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 For small, single files, use L</guestfs_write>.
263 To upload a single file, use L</guestfs_upload>. This call has no
264 limits on file content or size (even files larger than 4 GB).
266 To upload multiple files, see L</guestfs_tar_in> and L</guestfs_tgz_in>.
268 However the fastest way to upload I<large numbers of arbitrary files>
269 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
270 L<mkisofs(8)>), then attach this using L</guestfs_add_drive_ro>. If
271 you add the drive in a predictable way (eg. adding it last after all
272 other drives) then you can get the device name from
273 L</guestfs_list_devices> and mount it directly using
274 L</guestfs_mount_ro>. Note that squashfs images are sometimes
275 non-portable between kernel versions, and they don't support labels or
276 UUIDs. If you want to pre-build an image or you need to mount it
277 using a label or UUID, use an ISO image instead.
281 There are various different commands for copying between files and
282 devices and in and out of the guest filesystem. These are summarised
287 =item B<file> to B<file>
289 Use L</guestfs_cp> to copy a single file, or
290 L</guestfs_cp_a> to copy directories recursively.
292 =item B<file or device> to B<file or device>
294 Use L</guestfs_dd> which efficiently uses L<dd(1)>
295 to copy between files and devices in the guest.
297 Example: duplicate the contents of an LV:
299 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
301 The destination (C</dev/VG/Copy>) must be at least as large as the
302 source (C</dev/VG/Original>). To copy less than the whole
303 source device, use L</guestfs_copy_size>.
305 =item B<file on the host> to B<file or device>
307 Use L</guestfs_upload>. See L</UPLOADING> above.
309 =item B<file or device> to B<file on the host>
311 Use L</guestfs_download>. See L</DOWNLOADING> above.
317 L</guestfs_ll> is just designed for humans to read (mainly when using
318 the L<guestfish(1)>-equivalent command C<ll>).
320 L</guestfs_ls> is a quick way to get a list of files in a directory
321 from programs, as a flat list of strings.
323 L</guestfs_readdir> is a programmatic way to get a list of files in a
324 directory, plus additional information about each one. It is more
325 equivalent to using the L<readdir(3)> call on a local filesystem.
327 L</guestfs_find> and L</guestfs_find0> can be used to recursively list
330 =head2 RUNNING COMMANDS
332 Although libguestfs is a primarily an API for manipulating files
333 inside guest images, we also provide some limited facilities for
334 running commands inside guests.
336 There are many limitations to this:
342 The kernel version that the command runs under will be different
343 from what it expects.
347 If the command needs to communicate with daemons, then most likely
348 they won't be running.
352 The command will be running in limited memory.
356 Only supports Linux guests (not Windows, BSD, etc).
360 Architecture limitations (eg. won't work for a PPC guest on
365 For SELinux guests, you may need to enable SELinux and load policy
366 first. See L</SELINUX> in this manpage.
370 The two main API calls to run commands are L</guestfs_command> and
371 L</guestfs_sh> (there are also variations).
373 The difference is that L</guestfs_sh> runs commands using the shell, so
374 any shell globs, redirections, etc will work.
376 =head2 CONFIGURATION FILES
378 To read and write configuration files in Linux guest filesystems, we
379 strongly recommend using Augeas. For example, Augeas understands how
380 to read and write, say, a Linux shadow password file or X.org
381 configuration file, and so avoids you having to write that code.
383 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
384 don't document Augeas itself here because there is excellent
385 documentation on the L<http://augeas.net/> website.
387 If you don't want to use Augeas (you fool!) then try calling
388 L</guestfs_read_lines> to get the file as a list of lines which
389 you can iterate over.
393 We support SELinux guests. To ensure that labeling happens correctly
394 in SELinux guests, you need to enable SELinux and load the guest's
401 Before launching, do:
403 guestfs_set_selinux (g, 1);
407 After mounting the guest's filesystem(s), load the policy. This
408 is best done by running the L<load_policy(8)> command in the
411 guestfs_sh (g, "/usr/sbin/load_policy");
413 (Older versions of C<load_policy> require you to specify the
414 name of the policy file).
418 Optionally, set the security context for the API. The correct
419 security context to use can only be known by inspecting the
420 guest. As an example:
422 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
426 This will work for running commands and editing existing files.
428 When new files are created, you may need to label them explicitly,
429 for example by running the external command
430 C<restorecon pathname>.
434 Certain calls are affected by the current file mode creation mask (the
435 "umask"). In particular ones which create files or directories, such
436 as L</guestfs_touch>, L</guestfs_mknod> or L</guestfs_mkdir>. This
437 affects either the default mode that the file is created with or
438 modifies the mode that you supply.
440 The default umask is C<022>, so files are created with modes such as
441 C<0644> and directories with C<0755>.
443 There are two ways to avoid being affected by umask. Either set umask
444 to 0 (call C<guestfs_umask (g, 0)> early after launching). Or call
445 L</guestfs_chmod> after creating each file or directory.
447 For more information about umask, see L<umask(2)>.
449 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
451 Libguestfs can mount NTFS partitions. It does this using the
452 L<http://www.ntfs-3g.org/> driver.
454 DOS and Windows still use drive letters, and the filesystems are
455 always treated as case insensitive by Windows itself, and therefore
456 you might find a Windows configuration file referring to a path like
457 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
458 that directory might be referred to as C</WINDOWS/System32>.
460 Drive letter mappings are outside the scope of libguestfs. You have
461 to use libguestfs to read the appropriate Windows Registry and
462 configuration files, to determine yourself how drives are mapped (see
463 also L<virt-inspector(1)>).
465 Replacing backslash characters with forward slash characters is also
466 outside the scope of libguestfs, but something that you can easily do.
468 Where we can help is in resolving the case insensitivity of paths.
469 For this, call L</guestfs_case_sensitive_path>.
471 Libguestfs also provides some help for decoding Windows Registry
472 "hive" files, through the library C<hivex> which is part of the
473 libguestfs project although ships as a separate tarball. You have to
474 locate and download the hive file(s) yourself, and then pass them to
475 C<hivex> functions. See also the programs L<hivexml(1)>,
476 L<hivexsh(1)>, L<hivexregedit(1)> and L<virt-win-reg(1)> for more help
479 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
481 Although we don't want to discourage you from using the C API, we will
482 mention here that the same API is also available in other languages.
484 The API is broadly identical in all supported languages. This means
485 that the C call C<guestfs_mount(g,path)> is
486 C<$g-E<gt>mount($path)> in Perl, C<g.mount(path)> in Python,
487 and C<Guestfs.mount g path> in OCaml. In other words, a
488 straightforward, predictable isomorphism between each language.
490 Error messages are automatically transformed
491 into exceptions if the language supports it.
493 We don't try to "object orientify" parts of the API in OO languages,
494 although contributors are welcome to write higher level APIs above
495 what we provide in their favourite languages if they wish.
501 You can use the I<guestfs.h> header file from C++ programs. The C++
502 API is identical to the C API. C++ classes and exceptions are not
507 The C# bindings are highly experimental. Please read the warnings
508 at the top of C<csharp/Libguestfs.cs>.
512 This is the only language binding that is working but incomplete.
513 Only calls which return simple integers have been bound in Haskell,
514 and we are looking for help to complete this binding.
518 Full documentation is contained in the Javadoc which is distributed
523 For documentation see the file C<guestfs.mli>.
527 For documentation see L<Sys::Guestfs(3)>.
531 For documentation do:
539 Use the Guestfs module. There is no Ruby-specific documentation, but
540 you can find examples written in Ruby in the libguestfs source.
542 =item B<shell scripts>
544 For documentation see L<guestfish(1)>.
548 =head2 LIBGUESTFS GOTCHAS
550 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
551 system [...] that works in the way it is documented but is
552 counterintuitive and almost invites mistakes."
554 Since we developed libguestfs and the associated tools, there are
555 several things we would have designed differently, but are now stuck
556 with for backwards compatibility or other reasons. If there is ever a
557 libguestfs 2.0 release, you can expect these to change. Beware of
562 =item Autosync / forgetting to sync.
564 When modifying a filesystem from C or another language, you B<must>
565 unmount all filesystems and call L</guestfs_sync> explicitly before
566 you close the libguestfs handle. You can also call:
568 guestfs_set_autosync (g, 1);
570 to have the unmount/sync done automatically for you when the handle 'g'
571 is closed. (This feature is called "autosync", L</guestfs_set_autosync>
574 If you forget to do this, then it is entirely possible that your
575 changes won't be written out, or will be partially written, or (very
576 rarely) that you'll get disk corruption.
578 Note that in L<guestfish(3)> autosync is the default. So quick and
579 dirty guestfish scripts that forget to sync will work just fine, which
580 can make this very puzzling if you are trying to debug a problem.
582 =item Mount option C<-o sync> should not be the default.
584 If you use L</guestfs_mount>, then C<-o sync,noatime> are added
585 implicitly. However C<-o sync> does not add any reliability benefit,
586 but does have a very large performance impact.
588 The work around is to use L</guestfs_mount_options> and set the mount
589 options that you actually want to use.
591 =item Read-only should be the default.
593 In L<guestfish(3)>, I<--ro> should be the default, and you should
594 have to specify I<--rw> if you want to make changes to the image.
596 This would reduce the potential to corrupt live VM images.
598 Note that many filesystems change the disk when you just mount and
599 unmount, even if you didn't perform any writes. You need to use
600 L</guestfs_add_drive_ro> to guarantee that the disk is not changed.
602 =item guestfish command line is hard to use.
604 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
605 for examination). It tries to run a guestfish command C<disk.img>
606 which doesn't exist, so it fails. In earlier versions of guestfish
607 the error message was also unintuitive, but we have corrected this
608 since. Like the Bourne shell, we should have used C<guestfish -c
609 command> to run commands.
611 =item Protocol limit of 256 characters for error messages
613 This limit is both rather small and quite unnecessary. We should be
614 able to return error messages up to the length of the protocol message
617 Note that we cannot change the protocol without some breakage, because
618 there are distributions that repackage the Fedora appliance.
620 =item Protocol should return errno with error messages.
622 It would be a nice-to-have to be able to get the original value of
623 'errno' from inside the appliance along error paths (where set).
624 Currently L<guestmount(1)> goes through hoops to try to reverse the
625 error message string into an errno, see the function error() in
630 =head2 PROTOCOL LIMITS
632 Internally libguestfs uses a message-based protocol to pass API calls
633 and their responses to and from a small "appliance" (see L</INTERNALS>
634 for plenty more detail about this). The maximum message size used by
635 the protocol is slightly less than 4 MB. For some API calls you may
636 need to be aware of this limit. The API calls which may be affected
637 are individually documented, with a link back to this section of the
640 A simple call such as L</guestfs_cat> returns its result (the file
641 data) in a simple string. Because this string is at some point
642 internally encoded as a message, the maximum size that it can return
643 is slightly under 4 MB. If the requested file is larger than this
644 then you will get an error.
646 In order to transfer large files into and out of the guest filesystem,
647 you need to use particular calls that support this. The sections
648 L</UPLOADING> and L</DOWNLOADING> document how to do this.
650 You might also consider mounting the disk image using our FUSE
651 filesystem support (L<guestmount(1)>).
653 =head1 CONNECTION MANAGEMENT
657 C<guestfs_h> is the opaque type representing a connection handle.
658 Create a handle by calling L</guestfs_create>. Call L</guestfs_close>
659 to free the handle and release all resources used.
661 For information on using multiple handles and threads, see the section
662 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
664 =head2 guestfs_create
666 guestfs_h *guestfs_create (void);
668 Create a connection handle.
670 You have to call L</guestfs_add_drive> on the handle at least once.
672 This function returns a non-NULL pointer to a handle on success or
675 After configuring the handle, you have to call L</guestfs_launch>.
677 You may also want to configure error handling for the handle. See
678 L</ERROR HANDLING> section below.
682 void guestfs_close (guestfs_h *g);
684 This closes the connection handle and frees up all resources used.
686 =head1 ERROR HANDLING
688 The convention in all functions that return C<int> is that they return
689 C<-1> to indicate an error. You can get additional information on
690 errors by calling L</guestfs_last_error> and/or by setting up an error
691 handler with L</guestfs_set_error_handler>.
693 The default error handler prints the information string to C<stderr>.
695 Out of memory errors are handled differently. The default action is
696 to call L<abort(3)>. If this is undesirable, then you can set a
697 handler using L</guestfs_set_out_of_memory_handler>.
699 =head2 guestfs_last_error
701 const char *guestfs_last_error (guestfs_h *g);
703 This returns the last error message that happened on C<g>. If
704 there has not been an error since the handle was created, then this
707 The lifetime of the returned string is until the next error occurs, or
708 L</guestfs_close> is called.
710 The error string is not localized (ie. is always in English), because
711 this makes searching for error messages in search engines give the
712 largest number of results.
714 =head2 guestfs_set_error_handler
716 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
719 void guestfs_set_error_handler (guestfs_h *g,
720 guestfs_error_handler_cb cb,
723 The callback C<cb> will be called if there is an error. The
724 parameters passed to the callback are an opaque data pointer and the
725 error message string.
727 Note that the message string C<msg> is freed as soon as the callback
728 function returns, so if you want to stash it somewhere you must make
731 The default handler prints messages on C<stderr>.
733 If you set C<cb> to C<NULL> then I<no> handler is called.
735 =head2 guestfs_get_error_handler
737 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g,
740 Returns the current error handler callback.
742 =head2 guestfs_set_out_of_memory_handler
744 typedef void (*guestfs_abort_cb) (void);
745 int guestfs_set_out_of_memory_handler (guestfs_h *g,
748 The callback C<cb> will be called if there is an out of memory
749 situation. I<Note this callback must not return>.
751 The default is to call L<abort(3)>.
753 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
756 =head2 guestfs_get_out_of_memory_handler
758 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *g);
760 This returns the current out of memory handler.
764 Libguestfs needs a kernel and initrd.img, which it finds by looking
765 along an internal path.
767 By default it looks for these in the directory C<$libdir/guestfs>
768 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
770 Use L</guestfs_set_path> or set the environment variable
771 L</LIBGUESTFS_PATH> to change the directories that libguestfs will
772 search in. The value is a colon-separated list of paths. The current
773 directory is I<not> searched unless the path contains an empty element
774 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
775 search the current directory and then C</usr/lib/guestfs>.
777 =head1 HIGH-LEVEL API ACTIONS
781 We guarantee the libguestfs ABI (binary interface), for public,
782 high-level actions as outlined in this section. Although we will
783 deprecate some actions, for example if they get replaced by newer
784 calls, we will keep the old actions forever. This allows you the
785 developer to program in confidence against the libguestfs API.
795 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
797 Using L</guestfs_available> you can test availability of
798 the following groups of functions. This test queries the
799 appliance to see if the appliance you are currently using
800 supports the functionality.
804 =head2 SINGLE CALLS AT COMPILE TIME
806 If you need to test whether a single libguestfs function is
807 available at compile time, we recommend using build tools
808 such as autoconf or cmake. For example in autotools you could
811 AC_CHECK_LIB([guestfs],[guestfs_create])
812 AC_CHECK_FUNCS([guestfs_dd])
814 which would result in C<HAVE_GUESTFS_DD> being either defined
815 or not defined in your program.
817 =head2 SINGLE CALLS AT RUN TIME
819 Testing at compile time doesn't guarantee that a function really
820 exists in the library. The reason is that you might be dynamically
821 linked against a previous I<libguestfs.so> (dynamic library)
822 which doesn't have the call. This situation unfortunately results
823 in a segmentation fault, which is a shortcoming of the C dynamic
824 linking system itself.
826 You can use L<dlopen(3)> to test if a function is available
827 at run time, as in this example program (note that you still
828 need the compile time check as well):
840 #ifdef HAVE_GUESTFS_DD
844 /* Test if the function guestfs_dd is really available. */
845 dl = dlopen (NULL, RTLD_LAZY);
847 fprintf (stderr, "dlopen: %s\n", dlerror ());
850 has_function = dlsym (dl, "guestfs_dd") != NULL;
854 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
856 printf ("this libguestfs.so has guestfs_dd function\n");
857 /* Now it's safe to call
858 guestfs_dd (g, "foo", "bar");
862 printf ("guestfs_dd function was not found at compile time\n");
866 You may think the above is an awful lot of hassle, and it is.
867 There are other ways outside of the C linking system to ensure
868 that this kind of incompatibility never arises, such as using
871 Requires: libguestfs >= 1.0.80
875 <!-- old anchor for the next section -->
876 <a name="state_machine_and_low_level_event_api"/>
882 Internally, libguestfs is implemented by running an appliance (a
883 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
884 a child process of the main program.
890 | | child process / appliance
891 | | __________________________
893 +-------------------+ RPC | +-----------------+ |
894 | libguestfs <--------------------> guestfsd | |
895 | | | +-----------------+ |
896 \___________________/ | | Linux kernel | |
897 | +--^--------------+ |
898 \_________|________________/
906 The library, linked to the main program, creates the child process and
907 hence the appliance in the L</guestfs_launch> function.
909 Inside the appliance is a Linux kernel and a complete stack of
910 userspace tools (such as LVM and ext2 programs) and a small
911 controlling daemon called L</guestfsd>. The library talks to
912 L</guestfsd> using remote procedure calls (RPC). There is a mostly
913 one-to-one correspondence between libguestfs API calls and RPC calls
914 to the daemon. Lastly the disk image(s) are attached to the qemu
915 process which translates device access by the appliance's Linux kernel
916 into accesses to the image.
918 A common misunderstanding is that the appliance "is" the virtual
919 machine. Although the disk image you are attached to might also be
920 used by some virtual machine, libguestfs doesn't know or care about
921 this. (But you will care if both libguestfs's qemu process and your
922 virtual machine are trying to update the disk image at the same time,
923 since these usually results in massive disk corruption).
927 libguestfs uses a state machine to model the child process:
938 / | \ \ guestfs_launch
949 \______/ <------ \________/
951 The normal transitions are (1) CONFIG (when the handle is created, but
952 there is no child process), (2) LAUNCHING (when the child process is
953 booting up), (3) alternating between READY and BUSY as commands are
954 issued to, and carried out by, the child process.
956 The guest may be killed by L</guestfs_kill_subprocess>, or may die
957 asynchronously at any time (eg. due to some internal error), and that
958 causes the state to transition back to CONFIG.
960 Configuration commands for qemu such as L</guestfs_add_drive> can only
961 be issued when in the CONFIG state.
963 The high-level API offers two calls that go from CONFIG through
964 LAUNCHING to READY. L</guestfs_launch> blocks until the child process
965 is READY to accept commands (or until some failure or timeout).
966 L</guestfs_launch> internally moves the state from CONFIG to LAUNCHING
969 High-level API actions such as L</guestfs_mount> can only be issued
970 when in the READY state. These high-level API calls block waiting for
971 the command to be carried out (ie. the state to transition to BUSY and
972 then back to READY). But using the low-level event API, you get
973 non-blocking versions. (But you can still only carry out one
974 operation per handle at a time - that is a limitation of the
975 communications protocol we use).
977 Finally, the child process sends asynchronous messages back to the
978 main program, such as kernel log messages. Mostly these are ignored
979 by the high-level API, but using the low-level event API you can
980 register to receive these messages.
982 =head2 SETTING CALLBACKS TO HANDLE EVENTS
984 The child process generates events in some situations. Current events
985 include: receiving a log message, the child process exits.
987 Use the C<guestfs_set_*_callback> functions to set a callback for
988 different types of events.
990 Only I<one callback of each type> can be registered for each handle.
991 Calling C<guestfs_set_*_callback> again overwrites the previous
992 callback of that type. Cancel all callbacks of this type by calling
993 this function with C<cb> set to C<NULL>.
995 =head2 guestfs_set_log_message_callback
997 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
999 void guestfs_set_log_message_callback (guestfs_h *g,
1000 guestfs_log_message_cb cb,
1003 The callback function C<cb> will be called whenever qemu or the guest
1004 writes anything to the console.
1006 Use this function to capture kernel messages and similar.
1008 Normally there is no log message handler, and log messages are just
1011 =head2 guestfs_set_subprocess_quit_callback
1013 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
1014 void guestfs_set_subprocess_quit_callback (guestfs_h *g,
1015 guestfs_subprocess_quit_cb cb,
1018 The callback function C<cb> will be called when the child process
1019 quits, either asynchronously or if killed by
1020 L</guestfs_kill_subprocess>. (This corresponds to a transition from
1021 any state to the CONFIG state).
1023 =head2 guestfs_set_launch_done_callback
1025 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
1026 void guestfs_set_launch_done_callback (guestfs_h *g,
1027 guestfs_ready_cb cb,
1030 The callback function C<cb> will be called when the child process
1031 becomes ready first time after it has been launched. (This
1032 corresponds to a transition from LAUNCHING to the READY state).
1034 =head1 BLOCK DEVICE NAMING
1036 In the kernel there is now quite a profusion of schemata for naming
1037 block devices (in this context, by I<block device> I mean a physical
1038 or virtual hard drive). The original Linux IDE driver used names
1039 starting with C</dev/hd*>. SCSI devices have historically used a
1040 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
1041 driver became a popular replacement for the old IDE driver
1042 (particularly for SATA devices) those devices also used the
1043 C</dev/sd*> scheme. Additionally we now have virtual machines with
1044 paravirtualized drivers. This has created several different naming
1045 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
1048 As discussed above, libguestfs uses a qemu appliance running an
1049 embedded Linux kernel to access block devices. We can run a variety
1050 of appliances based on a variety of Linux kernels.
1052 This causes a problem for libguestfs because many API calls use device
1053 or partition names. Working scripts and the recipe (example) scripts
1054 that we make available over the internet could fail if the naming
1057 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
1058 scheme>. Internally C</dev/sd*> names are translated, if necessary,
1059 to other names as required. For example, under RHEL 5 which uses the
1060 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
1061 C</dev/hda2> transparently.
1063 Note that this I<only> applies to parameters. The
1064 L</guestfs_list_devices>, L</guestfs_list_partitions> and similar calls
1065 return the true names of the devices and partitions as known to the
1068 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
1070 Usually this translation is transparent. However in some (very rare)
1071 cases you may need to know the exact algorithm. Such cases include
1072 where you use L</guestfs_config> to add a mixture of virtio and IDE
1073 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
1074 and C</dev/vd*> devices.
1076 The algorithm is applied only to I<parameters> which are known to be
1077 either device or partition names. Return values from functions such
1078 as L</guestfs_list_devices> are never changed.
1084 Is the string a parameter which is a device or partition name?
1088 Does the string begin with C</dev/sd>?
1092 Does the named device exist? If so, we use that device.
1093 However if I<not> then we continue with this algorithm.
1097 Replace initial C</dev/sd> string with C</dev/hd>.
1099 For example, change C</dev/sda2> to C</dev/hda2>.
1101 If that named device exists, use it. If not, continue.
1105 Replace initial C</dev/sd> string with C</dev/vd>.
1107 If that named device exists, use it. If not, return an error.
1111 =head2 PORTABILITY CONCERNS
1113 Although the standard naming scheme and automatic translation is
1114 useful for simple programs and guestfish scripts, for larger programs
1115 it is best not to rely on this mechanism.
1117 Where possible for maximum future portability programs using
1118 libguestfs should use these future-proof techniques:
1124 Use L</guestfs_list_devices> or L</guestfs_list_partitions> to list
1125 actual device names, and then use those names directly.
1127 Since those device names exist by definition, they will never be
1132 Use higher level ways to identify filesystems, such as LVM names,
1133 UUIDs and filesystem labels.
1139 =head2 COMMUNICATION PROTOCOL
1141 Don't rely on using this protocol directly. This section documents
1142 how it currently works, but it may change at any time.
1144 The protocol used to talk between the library and the daemon running
1145 inside the qemu virtual machine is a simple RPC mechanism built on top
1146 of XDR (RFC 1014, RFC 1832, RFC 4506).
1148 The detailed format of structures is in C<src/guestfs_protocol.x>
1149 (note: this file is automatically generated).
1151 There are two broad cases, ordinary functions that don't have any
1152 C<FileIn> and C<FileOut> parameters, which are handled with very
1153 simple request/reply messages. Then there are functions that have any
1154 C<FileIn> or C<FileOut> parameters, which use the same request and
1155 reply messages, but they may also be followed by files sent using a
1158 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1160 For ordinary functions, the request message is:
1162 total length (header + arguments,
1163 but not including the length word itself)
1164 struct guestfs_message_header (encoded as XDR)
1165 struct guestfs_<foo>_args (encoded as XDR)
1167 The total length field allows the daemon to allocate a fixed size
1168 buffer into which it slurps the rest of the message. As a result, the
1169 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1170 4MB), which means the effective size of any request is limited to
1171 somewhere under this size.
1173 Note also that many functions don't take any arguments, in which case
1174 the C<guestfs_I<foo>_args> is completely omitted.
1176 The header contains the procedure number (C<guestfs_proc>) which is
1177 how the receiver knows what type of args structure to expect, or none
1180 The reply message for ordinary functions is:
1182 total length (header + ret,
1183 but not including the length word itself)
1184 struct guestfs_message_header (encoded as XDR)
1185 struct guestfs_<foo>_ret (encoded as XDR)
1187 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1188 for functions that return no formal return values.
1190 As above the total length of the reply is limited to
1191 C<GUESTFS_MESSAGE_MAX>.
1193 In the case of an error, a flag is set in the header, and the reply
1194 message is slightly changed:
1196 total length (header + error,
1197 but not including the length word itself)
1198 struct guestfs_message_header (encoded as XDR)
1199 struct guestfs_message_error (encoded as XDR)
1201 The C<guestfs_message_error> structure contains the error message as a
1204 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1206 A C<FileIn> parameter indicates that we transfer a file I<into> the
1207 guest. The normal request message is sent (see above). However this
1208 is followed by a sequence of file chunks.
1210 total length (header + arguments,
1211 but not including the length word itself,
1212 and not including the chunks)
1213 struct guestfs_message_header (encoded as XDR)
1214 struct guestfs_<foo>_args (encoded as XDR)
1215 sequence of chunks for FileIn param #0
1216 sequence of chunks for FileIn param #1 etc.
1218 The "sequence of chunks" is:
1220 length of chunk (not including length word itself)
1221 struct guestfs_chunk (encoded as XDR)
1223 struct guestfs_chunk (encoded as XDR)
1226 struct guestfs_chunk (with data.data_len == 0)
1228 The final chunk has the C<data_len> field set to zero. Additionally a
1229 flag is set in the final chunk to indicate either successful
1230 completion or early cancellation.
1232 At time of writing there are no functions that have more than one
1233 FileIn parameter. However this is (theoretically) supported, by
1234 sending the sequence of chunks for each FileIn parameter one after
1235 another (from left to right).
1237 Both the library (sender) I<and> the daemon (receiver) may cancel the
1238 transfer. The library does this by sending a chunk with a special
1239 flag set to indicate cancellation. When the daemon sees this, it
1240 cancels the whole RPC, does I<not> send any reply, and goes back to
1241 reading the next request.
1243 The daemon may also cancel. It does this by writing a special word
1244 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1245 during the transfer, and if it gets it, it will cancel the transfer
1246 (it sends a cancel chunk). The special word is chosen so that even if
1247 cancellation happens right at the end of the transfer (after the
1248 library has finished writing and has started listening for the reply),
1249 the "spurious" cancel flag will not be confused with the reply
1252 This protocol allows the transfer of arbitrary sized files (no 32 bit
1253 limit), and also files where the size is not known in advance
1254 (eg. from pipes or sockets). However the chunks are rather small
1255 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1256 daemon need to keep much in memory.
1258 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1260 The protocol for FileOut parameters is exactly the same as for FileIn
1261 parameters, but with the roles of daemon and library reversed.
1263 total length (header + ret,
1264 but not including the length word itself,
1265 and not including the chunks)
1266 struct guestfs_message_header (encoded as XDR)
1267 struct guestfs_<foo>_ret (encoded as XDR)
1268 sequence of chunks for FileOut param #0
1269 sequence of chunks for FileOut param #1 etc.
1271 =head3 INITIAL MESSAGE
1273 Because the underlying channel (QEmu -net channel) doesn't have any
1274 sort of connection control, when the daemon launches it sends an
1275 initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
1276 and daemon is alive. This is what L</guestfs_launch> waits for.
1278 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1280 All high-level libguestfs actions are synchronous. If you want
1281 to use libguestfs asynchronously then you must create a thread.
1283 Only use the handle from a single thread. Either use the handle
1284 exclusively from one thread, or provide your own mutex so that two
1285 threads cannot issue calls on the same handle at the same time.
1287 =head1 QEMU WRAPPERS
1289 If you want to compile your own qemu, run qemu from a non-standard
1290 location, or pass extra arguments to qemu, then you can write a
1291 shell-script wrapper around qemu.
1293 There is one important rule to remember: you I<must C<exec qemu>> as
1294 the last command in the shell script (so that qemu replaces the shell
1295 and becomes the direct child of the libguestfs-using program). If you
1296 don't do this, then the qemu process won't be cleaned up correctly.
1298 Here is an example of a wrapper, where I have built my own copy of
1302 qemudir=/home/rjones/d/qemu
1303 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1305 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1306 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1309 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1311 Note that libguestfs also calls qemu with the -help and -version
1312 options in order to determine features.
1314 =head1 LIBGUESTFS VERSION NUMBERS
1316 Since April 2010, libguestfs has started to make separate development
1317 and stable releases, along with corresponding branches in our git
1318 repository. These separate releases can be identified by version
1321 even numbers for stable: 1.2.x, 1.4.x, ...
1322 .-------- odd numbers for development: 1.3.x, 1.5.x, ...
1328 | `-------- sub-version
1330 `------ always '1' because we don't change the ABI
1332 Thus "1.3.5" is the 5th update to the development branch "1.3".
1334 As time passes we cherry pick fixes from the development branch and
1335 backport those into the stable branch, the effect being that the
1336 stable branch should get more stable and less buggy over time. So the
1337 stable releases are ideal for people who don't need new features but
1338 would just like the software to work.
1340 Our criteria for backporting changes are:
1346 Documentation changes which don't affect any code are
1347 backported unless the documentation refers to a future feature
1348 which is not in stable.
1352 Bug fixes which are not controversial, fix obvious problems, and
1353 have been well tested are backported.
1357 Simple rearrangements of code which shouldn't affect how it works get
1358 backported. This is so that the code in the two branches doesn't get
1359 too far out of step, allowing us to backport future fixes more easily.
1363 We I<don't> backport new features, new APIs, new tools etc, except in
1364 one exceptional case: the new feature is required in order to
1365 implement an important bug fix.
1369 A new stable branch starts when we think the new features in
1370 development are substantial and compelling enough over the current
1371 stable branch to warrant it. When that happens we create new stable
1372 and development versions 1.N.0 and 1.(N+1).0 [N is even]. The new
1373 dot-oh release won't necessarily be so stable at this point, but by
1374 backporting fixes from development, that branch will stabilize over
1377 =head1 ENVIRONMENT VARIABLES
1381 =item LIBGUESTFS_APPEND
1383 Pass additional options to the guest kernel.
1385 =item LIBGUESTFS_DEBUG
1387 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1388 has the same effect as calling C<guestfs_set_verbose (g, 1)>.
1390 =item LIBGUESTFS_MEMSIZE
1392 Set the memory allocated to the qemu process, in megabytes. For
1395 LIBGUESTFS_MEMSIZE=700
1397 =item LIBGUESTFS_PATH
1399 Set the path that libguestfs uses to search for kernel and initrd.img.
1400 See the discussion of paths in section PATH above.
1402 =item LIBGUESTFS_QEMU
1404 Set the default qemu binary that libguestfs uses. If not set, then
1405 the qemu which was found at compile time by the configure script is
1408 See also L</QEMU WRAPPERS> above.
1410 =item LIBGUESTFS_TRACE
1412 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1413 has the same effect as calling C<guestfs_set_trace (g, 1)>.
1417 Location of temporary directory, defaults to C</tmp>.
1419 If libguestfs was compiled to use the supermin appliance then each
1420 handle will require rather a large amount of space in this directory
1421 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1422 configure another directory to use in case C</tmp> is not large
1434 L<virt-inspector(1)>,
1435 L<virt-list-filesystems(1)>,
1436 L<virt-list-partitions(1)>,
1445 L<http://libguestfs.org/>.
1447 Tools with a similar purpose:
1456 To get a list of bugs against libguestfs use this link:
1458 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1460 To report a new bug against libguestfs use this link:
1462 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1464 When reporting a bug, please check:
1470 That the bug hasn't been reported already.
1474 That you are testing a recent version.
1478 Describe the bug accurately, and give a way to reproduce it.
1482 Run libguestfs-test-tool and paste the B<complete, unedited>
1483 output into the bug report.
1489 Richard W.M. Jones (C<rjones at redhat dot com>)
1493 Copyright (C) 2009-2010 Red Hat Inc.
1494 L<http://libguestfs.org/>
1496 This library is free software; you can redistribute it and/or
1497 modify it under the terms of the GNU Lesser General Public
1498 License as published by the Free Software Foundation; either
1499 version 2 of the License, or (at your option) any later version.
1501 This library is distributed in the hope that it will be useful,
1502 but WITHOUT ANY WARRANTY; without even the implied warranty of
1503 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1504 Lesser General Public License for more details.
1506 You should have received a copy of the GNU Lesser General Public
1507 License along with this library; if not, write to the Free Software
1508 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA