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 below.
63 Before you can use libguestfs calls, you have to create a handle.
64 Then you must add at least one disk image to the handle, followed by
65 launching the handle, then performing whatever operations you want,
66 and finally closing the handle. By convention we use the single
67 letter C<g> for the name of the handle variable, although of course
68 you can use any name you want.
70 The general structure of all libguestfs-using programs looks like
73 guestfs_h *g = guestfs_create ();
75 /* Call guestfs_add_drive additional times if there are
76 * multiple disk images.
78 guestfs_add_drive (g, "guest.img");
80 /* Most manipulation calls won't work until you've launched
81 * the handle 'g'. You have to do this _after_ adding drives
82 * and _before_ other commands.
86 /* Now you can examine what partitions, LVs etc are available.
88 char **partitions = guestfs_list_partitions (g);
89 char **logvols = guestfs_lvs (g);
91 /* To access a filesystem in the image, you must mount it.
93 guestfs_mount (g, "/dev/sda1", "/");
95 /* Now you can perform filesystem actions on the guest
98 guestfs_touch (g, "/hello");
100 /* You only need to call guestfs_sync if you have made
101 * changes to the guest image. (But if you've made changes
102 * then you *must* sync).
106 /* Close the handle 'g'. */
109 The code above doesn't include any error checking. In real code you
110 should check return values carefully for errors. In general all
111 functions that return integers return C<-1> on error, and all
112 functions that return pointers return C<NULL> on error. See section
113 L</ERROR HANDLING> below for how to handle errors, and consult the
114 documentation for each function call below to see precisely how they
115 return error indications.
119 The image filename (C<"guest.img"> in the example above) could be a
120 disk image from a virtual machine, a L<dd(1)> copy of a physical hard
121 disk, an actual block device, or simply an empty file of zeroes that
122 you have created through L<posix_fallocate(3)>. Libguestfs lets you
123 do useful things to all of these.
125 You can add a disk read-only using C<guestfs_add_drive_ro>, in which
126 case libguestfs won't modify the file.
128 Be extremely cautious if the disk image is in use, eg. if it is being
129 used by a virtual machine. Adding it read-write will almost certainly
130 cause disk corruption, but adding it read-only is safe.
132 You must add at least one disk image, and you may add multiple disk
133 images. In the API, the disk images are usually referred to as
134 C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
137 Once C<guestfs_launch> has been called you cannot add any more images.
138 You can call C<guestfs_list_devices> to get a list of the device
139 names, in the order that you added them. See also L</BLOCK DEVICE
144 Before you can read or write files, create directories and so on in a
145 disk image that contains filesystems, you have to mount those
146 filesystems using C<guestfs_mount>. If you already know that a disk
147 image contains (for example) one partition with a filesystem on that
148 partition, then you can mount it directly:
150 guestfs_mount (g, "/dev/sda1", "/");
152 where C</dev/sda1> means literally the first partition (C<1>) of the
153 first disk image that we added (C</dev/sda>). If the disk contains
154 Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).
156 If you are given a disk image and you don't know what it contains then
157 you have to find out. Libguestfs can do that too: use
158 C<guestfs_list_partitions> and C<guestfs_lvs> to list possible
159 partitions and LVs, and either try mounting each to see what is
160 mountable, or else examine them with C<guestfs_file>. But you might
161 find it easier to look at higher level programs built on top of
162 libguestfs, in particular L<virt-inspector(1)>.
164 To mount a disk image read-only, use C<guestfs_mount_ro>. There are
165 several other variations of the C<guestfs_mount_*> call.
167 =head2 FILESYSTEM ACCESS AND MODIFICATION
169 The majority of the libguestfs API consists of fairly low-level calls
170 for accessing and modifying the files, directories, symlinks etc on
171 mounted filesystems. There are over a hundred such calls which you
172 can find listed in detail below in this man page, and we don't even
173 pretend to cover them all in this overview.
175 Specify filenames as full paths including the mount point.
177 For example, if you mounted a filesystem at C<"/"> and you want to
178 read the file called C<"etc/passwd"> then you could do:
180 char *data = guestfs_cat (g, "/etc/passwd");
182 This would return C<data> as a newly allocated buffer containing the
183 full content of that file (with some conditions: see also
184 L</DOWNLOADING> below), or C<NULL> if there was an error.
186 As another example, to create a top-level directory on that filesystem
187 called C<"var"> you would do:
189 guestfs_mkdir (g, "/var");
191 To create a symlink you could do:
193 guestfs_ln_s (g, "/etc/init.d/portmap",
194 "/etc/rc3.d/S30portmap");
196 Libguestfs will reject attempts to use relative paths. There is no
197 concept of a current working directory. Libguestfs can return errors
198 in many situations: for example if the filesystem isn't writable, or
199 if a file or directory that you requested doesn't exist. If you are
200 using the C API (documented here) you have to check for those error
201 conditions after each call. (Other language bindings turn these
202 errors into exceptions).
204 File writes are affected by the per-handle umask, set by calling
205 C<guestfs_umask> and defaulting to 022. See L</UMASK>.
209 Libguestfs contains API calls to read, create and modify partition
210 tables on disk images.
212 In the common case where you want to create a single partition
213 covering the whole disk, you should use the C<guestfs_part_disk>
216 const char *parttype = "mbr";
217 if (disk_is_larger_than_2TB)
219 guestfs_part_disk (g, "/dev/sda", parttype);
221 Obviously this effectively wipes anything that was on that disk image
224 In general MBR partitions are both unnecessarily complicated and
225 depend on archaic details, namely the Cylinder-Head-Sector (CHS)
226 geometry of the disk. C<guestfs_sfdiskM> can be used to
227 create more complex arrangements where the relative sizes are
228 expressed in megabytes instead of cylinders, which is a small win.
229 C<guestfs_sfdiskM> will choose the nearest cylinder to approximate the
230 requested size. There's a lot of crazy stuff to do with IDE and
231 virtio disks having different, incompatible CHS geometries, that you
232 probably don't want to know about.
234 My advice: make a single partition to cover the whole disk, then use
239 Libguestfs provides access to a large part of the LVM2 API, such as
240 C<guestfs_lvcreate> and C<guestfs_vgremove>. It won't make much sense
241 unless you familiarize yourself with the concepts of physical volumes,
242 volume groups and logical volumes.
244 This author strongly recommends reading the LVM HOWTO, online at
245 L<http://tldp.org/HOWTO/LVM-HOWTO/>.
249 Use C<guestfs_cat> to download small, text only files. This call
250 is limited to files which are less than 2 MB and which cannot contain
251 any ASCII NUL (C<\0>) characters. However it has a very simple
254 C<guestfs_read_file> can be used to read files which contain
255 arbitrary 8 bit data, since it returns a (pointer, size) pair.
256 However it is still limited to "small" files, less than 2 MB.
258 C<guestfs_download> can be used to download any file, with no
259 limits on content or size (even files larger than 4 GB).
261 To download multiple files, see C<guestfs_tar_out> and
266 It's often the case that you want to write a file or files to the disk
269 For small, single files, use C<guestfs_write_file>. This call
270 currently contains a bug which limits the call to plain text files
271 (not containing ASCII NUL characters).
273 To upload a single file, use C<guestfs_upload>. This call has no
274 limits on file content or size (even files larger than 4 GB).
276 To upload multiple files, see C<guestfs_tar_in> and C<guestfs_tgz_in>.
278 However the fastest way to upload I<large numbers of arbitrary files>
279 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
280 L<mkisofs(8)>), then attach this using C<guestfs_add_drive_ro>. If
281 you add the drive in a predictable way (eg. adding it last after all
282 other drives) then you can get the device name from
283 C<guestfs_list_devices> and mount it directly using
284 C<guestfs_mount_ro>. Note that squashfs images are sometimes
285 non-portable between kernel versions, and they don't support labels or
286 UUIDs. If you want to pre-build an image or you need to mount it
287 using a label or UUID, use an ISO image instead.
291 There are various different commands for copying between files and
292 devices and in and out of the guest filesystem. These are summarised
297 =item B<file> to B<file>
299 Use L</guestfs_cp> to copy a single file, or
300 L</guestfs_cp_a> to copy directories recursively.
302 =item B<file or device> to B<file or device>
304 Use L</guestfs_dd> which efficiently uses L<dd(1)>
305 to copy between files and devices in the guest.
307 Example: duplicate the contents of an LV:
309 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
311 The destination (C</dev/VG/Copy>) must be at least as large as the
312 source (C</dev/VG/Original>).
314 =item B<file on the host> to B<file or device>
316 Use L</guestfs_upload>. See L</UPLOADING> above.
318 =item B<file or device> to B<file on the host>
320 Use L</guestfs_download>. See L</DOWNLOADING> above.
326 C<guestfs_ll> is just designed for humans to read (mainly when using
327 the L<guestfish(1)>-equivalent command C<ll>).
329 C<guestfs_ls> is a quick way to get a list of files in a directory
330 from programs, as a flat list of strings.
332 C<guestfs_readdir> is a programmatic way to get a list of files in a
333 directory, plus additional information about each one. It is more
334 equivalent to using the L<readdir(3)> call on a local filesystem.
336 C<guestfs_find> can be used to recursively list files.
338 =head2 RUNNING COMMANDS
340 Although libguestfs is a primarily an API for manipulating files
341 inside guest images, we also provide some limited facilities for
342 running commands inside guests.
344 There are many limitations to this:
350 The kernel version that the command runs under will be different
351 from what it expects.
355 If the command needs to communicate with daemons, then most likely
356 they won't be running.
360 The command will be running in limited memory.
364 Only supports Linux guests (not Windows, BSD, etc).
368 Architecture limitations (eg. won't work for a PPC guest on
373 For SELinux guests, you may need to enable SELinux and load policy
374 first. See L</SELINUX> in this manpage.
378 The two main API calls to run commands are C<guestfs_command> and
379 C<guestfs_sh> (there are also variations).
381 The difference is that C<guestfs_sh> runs commands using the shell, so
382 any shell globs, redirections, etc will work.
384 =head2 CONFIGURATION FILES
386 To read and write configuration files in Linux guest filesystems, we
387 strongly recommend using Augeas. For example, Augeas understands how
388 to read and write, say, a Linux shadow password file or X.org
389 configuration file, and so avoids you having to write that code.
391 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
392 don't document Augeas itself here because there is excellent
393 documentation on the L<http://augeas.net/> website.
395 If you don't want to use Augeas (you fool!) then try calling
396 C<guestfs_read_lines> to get the file as a list of lines which
397 you can iterate over.
401 We support SELinux guests. To ensure that labeling happens correctly
402 in SELinux guests, you need to enable SELinux and load the guest's
409 Before launching, do:
411 guestfs_set_selinux (g, 1);
415 After mounting the guest's filesystem(s), load the policy. This
416 is best done by running the L<load_policy(8)> command in the
419 guestfs_sh (g, "/usr/sbin/load_policy");
421 (Older versions of C<load_policy> require you to specify the
422 name of the policy file).
426 Optionally, set the security context for the API. The correct
427 security context to use can only be known by inspecting the
428 guest. As an example:
430 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
434 This will work for running commands and editing existing files.
436 When new files are created, you may need to label them explicitly,
437 for example by running the external command
438 C<restorecon pathname>.
442 Certain calls are affected by the current file mode creation mask (the
443 "umask"). In particular ones which create files or directories, such
444 as C<guestfs_touch>, C<guestfs_mknod> or C<guestfs_mkdir>. This
445 affects either the default mode that the file is created with or
446 modifies the mode that you supply.
448 The default umask is C<022>, so files are created with modes such as
449 C<0644> and directories with C<0755>.
451 There are two ways to avoid being affected by umask. Either set umask
452 to 0 (call C<guestfs_umask (g, 0)> early after launching). Or call
453 C<guestfs_chmod> after creating each file or directory.
455 For more information about umask, see L<umask(2)>.
457 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
459 Libguestfs can mount NTFS partitions. It does this using the
460 L<http://www.ntfs-3g.org/> driver.
462 DOS and Windows still use drive letters, and the filesystems are
463 always treated as case insensitive by Windows itself, and therefore
464 you might find a Windows configuration file referring to a path like
465 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
466 that directory might be referred to as C</WINDOWS/System32>.
468 Drive letter mappings are outside the scope of libguestfs. You have
469 to use libguestfs to read the appropriate Windows Registry and
470 configuration files, to determine yourself how drives are mapped (see
471 also L<virt-inspector(1)>).
473 Replacing backslash characters with forward slash characters is also
474 outside the scope of libguestfs, but something that you can easily do.
476 Where we can help is in resolving the case insensitivity of paths.
477 For this, call C<guestfs_case_sensitive_path>.
479 Libguestfs also provides some help for decoding Windows Registry
480 "hive" files, through the library C<hivex> which is part of the
481 libguestfs project. You have to locate and download the hive file(s)
482 yourself, and then pass them to C<hivex> functions. See also the
483 programs L<hivexml(1)>, L<hivexsh(1)> and L<virt-win-reg(1)> for more
486 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
488 Although we don't want to discourage you from using the C API, we will
489 mention here that the same API is also available in other languages.
491 The API is broadly identical in all supported languages. This means
492 that the C call C<guestfs_mount(g,path)> is
493 C<$g-E<gt>mount($path)> in Perl, C<g.mount(path)> in Python,
494 and C<Guestfs.mount g path> in OCaml. In other words, a
495 straightforward, predictable isomorphism between each language.
497 Error messages are automatically transformed
498 into exceptions if the language supports it.
500 We don't try to "object orientify" parts of the API in OO languages,
501 although contributors are welcome to write higher level APIs above
502 what we provide in their favourite languages if they wish.
508 You can use the I<guestfs.h> header file from C++ programs. The C++
509 API is identical to the C API. C++ classes and exceptions are
514 The C# bindings are highly experimental. Please read the warnings
515 at the top of C<csharp/Libguestfs.cs>.
519 This is the only language binding that working but incomplete. Only
520 calls which return simple integers have been bound in Haskell, and we
521 are looking for help to complete this binding.
525 Full documentation is contained in the Javadoc which is distributed
530 For documentation see the file C<guestfs.mli>.
534 For documentation see L<Sys::Guestfs(3)>.
538 For documentation do:
546 Use the Guestfs module. There is no Ruby-specific documentation, but
547 you can find examples written in Ruby in the libguestfs source.
549 =item B<shell scripts>
551 For documentation see L<guestfish(1)>.
555 =head2 LIBGUESTFS GOTCHAS
557 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
558 system [...] that works in the way it is documented but is
559 counterintuitive and almost invites mistakes."
561 Since we developed libguestfs and the associated tools, there are
562 several things we would have designed differently, but are now stuck
563 with for backwards compatibility or other reasons. If there is ever a
564 libguestfs 2.0 release, you can expect these to change. Beware of
569 =item Autosync / forgetting to sync.
571 When modifying a filesystem from C or another language, you B<must>
572 unmount all filesystems and call L</guestfs_sync> explicitly before
573 you close the libguestfs handle. You can also call:
575 guestfs_set_autosync (g, 1);
577 to have the unmount/sync done automatically for you when the handle 'g'
578 is closed. (This feature is called "autosync", L</guestfs_set_autosync>
581 If you forget to do this, then it is entirely possible that your
582 changes won't be written out, or will be partially written, or (very
583 rarely) that you'll get disk corruption.
585 Note that in L<guestfish(3)> I<autosync is the default>. So quick and
586 dirty guestfish scripts that forget to sync will work just fine, which
587 can make this extra-puzzling if you are trying to debug a problem.
589 =item Mount option C<-o sync> should not be the default.
591 If you use C<guestfs_mount>, then C<-o sync,noatime> are added
592 implicitly. However C<-o sync> does not add any reliability benefit,
593 but does have a very large performance impact.
595 The work around is to use C<guestfs_mount_options> and set the mount
596 options that you actually want to use.
598 =item Read-only should be the default.
600 In L<guestfish(3)>, I<--ro> should be the default, and you should
601 have to specify I<--rw> if you want to make changes to the image.
603 This would reduce the potential to corrupt live VM images.
605 Note that many filesystems change the disk when you just mount and
606 unmount, even if you didn't perform any writes. You need to use
607 C<guestfs_add_drive_ro> to guarantee that the disk is not changed.
609 =item guestfish command line is hard to use.
611 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
612 for examination). It tries to run a guestfish command C<disk.img>
613 which doesn't exist, so it fails, and it fails with a strange and
614 unintuitive error message. Like the Bourne shell, we should have used
615 C<guestfish -c command> to run commands.
619 =head2 PROTOCOL LIMITS
621 Internally libguestfs uses a message-based protocol to pass API calls
622 and their responses to and from a small "appliance" (see L</INTERNALS>
623 for plenty more detail about this). The maximum message size used by
624 the protocol is slightly less than 4 MB. For some API calls you may
625 need to be aware of this limit. The API calls which may be affected
626 are individually documented, with a link back to this section of the
629 A simple call such as C<guestfs_cat> returns its result (the file
630 data) in a simple string. Because this string is at some point
631 internally encoded as a message, the maximum size that it can return
632 is slightly under 4 MB. If the requested file is larger than this
633 then you will get an error.
635 In order to transfer large files into and out of the guest filesystem,
636 you need to use particular calls that support this. The sections
637 L</UPLOADING> and L</DOWNLOADING> document how to do this.
639 You might also consider mounting the disk image using our FUSE
640 filesystem support (L<guestmount(1)>).
642 =head1 CONNECTION MANAGEMENT
646 C<guestfs_h> is the opaque type representing a connection handle.
647 Create a handle by calling C<guestfs_create>. Call C<guestfs_close>
648 to free the handle and release all resources used.
650 For information on using multiple handles and threads, see the section
651 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
653 =head2 guestfs_create
655 guestfs_h *guestfs_create (void);
657 Create a connection handle.
659 You have to call C<guestfs_add_drive> on the handle at least once.
661 This function returns a non-NULL pointer to a handle on success or
664 After configuring the handle, you have to call C<guestfs_launch>.
666 You may also want to configure error handling for the handle. See
667 L</ERROR HANDLING> section below.
671 void guestfs_close (guestfs_h *g);
673 This closes the connection handle and frees up all resources used.
675 =head1 ERROR HANDLING
677 The convention in all functions that return C<int> is that they return
678 C<-1> to indicate an error. You can get additional information on
679 errors by calling C<guestfs_last_error> and/or by setting up an error
680 handler with C<guestfs_set_error_handler>.
682 The default error handler prints the information string to C<stderr>.
684 Out of memory errors are handled differently. The default action is
685 to call L<abort(3)>. If this is undesirable, then you can set a
686 handler using C<guestfs_set_out_of_memory_handler>.
688 =head2 guestfs_last_error
690 const char *guestfs_last_error (guestfs_h *g);
692 This returns the last error message that happened on C<g>. If
693 there has not been an error since the handle was created, then this
696 The lifetime of the returned string is until the next error occurs, or
697 C<guestfs_close> is called.
699 The error string is not localized (ie. is always in English), because
700 this makes searching for error messages in search engines give the
701 largest number of results.
703 =head2 guestfs_set_error_handler
705 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
708 void guestfs_set_error_handler (guestfs_h *g,
709 guestfs_error_handler_cb cb,
712 The callback C<cb> will be called if there is an error. The
713 parameters passed to the callback are an opaque data pointer and the
714 error message string.
716 Note that the message string C<msg> is freed as soon as the callback
717 function returns, so if you want to stash it somewhere you must make
720 The default handler prints messages on C<stderr>.
722 If you set C<cb> to C<NULL> then I<no> handler is called.
724 =head2 guestfs_get_error_handler
726 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g,
729 Returns the current error handler callback.
731 =head2 guestfs_set_out_of_memory_handler
733 typedef void (*guestfs_abort_cb) (void);
734 int guestfs_set_out_of_memory_handler (guestfs_h *g,
737 The callback C<cb> will be called if there is an out of memory
738 situation. I<Note this callback must not return>.
740 The default is to call L<abort(3)>.
742 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
745 =head2 guestfs_get_out_of_memory_handler
747 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *g);
749 This returns the current out of memory handler.
753 Libguestfs needs a kernel and initrd.img, which it finds by looking
754 along an internal path.
756 By default it looks for these in the directory C<$libdir/guestfs>
757 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
759 Use C<guestfs_set_path> or set the environment variable
760 C<LIBGUESTFS_PATH> to change the directories that libguestfs will
761 search in. The value is a colon-separated list of paths. The current
762 directory is I<not> searched unless the path contains an empty element
763 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
764 search the current directory and then C</usr/lib/guestfs>.
766 =head1 HIGH-LEVEL API ACTIONS
770 We guarantee the libguestfs ABI (binary interface), for public,
771 high-level actions as outlined in this section. Although we will
772 deprecate some actions, for example if they get replaced by newer
773 calls, we will keep the old actions forever. This allows you the
774 developer to program in confidence against libguestfs.
784 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
786 Using L</guestfs_available> you can test availability of
787 the following groups of functions. This test queries the
788 appliance to see if the appliance you are currently using
789 supports the functionality.
793 =head2 SINGLE CALLS AT COMPILE TIME
795 If you need to test whether a single libguestfs function is
796 available at compile time, we recommend using build tools
797 such as autoconf or cmake. For example in autotools you could
800 AC_CHECK_LIB([guestfs],[guestfs_create])
801 AC_CHECK_FUNCS([guestfs_dd])
803 which would result in C<HAVE_GUESTFS_DD> being either defined
804 or not defined in your program.
806 =head2 SINGLE CALLS AT RUN TIME
808 Testing at compile time doesn't guarantee that a function really
809 exists in the library. The reason is that you might be dynamically
810 linked against a previous I<libguestfs.so> (dynamic library)
811 which doesn't have the call. This situation unfortunately results
812 in a segmentation fault, which is a shortcoming of the C dynamic
813 linking system itself.
815 You can use L<dlopen(3)> to test if a function is available
816 at run time, as in this example program (note that you still
817 need the compile time check as well):
829 #ifdef HAVE_GUESTFS_DD
833 /* Test if the function guestfs_dd is really available. */
834 dl = dlopen (NULL, RTLD_LAZY);
836 fprintf (stderr, "dlopen: %s\n", dlerror ());
839 has_function = dlsym (dl, "guestfs_dd") != NULL;
843 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
845 printf ("this libguestfs.so has guestfs_dd function\n");
846 /* Now it's safe to call
847 guestfs_dd (g, "foo", "bar");
851 printf ("guestfs_dd function was not found at compile time\n");
855 You may think the above is an awful lot of hassle, and it is.
856 There are other ways outside of the C linking system to ensure
857 that this kind of incompatibility never arises, such as using
860 Requires: libguestfs >= 1.0.80
864 <!-- old anchor for the next section -->
865 <a name="state_machine_and_low_level_event_api"/>
871 Internally, libguestfs is implemented by running an appliance (a
872 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
873 a child process of the main program.
879 | | child process / appliance
880 | | __________________________
882 +-------------------+ RPC | +-----------------+ |
883 | libguestfs <--------------------> guestfsd | |
884 | | | +-----------------+ |
885 \___________________/ | | Linux kernel | |
886 | +--^--------------+ |
887 \_________|________________/
895 The library, linked to the main program, creates the child process and
896 hence the appliance in the L</guestfs_launch> function.
898 Inside the appliance is a Linux kernel and a complete stack of
899 userspace tools (such as LVM and ext2 programs) and a small
900 controlling daemon called C<guestfsd>. The library talks to
901 C<guestfsd> using remote procedure calls (RPC). There is a mostly
902 one-to-one correspondence between libguestfs API calls and RPC calls
903 to the daemon. Lastly the disk image(s) are attached to the qemu
904 process which translates device access by the appliance's Linux kernel
905 into accesses to the image.
907 A common misunderstanding is that the appliance "is" the virtual
908 machine. Although the disk image you are attached to might also be
909 used by some virtual machine, libguestfs doesn't know or care about
910 this. (But you will care if both libguestfs's qemu process and your
911 virtual machine are trying to update the disk image at the same time,
912 since these usually results in massive disk corruption).
916 libguestfs uses a state machine to model the child process:
927 / | \ \ guestfs_launch
938 \______/ <------ \________/
940 The normal transitions are (1) CONFIG (when the handle is created, but
941 there is no child process), (2) LAUNCHING (when the child process is
942 booting up), (3) alternating between READY and BUSY as commands are
943 issued to, and carried out by, the child process.
945 The guest may be killed by C<guestfs_kill_subprocess>, or may die
946 asynchronously at any time (eg. due to some internal error), and that
947 causes the state to transition back to CONFIG.
949 Configuration commands for qemu such as C<guestfs_add_drive> can only
950 be issued when in the CONFIG state.
952 The high-level API offers two calls that go from CONFIG through
953 LAUNCHING to READY. C<guestfs_launch> blocks until the child process
954 is READY to accept commands (or until some failure or timeout).
955 C<guestfs_launch> internally moves the state from CONFIG to LAUNCHING
958 High-level API actions such as C<guestfs_mount> can only be issued
959 when in the READY state. These high-level API calls block waiting for
960 the command to be carried out (ie. the state to transition to BUSY and
961 then back to READY). But using the low-level event API, you get
962 non-blocking versions. (But you can still only carry out one
963 operation per handle at a time - that is a limitation of the
964 communications protocol we use).
966 Finally, the child process sends asynchronous messages back to the
967 main program, such as kernel log messages. Mostly these are ignored
968 by the high-level API, but using the low-level event API you can
969 register to receive these messages.
971 =head2 SETTING CALLBACKS TO HANDLE EVENTS
973 The child process generates events in some situations. Current events
974 include: receiving a log message, the child process exits.
976 Use the C<guestfs_set_*_callback> functions to set a callback for
977 different types of events.
979 Only I<one callback of each type> can be registered for each handle.
980 Calling C<guestfs_set_*_callback> again overwrites the previous
981 callback of that type. Cancel all callbacks of this type by calling
982 this function with C<cb> set to C<NULL>.
984 =head2 guestfs_set_log_message_callback
986 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
988 void guestfs_set_log_message_callback (guestfs_h *g,
989 guestfs_log_message_cb cb,
992 The callback function C<cb> will be called whenever qemu or the guest
993 writes anything to the console.
995 Use this function to capture kernel messages and similar.
997 Normally there is no log message handler, and log messages are just
1000 =head2 guestfs_set_subprocess_quit_callback
1002 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
1003 void guestfs_set_subprocess_quit_callback (guestfs_h *g,
1004 guestfs_subprocess_quit_cb cb,
1007 The callback function C<cb> will be called when the child process
1008 quits, either asynchronously or if killed by
1009 C<guestfs_kill_subprocess>. (This corresponds to a transition from
1010 any state to the CONFIG state).
1012 =head2 guestfs_set_launch_done_callback
1014 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
1015 void guestfs_set_launch_done_callback (guestfs_h *g,
1016 guestfs_ready_cb cb,
1019 The callback function C<cb> will be called when the child process
1020 becomes ready first time after it has been launched. (This
1021 corresponds to a transition from LAUNCHING to the READY state).
1023 =head1 BLOCK DEVICE NAMING
1025 In the kernel there is now quite a profusion of schemata for naming
1026 block devices (in this context, by I<block device> I mean a physical
1027 or virtual hard drive). The original Linux IDE driver used names
1028 starting with C</dev/hd*>. SCSI devices have historically used a
1029 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
1030 driver became a popular replacement for the old IDE driver
1031 (particularly for SATA devices) those devices also used the
1032 C</dev/sd*> scheme. Additionally we now have virtual machines with
1033 paravirtualized drivers. This has created several different naming
1034 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
1037 As discussed above, libguestfs uses a qemu appliance running an
1038 embedded Linux kernel to access block devices. We can run a variety
1039 of appliances based on a variety of Linux kernels.
1041 This causes a problem for libguestfs because many API calls use device
1042 or partition names. Working scripts and the recipe (example) scripts
1043 that we make available over the internet could fail if the naming
1046 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
1047 scheme>. Internally C</dev/sd*> names are translated, if necessary,
1048 to other names as required. For example, under RHEL 5 which uses the
1049 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
1050 C</dev/hda2> transparently.
1052 Note that this I<only> applies to parameters. The
1053 C<guestfs_list_devices>, C<guestfs_list_partitions> and similar calls
1054 return the true names of the devices and partitions as known to the
1057 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
1059 Usually this translation is transparent. However in some (very rare)
1060 cases you may need to know the exact algorithm. Such cases include
1061 where you use C<guestfs_config> to add a mixture of virtio and IDE
1062 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
1063 and C</dev/vd*> devices.
1065 The algorithm is applied only to I<parameters> which are known to be
1066 either device or partition names. Return values from functions such
1067 as C<guestfs_list_devices> are never changed.
1073 Is the string a parameter which is a device or partition name?
1077 Does the string begin with C</dev/sd>?
1081 Does the named device exist? If so, we use that device.
1082 However if I<not> then we continue with this algorithm.
1086 Replace initial C</dev/sd> string with C</dev/hd>.
1088 For example, change C</dev/sda2> to C</dev/hda2>.
1090 If that named device exists, use it. If not, continue.
1094 Replace initial C</dev/sd> string with C</dev/vd>.
1096 If that named device exists, use it. If not, return an error.
1100 =head2 PORTABILITY CONCERNS
1102 Although the standard naming scheme and automatic translation is
1103 useful for simple programs and guestfish scripts, for larger programs
1104 it is best not to rely on this mechanism.
1106 Where possible for maximum future portability programs using
1107 libguestfs should use these future-proof techniques:
1113 Use C<guestfs_list_devices> or C<guestfs_list_partitions> to list
1114 actual device names, and then use those names directly.
1116 Since those device names exist by definition, they will never be
1121 Use higher level ways to identify filesystems, such as LVM names,
1122 UUIDs and filesystem labels.
1128 =head2 COMMUNICATION PROTOCOL
1130 Don't rely on using this protocol directly. This section documents
1131 how it currently works, but it may change at any time.
1133 The protocol used to talk between the library and the daemon running
1134 inside the qemu virtual machine is a simple RPC mechanism built on top
1135 of XDR (RFC 1014, RFC 1832, RFC 4506).
1137 The detailed format of structures is in C<src/guestfs_protocol.x>
1138 (note: this file is automatically generated).
1140 There are two broad cases, ordinary functions that don't have any
1141 C<FileIn> and C<FileOut> parameters, which are handled with very
1142 simple request/reply messages. Then there are functions that have any
1143 C<FileIn> or C<FileOut> parameters, which use the same request and
1144 reply messages, but they may also be followed by files sent using a
1147 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1149 For ordinary functions, the request message is:
1151 total length (header + arguments,
1152 but not including the length word itself)
1153 struct guestfs_message_header (encoded as XDR)
1154 struct guestfs_<foo>_args (encoded as XDR)
1156 The total length field allows the daemon to allocate a fixed size
1157 buffer into which it slurps the rest of the message. As a result, the
1158 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1159 4MB), which means the effective size of any request is limited to
1160 somewhere under this size.
1162 Note also that many functions don't take any arguments, in which case
1163 the C<guestfs_I<foo>_args> is completely omitted.
1165 The header contains the procedure number (C<guestfs_proc>) which is
1166 how the receiver knows what type of args structure to expect, or none
1169 The reply message for ordinary functions is:
1171 total length (header + ret,
1172 but not including the length word itself)
1173 struct guestfs_message_header (encoded as XDR)
1174 struct guestfs_<foo>_ret (encoded as XDR)
1176 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1177 for functions that return no formal return values.
1179 As above the total length of the reply is limited to
1180 C<GUESTFS_MESSAGE_MAX>.
1182 In the case of an error, a flag is set in the header, and the reply
1183 message is slightly changed:
1185 total length (header + error,
1186 but not including the length word itself)
1187 struct guestfs_message_header (encoded as XDR)
1188 struct guestfs_message_error (encoded as XDR)
1190 The C<guestfs_message_error> structure contains the error message as a
1193 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1195 A C<FileIn> parameter indicates that we transfer a file I<into> the
1196 guest. The normal request message is sent (see above). However this
1197 is followed by a sequence of file chunks.
1199 total length (header + arguments,
1200 but not including the length word itself,
1201 and not including the chunks)
1202 struct guestfs_message_header (encoded as XDR)
1203 struct guestfs_<foo>_args (encoded as XDR)
1204 sequence of chunks for FileIn param #0
1205 sequence of chunks for FileIn param #1 etc.
1207 The "sequence of chunks" is:
1209 length of chunk (not including length word itself)
1210 struct guestfs_chunk (encoded as XDR)
1212 struct guestfs_chunk (encoded as XDR)
1215 struct guestfs_chunk (with data.data_len == 0)
1217 The final chunk has the C<data_len> field set to zero. Additionally a
1218 flag is set in the final chunk to indicate either successful
1219 completion or early cancellation.
1221 At time of writing there are no functions that have more than one
1222 FileIn parameter. However this is (theoretically) supported, by
1223 sending the sequence of chunks for each FileIn parameter one after
1224 another (from left to right).
1226 Both the library (sender) I<and> the daemon (receiver) may cancel the
1227 transfer. The library does this by sending a chunk with a special
1228 flag set to indicate cancellation. When the daemon sees this, it
1229 cancels the whole RPC, does I<not> send any reply, and goes back to
1230 reading the next request.
1232 The daemon may also cancel. It does this by writing a special word
1233 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1234 during the transfer, and if it gets it, it will cancel the transfer
1235 (it sends a cancel chunk). The special word is chosen so that even if
1236 cancellation happens right at the end of the transfer (after the
1237 library has finished writing and has started listening for the reply),
1238 the "spurious" cancel flag will not be confused with the reply
1241 This protocol allows the transfer of arbitrary sized files (no 32 bit
1242 limit), and also files where the size is not known in advance
1243 (eg. from pipes or sockets). However the chunks are rather small
1244 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1245 daemon need to keep much in memory.
1247 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1249 The protocol for FileOut parameters is exactly the same as for FileIn
1250 parameters, but with the roles of daemon and library reversed.
1252 total length (header + ret,
1253 but not including the length word itself,
1254 and not including the chunks)
1255 struct guestfs_message_header (encoded as XDR)
1256 struct guestfs_<foo>_ret (encoded as XDR)
1257 sequence of chunks for FileOut param #0
1258 sequence of chunks for FileOut param #1 etc.
1260 =head3 INITIAL MESSAGE
1262 Because the underlying channel (QEmu -net channel) doesn't have any
1263 sort of connection control, when the daemon launches it sends an
1264 initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
1265 and daemon is alive. This is what C<guestfs_launch> waits for.
1267 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1269 All high-level libguestfs actions are synchronous. If you want
1270 to use libguestfs asynchronously then you must create a thread.
1272 Only use the handle from a single thread. Either use the handle
1273 exclusively from one thread, or provide your own mutex so that two
1274 threads cannot issue calls on the same handle at the same time.
1276 =head1 QEMU WRAPPERS
1278 If you want to compile your own qemu, run qemu from a non-standard
1279 location, or pass extra arguments to qemu, then you can write a
1280 shell-script wrapper around qemu.
1282 There is one important rule to remember: you I<must C<exec qemu>> as
1283 the last command in the shell script (so that qemu replaces the shell
1284 and becomes the direct child of the libguestfs-using program). If you
1285 don't do this, then the qemu process won't be cleaned up correctly.
1287 Here is an example of a wrapper, where I have built my own copy of
1291 qemudir=/home/rjones/d/qemu
1292 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1294 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1295 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1298 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1300 Note that libguestfs also calls qemu with the -help and -version
1301 options in order to determine features.
1303 =head1 ENVIRONMENT VARIABLES
1307 =item LIBGUESTFS_APPEND
1309 Pass additional options to the guest kernel.
1311 =item LIBGUESTFS_DEBUG
1313 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1314 has the same effect as calling C<guestfs_set_verbose (g, 1)>.
1316 =item LIBGUESTFS_MEMSIZE
1318 Set the memory allocated to the qemu process, in megabytes. For
1321 LIBGUESTFS_MEMSIZE=700
1323 =item LIBGUESTFS_PATH
1325 Set the path that libguestfs uses to search for kernel and initrd.img.
1326 See the discussion of paths in section PATH above.
1328 =item LIBGUESTFS_QEMU
1330 Set the default qemu binary that libguestfs uses. If not set, then
1331 the qemu which was found at compile time by the configure script is
1334 See also L</QEMU WRAPPERS> above.
1336 =item LIBGUESTFS_TRACE
1338 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1339 has the same effect as calling C<guestfs_set_trace (g, 1)>.
1343 Location of temporary directory, defaults to C</tmp>.
1345 If libguestfs was compiled to use the supermin appliance then each
1346 handle will require rather a large amount of space in this directory
1347 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1348 configure another directory to use in case C</tmp> is not large
1360 L<virt-inspector(1)>,
1361 L<virt-list-filesystems(1)>,
1362 L<virt-list-partitions(1)>,
1371 L<http://libguestfs.org/>.
1373 Tools with a similar purpose:
1382 To get a list of bugs against libguestfs use this link:
1384 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1386 To report a new bug against libguestfs use this link:
1388 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1390 When reporting a bug, please check:
1396 That the bug hasn't been reported already.
1400 That you are testing a recent version.
1404 Describe the bug accurately, and give a way to reproduce it.
1408 Run libguestfs-test-tool and paste the B<complete, unedited>
1409 output into the bug report.
1415 Richard W.M. Jones (C<rjones at redhat dot com>)
1419 Copyright (C) 2009 Red Hat Inc.
1420 L<http://libguestfs.org/>
1422 This library is free software; you can redistribute it and/or
1423 modify it under the terms of the GNU Lesser General Public
1424 License as published by the Free Software Foundation; either
1425 version 2 of the License, or (at your option) any later version.
1427 This library is distributed in the hope that it will be useful,
1428 but WITHOUT ANY WARRANTY; without even the implied warranty of
1429 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1430 Lesser General Public License for more details.
1432 You should have received a copy of the GNU Lesser General Public
1433 License along with this library; if not, write to the Free Software
1434 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA