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, 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.
57 This section provides a gentler overview of the libguestfs API. We
58 also try to group API calls together, where that may not be obvious
59 from reading about the individual calls in the main section of this
64 Before you can use libguestfs calls, you have to create a handle.
65 Then you must add at least one disk image to the handle, followed by
66 launching the handle, then performing whatever operations you want,
67 and finally closing the handle. By convention we use the single
68 letter C<g> for the name of the handle variable, although of course
69 you can use any name you want.
71 The general structure of all libguestfs-using programs looks like
74 guestfs_h *g = guestfs_create ();
76 /* Call guestfs_add_drive additional times if there are
77 * multiple disk images.
79 guestfs_add_drive (g, "guest.img");
81 /* Most manipulation calls won't work until you've launched
82 * the handle 'g'. You have to do this _after_ adding drives
83 * and _before_ other commands.
87 /* Now you can examine what partitions, LVs etc are available.
89 char **partitions = guestfs_list_partitions (g);
90 char **logvols = guestfs_lvs (g);
92 /* To access a filesystem in the image, you must mount it.
94 guestfs_mount (g, "/dev/sda1", "/");
96 /* Now you can perform filesystem actions on the guest
99 guestfs_touch (g, "/hello");
101 /* You only need to call guestfs_sync if you have made
102 * changes to the guest image. (But if you've made changes
103 * then you *must* sync). See also: guestfs_umount and
104 * guestfs_umount_all calls.
108 /* Close the handle 'g'. */
111 The code above doesn't include any error checking. In real code you
112 should check return values carefully for errors. In general all
113 functions that return integers return C<-1> on error, and all
114 functions that return pointers return C<NULL> on error. See section
115 L</ERROR HANDLING> below for how to handle errors, and consult the
116 documentation for each function call below to see precisely how they
117 return error indications.
121 The image filename (C<"guest.img"> in the example above) could be a
122 disk image from a virtual machine, a L<dd(1)> copy of a physical hard
123 disk, an actual block device, or simply an empty file of zeroes that
124 you have created through L<posix_fallocate(3)>. Libguestfs lets you
125 do useful things to all of these.
127 You can add a disk read-only using L</guestfs_add_drive_ro>, in which
128 case libguestfs won't modify the file.
130 Be extremely cautious if the disk image is in use, eg. if it is being
131 used by a virtual machine. Adding it read-write will almost certainly
132 cause disk corruption, but adding it read-only is safe.
134 You must add at least one disk image, and you may add multiple disk
135 images. In the API, the disk images are usually referred to as
136 C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
139 Once L</guestfs_launch> has been called you cannot add any more images.
140 You can call L</guestfs_list_devices> to get a list of the device
141 names, in the order that you added them. See also L</BLOCK DEVICE
146 Before you can read or write files, create directories and so on in a
147 disk image that contains filesystems, you have to mount those
148 filesystems using L</guestfs_mount>. If you already know that a disk
149 image contains (for example) one partition with a filesystem on that
150 partition, then you can mount it directly:
152 guestfs_mount (g, "/dev/sda1", "/");
154 where C</dev/sda1> means literally the first partition (C<1>) of the
155 first disk image that we added (C</dev/sda>). If the disk contains
156 Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).
158 If you are given a disk image and you don't know what it contains then
159 you have to find out. Libguestfs can do that too: use
160 L</guestfs_list_partitions> and L</guestfs_lvs> to list possible
161 partitions and LVs, and either try mounting each to see what is
162 mountable, or else examine them with L</guestfs_vfs_type> or
163 L</guestfs_file>. But you might find it easier to look at higher level
164 programs built on top of libguestfs, in particular
165 L<virt-inspector(1)>.
167 To mount a disk image read-only, use L</guestfs_mount_ro>. There are
168 several other variations of the C<guestfs_mount_*> call.
170 =head2 FILESYSTEM ACCESS AND MODIFICATION
172 The majority of the libguestfs API consists of fairly low-level calls
173 for accessing and modifying the files, directories, symlinks etc on
174 mounted filesystems. There are over a hundred such calls which you
175 can find listed in detail below in this man page, and we don't even
176 pretend to cover them all in this overview.
178 Specify filenames as full paths, starting with C<"/"> and including
181 For example, if you mounted a filesystem at C<"/"> and you want to
182 read the file called C<"etc/passwd"> then you could do:
184 char *data = guestfs_cat (g, "/etc/passwd");
186 This would return C<data> as a newly allocated buffer containing the
187 full content of that file (with some conditions: see also
188 L</DOWNLOADING> below), or C<NULL> if there was an error.
190 As another example, to create a top-level directory on that filesystem
191 called C<"var"> you would do:
193 guestfs_mkdir (g, "/var");
195 To create a symlink you could do:
197 guestfs_ln_s (g, "/etc/init.d/portmap",
198 "/etc/rc3.d/S30portmap");
200 Libguestfs will reject attempts to use relative paths and there is no
201 concept of a current working directory.
203 Libguestfs can return errors in many situations: for example if the
204 filesystem isn't writable, or if a file or directory that you
205 requested doesn't exist. If you are using the C API (documented here)
206 you have to check for those error conditions after each call. (Other
207 language bindings turn these errors into exceptions).
209 File writes are affected by the per-handle umask, set by calling
210 L</guestfs_umask> and defaulting to 022. See L</UMASK>.
214 Libguestfs contains API calls to read, create and modify partition
215 tables on disk images.
217 In the common case where you want to create a single partition
218 covering the whole disk, you should use the L</guestfs_part_disk>
221 const char *parttype = "mbr";
222 if (disk_is_larger_than_2TB)
224 guestfs_part_disk (g, "/dev/sda", parttype);
226 Obviously this effectively wipes anything that was on that disk image
231 Libguestfs provides access to a large part of the LVM2 API, such as
232 L</guestfs_lvcreate> and L</guestfs_vgremove>. It won't make much sense
233 unless you familiarize yourself with the concepts of physical volumes,
234 volume groups and logical volumes.
236 This author strongly recommends reading the LVM HOWTO, online at
237 L<http://tldp.org/HOWTO/LVM-HOWTO/>.
241 Use L</guestfs_cat> to download small, text only files. This call
242 is limited to files which are less than 2 MB and which cannot contain
243 any ASCII NUL (C<\0>) characters. However it has a very simple
246 L</guestfs_read_file> can be used to read files which contain
247 arbitrary 8 bit data, since it returns a (pointer, size) pair.
248 However it is still limited to "small" files, less than 2 MB.
250 L</guestfs_download> can be used to download any file, with no
251 limits on content or size (even files larger than 4 GB).
253 To download multiple files, see L</guestfs_tar_out> and
258 It's often the case that you want to write a file or files to the disk
261 To write a small file with fixed content, use L</guestfs_write>. To
262 create a file of all zeroes, use L</guestfs_truncate_size> (sparse) or
263 L</guestfs_fallocate64> (with all disk blocks allocated). There are a
264 variety of other functions for creating test files, for example
265 L</guestfs_fill> and L</guestfs_fill_pattern>.
267 To upload a single file, use L</guestfs_upload>. This call has no
268 limits on file content or size (even files larger than 4 GB).
270 To upload multiple files, see L</guestfs_tar_in> and L</guestfs_tgz_in>.
272 However the fastest way to upload I<large numbers of arbitrary files>
273 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
274 L<mkisofs(8)>), then attach this using L</guestfs_add_drive_ro>. If
275 you add the drive in a predictable way (eg. adding it last after all
276 other drives) then you can get the device name from
277 L</guestfs_list_devices> and mount it directly using
278 L</guestfs_mount_ro>. Note that squashfs images are sometimes
279 non-portable between kernel versions, and they don't support labels or
280 UUIDs. If you want to pre-build an image or you need to mount it
281 using a label or UUID, use an ISO image instead.
285 There are various different commands for copying between files and
286 devices and in and out of the guest filesystem. These are summarised
291 =item B<file> to B<file>
293 Use L</guestfs_cp> to copy a single file, or
294 L</guestfs_cp_a> to copy directories recursively.
296 =item B<file or device> to B<file or device>
298 Use L</guestfs_dd> which efficiently uses L<dd(1)>
299 to copy between files and devices in the guest.
301 Example: duplicate the contents of an LV:
303 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
305 The destination (C</dev/VG/Copy>) must be at least as large as the
306 source (C</dev/VG/Original>). To copy less than the whole
307 source device, use L</guestfs_copy_size>.
309 =item B<file on the host> to B<file or device>
311 Use L</guestfs_upload>. See L</UPLOADING> above.
313 =item B<file or device> to B<file on the host>
315 Use L</guestfs_download>. See L</DOWNLOADING> above.
321 L</guestfs_ll> is just designed for humans to read (mainly when using
322 the L<guestfish(1)>-equivalent command C<ll>).
324 L</guestfs_ls> is a quick way to get a list of files in a directory
325 from programs, as a flat list of strings.
327 L</guestfs_readdir> is a programmatic way to get a list of files in a
328 directory, plus additional information about each one. It is more
329 equivalent to using the L<readdir(3)> call on a local filesystem.
331 L</guestfs_find> and L</guestfs_find0> can be used to recursively list
334 =head2 RUNNING COMMANDS
336 Although libguestfs is a primarily an API for manipulating files
337 inside guest images, we also provide some limited facilities for
338 running commands inside guests.
340 There are many limitations to this:
346 The kernel version that the command runs under will be different
347 from what it expects.
351 If the command needs to communicate with daemons, then most likely
352 they won't be running.
356 The command will be running in limited memory.
360 Only supports Linux guests (not Windows, BSD, etc).
364 Architecture limitations (eg. won't work for a PPC guest on
369 For SELinux guests, you may need to enable SELinux and load policy
370 first. See L</SELINUX> in this manpage.
374 The two main API calls to run commands are L</guestfs_command> and
375 L</guestfs_sh> (there are also variations).
377 The difference is that L</guestfs_sh> runs commands using the shell, so
378 any shell globs, redirections, etc will work.
380 =head2 CONFIGURATION FILES
382 To read and write configuration files in Linux guest filesystems, we
383 strongly recommend using Augeas. For example, Augeas understands how
384 to read and write, say, a Linux shadow password file or X.org
385 configuration file, and so avoids you having to write that code.
387 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
388 don't document Augeas itself here because there is excellent
389 documentation on the L<http://augeas.net/> website.
391 If you don't want to use Augeas (you fool!) then try calling
392 L</guestfs_read_lines> to get the file as a list of lines which
393 you can iterate over.
397 We support SELinux guests. To ensure that labeling happens correctly
398 in SELinux guests, you need to enable SELinux and load the guest's
405 Before launching, do:
407 guestfs_set_selinux (g, 1);
411 After mounting the guest's filesystem(s), load the policy. This
412 is best done by running the L<load_policy(8)> command in the
415 guestfs_sh (g, "/usr/sbin/load_policy");
417 (Older versions of C<load_policy> require you to specify the
418 name of the policy file).
422 Optionally, set the security context for the API. The correct
423 security context to use can only be known by inspecting the
424 guest. As an example:
426 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
430 This will work for running commands and editing existing files.
432 When new files are created, you may need to label them explicitly,
433 for example by running the external command
434 C<restorecon pathname>.
438 Certain calls are affected by the current file mode creation mask (the
439 "umask"). In particular ones which create files or directories, such
440 as L</guestfs_touch>, L</guestfs_mknod> or L</guestfs_mkdir>. This
441 affects either the default mode that the file is created with or
442 modifies the mode that you supply.
444 The default umask is C<022>, so files are created with modes such as
445 C<0644> and directories with C<0755>.
447 There are two ways to avoid being affected by umask. Either set umask
448 to 0 (call C<guestfs_umask (g, 0)> early after launching). Or call
449 L</guestfs_chmod> after creating each file or directory.
451 For more information about umask, see L<umask(2)>.
453 =head2 ENCRYPTED DISKS
455 Libguestfs allows you to access Linux guests which have been
456 encrypted using whole disk encryption that conforms to the
457 Linux Unified Key Setup (LUKS) standard. This includes
458 nearly all whole disk encryption systems used by modern
461 Use L</guestfs_vfs_type> to identify LUKS-encrypted block
462 devices (it returns the string C<crypto_LUKS>).
464 Then open these devices by calling L</guestfs_luks_open>.
465 Obviously you will require the passphrase!
467 Opening a LUKS device creates a new device mapper device
468 called C</dev/mapper/mapname> (where C<mapname> is the
469 string you supply to L</guestfs_luks_open>).
470 Reads and writes to this mapper device are decrypted from and
471 encrypted to the underlying block device respectively.
473 LVM volume groups on the device can be made visible by calling
474 L</guestfs_vgscan> followed by L</guestfs_vg_activate_all>.
475 The logical volume(s) can now be mounted in the usual way.
477 Use the reverse process to close a LUKS device. Unmount
478 any logical volumes on it, deactivate the volume groups
479 by caling C<guestfs_vg_activate (g, 0, ["/dev/VG"])>.
480 Then close the mapper device by calling
481 L</guestfs_luks_close> on the C</dev/mapper/mapname>
482 device (I<not> the underlying encrypted block device).
484 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
486 Libguestfs can mount NTFS partitions. It does this using the
487 L<http://www.ntfs-3g.org/> driver.
489 DOS and Windows still use drive letters, and the filesystems are
490 always treated as case insensitive by Windows itself, and therefore
491 you might find a Windows configuration file referring to a path like
492 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
493 that directory might be referred to as C</WINDOWS/System32>.
495 Drive letter mappings are outside the scope of libguestfs. You have
496 to use libguestfs to read the appropriate Windows Registry and
497 configuration files, to determine yourself how drives are mapped (see
498 also L<virt-inspector(1)>).
500 Replacing backslash characters with forward slash characters is also
501 outside the scope of libguestfs, but something that you can easily do.
503 Where we can help is in resolving the case insensitivity of paths.
504 For this, call L</guestfs_case_sensitive_path>.
506 Libguestfs also provides some help for decoding Windows Registry
507 "hive" files, through the library C<hivex> which is part of the
508 libguestfs project although ships as a separate tarball. You have to
509 locate and download the hive file(s) yourself, and then pass them to
510 C<hivex> functions. See also the programs L<hivexml(1)>,
511 L<hivexsh(1)>, L<hivexregedit(1)> and L<virt-win-reg(1)> for more help
514 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
516 Although we don't want to discourage you from using the C API, we will
517 mention here that the same API is also available in other languages.
519 The API is broadly identical in all supported languages. This means
520 that the C call C<guestfs_mount(g,path)> is
521 C<$g-E<gt>mount($path)> in Perl, C<g.mount(path)> in Python,
522 and C<Guestfs.mount g path> in OCaml. In other words, a
523 straightforward, predictable isomorphism between each language.
525 Error messages are automatically transformed
526 into exceptions if the language supports it.
528 We don't try to "object orientify" parts of the API in OO languages,
529 although contributors are welcome to write higher level APIs above
530 what we provide in their favourite languages if they wish.
536 You can use the I<guestfs.h> header file from C++ programs. The C++
537 API is identical to the C API. C++ classes and exceptions are not
542 The C# bindings are highly experimental. Please read the warnings
543 at the top of C<csharp/Libguestfs.cs>.
547 This is the only language binding that is working but incomplete.
548 Only calls which return simple integers have been bound in Haskell,
549 and we are looking for help to complete this binding.
553 Full documentation is contained in the Javadoc which is distributed
558 For documentation see the file C<guestfs.mli>.
562 For documentation see L<Sys::Guestfs(3)>.
566 For documentation do:
574 Use the Guestfs module. There is no Ruby-specific documentation, but
575 you can find examples written in Ruby in the libguestfs source.
577 =item B<shell scripts>
579 For documentation see L<guestfish(1)>.
583 =head2 LIBGUESTFS GOTCHAS
585 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
586 system [...] that works in the way it is documented but is
587 counterintuitive and almost invites mistakes."
589 Since we developed libguestfs and the associated tools, there are
590 several things we would have designed differently, but are now stuck
591 with for backwards compatibility or other reasons. If there is ever a
592 libguestfs 2.0 release, you can expect these to change. Beware of
597 =item Autosync / forgetting to sync.
599 When modifying a filesystem from C or another language, you B<must>
600 unmount all filesystems and call L</guestfs_sync> explicitly before
601 you close the libguestfs handle. You can also call:
603 guestfs_set_autosync (g, 1);
605 to have the unmount/sync done automatically for you when the handle 'g'
606 is closed. (This feature is called "autosync", L</guestfs_set_autosync>
609 If you forget to do this, then it is entirely possible that your
610 changes won't be written out, or will be partially written, or (very
611 rarely) that you'll get disk corruption.
613 Note that in L<guestfish(3)> autosync is the default. So quick and
614 dirty guestfish scripts that forget to sync will work just fine, which
615 can make this very puzzling if you are trying to debug a problem.
617 =item Mount option C<-o sync> should not be the default.
619 If you use L</guestfs_mount>, then C<-o sync,noatime> are added
620 implicitly. However C<-o sync> does not add any reliability benefit,
621 but does have a very large performance impact.
623 The work around is to use L</guestfs_mount_options> and set the mount
624 options that you actually want to use.
626 =item Read-only should be the default.
628 In L<guestfish(3)>, I<--ro> should be the default, and you should
629 have to specify I<--rw> if you want to make changes to the image.
631 This would reduce the potential to corrupt live VM images.
633 Note that many filesystems change the disk when you just mount and
634 unmount, even if you didn't perform any writes. You need to use
635 L</guestfs_add_drive_ro> to guarantee that the disk is not changed.
637 =item guestfish command line is hard to use.
639 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
640 for examination). It tries to run a guestfish command C<disk.img>
641 which doesn't exist, so it fails. In earlier versions of guestfish
642 the error message was also unintuitive, but we have corrected this
643 since. Like the Bourne shell, we should have used C<guestfish -c
644 command> to run commands.
646 =item guestfish megabyte modifiers don't work right on all commands
648 In recent guestfish you can use C<1M> to mean 1 megabyte (and
649 similarly for other modifiers). What guestfish actually does is to
650 multiply the number part by the modifier part and pass the result to
651 the C API. However this doesn't work for a few APIs which aren't
652 expecting bytes, but are already expecting some other unit
655 The most common is L</guestfs_lvcreate>. The guestfish command:
659 does not do what you might expect. Instead because
660 L</guestfs_lvcreate> is already expecting megabytes, this tries to
661 create a 100 I<terabyte> (100 megabytes * megabytes) logical volume.
662 The error message you get from this is also a little obscure.
664 This could be fixed in the generator by specially marking parameters
665 and return values which take bytes or other units.
667 =item Protocol limit of 256 characters for error messages
669 This limit is both rather small and quite unnecessary. We should be
670 able to return error messages up to the length of the protocol message
673 Note that we cannot change the protocol without some breakage, because
674 there are distributions that repackage the Fedora appliance.
676 =item Protocol should return errno with error messages.
678 It would be a nice-to-have to be able to get the original value of
679 'errno' from inside the appliance along error paths (where set).
680 Currently L<guestmount(1)> goes through hoops to try to reverse the
681 error message string into an errno, see the function error() in
686 =head2 PROTOCOL LIMITS
688 Internally libguestfs uses a message-based protocol to pass API calls
689 and their responses to and from a small "appliance" (see L</INTERNALS>
690 for plenty more detail about this). The maximum message size used by
691 the protocol is slightly less than 4 MB. For some API calls you may
692 need to be aware of this limit. The API calls which may be affected
693 are individually documented, with a link back to this section of the
696 A simple call such as L</guestfs_cat> returns its result (the file
697 data) in a simple string. Because this string is at some point
698 internally encoded as a message, the maximum size that it can return
699 is slightly under 4 MB. If the requested file is larger than this
700 then you will get an error.
702 In order to transfer large files into and out of the guest filesystem,
703 you need to use particular calls that support this. The sections
704 L</UPLOADING> and L</DOWNLOADING> document how to do this.
706 You might also consider mounting the disk image using our FUSE
707 filesystem support (L<guestmount(1)>).
709 =head2 KEYS AND PASSPHRASES
711 Certain libguestfs calls take a parameter that contains sensitive key
712 material, passed in as a C string.
714 In the future we would hope to change the libguestfs implementation so
715 that keys are L<mlock(2)>-ed into physical RAM, and thus can never end
716 up in swap. However this is I<not> done at the moment, because of the
717 complexity of such an implementation.
719 Therefore you should be aware that any key parameter you pass to
720 libguestfs might end up being written out to the swap partition. If
721 this is a concern, scrub the swap partition or don't use libguestfs on
724 =head1 CONNECTION MANAGEMENT
728 C<guestfs_h> is the opaque type representing a connection handle.
729 Create a handle by calling L</guestfs_create>. Call L</guestfs_close>
730 to free the handle and release all resources used.
732 For information on using multiple handles and threads, see the section
733 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
735 =head2 guestfs_create
737 guestfs_h *guestfs_create (void);
739 Create a connection handle.
741 You have to call L</guestfs_add_drive> on the handle at least once.
743 This function returns a non-NULL pointer to a handle on success or
746 After configuring the handle, you have to call L</guestfs_launch>.
748 You may also want to configure error handling for the handle. See
749 L</ERROR HANDLING> section below.
753 void guestfs_close (guestfs_h *g);
755 This closes the connection handle and frees up all resources used.
757 =head1 ERROR HANDLING
759 The convention in all functions that return C<int> is that they return
760 C<-1> to indicate an error. You can get additional information on
761 errors by calling L</guestfs_last_error> and/or by setting up an error
762 handler with L</guestfs_set_error_handler>.
764 The default error handler prints the information string to C<stderr>.
766 Out of memory errors are handled differently. The default action is
767 to call L<abort(3)>. If this is undesirable, then you can set a
768 handler using L</guestfs_set_out_of_memory_handler>.
770 =head2 guestfs_last_error
772 const char *guestfs_last_error (guestfs_h *g);
774 This returns the last error message that happened on C<g>. If
775 there has not been an error since the handle was created, then this
778 The lifetime of the returned string is until the next error occurs, or
779 L</guestfs_close> is called.
781 The error string is not localized (ie. is always in English), because
782 this makes searching for error messages in search engines give the
783 largest number of results.
785 =head2 guestfs_set_error_handler
787 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
790 void guestfs_set_error_handler (guestfs_h *g,
791 guestfs_error_handler_cb cb,
794 The callback C<cb> will be called if there is an error. The
795 parameters passed to the callback are an opaque data pointer and the
796 error message string.
798 Note that the message string C<msg> is freed as soon as the callback
799 function returns, so if you want to stash it somewhere you must make
802 The default handler prints messages on C<stderr>.
804 If you set C<cb> to C<NULL> then I<no> handler is called.
806 =head2 guestfs_get_error_handler
808 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g,
811 Returns the current error handler callback.
813 =head2 guestfs_set_out_of_memory_handler
815 typedef void (*guestfs_abort_cb) (void);
816 int guestfs_set_out_of_memory_handler (guestfs_h *g,
819 The callback C<cb> will be called if there is an out of memory
820 situation. I<Note this callback must not return>.
822 The default is to call L<abort(3)>.
824 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
827 =head2 guestfs_get_out_of_memory_handler
829 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *g);
831 This returns the current out of memory handler.
835 Libguestfs needs a kernel and initrd.img, which it finds by looking
836 along an internal path.
838 By default it looks for these in the directory C<$libdir/guestfs>
839 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
841 Use L</guestfs_set_path> or set the environment variable
842 L</LIBGUESTFS_PATH> to change the directories that libguestfs will
843 search in. The value is a colon-separated list of paths. The current
844 directory is I<not> searched unless the path contains an empty element
845 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
846 search the current directory and then C</usr/lib/guestfs>.
848 =head1 HIGH-LEVEL API ACTIONS
852 We guarantee the libguestfs ABI (binary interface), for public,
853 high-level actions as outlined in this section. Although we will
854 deprecate some actions, for example if they get replaced by newer
855 calls, we will keep the old actions forever. This allows you the
856 developer to program in confidence against the libguestfs API.
866 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
868 Using L</guestfs_available> you can test availability of
869 the following groups of functions. This test queries the
870 appliance to see if the appliance you are currently using
871 supports the functionality.
875 =head2 GUESTFISH supported COMMAND
877 In L<guestfish(3)> there is a handy interactive command
878 C<supported> which prints out the available groups and
879 whether they are supported by this build of libguestfs.
880 Note however that you have to do C<run> first.
882 =head2 SINGLE CALLS AT COMPILE TIME
884 If you need to test whether a single libguestfs function is
885 available at compile time, we recommend using build tools
886 such as autoconf or cmake. For example in autotools you could
889 AC_CHECK_LIB([guestfs],[guestfs_create])
890 AC_CHECK_FUNCS([guestfs_dd])
892 which would result in C<HAVE_GUESTFS_DD> being either defined
893 or not defined in your program.
895 =head2 SINGLE CALLS AT RUN TIME
897 Testing at compile time doesn't guarantee that a function really
898 exists in the library. The reason is that you might be dynamically
899 linked against a previous I<libguestfs.so> (dynamic library)
900 which doesn't have the call. This situation unfortunately results
901 in a segmentation fault, which is a shortcoming of the C dynamic
902 linking system itself.
904 You can use L<dlopen(3)> to test if a function is available
905 at run time, as in this example program (note that you still
906 need the compile time check as well):
918 #ifdef HAVE_GUESTFS_DD
922 /* Test if the function guestfs_dd is really available. */
923 dl = dlopen (NULL, RTLD_LAZY);
925 fprintf (stderr, "dlopen: %s\n", dlerror ());
928 has_function = dlsym (dl, "guestfs_dd") != NULL;
932 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
934 printf ("this libguestfs.so has guestfs_dd function\n");
935 /* Now it's safe to call
936 guestfs_dd (g, "foo", "bar");
940 printf ("guestfs_dd function was not found at compile time\n");
944 You may think the above is an awful lot of hassle, and it is.
945 There are other ways outside of the C linking system to ensure
946 that this kind of incompatibility never arises, such as using
949 Requires: libguestfs >= 1.0.80
953 <!-- old anchor for the next section -->
954 <a name="state_machine_and_low_level_event_api"/>
960 Internally, libguestfs is implemented by running an appliance (a
961 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
962 a child process of the main program.
968 | | child process / appliance
969 | | __________________________
971 +-------------------+ RPC | +-----------------+ |
972 | libguestfs <--------------------> guestfsd | |
973 | | | +-----------------+ |
974 \___________________/ | | Linux kernel | |
975 | +--^--------------+ |
976 \_________|________________/
984 The library, linked to the main program, creates the child process and
985 hence the appliance in the L</guestfs_launch> function.
987 Inside the appliance is a Linux kernel and a complete stack of
988 userspace tools (such as LVM and ext2 programs) and a small
989 controlling daemon called L</guestfsd>. The library talks to
990 L</guestfsd> using remote procedure calls (RPC). There is a mostly
991 one-to-one correspondence between libguestfs API calls and RPC calls
992 to the daemon. Lastly the disk image(s) are attached to the qemu
993 process which translates device access by the appliance's Linux kernel
994 into accesses to the image.
996 A common misunderstanding is that the appliance "is" the virtual
997 machine. Although the disk image you are attached to might also be
998 used by some virtual machine, libguestfs doesn't know or care about
999 this. (But you will care if both libguestfs's qemu process and your
1000 virtual machine are trying to update the disk image at the same time,
1001 since these usually results in massive disk corruption).
1003 =head1 STATE MACHINE
1005 libguestfs uses a state machine to model the child process:
1016 / | \ \ guestfs_launch
1027 \______/ <------ \________/
1029 The normal transitions are (1) CONFIG (when the handle is created, but
1030 there is no child process), (2) LAUNCHING (when the child process is
1031 booting up), (3) alternating between READY and BUSY as commands are
1032 issued to, and carried out by, the child process.
1034 The guest may be killed by L</guestfs_kill_subprocess>, or may die
1035 asynchronously at any time (eg. due to some internal error), and that
1036 causes the state to transition back to CONFIG.
1038 Configuration commands for qemu such as L</guestfs_add_drive> can only
1039 be issued when in the CONFIG state.
1041 The high-level API offers two calls that go from CONFIG through
1042 LAUNCHING to READY. L</guestfs_launch> blocks until the child process
1043 is READY to accept commands (or until some failure or timeout).
1044 L</guestfs_launch> internally moves the state from CONFIG to LAUNCHING
1045 while it is running.
1047 High-level API actions such as L</guestfs_mount> can only be issued
1048 when in the READY state. These high-level API calls block waiting for
1049 the command to be carried out (ie. the state to transition to BUSY and
1050 then back to READY). But using the low-level event API, you get
1051 non-blocking versions. (But you can still only carry out one
1052 operation per handle at a time - that is a limitation of the
1053 communications protocol we use).
1055 Finally, the child process sends asynchronous messages back to the
1056 main program, such as kernel log messages. Mostly these are ignored
1057 by the high-level API, but using the low-level event API you can
1058 register to receive these messages.
1060 =head2 SETTING CALLBACKS TO HANDLE EVENTS
1062 The child process generates events in some situations. Current events
1063 include: receiving a log message, the child process exits.
1065 Use the C<guestfs_set_*_callback> functions to set a callback for
1066 different types of events.
1068 Only I<one callback of each type> can be registered for each handle.
1069 Calling C<guestfs_set_*_callback> again overwrites the previous
1070 callback of that type. Cancel all callbacks of this type by calling
1071 this function with C<cb> set to C<NULL>.
1073 =head2 guestfs_set_log_message_callback
1075 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
1076 char *buf, int len);
1077 void guestfs_set_log_message_callback (guestfs_h *g,
1078 guestfs_log_message_cb cb,
1081 The callback function C<cb> will be called whenever qemu or the guest
1082 writes anything to the console.
1084 Use this function to capture kernel messages and similar.
1086 Normally there is no log message handler, and log messages are just
1089 =head2 guestfs_set_subprocess_quit_callback
1091 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
1092 void guestfs_set_subprocess_quit_callback (guestfs_h *g,
1093 guestfs_subprocess_quit_cb cb,
1096 The callback function C<cb> will be called when the child process
1097 quits, either asynchronously or if killed by
1098 L</guestfs_kill_subprocess>. (This corresponds to a transition from
1099 any state to the CONFIG state).
1101 =head2 guestfs_set_launch_done_callback
1103 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
1104 void guestfs_set_launch_done_callback (guestfs_h *g,
1105 guestfs_launch_done_cb cb,
1108 The callback function C<cb> will be called when the child process
1109 becomes ready first time after it has been launched. (This
1110 corresponds to a transition from LAUNCHING to the READY state).
1112 =head2 guestfs_set_close_callback
1114 typedef void (*guestfs_close_cb) (guestfs_h *g, void *opaque);
1115 void guestfs_set_close_callback (guestfs_h *g,
1116 guestfs_close_cb cb,
1119 The callback function C<cb> will be called while the handle
1120 is being closed (synchronously from L</guestfs_close>).
1122 Note that libguestfs installs an L<atexit(3)> handler to try to
1123 clean up handles that are open when the program exits. This
1124 means that this callback might be called indirectly from
1125 L<exit(3)>, which can cause unexpected problems in higher-level
1126 languages (eg. if your HLL interpreter has already been cleaned
1127 up by the time this is called, and if your callback then jumps
1128 into some HLL function).
1130 =head1 BLOCK DEVICE NAMING
1132 In the kernel there is now quite a profusion of schemata for naming
1133 block devices (in this context, by I<block device> I mean a physical
1134 or virtual hard drive). The original Linux IDE driver used names
1135 starting with C</dev/hd*>. SCSI devices have historically used a
1136 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
1137 driver became a popular replacement for the old IDE driver
1138 (particularly for SATA devices) those devices also used the
1139 C</dev/sd*> scheme. Additionally we now have virtual machines with
1140 paravirtualized drivers. This has created several different naming
1141 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
1144 As discussed above, libguestfs uses a qemu appliance running an
1145 embedded Linux kernel to access block devices. We can run a variety
1146 of appliances based on a variety of Linux kernels.
1148 This causes a problem for libguestfs because many API calls use device
1149 or partition names. Working scripts and the recipe (example) scripts
1150 that we make available over the internet could fail if the naming
1153 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
1154 scheme>. Internally C</dev/sd*> names are translated, if necessary,
1155 to other names as required. For example, under RHEL 5 which uses the
1156 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
1157 C</dev/hda2> transparently.
1159 Note that this I<only> applies to parameters. The
1160 L</guestfs_list_devices>, L</guestfs_list_partitions> and similar calls
1161 return the true names of the devices and partitions as known to the
1164 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
1166 Usually this translation is transparent. However in some (very rare)
1167 cases you may need to know the exact algorithm. Such cases include
1168 where you use L</guestfs_config> to add a mixture of virtio and IDE
1169 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
1170 and C</dev/vd*> devices.
1172 The algorithm is applied only to I<parameters> which are known to be
1173 either device or partition names. Return values from functions such
1174 as L</guestfs_list_devices> are never changed.
1180 Is the string a parameter which is a device or partition name?
1184 Does the string begin with C</dev/sd>?
1188 Does the named device exist? If so, we use that device.
1189 However if I<not> then we continue with this algorithm.
1193 Replace initial C</dev/sd> string with C</dev/hd>.
1195 For example, change C</dev/sda2> to C</dev/hda2>.
1197 If that named device exists, use it. If not, continue.
1201 Replace initial C</dev/sd> string with C</dev/vd>.
1203 If that named device exists, use it. If not, return an error.
1207 =head2 PORTABILITY CONCERNS
1209 Although the standard naming scheme and automatic translation is
1210 useful for simple programs and guestfish scripts, for larger programs
1211 it is best not to rely on this mechanism.
1213 Where possible for maximum future portability programs using
1214 libguestfs should use these future-proof techniques:
1220 Use L</guestfs_list_devices> or L</guestfs_list_partitions> to list
1221 actual device names, and then use those names directly.
1223 Since those device names exist by definition, they will never be
1228 Use higher level ways to identify filesystems, such as LVM names,
1229 UUIDs and filesystem labels.
1235 =head2 COMMUNICATION PROTOCOL
1237 Don't rely on using this protocol directly. This section documents
1238 how it currently works, but it may change at any time.
1240 The protocol used to talk between the library and the daemon running
1241 inside the qemu virtual machine is a simple RPC mechanism built on top
1242 of XDR (RFC 1014, RFC 1832, RFC 4506).
1244 The detailed format of structures is in C<src/guestfs_protocol.x>
1245 (note: this file is automatically generated).
1247 There are two broad cases, ordinary functions that don't have any
1248 C<FileIn> and C<FileOut> parameters, which are handled with very
1249 simple request/reply messages. Then there are functions that have any
1250 C<FileIn> or C<FileOut> parameters, which use the same request and
1251 reply messages, but they may also be followed by files sent using a
1254 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1256 For ordinary functions, the request message is:
1258 total length (header + arguments,
1259 but not including the length word itself)
1260 struct guestfs_message_header (encoded as XDR)
1261 struct guestfs_<foo>_args (encoded as XDR)
1263 The total length field allows the daemon to allocate a fixed size
1264 buffer into which it slurps the rest of the message. As a result, the
1265 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1266 4MB), which means the effective size of any request is limited to
1267 somewhere under this size.
1269 Note also that many functions don't take any arguments, in which case
1270 the C<guestfs_I<foo>_args> is completely omitted.
1272 The header contains the procedure number (C<guestfs_proc>) which is
1273 how the receiver knows what type of args structure to expect, or none
1276 The reply message for ordinary functions is:
1278 total length (header + ret,
1279 but not including the length word itself)
1280 struct guestfs_message_header (encoded as XDR)
1281 struct guestfs_<foo>_ret (encoded as XDR)
1283 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1284 for functions that return no formal return values.
1286 As above the total length of the reply is limited to
1287 C<GUESTFS_MESSAGE_MAX>.
1289 In the case of an error, a flag is set in the header, and the reply
1290 message is slightly changed:
1292 total length (header + error,
1293 but not including the length word itself)
1294 struct guestfs_message_header (encoded as XDR)
1295 struct guestfs_message_error (encoded as XDR)
1297 The C<guestfs_message_error> structure contains the error message as a
1300 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1302 A C<FileIn> parameter indicates that we transfer a file I<into> the
1303 guest. The normal request message is sent (see above). However this
1304 is followed by a sequence of file chunks.
1306 total length (header + arguments,
1307 but not including the length word itself,
1308 and not including the chunks)
1309 struct guestfs_message_header (encoded as XDR)
1310 struct guestfs_<foo>_args (encoded as XDR)
1311 sequence of chunks for FileIn param #0
1312 sequence of chunks for FileIn param #1 etc.
1314 The "sequence of chunks" is:
1316 length of chunk (not including length word itself)
1317 struct guestfs_chunk (encoded as XDR)
1319 struct guestfs_chunk (encoded as XDR)
1322 struct guestfs_chunk (with data.data_len == 0)
1324 The final chunk has the C<data_len> field set to zero. Additionally a
1325 flag is set in the final chunk to indicate either successful
1326 completion or early cancellation.
1328 At time of writing there are no functions that have more than one
1329 FileIn parameter. However this is (theoretically) supported, by
1330 sending the sequence of chunks for each FileIn parameter one after
1331 another (from left to right).
1333 Both the library (sender) I<and> the daemon (receiver) may cancel the
1334 transfer. The library does this by sending a chunk with a special
1335 flag set to indicate cancellation. When the daemon sees this, it
1336 cancels the whole RPC, does I<not> send any reply, and goes back to
1337 reading the next request.
1339 The daemon may also cancel. It does this by writing a special word
1340 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1341 during the transfer, and if it gets it, it will cancel the transfer
1342 (it sends a cancel chunk). The special word is chosen so that even if
1343 cancellation happens right at the end of the transfer (after the
1344 library has finished writing and has started listening for the reply),
1345 the "spurious" cancel flag will not be confused with the reply
1348 This protocol allows the transfer of arbitrary sized files (no 32 bit
1349 limit), and also files where the size is not known in advance
1350 (eg. from pipes or sockets). However the chunks are rather small
1351 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1352 daemon need to keep much in memory.
1354 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1356 The protocol for FileOut parameters is exactly the same as for FileIn
1357 parameters, but with the roles of daemon and library reversed.
1359 total length (header + ret,
1360 but not including the length word itself,
1361 and not including the chunks)
1362 struct guestfs_message_header (encoded as XDR)
1363 struct guestfs_<foo>_ret (encoded as XDR)
1364 sequence of chunks for FileOut param #0
1365 sequence of chunks for FileOut param #1 etc.
1367 =head3 INITIAL MESSAGE
1369 Because the underlying channel (QEmu -net channel) doesn't have any
1370 sort of connection control, when the daemon launches it sends an
1371 initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
1372 and daemon is alive. This is what L</guestfs_launch> waits for.
1374 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1376 All high-level libguestfs actions are synchronous. If you want
1377 to use libguestfs asynchronously then you must create a thread.
1379 Only use the handle from a single thread. Either use the handle
1380 exclusively from one thread, or provide your own mutex so that two
1381 threads cannot issue calls on the same handle at the same time.
1383 =head1 QEMU WRAPPERS
1385 If you want to compile your own qemu, run qemu from a non-standard
1386 location, or pass extra arguments to qemu, then you can write a
1387 shell-script wrapper around qemu.
1389 There is one important rule to remember: you I<must C<exec qemu>> as
1390 the last command in the shell script (so that qemu replaces the shell
1391 and becomes the direct child of the libguestfs-using program). If you
1392 don't do this, then the qemu process won't be cleaned up correctly.
1394 Here is an example of a wrapper, where I have built my own copy of
1398 qemudir=/home/rjones/d/qemu
1399 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1401 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1402 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1405 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1407 Note that libguestfs also calls qemu with the -help and -version
1408 options in order to determine features.
1410 =head1 LIBGUESTFS VERSION NUMBERS
1412 Since April 2010, libguestfs has started to make separate development
1413 and stable releases, along with corresponding branches in our git
1414 repository. These separate releases can be identified by version
1417 even numbers for stable: 1.2.x, 1.4.x, ...
1418 .-------- odd numbers for development: 1.3.x, 1.5.x, ...
1424 | `-------- sub-version
1426 `------ always '1' because we don't change the ABI
1428 Thus "1.3.5" is the 5th update to the development branch "1.3".
1430 As time passes we cherry pick fixes from the development branch and
1431 backport those into the stable branch, the effect being that the
1432 stable branch should get more stable and less buggy over time. So the
1433 stable releases are ideal for people who don't need new features but
1434 would just like the software to work.
1436 Our criteria for backporting changes are:
1442 Documentation changes which don't affect any code are
1443 backported unless the documentation refers to a future feature
1444 which is not in stable.
1448 Bug fixes which are not controversial, fix obvious problems, and
1449 have been well tested are backported.
1453 Simple rearrangements of code which shouldn't affect how it works get
1454 backported. This is so that the code in the two branches doesn't get
1455 too far out of step, allowing us to backport future fixes more easily.
1459 We I<don't> backport new features, new APIs, new tools etc, except in
1460 one exceptional case: the new feature is required in order to
1461 implement an important bug fix.
1465 A new stable branch starts when we think the new features in
1466 development are substantial and compelling enough over the current
1467 stable branch to warrant it. When that happens we create new stable
1468 and development versions 1.N.0 and 1.(N+1).0 [N is even]. The new
1469 dot-oh release won't necessarily be so stable at this point, but by
1470 backporting fixes from development, that branch will stabilize over
1473 =head1 ENVIRONMENT VARIABLES
1477 =item LIBGUESTFS_APPEND
1479 Pass additional options to the guest kernel.
1481 =item LIBGUESTFS_DEBUG
1483 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1484 has the same effect as calling C<guestfs_set_verbose (g, 1)>.
1486 =item LIBGUESTFS_MEMSIZE
1488 Set the memory allocated to the qemu process, in megabytes. For
1491 LIBGUESTFS_MEMSIZE=700
1493 =item LIBGUESTFS_PATH
1495 Set the path that libguestfs uses to search for kernel and initrd.img.
1496 See the discussion of paths in section PATH above.
1498 =item LIBGUESTFS_QEMU
1500 Set the default qemu binary that libguestfs uses. If not set, then
1501 the qemu which was found at compile time by the configure script is
1504 See also L</QEMU WRAPPERS> above.
1506 =item LIBGUESTFS_TRACE
1508 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1509 has the same effect as calling C<guestfs_set_trace (g, 1)>.
1513 Location of temporary directory, defaults to C</tmp>.
1515 If libguestfs was compiled to use the supermin appliance then each
1516 handle will require rather a large amount of space in this directory
1517 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1518 configure another directory to use in case C</tmp> is not large
1530 L<virt-inspector(1)>,
1531 L<virt-list-filesystems(1)>,
1532 L<virt-list-partitions(1)>,
1541 L<http://libguestfs.org/>.
1543 Tools with a similar purpose:
1552 To get a list of bugs against libguestfs use this link:
1554 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1556 To report a new bug against libguestfs use this link:
1558 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1560 When reporting a bug, please check:
1566 That the bug hasn't been reported already.
1570 That you are testing a recent version.
1574 Describe the bug accurately, and give a way to reproduce it.
1578 Run libguestfs-test-tool and paste the B<complete, unedited>
1579 output into the bug report.
1585 Richard W.M. Jones (C<rjones at redhat dot com>)
1589 Copyright (C) 2009-2010 Red Hat Inc.
1590 L<http://libguestfs.org/>
1592 This library is free software; you can redistribute it and/or
1593 modify it under the terms of the GNU Lesser General Public
1594 License as published by the Free Software Foundation; either
1595 version 2 of the License, or (at your option) any later version.
1597 This library is distributed in the hope that it will be useful,
1598 but WITHOUT ANY WARRANTY; without even the implied warranty of
1599 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1600 Lesser General Public License for more details.
1602 You should have received a copy of the GNU Lesser General Public
1603 License along with this library; if not, write to the Free Software
1604 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA