5 guestfs - Library for accessing and modifying virtual machine images
11 guestfs_h *g = guestfs_create ();
12 guestfs_add_drive (g, "guest.img");
14 guestfs_mount (g, "/dev/sda1", "/");
15 guestfs_touch (g, "/hello");
16 guestfs_umount (g, "/");
20 cc prog.c -o prog -lguestfs
22 cc prog.c -o prog `pkg-config libguestfs --cflags --libs`
26 Libguestfs is a library for accessing and modifying guest disk images.
27 Amongst the things this is good for: making batch configuration
28 changes to guests, getting disk used/free statistics (see also:
29 virt-df), migrating between virtualization systems (see also:
30 virt-p2v), performing partial backups, performing partial guest
31 clones, cloning guests and changing registry/UUID/hostname info, and
34 Libguestfs uses Linux kernel and qemu code, and can access any type of
35 guest filesystem that Linux and qemu can, including but not limited
36 to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition
37 schemes, qcow, qcow2, vmdk.
39 Libguestfs provides ways to enumerate guest storage (eg. partitions,
40 LVs, what filesystem is in each LV, etc.). It can also run commands
41 in the context of the guest. Also you can access filesystems over
44 Libguestfs is a library that can be linked with C and C++ management
45 programs (or management programs written in OCaml, Perl, Python, Ruby,
46 Java, Haskell or C#). You can also use it from shell scripts or the
49 You don't need to be root to use libguestfs, although obviously you do
50 need enough permissions to access the disk images.
52 Libguestfs is a large API because it can do many things. For a gentle
53 introduction, please read the L</API OVERVIEW> section next.
57 This section provides a gentler overview of the libguestfs API. We
58 also try to group API calls together, where that may not be obvious
59 from reading about the individual calls in the main section of this
64 Before you can use libguestfs calls, you have to create a handle.
65 Then you must add at least one disk image to the handle, followed by
66 launching the handle, then performing whatever operations you want,
67 and finally closing the handle. By convention we use the single
68 letter C<g> for the name of the handle variable, although of course
69 you can use any name you want.
71 The general structure of all libguestfs-using programs looks like
74 guestfs_h *g = guestfs_create ();
76 /* Call guestfs_add_drive additional times if there are
77 * multiple disk images.
79 guestfs_add_drive (g, "guest.img");
81 /* Most manipulation calls won't work until you've launched
82 * the handle 'g'. You have to do this _after_ adding drives
83 * and _before_ other commands.
87 /* Now you can examine what partitions, LVs etc are available.
89 char **partitions = guestfs_list_partitions (g);
90 char **logvols = guestfs_lvs (g);
92 /* To access a filesystem in the image, you must mount it.
94 guestfs_mount (g, "/dev/sda1", "/");
96 /* Now you can perform filesystem actions on the guest
99 guestfs_touch (g, "/hello");
101 /* You only need to call guestfs_sync if you have made
102 * changes to the guest image. (But if you've made changes
103 * then you *must* sync). See also: guestfs_umount and
104 * guestfs_umount_all calls.
108 /* Close the handle 'g'. */
111 The code above doesn't include any error checking. In real code you
112 should check return values carefully for errors. In general all
113 functions that return integers return C<-1> on error, and all
114 functions that return pointers return C<NULL> on error. See section
115 L</ERROR HANDLING> below for how to handle errors, and consult the
116 documentation for each function call below to see precisely how they
117 return error indications.
121 The image filename (C<"guest.img"> in the example above) could be a
122 disk image from a virtual machine, a L<dd(1)> copy of a physical hard
123 disk, an actual block device, or simply an empty file of zeroes that
124 you have created through L<posix_fallocate(3)>. Libguestfs lets you
125 do useful things to all of these.
127 You can add a disk read-only using L</guestfs_add_drive_ro>, in which
128 case libguestfs won't modify the file.
130 Be extremely cautious if the disk image is in use, eg. if it is being
131 used by a virtual machine. Adding it read-write will almost certainly
132 cause disk corruption, but adding it read-only is safe.
134 You must add at least one disk image, and you may add multiple disk
135 images. In the API, the disk images are usually referred to as
136 C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
139 Once L</guestfs_launch> has been called you cannot add any more images.
140 You can call L</guestfs_list_devices> to get a list of the device
141 names, in the order that you added them. See also L</BLOCK DEVICE
146 Before you can read or write files, create directories and so on in a
147 disk image that contains filesystems, you have to mount those
148 filesystems using L</guestfs_mount>. If you already know that a disk
149 image contains (for example) one partition with a filesystem on that
150 partition, then you can mount it directly:
152 guestfs_mount (g, "/dev/sda1", "/");
154 where C</dev/sda1> means literally the first partition (C<1>) of the
155 first disk image that we added (C</dev/sda>). If the disk contains
156 Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).
158 If you are given a disk image and you don't know what it contains then
159 you have to find out. Libguestfs can do that too: use
160 L</guestfs_list_partitions> and L</guestfs_lvs> to list possible
161 partitions and LVs, and either try mounting each to see what is
162 mountable, or else examine them with L</guestfs_vfs_type> or
163 L</guestfs_file>. But you might find it easier to look at higher level
164 programs built on top of libguestfs, in particular
165 L<virt-inspector(1)>.
167 To mount a disk image read-only, use L</guestfs_mount_ro>. There are
168 several other variations of the C<guestfs_mount_*> call.
170 =head2 FILESYSTEM ACCESS AND MODIFICATION
172 The majority of the libguestfs API consists of fairly low-level calls
173 for accessing and modifying the files, directories, symlinks etc on
174 mounted filesystems. There are over a hundred such calls which you
175 can find listed in detail below in this man page, and we don't even
176 pretend to cover them all in this overview.
178 Specify filenames as full paths, starting with C<"/"> and including
181 For example, if you mounted a filesystem at C<"/"> and you want to
182 read the file called C<"etc/passwd"> then you could do:
184 char *data = guestfs_cat (g, "/etc/passwd");
186 This would return C<data> as a newly allocated buffer containing the
187 full content of that file (with some conditions: see also
188 L</DOWNLOADING> below), or C<NULL> if there was an error.
190 As another example, to create a top-level directory on that filesystem
191 called C<"var"> you would do:
193 guestfs_mkdir (g, "/var");
195 To create a symlink you could do:
197 guestfs_ln_s (g, "/etc/init.d/portmap",
198 "/etc/rc3.d/S30portmap");
200 Libguestfs will reject attempts to use relative paths and there is no
201 concept of a current working directory.
203 Libguestfs can return errors in many situations: for example if the
204 filesystem isn't writable, or if a file or directory that you
205 requested doesn't exist. If you are using the C API (documented here)
206 you have to check for those error conditions after each call. (Other
207 language bindings turn these errors into exceptions).
209 File writes are affected by the per-handle umask, set by calling
210 L</guestfs_umask> and defaulting to 022. See L</UMASK>.
214 Libguestfs contains API calls to read, create and modify partition
215 tables on disk images.
217 In the common case where you want to create a single partition
218 covering the whole disk, you should use the L</guestfs_part_disk>
221 const char *parttype = "mbr";
222 if (disk_is_larger_than_2TB)
224 guestfs_part_disk (g, "/dev/sda", parttype);
226 Obviously this effectively wipes anything that was on that disk image
231 Libguestfs provides access to a large part of the LVM2 API, such as
232 L</guestfs_lvcreate> and L</guestfs_vgremove>. It won't make much sense
233 unless you familiarize yourself with the concepts of physical volumes,
234 volume groups and logical volumes.
236 This author strongly recommends reading the LVM HOWTO, online at
237 L<http://tldp.org/HOWTO/LVM-HOWTO/>.
241 Use L</guestfs_cat> to download small, text only files. This call
242 is limited to files which are less than 2 MB and which cannot contain
243 any ASCII NUL (C<\0>) characters. However it has a very simple
246 L</guestfs_read_file> can be used to read files which contain
247 arbitrary 8 bit data, since it returns a (pointer, size) pair.
248 However it is still limited to "small" files, less than 2 MB.
250 L</guestfs_download> can be used to download any file, with no
251 limits on content or size (even files larger than 4 GB).
253 To download multiple files, see L</guestfs_tar_out> and
258 It's often the case that you want to write a file or files to the disk
261 To write a small file with fixed content, use L</guestfs_write>. To
262 create a file of all zeroes, use L</guestfs_truncate_size> (sparse) or
263 L</guestfs_fallocate64> (with all disk blocks allocated). There are a
264 variety of other functions for creating test files, for example
265 L</guestfs_fill> and L</guestfs_fill_pattern>.
267 To upload a single file, use L</guestfs_upload>. This call has no
268 limits on file content or size (even files larger than 4 GB).
270 To upload multiple files, see L</guestfs_tar_in> and L</guestfs_tgz_in>.
272 However the fastest way to upload I<large numbers of arbitrary files>
273 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
274 L<mkisofs(8)>), then attach this using L</guestfs_add_drive_ro>. If
275 you add the drive in a predictable way (eg. adding it last after all
276 other drives) then you can get the device name from
277 L</guestfs_list_devices> and mount it directly using
278 L</guestfs_mount_ro>. Note that squashfs images are sometimes
279 non-portable between kernel versions, and they don't support labels or
280 UUIDs. If you want to pre-build an image or you need to mount it
281 using a label or UUID, use an ISO image instead.
285 There are various different commands for copying between files and
286 devices and in and out of the guest filesystem. These are summarised
291 =item B<file> to B<file>
293 Use L</guestfs_cp> to copy a single file, or
294 L</guestfs_cp_a> to copy directories recursively.
296 =item B<file or device> to B<file or device>
298 Use L</guestfs_dd> which efficiently uses L<dd(1)>
299 to copy between files and devices in the guest.
301 Example: duplicate the contents of an LV:
303 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
305 The destination (C</dev/VG/Copy>) must be at least as large as the
306 source (C</dev/VG/Original>). To copy less than the whole
307 source device, use L</guestfs_copy_size>.
309 =item B<file on the host> to B<file or device>
311 Use L</guestfs_upload>. See L</UPLOADING> above.
313 =item B<file or device> to B<file on the host>
315 Use L</guestfs_download>. See L</DOWNLOADING> above.
321 L</guestfs_ll> is just designed for humans to read (mainly when using
322 the L<guestfish(1)>-equivalent command C<ll>).
324 L</guestfs_ls> is a quick way to get a list of files in a directory
325 from programs, as a flat list of strings.
327 L</guestfs_readdir> is a programmatic way to get a list of files in a
328 directory, plus additional information about each one. It is more
329 equivalent to using the L<readdir(3)> call on a local filesystem.
331 L</guestfs_find> and L</guestfs_find0> can be used to recursively list
334 =head2 RUNNING COMMANDS
336 Although libguestfs is a primarily an API for manipulating files
337 inside guest images, we also provide some limited facilities for
338 running commands inside guests.
340 There are many limitations to this:
346 The kernel version that the command runs under will be different
347 from what it expects.
351 If the command needs to communicate with daemons, then most likely
352 they won't be running.
356 The command will be running in limited memory.
360 Only supports Linux guests (not Windows, BSD, etc).
364 Architecture limitations (eg. won't work for a PPC guest on
369 For SELinux guests, you may need to enable SELinux and load policy
370 first. See L</SELINUX> in this manpage.
374 The two main API calls to run commands are L</guestfs_command> and
375 L</guestfs_sh> (there are also variations).
377 The difference is that L</guestfs_sh> runs commands using the shell, so
378 any shell globs, redirections, etc will work.
380 =head2 CONFIGURATION FILES
382 To read and write configuration files in Linux guest filesystems, we
383 strongly recommend using Augeas. For example, Augeas understands how
384 to read and write, say, a Linux shadow password file or X.org
385 configuration file, and so avoids you having to write that code.
387 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
388 don't document Augeas itself here because there is excellent
389 documentation on the L<http://augeas.net/> website.
391 If you don't want to use Augeas (you fool!) then try calling
392 L</guestfs_read_lines> to get the file as a list of lines which
393 you can iterate over.
397 We support SELinux guests. To ensure that labeling happens correctly
398 in SELinux guests, you need to enable SELinux and load the guest's
405 Before launching, do:
407 guestfs_set_selinux (g, 1);
411 After mounting the guest's filesystem(s), load the policy. This
412 is best done by running the L<load_policy(8)> command in the
415 guestfs_sh (g, "/usr/sbin/load_policy");
417 (Older versions of C<load_policy> require you to specify the
418 name of the policy file).
422 Optionally, set the security context for the API. The correct
423 security context to use can only be known by inspecting the
424 guest. As an example:
426 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
430 This will work for running commands and editing existing files.
432 When new files are created, you may need to label them explicitly,
433 for example by running the external command
434 C<restorecon pathname>.
438 Certain calls are affected by the current file mode creation mask (the
439 "umask"). In particular ones which create files or directories, such
440 as L</guestfs_touch>, L</guestfs_mknod> or L</guestfs_mkdir>. This
441 affects either the default mode that the file is created with or
442 modifies the mode that you supply.
444 The default umask is C<022>, so files are created with modes such as
445 C<0644> and directories with C<0755>.
447 There are two ways to avoid being affected by umask. Either set umask
448 to 0 (call C<guestfs_umask (g, 0)> early after launching). Or call
449 L</guestfs_chmod> after creating each file or directory.
451 For more information about umask, see L<umask(2)>.
453 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
455 Libguestfs can mount NTFS partitions. It does this using the
456 L<http://www.ntfs-3g.org/> driver.
458 DOS and Windows still use drive letters, and the filesystems are
459 always treated as case insensitive by Windows itself, and therefore
460 you might find a Windows configuration file referring to a path like
461 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
462 that directory might be referred to as C</WINDOWS/System32>.
464 Drive letter mappings are outside the scope of libguestfs. You have
465 to use libguestfs to read the appropriate Windows Registry and
466 configuration files, to determine yourself how drives are mapped (see
467 also L<virt-inspector(1)>).
469 Replacing backslash characters with forward slash characters is also
470 outside the scope of libguestfs, but something that you can easily do.
472 Where we can help is in resolving the case insensitivity of paths.
473 For this, call L</guestfs_case_sensitive_path>.
475 Libguestfs also provides some help for decoding Windows Registry
476 "hive" files, through the library C<hivex> which is part of the
477 libguestfs project although ships as a separate tarball. You have to
478 locate and download the hive file(s) yourself, and then pass them to
479 C<hivex> functions. See also the programs L<hivexml(1)>,
480 L<hivexsh(1)>, L<hivexregedit(1)> and L<virt-win-reg(1)> for more help
483 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
485 Although we don't want to discourage you from using the C API, we will
486 mention here that the same API is also available in other languages.
488 The API is broadly identical in all supported languages. This means
489 that the C call C<guestfs_mount(g,path)> is
490 C<$g-E<gt>mount($path)> in Perl, C<g.mount(path)> in Python,
491 and C<Guestfs.mount g path> in OCaml. In other words, a
492 straightforward, predictable isomorphism between each language.
494 Error messages are automatically transformed
495 into exceptions if the language supports it.
497 We don't try to "object orientify" parts of the API in OO languages,
498 although contributors are welcome to write higher level APIs above
499 what we provide in their favourite languages if they wish.
505 You can use the I<guestfs.h> header file from C++ programs. The C++
506 API is identical to the C API. C++ classes and exceptions are not
511 The C# bindings are highly experimental. Please read the warnings
512 at the top of C<csharp/Libguestfs.cs>.
516 This is the only language binding that is working but incomplete.
517 Only calls which return simple integers have been bound in Haskell,
518 and we are looking for help to complete this binding.
522 Full documentation is contained in the Javadoc which is distributed
527 For documentation see the file C<guestfs.mli>.
531 For documentation see L<Sys::Guestfs(3)>.
535 For documentation do:
543 Use the Guestfs module. There is no Ruby-specific documentation, but
544 you can find examples written in Ruby in the libguestfs source.
546 =item B<shell scripts>
548 For documentation see L<guestfish(1)>.
552 =head2 LIBGUESTFS GOTCHAS
554 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
555 system [...] that works in the way it is documented but is
556 counterintuitive and almost invites mistakes."
558 Since we developed libguestfs and the associated tools, there are
559 several things we would have designed differently, but are now stuck
560 with for backwards compatibility or other reasons. If there is ever a
561 libguestfs 2.0 release, you can expect these to change. Beware of
566 =item Autosync / forgetting to sync.
568 When modifying a filesystem from C or another language, you B<must>
569 unmount all filesystems and call L</guestfs_sync> explicitly before
570 you close the libguestfs handle. You can also call:
572 guestfs_set_autosync (g, 1);
574 to have the unmount/sync done automatically for you when the handle 'g'
575 is closed. (This feature is called "autosync", L</guestfs_set_autosync>
578 If you forget to do this, then it is entirely possible that your
579 changes won't be written out, or will be partially written, or (very
580 rarely) that you'll get disk corruption.
582 Note that in L<guestfish(3)> autosync is the default. So quick and
583 dirty guestfish scripts that forget to sync will work just fine, which
584 can make this very puzzling if you are trying to debug a problem.
586 =item Mount option C<-o sync> should not be the default.
588 If you use L</guestfs_mount>, then C<-o sync,noatime> are added
589 implicitly. However C<-o sync> does not add any reliability benefit,
590 but does have a very large performance impact.
592 The work around is to use L</guestfs_mount_options> and set the mount
593 options that you actually want to use.
595 =item Read-only should be the default.
597 In L<guestfish(3)>, I<--ro> should be the default, and you should
598 have to specify I<--rw> if you want to make changes to the image.
600 This would reduce the potential to corrupt live VM images.
602 Note that many filesystems change the disk when you just mount and
603 unmount, even if you didn't perform any writes. You need to use
604 L</guestfs_add_drive_ro> to guarantee that the disk is not changed.
606 =item guestfish command line is hard to use.
608 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
609 for examination). It tries to run a guestfish command C<disk.img>
610 which doesn't exist, so it fails. In earlier versions of guestfish
611 the error message was also unintuitive, but we have corrected this
612 since. Like the Bourne shell, we should have used C<guestfish -c
613 command> to run commands.
615 =item Protocol limit of 256 characters for error messages
617 This limit is both rather small and quite unnecessary. We should be
618 able to return error messages up to the length of the protocol message
621 Note that we cannot change the protocol without some breakage, because
622 there are distributions that repackage the Fedora appliance.
624 =item Protocol should return errno with error messages.
626 It would be a nice-to-have to be able to get the original value of
627 'errno' from inside the appliance along error paths (where set).
628 Currently L<guestmount(1)> goes through hoops to try to reverse the
629 error message string into an errno, see the function error() in
634 =head2 PROTOCOL LIMITS
636 Internally libguestfs uses a message-based protocol to pass API calls
637 and their responses to and from a small "appliance" (see L</INTERNALS>
638 for plenty more detail about this). The maximum message size used by
639 the protocol is slightly less than 4 MB. For some API calls you may
640 need to be aware of this limit. The API calls which may be affected
641 are individually documented, with a link back to this section of the
644 A simple call such as L</guestfs_cat> returns its result (the file
645 data) in a simple string. Because this string is at some point
646 internally encoded as a message, the maximum size that it can return
647 is slightly under 4 MB. If the requested file is larger than this
648 then you will get an error.
650 In order to transfer large files into and out of the guest filesystem,
651 you need to use particular calls that support this. The sections
652 L</UPLOADING> and L</DOWNLOADING> document how to do this.
654 You might also consider mounting the disk image using our FUSE
655 filesystem support (L<guestmount(1)>).
657 =head1 CONNECTION MANAGEMENT
661 C<guestfs_h> is the opaque type representing a connection handle.
662 Create a handle by calling L</guestfs_create>. Call L</guestfs_close>
663 to free the handle and release all resources used.
665 For information on using multiple handles and threads, see the section
666 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
668 =head2 guestfs_create
670 guestfs_h *guestfs_create (void);
672 Create a connection handle.
674 You have to call L</guestfs_add_drive> on the handle at least once.
676 This function returns a non-NULL pointer to a handle on success or
679 After configuring the handle, you have to call L</guestfs_launch>.
681 You may also want to configure error handling for the handle. See
682 L</ERROR HANDLING> section below.
686 void guestfs_close (guestfs_h *g);
688 This closes the connection handle and frees up all resources used.
690 =head1 ERROR HANDLING
692 The convention in all functions that return C<int> is that they return
693 C<-1> to indicate an error. You can get additional information on
694 errors by calling L</guestfs_last_error> and/or by setting up an error
695 handler with L</guestfs_set_error_handler>.
697 The default error handler prints the information string to C<stderr>.
699 Out of memory errors are handled differently. The default action is
700 to call L<abort(3)>. If this is undesirable, then you can set a
701 handler using L</guestfs_set_out_of_memory_handler>.
703 =head2 guestfs_last_error
705 const char *guestfs_last_error (guestfs_h *g);
707 This returns the last error message that happened on C<g>. If
708 there has not been an error since the handle was created, then this
711 The lifetime of the returned string is until the next error occurs, or
712 L</guestfs_close> is called.
714 The error string is not localized (ie. is always in English), because
715 this makes searching for error messages in search engines give the
716 largest number of results.
718 =head2 guestfs_set_error_handler
720 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
723 void guestfs_set_error_handler (guestfs_h *g,
724 guestfs_error_handler_cb cb,
727 The callback C<cb> will be called if there is an error. The
728 parameters passed to the callback are an opaque data pointer and the
729 error message string.
731 Note that the message string C<msg> is freed as soon as the callback
732 function returns, so if you want to stash it somewhere you must make
735 The default handler prints messages on C<stderr>.
737 If you set C<cb> to C<NULL> then I<no> handler is called.
739 =head2 guestfs_get_error_handler
741 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g,
744 Returns the current error handler callback.
746 =head2 guestfs_set_out_of_memory_handler
748 typedef void (*guestfs_abort_cb) (void);
749 int guestfs_set_out_of_memory_handler (guestfs_h *g,
752 The callback C<cb> will be called if there is an out of memory
753 situation. I<Note this callback must not return>.
755 The default is to call L<abort(3)>.
757 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
760 =head2 guestfs_get_out_of_memory_handler
762 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *g);
764 This returns the current out of memory handler.
768 Libguestfs needs a kernel and initrd.img, which it finds by looking
769 along an internal path.
771 By default it looks for these in the directory C<$libdir/guestfs>
772 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
774 Use L</guestfs_set_path> or set the environment variable
775 L</LIBGUESTFS_PATH> to change the directories that libguestfs will
776 search in. The value is a colon-separated list of paths. The current
777 directory is I<not> searched unless the path contains an empty element
778 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
779 search the current directory and then C</usr/lib/guestfs>.
781 =head1 HIGH-LEVEL API ACTIONS
785 We guarantee the libguestfs ABI (binary interface), for public,
786 high-level actions as outlined in this section. Although we will
787 deprecate some actions, for example if they get replaced by newer
788 calls, we will keep the old actions forever. This allows you the
789 developer to program in confidence against the libguestfs API.
799 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
801 Using L</guestfs_available> you can test availability of
802 the following groups of functions. This test queries the
803 appliance to see if the appliance you are currently using
804 supports the functionality.
808 =head2 GUESTFISH supported COMMAND
810 In L<guestfish(3)> there is a handy interactive command
811 C<supported> which prints out the available groups and
812 whether they are supported by this build of libguestfs.
813 Note however that you have to do C<run> first.
815 =head2 SINGLE CALLS AT COMPILE TIME
817 If you need to test whether a single libguestfs function is
818 available at compile time, we recommend using build tools
819 such as autoconf or cmake. For example in autotools you could
822 AC_CHECK_LIB([guestfs],[guestfs_create])
823 AC_CHECK_FUNCS([guestfs_dd])
825 which would result in C<HAVE_GUESTFS_DD> being either defined
826 or not defined in your program.
828 =head2 SINGLE CALLS AT RUN TIME
830 Testing at compile time doesn't guarantee that a function really
831 exists in the library. The reason is that you might be dynamically
832 linked against a previous I<libguestfs.so> (dynamic library)
833 which doesn't have the call. This situation unfortunately results
834 in a segmentation fault, which is a shortcoming of the C dynamic
835 linking system itself.
837 You can use L<dlopen(3)> to test if a function is available
838 at run time, as in this example program (note that you still
839 need the compile time check as well):
851 #ifdef HAVE_GUESTFS_DD
855 /* Test if the function guestfs_dd is really available. */
856 dl = dlopen (NULL, RTLD_LAZY);
858 fprintf (stderr, "dlopen: %s\n", dlerror ());
861 has_function = dlsym (dl, "guestfs_dd") != NULL;
865 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
867 printf ("this libguestfs.so has guestfs_dd function\n");
868 /* Now it's safe to call
869 guestfs_dd (g, "foo", "bar");
873 printf ("guestfs_dd function was not found at compile time\n");
877 You may think the above is an awful lot of hassle, and it is.
878 There are other ways outside of the C linking system to ensure
879 that this kind of incompatibility never arises, such as using
882 Requires: libguestfs >= 1.0.80
886 <!-- old anchor for the next section -->
887 <a name="state_machine_and_low_level_event_api"/>
893 Internally, libguestfs is implemented by running an appliance (a
894 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
895 a child process of the main program.
901 | | child process / appliance
902 | | __________________________
904 +-------------------+ RPC | +-----------------+ |
905 | libguestfs <--------------------> guestfsd | |
906 | | | +-----------------+ |
907 \___________________/ | | Linux kernel | |
908 | +--^--------------+ |
909 \_________|________________/
917 The library, linked to the main program, creates the child process and
918 hence the appliance in the L</guestfs_launch> function.
920 Inside the appliance is a Linux kernel and a complete stack of
921 userspace tools (such as LVM and ext2 programs) and a small
922 controlling daemon called L</guestfsd>. The library talks to
923 L</guestfsd> using remote procedure calls (RPC). There is a mostly
924 one-to-one correspondence between libguestfs API calls and RPC calls
925 to the daemon. Lastly the disk image(s) are attached to the qemu
926 process which translates device access by the appliance's Linux kernel
927 into accesses to the image.
929 A common misunderstanding is that the appliance "is" the virtual
930 machine. Although the disk image you are attached to might also be
931 used by some virtual machine, libguestfs doesn't know or care about
932 this. (But you will care if both libguestfs's qemu process and your
933 virtual machine are trying to update the disk image at the same time,
934 since these usually results in massive disk corruption).
938 libguestfs uses a state machine to model the child process:
949 / | \ \ guestfs_launch
960 \______/ <------ \________/
962 The normal transitions are (1) CONFIG (when the handle is created, but
963 there is no child process), (2) LAUNCHING (when the child process is
964 booting up), (3) alternating between READY and BUSY as commands are
965 issued to, and carried out by, the child process.
967 The guest may be killed by L</guestfs_kill_subprocess>, or may die
968 asynchronously at any time (eg. due to some internal error), and that
969 causes the state to transition back to CONFIG.
971 Configuration commands for qemu such as L</guestfs_add_drive> can only
972 be issued when in the CONFIG state.
974 The high-level API offers two calls that go from CONFIG through
975 LAUNCHING to READY. L</guestfs_launch> blocks until the child process
976 is READY to accept commands (or until some failure or timeout).
977 L</guestfs_launch> internally moves the state from CONFIG to LAUNCHING
980 High-level API actions such as L</guestfs_mount> can only be issued
981 when in the READY state. These high-level API calls block waiting for
982 the command to be carried out (ie. the state to transition to BUSY and
983 then back to READY). But using the low-level event API, you get
984 non-blocking versions. (But you can still only carry out one
985 operation per handle at a time - that is a limitation of the
986 communications protocol we use).
988 Finally, the child process sends asynchronous messages back to the
989 main program, such as kernel log messages. Mostly these are ignored
990 by the high-level API, but using the low-level event API you can
991 register to receive these messages.
993 =head2 SETTING CALLBACKS TO HANDLE EVENTS
995 The child process generates events in some situations. Current events
996 include: receiving a log message, the child process exits.
998 Use the C<guestfs_set_*_callback> functions to set a callback for
999 different types of events.
1001 Only I<one callback of each type> can be registered for each handle.
1002 Calling C<guestfs_set_*_callback> again overwrites the previous
1003 callback of that type. Cancel all callbacks of this type by calling
1004 this function with C<cb> set to C<NULL>.
1006 =head2 guestfs_set_log_message_callback
1008 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
1009 char *buf, int len);
1010 void guestfs_set_log_message_callback (guestfs_h *g,
1011 guestfs_log_message_cb cb,
1014 The callback function C<cb> will be called whenever qemu or the guest
1015 writes anything to the console.
1017 Use this function to capture kernel messages and similar.
1019 Normally there is no log message handler, and log messages are just
1022 =head2 guestfs_set_subprocess_quit_callback
1024 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
1025 void guestfs_set_subprocess_quit_callback (guestfs_h *g,
1026 guestfs_subprocess_quit_cb cb,
1029 The callback function C<cb> will be called when the child process
1030 quits, either asynchronously or if killed by
1031 L</guestfs_kill_subprocess>. (This corresponds to a transition from
1032 any state to the CONFIG state).
1034 =head2 guestfs_set_launch_done_callback
1036 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
1037 void guestfs_set_launch_done_callback (guestfs_h *g,
1038 guestfs_launch_done_cb cb,
1041 The callback function C<cb> will be called when the child process
1042 becomes ready first time after it has been launched. (This
1043 corresponds to a transition from LAUNCHING to the READY state).
1045 =head2 guestfs_set_close_callback
1047 typedef void (*guestfs_close_cb) (guestfs_h *g, void *opaque);
1048 void guestfs_set_close_callback (guestfs_h *g,
1049 guestfs_close_cb cb,
1052 The callback function C<cb> will be called while the handle
1053 is being closed (synchronously from L</guestfs_close>).
1055 Note that libguestfs installs an L<atexit(3)> handler to try to
1056 clean up handles that are open when the program exits. This
1057 means that this callback might be called indirectly from
1058 L<exit(3)>, which can cause unexpected problems in higher-level
1059 languages (eg. if your HLL interpreter has already been cleaned
1060 up by the time this is called, and if your callback then jumps
1061 into some HLL function).
1063 =head1 BLOCK DEVICE NAMING
1065 In the kernel there is now quite a profusion of schemata for naming
1066 block devices (in this context, by I<block device> I mean a physical
1067 or virtual hard drive). The original Linux IDE driver used names
1068 starting with C</dev/hd*>. SCSI devices have historically used a
1069 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
1070 driver became a popular replacement for the old IDE driver
1071 (particularly for SATA devices) those devices also used the
1072 C</dev/sd*> scheme. Additionally we now have virtual machines with
1073 paravirtualized drivers. This has created several different naming
1074 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
1077 As discussed above, libguestfs uses a qemu appliance running an
1078 embedded Linux kernel to access block devices. We can run a variety
1079 of appliances based on a variety of Linux kernels.
1081 This causes a problem for libguestfs because many API calls use device
1082 or partition names. Working scripts and the recipe (example) scripts
1083 that we make available over the internet could fail if the naming
1086 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
1087 scheme>. Internally C</dev/sd*> names are translated, if necessary,
1088 to other names as required. For example, under RHEL 5 which uses the
1089 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
1090 C</dev/hda2> transparently.
1092 Note that this I<only> applies to parameters. The
1093 L</guestfs_list_devices>, L</guestfs_list_partitions> and similar calls
1094 return the true names of the devices and partitions as known to the
1097 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
1099 Usually this translation is transparent. However in some (very rare)
1100 cases you may need to know the exact algorithm. Such cases include
1101 where you use L</guestfs_config> to add a mixture of virtio and IDE
1102 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
1103 and C</dev/vd*> devices.
1105 The algorithm is applied only to I<parameters> which are known to be
1106 either device or partition names. Return values from functions such
1107 as L</guestfs_list_devices> are never changed.
1113 Is the string a parameter which is a device or partition name?
1117 Does the string begin with C</dev/sd>?
1121 Does the named device exist? If so, we use that device.
1122 However if I<not> then we continue with this algorithm.
1126 Replace initial C</dev/sd> string with C</dev/hd>.
1128 For example, change C</dev/sda2> to C</dev/hda2>.
1130 If that named device exists, use it. If not, continue.
1134 Replace initial C</dev/sd> string with C</dev/vd>.
1136 If that named device exists, use it. If not, return an error.
1140 =head2 PORTABILITY CONCERNS
1142 Although the standard naming scheme and automatic translation is
1143 useful for simple programs and guestfish scripts, for larger programs
1144 it is best not to rely on this mechanism.
1146 Where possible for maximum future portability programs using
1147 libguestfs should use these future-proof techniques:
1153 Use L</guestfs_list_devices> or L</guestfs_list_partitions> to list
1154 actual device names, and then use those names directly.
1156 Since those device names exist by definition, they will never be
1161 Use higher level ways to identify filesystems, such as LVM names,
1162 UUIDs and filesystem labels.
1168 =head2 COMMUNICATION PROTOCOL
1170 Don't rely on using this protocol directly. This section documents
1171 how it currently works, but it may change at any time.
1173 The protocol used to talk between the library and the daemon running
1174 inside the qemu virtual machine is a simple RPC mechanism built on top
1175 of XDR (RFC 1014, RFC 1832, RFC 4506).
1177 The detailed format of structures is in C<src/guestfs_protocol.x>
1178 (note: this file is automatically generated).
1180 There are two broad cases, ordinary functions that don't have any
1181 C<FileIn> and C<FileOut> parameters, which are handled with very
1182 simple request/reply messages. Then there are functions that have any
1183 C<FileIn> or C<FileOut> parameters, which use the same request and
1184 reply messages, but they may also be followed by files sent using a
1187 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1189 For ordinary functions, the request message is:
1191 total length (header + arguments,
1192 but not including the length word itself)
1193 struct guestfs_message_header (encoded as XDR)
1194 struct guestfs_<foo>_args (encoded as XDR)
1196 The total length field allows the daemon to allocate a fixed size
1197 buffer into which it slurps the rest of the message. As a result, the
1198 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1199 4MB), which means the effective size of any request is limited to
1200 somewhere under this size.
1202 Note also that many functions don't take any arguments, in which case
1203 the C<guestfs_I<foo>_args> is completely omitted.
1205 The header contains the procedure number (C<guestfs_proc>) which is
1206 how the receiver knows what type of args structure to expect, or none
1209 The reply message for ordinary functions is:
1211 total length (header + ret,
1212 but not including the length word itself)
1213 struct guestfs_message_header (encoded as XDR)
1214 struct guestfs_<foo>_ret (encoded as XDR)
1216 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1217 for functions that return no formal return values.
1219 As above the total length of the reply is limited to
1220 C<GUESTFS_MESSAGE_MAX>.
1222 In the case of an error, a flag is set in the header, and the reply
1223 message is slightly changed:
1225 total length (header + error,
1226 but not including the length word itself)
1227 struct guestfs_message_header (encoded as XDR)
1228 struct guestfs_message_error (encoded as XDR)
1230 The C<guestfs_message_error> structure contains the error message as a
1233 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1235 A C<FileIn> parameter indicates that we transfer a file I<into> the
1236 guest. The normal request message is sent (see above). However this
1237 is followed by a sequence of file chunks.
1239 total length (header + arguments,
1240 but not including the length word itself,
1241 and not including the chunks)
1242 struct guestfs_message_header (encoded as XDR)
1243 struct guestfs_<foo>_args (encoded as XDR)
1244 sequence of chunks for FileIn param #0
1245 sequence of chunks for FileIn param #1 etc.
1247 The "sequence of chunks" is:
1249 length of chunk (not including length word itself)
1250 struct guestfs_chunk (encoded as XDR)
1252 struct guestfs_chunk (encoded as XDR)
1255 struct guestfs_chunk (with data.data_len == 0)
1257 The final chunk has the C<data_len> field set to zero. Additionally a
1258 flag is set in the final chunk to indicate either successful
1259 completion or early cancellation.
1261 At time of writing there are no functions that have more than one
1262 FileIn parameter. However this is (theoretically) supported, by
1263 sending the sequence of chunks for each FileIn parameter one after
1264 another (from left to right).
1266 Both the library (sender) I<and> the daemon (receiver) may cancel the
1267 transfer. The library does this by sending a chunk with a special
1268 flag set to indicate cancellation. When the daemon sees this, it
1269 cancels the whole RPC, does I<not> send any reply, and goes back to
1270 reading the next request.
1272 The daemon may also cancel. It does this by writing a special word
1273 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1274 during the transfer, and if it gets it, it will cancel the transfer
1275 (it sends a cancel chunk). The special word is chosen so that even if
1276 cancellation happens right at the end of the transfer (after the
1277 library has finished writing and has started listening for the reply),
1278 the "spurious" cancel flag will not be confused with the reply
1281 This protocol allows the transfer of arbitrary sized files (no 32 bit
1282 limit), and also files where the size is not known in advance
1283 (eg. from pipes or sockets). However the chunks are rather small
1284 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1285 daemon need to keep much in memory.
1287 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1289 The protocol for FileOut parameters is exactly the same as for FileIn
1290 parameters, but with the roles of daemon and library reversed.
1292 total length (header + ret,
1293 but not including the length word itself,
1294 and not including the chunks)
1295 struct guestfs_message_header (encoded as XDR)
1296 struct guestfs_<foo>_ret (encoded as XDR)
1297 sequence of chunks for FileOut param #0
1298 sequence of chunks for FileOut param #1 etc.
1300 =head3 INITIAL MESSAGE
1302 Because the underlying channel (QEmu -net channel) doesn't have any
1303 sort of connection control, when the daemon launches it sends an
1304 initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
1305 and daemon is alive. This is what L</guestfs_launch> waits for.
1307 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1309 All high-level libguestfs actions are synchronous. If you want
1310 to use libguestfs asynchronously then you must create a thread.
1312 Only use the handle from a single thread. Either use the handle
1313 exclusively from one thread, or provide your own mutex so that two
1314 threads cannot issue calls on the same handle at the same time.
1316 =head1 QEMU WRAPPERS
1318 If you want to compile your own qemu, run qemu from a non-standard
1319 location, or pass extra arguments to qemu, then you can write a
1320 shell-script wrapper around qemu.
1322 There is one important rule to remember: you I<must C<exec qemu>> as
1323 the last command in the shell script (so that qemu replaces the shell
1324 and becomes the direct child of the libguestfs-using program). If you
1325 don't do this, then the qemu process won't be cleaned up correctly.
1327 Here is an example of a wrapper, where I have built my own copy of
1331 qemudir=/home/rjones/d/qemu
1332 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1334 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1335 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1338 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1340 Note that libguestfs also calls qemu with the -help and -version
1341 options in order to determine features.
1343 =head1 LIBGUESTFS VERSION NUMBERS
1345 Since April 2010, libguestfs has started to make separate development
1346 and stable releases, along with corresponding branches in our git
1347 repository. These separate releases can be identified by version
1350 even numbers for stable: 1.2.x, 1.4.x, ...
1351 .-------- odd numbers for development: 1.3.x, 1.5.x, ...
1357 | `-------- sub-version
1359 `------ always '1' because we don't change the ABI
1361 Thus "1.3.5" is the 5th update to the development branch "1.3".
1363 As time passes we cherry pick fixes from the development branch and
1364 backport those into the stable branch, the effect being that the
1365 stable branch should get more stable and less buggy over time. So the
1366 stable releases are ideal for people who don't need new features but
1367 would just like the software to work.
1369 Our criteria for backporting changes are:
1375 Documentation changes which don't affect any code are
1376 backported unless the documentation refers to a future feature
1377 which is not in stable.
1381 Bug fixes which are not controversial, fix obvious problems, and
1382 have been well tested are backported.
1386 Simple rearrangements of code which shouldn't affect how it works get
1387 backported. This is so that the code in the two branches doesn't get
1388 too far out of step, allowing us to backport future fixes more easily.
1392 We I<don't> backport new features, new APIs, new tools etc, except in
1393 one exceptional case: the new feature is required in order to
1394 implement an important bug fix.
1398 A new stable branch starts when we think the new features in
1399 development are substantial and compelling enough over the current
1400 stable branch to warrant it. When that happens we create new stable
1401 and development versions 1.N.0 and 1.(N+1).0 [N is even]. The new
1402 dot-oh release won't necessarily be so stable at this point, but by
1403 backporting fixes from development, that branch will stabilize over
1406 =head1 ENVIRONMENT VARIABLES
1410 =item LIBGUESTFS_APPEND
1412 Pass additional options to the guest kernel.
1414 =item LIBGUESTFS_DEBUG
1416 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1417 has the same effect as calling C<guestfs_set_verbose (g, 1)>.
1419 =item LIBGUESTFS_MEMSIZE
1421 Set the memory allocated to the qemu process, in megabytes. For
1424 LIBGUESTFS_MEMSIZE=700
1426 =item LIBGUESTFS_PATH
1428 Set the path that libguestfs uses to search for kernel and initrd.img.
1429 See the discussion of paths in section PATH above.
1431 =item LIBGUESTFS_QEMU
1433 Set the default qemu binary that libguestfs uses. If not set, then
1434 the qemu which was found at compile time by the configure script is
1437 See also L</QEMU WRAPPERS> above.
1439 =item LIBGUESTFS_TRACE
1441 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1442 has the same effect as calling C<guestfs_set_trace (g, 1)>.
1446 Location of temporary directory, defaults to C</tmp>.
1448 If libguestfs was compiled to use the supermin appliance then each
1449 handle will require rather a large amount of space in this directory
1450 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1451 configure another directory to use in case C</tmp> is not large
1463 L<virt-inspector(1)>,
1464 L<virt-list-filesystems(1)>,
1465 L<virt-list-partitions(1)>,
1474 L<http://libguestfs.org/>.
1476 Tools with a similar purpose:
1485 To get a list of bugs against libguestfs use this link:
1487 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1489 To report a new bug against libguestfs use this link:
1491 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1493 When reporting a bug, please check:
1499 That the bug hasn't been reported already.
1503 That you are testing a recent version.
1507 Describe the bug accurately, and give a way to reproduce it.
1511 Run libguestfs-test-tool and paste the B<complete, unedited>
1512 output into the bug report.
1518 Richard W.M. Jones (C<rjones at redhat dot com>)
1522 Copyright (C) 2009-2010 Red Hat Inc.
1523 L<http://libguestfs.org/>
1525 This library is free software; you can redistribute it and/or
1526 modify it under the terms of the GNU Lesser General Public
1527 License as published by the Free Software Foundation; either
1528 version 2 of the License, or (at your option) any later version.
1530 This library is distributed in the hope that it will be useful,
1531 but WITHOUT ANY WARRANTY; without even the implied warranty of
1532 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1533 Lesser General Public License for more details.
1535 You should have received a copy of the GNU Lesser General Public
1536 License along with this library; if not, write to the Free Software
1537 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA