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_file>. This call
262 currently contains a bug which limits the call to plain text files
263 (not containing ASCII NUL characters).
265 To upload a single file, use L</guestfs_upload>. This call has no
266 limits on file content or size (even files larger than 4 GB).
268 To upload multiple files, see L</guestfs_tar_in> and L</guestfs_tgz_in>.
270 However the fastest way to upload I<large numbers of arbitrary files>
271 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
272 L<mkisofs(8)>), then attach this using L</guestfs_add_drive_ro>. If
273 you add the drive in a predictable way (eg. adding it last after all
274 other drives) then you can get the device name from
275 L</guestfs_list_devices> and mount it directly using
276 L</guestfs_mount_ro>. Note that squashfs images are sometimes
277 non-portable between kernel versions, and they don't support labels or
278 UUIDs. If you want to pre-build an image or you need to mount it
279 using a label or UUID, use an ISO image instead.
283 There are various different commands for copying between files and
284 devices and in and out of the guest filesystem. These are summarised
289 =item B<file> to B<file>
291 Use L</guestfs_cp> to copy a single file, or
292 L</guestfs_cp_a> to copy directories recursively.
294 =item B<file or device> to B<file or device>
296 Use L</guestfs_dd> which efficiently uses L<dd(1)>
297 to copy between files and devices in the guest.
299 Example: duplicate the contents of an LV:
301 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
303 The destination (C</dev/VG/Copy>) must be at least as large as the
304 source (C</dev/VG/Original>). To copy less than the whole
305 source device, use L</guestfs_copy_size>.
307 =item B<file on the host> to B<file or device>
309 Use L</guestfs_upload>. See L</UPLOADING> above.
311 =item B<file or device> to B<file on the host>
313 Use L</guestfs_download>. See L</DOWNLOADING> above.
319 L</guestfs_ll> is just designed for humans to read (mainly when using
320 the L<guestfish(1)>-equivalent command C<ll>).
322 L</guestfs_ls> is a quick way to get a list of files in a directory
323 from programs, as a flat list of strings.
325 L</guestfs_readdir> is a programmatic way to get a list of files in a
326 directory, plus additional information about each one. It is more
327 equivalent to using the L<readdir(3)> call on a local filesystem.
329 L</guestfs_find> and L</guestfs_find0> can be used to recursively list
332 =head2 RUNNING COMMANDS
334 Although libguestfs is primarily an API for manipulating files
335 inside guest images, we also provide some limited facilities for
336 running commands inside guests.
338 There are many limitations to this:
344 The kernel version that the command runs under will be different
345 from what it expects.
349 If the command needs to communicate with daemons, then most likely
350 they won't be running.
354 The command will be running in limited memory.
358 Only supports Linux guests (not Windows, BSD, etc).
362 Architecture limitations (eg. won't work for a PPC guest on
367 For SELinux guests, you may need to enable SELinux and load policy
368 first. See L</SELINUX> in this manpage.
372 The two main API calls to run commands are L</guestfs_command> and
373 L</guestfs_sh> (there are also variations).
375 The difference is that L</guestfs_sh> runs commands using the shell, so
376 any shell globs, redirections, etc will work.
378 =head2 CONFIGURATION FILES
380 To read and write configuration files in Linux guest filesystems, we
381 strongly recommend using Augeas. For example, Augeas understands how
382 to read and write, say, a Linux shadow password file or X.org
383 configuration file, and so avoids you having to write that code.
385 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
386 don't document Augeas itself here because there is excellent
387 documentation on the L<http://augeas.net/> website.
389 If you don't want to use Augeas (you fool!) then try calling
390 L</guestfs_read_lines> to get the file as a list of lines which
391 you can iterate over.
395 We support SELinux guests. To ensure that labeling happens correctly
396 in SELinux guests, you need to enable SELinux and load the guest's
403 Before launching, do:
405 guestfs_set_selinux (g, 1);
409 After mounting the guest's filesystem(s), load the policy. This
410 is best done by running the L<load_policy(8)> command in the
413 guestfs_sh (g, "/usr/sbin/load_policy");
415 (Older versions of C<load_policy> require you to specify the
416 name of the policy file).
420 Optionally, set the security context for the API. The correct
421 security context to use can only be known by inspecting the
422 guest. As an example:
424 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
428 This will work for running commands and editing existing files.
430 When new files are created, you may need to label them explicitly,
431 for example by running the external command
432 C<restorecon pathname>.
436 Certain calls are affected by the current file mode creation mask (the
437 "umask"). In particular ones which create files or directories, such
438 as L</guestfs_touch>, L</guestfs_mknod> or L</guestfs_mkdir>. This
439 affects either the default mode that the file is created with or
440 modifies the mode that you supply.
442 The default umask is C<022>, so files are created with modes such as
443 C<0644> and directories with C<0755>.
445 There are two ways to avoid being affected by umask. Either set umask
446 to 0 (call C<guestfs_umask (g, 0)> early after launching). Or call
447 L</guestfs_chmod> after creating each file or directory.
449 For more information about umask, see L<umask(2)>.
451 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
453 Libguestfs can mount NTFS partitions. It does this using the
454 L<http://www.ntfs-3g.org/> driver.
456 DOS and Windows still use drive letters, and the filesystems are
457 always treated as case insensitive by Windows itself, and therefore
458 you might find a Windows configuration file referring to a path like
459 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
460 that directory might be referred to as C</WINDOWS/System32>.
462 Drive letter mappings are outside the scope of libguestfs. You have
463 to use libguestfs to read the appropriate Windows Registry and
464 configuration files, to determine yourself how drives are mapped (see
465 also L<virt-inspector(1)>).
467 Replacing backslash characters with forward slash characters is also
468 outside the scope of libguestfs, but something that you can easily do.
470 Where we can help is in resolving the case insensitivity of paths.
471 For this, call L</guestfs_case_sensitive_path>.
473 Libguestfs also provides some help for decoding Windows Registry
474 "hive" files, through the library C<hivex> which is part of the
475 libguestfs project although ships as a separate tarball. You have to
476 locate and download the hive file(s) yourself, and then pass them to
477 C<hivex> functions. See also the programs L<hivexml(1)>,
478 L<hivexsh(1)>, L<hivexregedit(1)> and L<virt-win-reg(1)> for more help
481 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
483 Although we don't want to discourage you from using the C API, we will
484 mention here that the same API is also available in other languages.
486 The API is broadly identical in all supported languages. This means
487 that the C call C<guestfs_mount(g,path)> is
488 C<$g-E<gt>mount($path)> in Perl, C<g.mount(path)> in Python,
489 and C<Guestfs.mount g path> in OCaml. In other words, a
490 straightforward, predictable isomorphism between each language.
492 Error messages are automatically transformed
493 into exceptions if the language supports it.
495 We don't try to "object orientify" parts of the API in OO languages,
496 although contributors are welcome to write higher level APIs above
497 what we provide in their favourite languages if they wish.
503 You can use the I<guestfs.h> header file from C++ programs. The C++
504 API is identical to the C API. C++ classes and exceptions are not
509 The C# bindings are highly experimental. Please read the warnings
510 at the top of C<csharp/Libguestfs.cs>.
514 This is the only language binding that is working but incomplete.
515 Only calls which return simple integers have been bound in Haskell,
516 and we are looking for help to complete this binding.
520 Full documentation is contained in the Javadoc which is distributed
525 For documentation see the file C<guestfs.mli>.
529 For documentation see L<Sys::Guestfs(3)>.
533 For documentation do:
541 Use the Guestfs module. There is no Ruby-specific documentation, but
542 you can find examples written in Ruby in the libguestfs source.
544 =item B<shell scripts>
546 For documentation see L<guestfish(1)>.
550 =head2 LIBGUESTFS GOTCHAS
552 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
553 system [...] that works in the way it is documented but is
554 counterintuitive and almost invites mistakes."
556 Since we developed libguestfs and the associated tools, there are
557 several things we would have designed differently, but are now stuck
558 with for backwards compatibility or other reasons. If there is ever a
559 libguestfs 2.0 release, you can expect these to change. Beware of
564 =item Autosync / forgetting to sync.
566 When modifying a filesystem from C or another language, you B<must>
567 unmount all filesystems and call L</guestfs_sync> explicitly before
568 you close the libguestfs handle. You can also call:
570 guestfs_set_autosync (g, 1);
572 to have the unmount/sync done automatically for you when the handle 'g'
573 is closed. (This feature is called "autosync", L</guestfs_set_autosync>
576 If you forget to do this, then it is entirely possible that your
577 changes won't be written out, or will be partially written, or (very
578 rarely) that you'll get disk corruption.
580 Note that in L<guestfish(3)> autosync is the default. So quick and
581 dirty guestfish scripts that forget to sync will work just fine, which
582 can make this very puzzling if you are trying to debug a problem.
584 =item Mount option C<-o sync> should not be the default.
586 If you use L</guestfs_mount>, then C<-o sync,noatime> are added
587 implicitly. However C<-o sync> does not add any reliability benefit,
588 but does have a very large performance impact.
590 The work around is to use L</guestfs_mount_options> and set the mount
591 options that you actually want to use.
593 =item Read-only should be the default.
595 In L<guestfish(3)>, I<--ro> should be the default, and you should
596 have to specify I<--rw> if you want to make changes to the image.
598 This would reduce the potential to corrupt live VM images.
600 Note that many filesystems change the disk when you just mount and
601 unmount, even if you didn't perform any writes. You need to use
602 L</guestfs_add_drive_ro> to guarantee that the disk is not changed.
604 =item guestfish command line is hard to use.
606 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
607 for examination). It tries to run a guestfish command C<disk.img>
608 which doesn't exist, so it fails. In earlier versions of guestfish
609 the error message was also unintuitive, but we have corrected this
610 since. Like the Bourne shell, we should have used C<guestfish -c
611 command> to run commands.
613 =item Protocol limit of 256 characters for error messages
615 This limit is both rather small and quite unnecessary. We should be
616 able to return error messages up to the length of the protocol message
619 Note that we cannot change the protocol without some breakage, because
620 there are distributions that repackage the Fedora appliance.
622 =item Protocol should return errno with error messages.
624 It would be a nice-to-have to be able to get the original value of
625 'errno' from inside the appliance along error paths (where set).
626 Currently L<guestmount(1)> goes through hoops to try to reverse the
627 error message string into an errno, see the function error() in
632 =head2 PROTOCOL LIMITS
634 Internally libguestfs uses a message-based protocol to pass API calls
635 and their responses to and from a small "appliance" (see L</INTERNALS>
636 for plenty more detail about this). The maximum message size used by
637 the protocol is slightly less than 4 MB. For some API calls you may
638 need to be aware of this limit. The API calls which may be affected
639 are individually documented, with a link back to this section of the
642 A simple call such as L</guestfs_cat> returns its result (the file
643 data) in a simple string. Because this string is at some point
644 internally encoded as a message, the maximum size that it can return
645 is slightly under 4 MB. If the requested file is larger than this
646 then you will get an error.
648 In order to transfer large files into and out of the guest filesystem,
649 you need to use particular calls that support this. The sections
650 L</UPLOADING> and L</DOWNLOADING> document how to do this.
652 You might also consider mounting the disk image using our FUSE
653 filesystem support (L<guestmount(1)>).
655 =head1 CONNECTION MANAGEMENT
659 C<guestfs_h> is the opaque type representing a connection handle.
660 Create a handle by calling L</guestfs_create>. Call L</guestfs_close>
661 to free the handle and release all resources used.
663 For information on using multiple handles and threads, see the section
664 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
666 =head2 guestfs_create
668 guestfs_h *guestfs_create (void);
670 Create a connection handle.
672 You have to call L</guestfs_add_drive> on the handle at least once.
674 This function returns a non-NULL pointer to a handle on success or
677 After configuring the handle, you have to call L</guestfs_launch>.
679 You may also want to configure error handling for the handle. See
680 L</ERROR HANDLING> section below.
684 void guestfs_close (guestfs_h *g);
686 This closes the connection handle and frees up all resources used.
688 =head1 ERROR HANDLING
690 The convention in all functions that return C<int> is that they return
691 C<-1> to indicate an error. You can get additional information on
692 errors by calling L</guestfs_last_error> and/or by setting up an error
693 handler with L</guestfs_set_error_handler>.
695 The default error handler prints the information string to C<stderr>.
697 Out of memory errors are handled differently. The default action is
698 to call L<abort(3)>. If this is undesirable, then you can set a
699 handler using L</guestfs_set_out_of_memory_handler>.
701 =head2 guestfs_last_error
703 const char *guestfs_last_error (guestfs_h *g);
705 This returns the last error message that happened on C<g>. If
706 there has not been an error since the handle was created, then this
709 The lifetime of the returned string is until the next error occurs, or
710 L</guestfs_close> is called.
712 The error string is not localized (ie. is always in English), because
713 this makes searching for error messages in search engines give the
714 largest number of results.
716 =head2 guestfs_set_error_handler
718 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
721 void guestfs_set_error_handler (guestfs_h *g,
722 guestfs_error_handler_cb cb,
725 The callback C<cb> will be called if there is an error. The
726 parameters passed to the callback are an opaque data pointer and the
727 error message string.
729 Note that the message string C<msg> is freed as soon as the callback
730 function returns, so if you want to stash it somewhere you must make
733 The default handler prints messages on C<stderr>.
735 If you set C<cb> to C<NULL> then I<no> handler is called.
737 =head2 guestfs_get_error_handler
739 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g,
742 Returns the current error handler callback.
744 =head2 guestfs_set_out_of_memory_handler
746 typedef void (*guestfs_abort_cb) (void);
747 int guestfs_set_out_of_memory_handler (guestfs_h *g,
750 The callback C<cb> will be called if there is an out of memory
751 situation. I<Note this callback must not return>.
753 The default is to call L<abort(3)>.
755 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
758 =head2 guestfs_get_out_of_memory_handler
760 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *g);
762 This returns the current out of memory handler.
766 Libguestfs needs a kernel and initrd.img, which it finds by looking
767 along an internal path.
769 By default it looks for these in the directory C<$libdir/guestfs>
770 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
772 Use L</guestfs_set_path> or set the environment variable
773 L</LIBGUESTFS_PATH> to change the directories that libguestfs will
774 search in. The value is a colon-separated list of paths. The current
775 directory is I<not> searched unless the path contains an empty element
776 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
777 search the current directory and then C</usr/lib/guestfs>.
779 =head1 HIGH-LEVEL API ACTIONS
783 We guarantee the libguestfs ABI (binary interface), for public,
784 high-level actions as outlined in this section. Although we will
785 deprecate some actions, for example if they get replaced by newer
786 calls, we will keep the old actions forever. This allows you the
787 developer to program in confidence against the libguestfs API.
797 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
799 Using L</guestfs_available> you can test availability of
800 the following groups of functions. This test queries the
801 appliance to see if the appliance you are currently using
802 supports the functionality.
806 =head2 SINGLE CALLS AT COMPILE TIME
808 If you need to test whether a single libguestfs function is
809 available at compile time, we recommend using build tools
810 such as autoconf or cmake. For example in autotools you could
813 AC_CHECK_LIB([guestfs],[guestfs_create])
814 AC_CHECK_FUNCS([guestfs_dd])
816 which would result in C<HAVE_GUESTFS_DD> being either defined
817 or not defined in your program.
819 =head2 SINGLE CALLS AT RUN TIME
821 Testing at compile time doesn't guarantee that a function really
822 exists in the library. The reason is that you might be dynamically
823 linked against a previous I<libguestfs.so> (dynamic library)
824 which doesn't have the call. This situation unfortunately results
825 in a segmentation fault, which is a shortcoming of the C dynamic
826 linking system itself.
828 You can use L<dlopen(3)> to test if a function is available
829 at run time, as in this example program (note that you still
830 need the compile time check as well):
842 #ifdef HAVE_GUESTFS_DD
846 /* Test if the function guestfs_dd is really available. */
847 dl = dlopen (NULL, RTLD_LAZY);
849 fprintf (stderr, "dlopen: %s\n", dlerror ());
852 has_function = dlsym (dl, "guestfs_dd") != NULL;
856 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
858 printf ("this libguestfs.so has guestfs_dd function\n");
859 /* Now it's safe to call
860 guestfs_dd (g, "foo", "bar");
864 printf ("guestfs_dd function was not found at compile time\n");
868 You may think the above is an awful lot of hassle, and it is.
869 There are other ways outside of the C linking system to ensure
870 that this kind of incompatibility never arises, such as using
873 Requires: libguestfs >= 1.0.80
877 <!-- old anchor for the next section -->
878 <a name="state_machine_and_low_level_event_api"/>
884 Internally, libguestfs is implemented by running an appliance (a
885 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
886 a child process of the main program.
892 | | child process / appliance
893 | | __________________________
895 +-------------------+ RPC | +-----------------+ |
896 | libguestfs <--------------------> guestfsd | |
897 | | | +-----------------+ |
898 \___________________/ | | Linux kernel | |
899 | +--^--------------+ |
900 \_________|________________/
908 The library, linked to the main program, creates the child process and
909 hence the appliance in the L</guestfs_launch> function.
911 Inside the appliance is a Linux kernel and a complete stack of
912 userspace tools (such as LVM and ext2 programs) and a small
913 controlling daemon called L</guestfsd>. The library talks to
914 L</guestfsd> using remote procedure calls (RPC). There is a mostly
915 one-to-one correspondence between libguestfs API calls and RPC calls
916 to the daemon. Lastly the disk image(s) are attached to the qemu
917 process which translates device access by the appliance's Linux kernel
918 into accesses to the image.
920 A common misunderstanding is that the appliance "is" the virtual
921 machine. Although the disk image you are attached to might also be
922 used by some virtual machine, libguestfs doesn't know or care about
923 this. (But you will care if both libguestfs's qemu process and your
924 virtual machine are trying to update the disk image at the same time,
925 since these usually results in massive disk corruption).
929 libguestfs uses a state machine to model the child process:
940 / | \ \ guestfs_launch
951 \______/ <------ \________/
953 The normal transitions are (1) CONFIG (when the handle is created, but
954 there is no child process), (2) LAUNCHING (when the child process is
955 booting up), (3) alternating between READY and BUSY as commands are
956 issued to, and carried out by, the child process.
958 The guest may be killed by L</guestfs_kill_subprocess>, or may die
959 asynchronously at any time (eg. due to some internal error), and that
960 causes the state to transition back to CONFIG.
962 Configuration commands for qemu such as L</guestfs_add_drive> can only
963 be issued when in the CONFIG state.
965 The API offers one call that goes from CONFIG through LAUNCHING to
966 READY. L</guestfs_launch> blocks until the child process is READY to
967 accept commands (or until some failure or timeout).
968 L</guestfs_launch> internally moves the state from CONFIG to LAUNCHING
971 API actions such as L</guestfs_mount> can only be issued when in the
972 READY state. These API calls block waiting for the command to be
973 carried out (ie. the state to transition to BUSY and then back to
974 READY). There are no non-blocking versions, and no way to issue more
975 than one command per handle at the same time.
977 Finally, the child process sends asynchronous messages back to the
978 main program, such as kernel log messages. You can register a
979 callback to receive these messages.
981 =head2 SETTING CALLBACKS TO HANDLE EVENTS
983 The child process generates events in some situations. Current events
984 include: receiving a log message, the child process exits.
986 Use the C<guestfs_set_*_callback> functions to set a callback for
987 different types of events.
989 Only I<one callback of each type> can be registered for each handle.
990 Calling C<guestfs_set_*_callback> again overwrites the previous
991 callback of that type. Cancel all callbacks of this type by calling
992 this function with C<cb> set to C<NULL>.
994 =head2 guestfs_set_log_message_callback
996 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
998 void guestfs_set_log_message_callback (guestfs_h *g,
999 guestfs_log_message_cb cb,
1002 The callback function C<cb> will be called whenever qemu or the guest
1003 writes anything to the console.
1005 Use this function to capture kernel messages and similar.
1007 Normally there is no log message handler, and log messages are just
1010 =head2 guestfs_set_subprocess_quit_callback
1012 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
1013 void guestfs_set_subprocess_quit_callback (guestfs_h *g,
1014 guestfs_subprocess_quit_cb cb,
1017 The callback function C<cb> will be called when the child process
1018 quits, either asynchronously or if killed by
1019 L</guestfs_kill_subprocess>. (This corresponds to a transition from
1020 any state to the CONFIG state).
1022 =head2 guestfs_set_launch_done_callback
1024 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
1025 void guestfs_set_launch_done_callback (guestfs_h *g,
1026 guestfs_launch_done_cb cb,
1029 The callback function C<cb> will be called when the child process
1030 becomes ready first time after it has been launched. (This
1031 corresponds to a transition from LAUNCHING to the READY state).
1033 =head1 BLOCK DEVICE NAMING
1035 In the kernel there is now quite a profusion of schemata for naming
1036 block devices (in this context, by I<block device> I mean a physical
1037 or virtual hard drive). The original Linux IDE driver used names
1038 starting with C</dev/hd*>. SCSI devices have historically used a
1039 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
1040 driver became a popular replacement for the old IDE driver
1041 (particularly for SATA devices) those devices also used the
1042 C</dev/sd*> scheme. Additionally we now have virtual machines with
1043 paravirtualized drivers. This has created several different naming
1044 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
1047 As discussed above, libguestfs uses a qemu appliance running an
1048 embedded Linux kernel to access block devices. We can run a variety
1049 of appliances based on a variety of Linux kernels.
1051 This causes a problem for libguestfs because many API calls use device
1052 or partition names. Working scripts and the recipe (example) scripts
1053 that we make available over the internet could fail if the naming
1056 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
1057 scheme>. Internally C</dev/sd*> names are translated, if necessary,
1058 to other names as required. For example, under RHEL 5 which uses the
1059 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
1060 C</dev/hda2> transparently.
1062 Note that this I<only> applies to parameters. The
1063 L</guestfs_list_devices>, L</guestfs_list_partitions> and similar calls
1064 return the true names of the devices and partitions as known to the
1067 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
1069 Usually this translation is transparent. However in some (very rare)
1070 cases you may need to know the exact algorithm. Such cases include
1071 where you use L</guestfs_config> to add a mixture of virtio and IDE
1072 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
1073 and C</dev/vd*> devices.
1075 The algorithm is applied only to I<parameters> which are known to be
1076 either device or partition names. Return values from functions such
1077 as L</guestfs_list_devices> are never changed.
1083 Is the string a parameter which is a device or partition name?
1087 Does the string begin with C</dev/sd>?
1091 Does the named device exist? If so, we use that device.
1092 However if I<not> then we continue with this algorithm.
1096 Replace initial C</dev/sd> string with C</dev/hd>.
1098 For example, change C</dev/sda2> to C</dev/hda2>.
1100 If that named device exists, use it. If not, continue.
1104 Replace initial C</dev/sd> string with C</dev/vd>.
1106 If that named device exists, use it. If not, return an error.
1110 =head2 PORTABILITY CONCERNS
1112 Although the standard naming scheme and automatic translation is
1113 useful for simple programs and guestfish scripts, for larger programs
1114 it is best not to rely on this mechanism.
1116 Where possible for maximum future portability programs using
1117 libguestfs should use these future-proof techniques:
1123 Use L</guestfs_list_devices> or L</guestfs_list_partitions> to list
1124 actual device names, and then use those names directly.
1126 Since those device names exist by definition, they will never be
1131 Use higher level ways to identify filesystems, such as LVM names,
1132 UUIDs and filesystem labels.
1138 =head2 COMMUNICATION PROTOCOL
1140 Don't rely on using this protocol directly. This section documents
1141 how it currently works, but it may change at any time.
1143 The protocol used to talk between the library and the daemon running
1144 inside the qemu virtual machine is a simple RPC mechanism built on top
1145 of XDR (RFC 1014, RFC 1832, RFC 4506).
1147 The detailed format of structures is in C<src/guestfs_protocol.x>
1148 (note: this file is automatically generated).
1150 There are two broad cases, ordinary functions that don't have any
1151 C<FileIn> and C<FileOut> parameters, which are handled with very
1152 simple request/reply messages. Then there are functions that have any
1153 C<FileIn> or C<FileOut> parameters, which use the same request and
1154 reply messages, but they may also be followed by files sent using a
1157 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1159 For ordinary functions, the request message is:
1161 total length (header + arguments,
1162 but not including the length word itself)
1163 struct guestfs_message_header (encoded as XDR)
1164 struct guestfs_<foo>_args (encoded as XDR)
1166 The total length field allows the daemon to allocate a fixed size
1167 buffer into which it slurps the rest of the message. As a result, the
1168 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1169 4MB), which means the effective size of any request is limited to
1170 somewhere under this size.
1172 Note also that many functions don't take any arguments, in which case
1173 the C<guestfs_I<foo>_args> is completely omitted.
1175 The header contains the procedure number (C<guestfs_proc>) which is
1176 how the receiver knows what type of args structure to expect, or none
1179 The reply message for ordinary functions is:
1181 total length (header + ret,
1182 but not including the length word itself)
1183 struct guestfs_message_header (encoded as XDR)
1184 struct guestfs_<foo>_ret (encoded as XDR)
1186 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1187 for functions that return no formal return values.
1189 As above the total length of the reply is limited to
1190 C<GUESTFS_MESSAGE_MAX>.
1192 In the case of an error, a flag is set in the header, and the reply
1193 message is slightly changed:
1195 total length (header + error,
1196 but not including the length word itself)
1197 struct guestfs_message_header (encoded as XDR)
1198 struct guestfs_message_error (encoded as XDR)
1200 The C<guestfs_message_error> structure contains the error message as a
1203 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1205 A C<FileIn> parameter indicates that we transfer a file I<into> the
1206 guest. The normal request message is sent (see above). However this
1207 is followed by a sequence of file chunks.
1209 total length (header + arguments,
1210 but not including the length word itself,
1211 and not including the chunks)
1212 struct guestfs_message_header (encoded as XDR)
1213 struct guestfs_<foo>_args (encoded as XDR)
1214 sequence of chunks for FileIn param #0
1215 sequence of chunks for FileIn param #1 etc.
1217 The "sequence of chunks" is:
1219 length of chunk (not including length word itself)
1220 struct guestfs_chunk (encoded as XDR)
1222 struct guestfs_chunk (encoded as XDR)
1225 struct guestfs_chunk (with data.data_len == 0)
1227 The final chunk has the C<data_len> field set to zero. Additionally a
1228 flag is set in the final chunk to indicate either successful
1229 completion or early cancellation.
1231 At time of writing there are no functions that have more than one
1232 FileIn parameter. However this is (theoretically) supported, by
1233 sending the sequence of chunks for each FileIn parameter one after
1234 another (from left to right).
1236 Both the library (sender) I<and> the daemon (receiver) may cancel the
1237 transfer. The library does this by sending a chunk with a special
1238 flag set to indicate cancellation. When the daemon sees this, it
1239 cancels the whole RPC, does I<not> send any reply, and goes back to
1240 reading the next request.
1242 The daemon may also cancel. It does this by writing a special word
1243 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1244 during the transfer, and if it gets it, it will cancel the transfer
1245 (it sends a cancel chunk). The special word is chosen so that even if
1246 cancellation happens right at the end of the transfer (after the
1247 library has finished writing and has started listening for the reply),
1248 the "spurious" cancel flag will not be confused with the reply
1251 This protocol allows the transfer of arbitrary sized files (no 32 bit
1252 limit), and also files where the size is not known in advance
1253 (eg. from pipes or sockets). However the chunks are rather small
1254 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1255 daemon need to keep much in memory.
1257 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1259 The protocol for FileOut parameters is exactly the same as for FileIn
1260 parameters, but with the roles of daemon and library reversed.
1262 total length (header + ret,
1263 but not including the length word itself,
1264 and not including the chunks)
1265 struct guestfs_message_header (encoded as XDR)
1266 struct guestfs_<foo>_ret (encoded as XDR)
1267 sequence of chunks for FileOut param #0
1268 sequence of chunks for FileOut param #1 etc.
1270 =head3 INITIAL MESSAGE
1272 When the daemon launches it sends an initial word
1273 (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest and daemon is
1274 alive. This is what L</guestfs_launch> waits for.
1276 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1278 All high-level libguestfs actions are synchronous. If you want
1279 to use libguestfs asynchronously then you must create a thread.
1281 Only use the handle from a single thread. Either use the handle
1282 exclusively from one thread, or provide your own mutex so that two
1283 threads cannot issue calls on the same handle at the same time.
1285 See the graphical program guestfs-browser for one possible
1286 architecture for multithreaded programs using libvirt and libguestfs.
1288 =head1 QEMU WRAPPERS
1290 If you want to compile your own qemu, run qemu from a non-standard
1291 location, or pass extra arguments to qemu, then you can write a
1292 shell-script wrapper around qemu.
1294 There is one important rule to remember: you I<must C<exec qemu>> as
1295 the last command in the shell script (so that qemu replaces the shell
1296 and becomes the direct child of the libguestfs-using program). If you
1297 don't do this, then the qemu process won't be cleaned up correctly.
1299 Here is an example of a wrapper, where I have built my own copy of
1303 qemudir=/home/rjones/d/qemu
1304 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1306 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1307 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1310 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1312 Note that libguestfs also calls qemu with the -help and -version
1313 options in order to determine features.
1315 =head1 LIBGUESTFS VERSION NUMBERS
1317 Since April 2010, libguestfs has started to make separate development
1318 and stable releases, along with corresponding branches in our git
1319 repository. These separate releases can be identified by version
1322 even numbers for stable: 1.2.x, 1.4.x, ...
1323 .-------- odd numbers for development: 1.3.x, 1.5.x, ...
1329 | `-------- sub-version
1331 `------ always '1' because we don't change the ABI
1333 Thus "1.3.5" is the 5th update to the development branch "1.3".
1335 As time passes we cherry pick fixes from the development branch and
1336 backport those into the stable branch, the effect being that the
1337 stable branch should get more stable and less buggy over time. So the
1338 stable releases are ideal for people who don't need new features but
1339 would just like the software to work.
1341 Our criteria for backporting changes are:
1347 Documentation changes which don't affect any code are
1348 backported unless the documentation refers to a future feature
1349 which is not in stable.
1353 Bug fixes which are not controversial, fix obvious problems, and
1354 have been well tested are backported.
1358 Simple rearrangements of code which shouldn't affect how it works get
1359 backported. This is so that the code in the two branches doesn't get
1360 too far out of step, allowing us to backport future fixes more easily.
1364 We I<don't> backport new features, new APIs, new tools etc, except in
1365 one exceptional case: the new feature is required in order to
1366 implement an important bug fix.
1370 A new stable branch starts when we think the new features in
1371 development are substantial and compelling enough over the current
1372 stable branch to warrant it. When that happens we create new stable
1373 and development versions 1.N.0 and 1.(N+1).0 [N is even]. The new
1374 dot-oh release won't necessarily be so stable at this point, but by
1375 backporting fixes from development, that branch will stabilize over
1378 =head1 ENVIRONMENT VARIABLES
1382 =item LIBGUESTFS_APPEND
1384 Pass additional options to the guest kernel.
1386 =item LIBGUESTFS_DEBUG
1388 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1389 has the same effect as calling C<guestfs_set_verbose (g, 1)>.
1391 =item LIBGUESTFS_MEMSIZE
1393 Set the memory allocated to the qemu process, in megabytes. For
1396 LIBGUESTFS_MEMSIZE=700
1398 =item LIBGUESTFS_PATH
1400 Set the path that libguestfs uses to search for kernel and initrd.img.
1401 See the discussion of paths in section PATH above.
1403 =item LIBGUESTFS_QEMU
1405 Set the default qemu binary that libguestfs uses. If not set, then
1406 the qemu which was found at compile time by the configure script is
1409 See also L</QEMU WRAPPERS> above.
1411 =item LIBGUESTFS_TRACE
1413 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1414 has the same effect as calling C<guestfs_set_trace (g, 1)>.
1418 Location of temporary directory, defaults to C</tmp>.
1420 If libguestfs was compiled to use the supermin appliance then each
1421 handle will require rather a large amount of space in this directory
1422 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1423 configure another directory to use in case C</tmp> is not large
1435 L<virt-inspector(1)>,
1436 L<virt-list-filesystems(1)>,
1437 L<virt-list-partitions(1)>,
1446 L<http://libguestfs.org/>.
1448 Tools with a similar purpose:
1457 To get a list of bugs against libguestfs use this link:
1459 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1461 To report a new bug against libguestfs use this link:
1463 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1465 When reporting a bug, please check:
1471 That the bug hasn't been reported already.
1475 That you are testing a recent version.
1479 Describe the bug accurately, and give a way to reproduce it.
1483 Run libguestfs-test-tool and paste the B<complete, unedited>
1484 output into the bug report.
1490 Richard W.M. Jones (C<rjones at redhat dot com>)
1494 Copyright (C) 2009-2010 Red Hat Inc.
1495 L<http://libguestfs.org/>
1497 This library is free software; you can redistribute it and/or
1498 modify it under the terms of the GNU Lesser General Public
1499 License as published by the Free Software Foundation; either
1500 version 2 of the License, or (at your option) any later version.
1502 This library is distributed in the hope that it will be useful,
1503 but WITHOUT ANY WARRANTY; without even the implied warranty of
1504 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1505 Lesser General Public License for more details.
1507 You should have received a copy of the GNU Lesser General Public
1508 License along with this library; if not, write to the Free Software
1509 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA