows the daemon to allocate a fixed size
1851 buffer into which it slurps the rest of the message. As a result, the
1852 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1853 4MB), which means the effective size of any request is limited to
1854 somewhere under this size.
1856 Note also that many functions don't take any arguments, in which case
1857 the C<guestfs_I<foo>_args> is completely omitted.
1859 The header contains the procedure number (C<guestfs_proc>) which is
1860 how the receiver knows what type of args structure to expect, or none
1863 The reply message for ordinary functions is:
1865 total length (header + ret,
1866 but not including the length word itself)
1867 struct guestfs_message_header (encoded as XDR)
1868 struct guestfs_<foo>_ret (encoded as XDR)
1870 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1871 for functions that return no formal return values.
1873 As above the total length of the reply is limited to
1874 C<GUESTFS_MESSAGE_MAX>.
1876 In the case of an error, a flag is set in the header, and the reply
1877 message is slightly changed:
1879 total length (header + error,
1880 but not including the length word itself)
1881 struct guestfs_message_header (encoded as XDR)
1882 struct guestfs_message_error (encoded as XDR)
1884 The C<guestfs_message_error> structure contains the error message as a
1887 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1889 A C<FileIn> parameter indicates that we transfer a file I<into> the
1890 guest. The normal request message is sent (see above). However this
1891 is followed by a sequence of file chunks.
1893 total length (header + arguments,
1894 but not including the length word itself,
1895 and not including the chunks)
1896 struct guestfs_message_header (encoded as XDR)
1897 struct guestfs_<foo>_args (encoded as XDR)
1898 sequence of chunks for FileIn param #0
1899 sequence of chunks for FileIn param #1 etc.
1901 The "sequence of chunks" is:
1903 length of chunk (not including length word itself)
1904 struct guestfs_chunk (encoded as XDR)
1906 struct guestfs_chunk (encoded as XDR)
1909 struct guestfs_chunk (with data.data_len == 0)
1911 The final chunk has the C<data_len> field set to zero. Additionally a
1912 flag is set in the final chunk to indicate either successful
1913 completion or early cancellation.
1915 At time of writing there are no functions that have more than one
1916 FileIn parameter. However this is (theoretically) supported, by
1917 sending the sequence of chunks for each FileIn parameter one after
1918 another (from left to right).
1920 Both the library (sender) I<and> the daemon (receiver) may cancel the
1921 transfer. The library does this by sending a chunk with a special
1922 flag set to indicate cancellation. When the daemon sees this, it
1923 cancels the whole RPC, does I<not> send any reply, and goes back to
1924 reading the next request.
1926 The daemon may also cancel. It does this by writing a special word
1927 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1928 during the transfer, and if it gets it, it will cancel the transfer
1929 (it sends a cancel chunk). The special word is chosen so that even if
1930 cancellation happens right at the end of the transfer (after the
1931 library has finished writing and has started listening for the reply),
1932 the "spurious" cancel flag will not be confused with the reply
1935 This protocol allows the transfer of arbitrary sized files (no 32 bit
1936 limit), and also files where the size is not known in advance
1937 (eg. from pipes or sockets). However the chunks are rather small
1938 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1939 daemon need to keep much in memory.
1941 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1943 The protocol for FileOut parameters is exactly the same as for FileIn
1944 parameters, but with the roles of daemon and library reversed.
1946 total length (header + ret,
1947 but not including the length word itself,
1948 and not including the chunks)
1949 struct guestfs_message_header (encoded as XDR)
1950 struct guestfs_<foo>_ret (encoded as XDR)
1951 sequence of chunks for FileOut param #0
1952 sequence of chunks for FileOut param #1 etc.
1954 =head3 INITIAL MESSAGE
1956 When the daemon launches it sends an initial word
1957 (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest and daemon is
1958 alive. This is what L</guestfs_launch> waits for.
1960 =head3 PROGRESS NOTIFICATION MESSAGES
1962 The daemon may send progress notification messages at any time. These
1963 are distinguished by the normal length word being replaced by
1964 C<GUESTFS_PROGRESS_FLAG>, followed by a fixed size progress message.
1966 The library turns them into progress callbacks (see
1967 C<guestfs_set_progress_callback>) if there is a callback registered,
1968 or discards them if not.
1970 The daemon self-limits the frequency of progress messages it sends
1971 (see C<daemon/proto.c:notify_progress>). Not all calls generate
1974 =head1 LIBGUESTFS VERSION NUMBERS
1976 Since April 2010, libguestfs has started to make separate development
1977 and stable releases, along with corresponding branches in our git
1978 repository. These separate releases can be identified by version
1981 even numbers for stable: 1.2.x, 1.4.x, ...
1982 .-------- odd numbers for development: 1.3.x, 1.5.x, ...
1988 | `-------- sub-version
1990 `------ always '1' because we don't change the ABI
1992 Thus "1.3.5" is the 5th update to the development branch "1.3".
1994 As time passes we cherry pick fixes from the development branch and
1995 backport those into the stable branch, the effect being that the
1996 stable branch should get more stable and less buggy over time. So the
1997 stable releases are ideal for people who don't need new features but
1998 would just like the software to work.
2000 Our criteria for backporting changes are:
2006 Documentation changes which don't affect any code are
2007 backported unless the documentation refers to a future feature
2008 which is not in stable.
2012 Bug fixes which are not controversial, fix obvious problems, and
2013 have been well tested are backported.
2017 Simple rearrangements of code which shouldn't affect how it works get
2018 backported. This is so that the code in the two branches doesn't get
2019 too far out of step, allowing us to backport future fixes more easily.
2023 We I<don't> backport new features, new APIs, new tools etc, except in
2024 one exceptional case: the new feature is required in order to
2025 implement an important bug fix.
2029 A new stable branch starts when we think the new features in
2030 development are substantial and compelling enough over the current
2031 stable branch to warrant it. When that happens we create new stable
2032 and development versions 1.N.0 and 1.(N+1).0 [N is even]. The new
2033 dot-oh release won't necessarily be so stable at this point, but by
2034 backporting fixes from development, that branch will stabilize over
2037 =head1 ENVIRONMENT VARIABLES
2041 =item LIBGUESTFS_APPEND
2043 Pass additional options to the guest kernel.
2045 =item LIBGUESTFS_DEBUG
2047 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
2048 has the same effect as calling C<guestfs_set_verbose (g, 1)>.
2050 =item LIBGUESTFS_MEMSIZE
2052 Set the memory allocated to the qemu process, in megabytes. For
2055 LIBGUESTFS_MEMSIZE=700
2057 =item LIBGUESTFS_PATH
2059 Set the path that libguestfs uses to search for kernel and initrd.img.
2060 See the discussion of paths in section PATH above.
2062 =item LIBGUESTFS_QEMU
2064 Set the default qemu binary that libguestfs uses. If not set, then
2065 the qemu which was found at compile time by the configure script is
2068 See also L</QEMU WRAPPERS> above.
2070 =item LIBGUESTFS_TRACE
2072 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
2073 has the same effect as calling C<guestfs_set_trace (g, 1)>.
2077 Location of temporary directory, defaults to C</tmp>.
2079 If libguestfs was compiled to use the supermin appliance then the
2080 real appliance is cached in this directory, shared between all
2081 handles belonging to the same EUID. You can use C<$TMPDIR> to
2082 configure another directory to use in case C</tmp> is not large
2089 L<guestfs-examples(3)>,
2095 L<virt-filesystems(1)>,
2096 L<virt-inspector(1)>,
2097 L<virt-list-filesystems(1)>,
2098 L<virt-list-partitions(1)>,
2107 L<http://libguestfs.org/>.
2109 Tools with a similar purpose:
2118 To get a list of bugs against libguestfs use this link:
2120 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
2122 To report a new bug against libguestfs use this link:
2124 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
2126 When reporting a bug, please check:
2132 That the bug hasn't been reported already.
2136 That you are testing a recent version.
2140 Describe the bug accurately, and give a way to reproduce it.
2144 Run libguestfs-test-tool and paste the B<complete, unedited>
2145 output into the bug report.
2151 Richard W.M. Jones (C<rjones at redhat dot com>)
2155 Copyright (C) 2009-2010 Red Hat Inc.
2156 L<http://libguestfs.org/>
2158 This library is free software; you can redistribute it and/or
2159 modify it under the terms of the GNU Lesser General Public
2160 License as published by the Free Software Foundation; either
2161 version 2 of the License, or (at your option) any later version.
2163 This library is distributed in the hope that it will be useful,
2164 but WITHOUT ANY WARRANTY; without even the implied warranty of
2165 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
2166 Lesser General Public License for more details.
2168 You should have received a copy of the GNU Lesser General Public
2169 License along with this library; if not, write to the Free Software
2170 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA