5 guestfs - Library for accessing and modifying virtual machine images
11 guestfs_h *handle = guestfs_create ();
12 guestfs_add_drive (handle, "guest.img");
13 guestfs_launch (handle);
14 guestfs_mount (handle, "/dev/sda1", "/");
15 guestfs_touch (handle, "/hello");
16 guestfs_sync (handle);
17 guestfs_close (handle);
21 Libguestfs is a library for accessing and modifying guest disk images.
22 Amongst the things this is good for: making batch configuration
23 changes to guests, getting disk used/free statistics (see also:
24 virt-df), migrating between virtualization systems (see also:
25 virt-p2v), performing partial backups, performing partial guest
26 clones, cloning guests and changing registry/UUID/hostname info, and
29 Libguestfs uses Linux kernel and qemu code, and can access any type of
30 guest filesystem that Linux and qemu can, including but not limited
31 to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition
32 schemes, qcow, qcow2, vmdk.
34 Libguestfs provides ways to enumerate guest storage (eg. partitions,
35 LVs, what filesystem is in each LV, etc.). It can also run commands
36 in the context of the guest. Also you can access filesystems over FTP.
38 Libguestfs is a library that can be linked with C and C++ management
39 programs (or management programs written in OCaml, Perl, Python, Ruby,
40 Java, Haskell or C#). You can also use it from shell scripts or the
43 You don't need to be root to use libguestfs, although obviously you do
44 need enough permissions to access the disk images.
46 Libguestfs is a large API because it can do many things. For a gentle
47 introduction, please read the L</API OVERVIEW> section next.
51 This section provides a gentler overview of the libguestfs API. We
52 also try to group API calls together, where that may not be obvious
53 from reading about the individual calls below.
57 Before you can use libguestfs calls, you have to create a handle.
58 Then you must add at least one disk image to the handle, followed by
59 launching the handle, then performing whatever operations you want,
60 and finally closing the handle. So the general structure of all
61 libguestfs-using programs looks like this:
63 guestfs_h *handle = guestfs_create ();
65 /* Call guestfs_add_drive additional times if there are
66 * multiple disk images.
68 guestfs_add_drive (handle, "guest.img");
70 /* Most manipulation calls won't work until you've launched
71 * the handle. You have to do this _after_ adding drives
72 * and _before_ other commands.
74 guestfs_launch (handle);
76 /* Now you can examine what partitions, LVs etc are available.
78 char **partitions = guestfs_list_partitions (handle);
79 char **logvols = guestfs_lvs (handle);
81 /* To access a filesystem in the image, you must mount it.
83 guestfs_mount (handle, "/dev/sda1", "/");
85 /* Now you can perform filesystem actions on the guest
88 guestfs_touch (handle, "/hello");
90 /* You only need to call guestfs_sync if you have made
91 * changes to the guest image.
93 guestfs_sync (handle);
95 /* Close the handle. */
96 guestfs_close (handle);
98 The code above doesn't include any error checking. In real code you
99 should check return values carefully for errors. In general all
100 functions that return integers return C<-1> on error, and all
101 functions that return pointers return C<NULL> on error. See section
102 L</ERROR HANDLING> below for how to handle errors, and consult the
103 documentation for each function call below to see precisely how they
104 return error indications.
108 The image filename (C<"guest.img"> in the example above) could be a
109 disk image from a virtual machine, a L<dd(1)> copy of a physical hard
110 disk, an actual block device, or simply an empty file of zeroes that
111 you have created through L<posix_fallocate(3)>. Libguestfs lets you
112 do useful things to all of these.
114 You can add a disk read-only using C<guestfs_add_drive_ro>, in which
115 case libguestfs won't modify the file.
117 Be extremely cautious if the disk image is in use, eg. if it is being
118 used by a virtual machine. Adding it read-write will almost certainly
119 cause disk corruption, but adding it read-only is safe.
121 You must add at least one disk image, and you may add multiple disk
122 images. In the API, the disk images are usually referred to as
123 C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
126 Once C<guestfs_launch> has been called you cannot add any more images.
127 You can call C<guestfs_list_devices> to get a list of the device
128 names, in the order that you added them. See also L</BLOCK DEVICE
133 Before you can read or write files, create directories and so on in a
134 disk image that contains filesystems, you have to mount those
135 filesystems using C<guestfs_mount>. If you already know that a disk
136 image contains (for example) one partition with a filesystem on that
137 partition, then you can mount it directly:
139 guestfs_mount (handle, "/dev/sda1", "/");
141 where C</dev/sda1> means literally the first partition (C<1>) of the
142 first disk image that we added (C</dev/sda>). If the disk contains
143 Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).
145 If you are given a disk image and you don't know what it contains then
146 you have to find out. Libguestfs can do that too: use
147 C<guestfs_list_partitions> and C<guestfs_lvs> to list possible
148 partitions and LVs, and either try mounting each to see what is
149 mountable, or else examine them with C<guestfs_file>. But you might
150 find it easier to look at higher level programs built on top of
151 libguestfs, in particular L<virt-inspector(1)>.
153 To mount a disk image read-only, use C<guestfs_mount_ro>. There are
154 several other variations of the C<guestfs_mount_*> call.
156 =head2 FILESYSTEM ACCESS AND MODIFICATION
158 The majority of the libguestfs API consists of fairly low-level calls
159 for accessing and modifying the files, directories, symlinks etc on
160 mounted filesystems. There are over a hundred such calls which you
161 can find listed in detail below in this man page, and we don't even
162 pretend to cover them all in this overview.
164 Specify filenames as full paths including the mount point.
166 For example, if you mounted a filesystem at C<"/"> and you want to
167 read the file called C<"etc/passwd"> then you could do:
169 char *data = guestfs_cat (handle, "/etc/passwd");
171 This would return C<data> as a newly allocated buffer containing the
172 full content of that file (with some conditions: see also
173 L</DOWNLOADING> below), or C<NULL> if there was an error.
175 As another example, to create a top-level directory on that filesystem
176 called C<"var"> you would do:
178 guestfs_mkdir (handle, "/var");
180 To create a symlink you could do:
182 guestfs_ln_s (handle, "/etc/init.d/portmap",
183 "/etc/rc3.d/S30portmap");
185 Libguestfs will reject attempts to use relative paths. There is no
186 concept of a current working directory. Libguestfs can return errors
187 in many situations: for example if the filesystem isn't writable, or
188 if a file or directory that you requested doesn't exist. If you are
189 using the C API (documented here) you have to check for those error
190 conditions after each call. (Other language bindings turn these
191 errors into exceptions).
193 File writes are affected by the per-handle umask, set by calling
194 C<guestfs_umask> and defaulting to 022.
198 Libguestfs contains API calls to read, create and modify partition
199 tables on disk images.
201 In the common case where you want to create a single partition
202 covering the whole disk, you should use the C<guestfs_part_disk>
205 const char *parttype = "mbr";
206 if (disk_is_larger_than_2TB)
208 guestfs_part_disk (g, "/dev/sda", parttype);
210 Obviously this effectively wipes anything that was on that disk image
213 In general MBR partitions are both unnecessarily complicated and
214 depend on archaic details, namely the Cylinder-Head-Sector (CHS)
215 geometry of the disk. C<guestfs_sfdiskM> can be used to
216 create more complex arrangements where the relative sizes are
217 expressed in megabytes instead of cylinders, which is a small win.
218 C<guestfs_sfdiskM> will choose the nearest cylinder to approximate the
219 requested size. There's a lot of crazy stuff to do with IDE and
220 virtio disks having different, incompatible CHS geometries, that you
221 probably don't want to know about.
223 My advice: make a single partition to cover the whole disk, then use
228 Libguestfs provides access to a large part of the LVM2 API, such as
229 C<guestfs_lvcreate> and C<guestfs_vgremove>. It won't make much sense
230 unless you familiarize yourself with the concepts of physical volumes,
231 volume groups and logical volumes.
233 This author strongly recommends reading the LVM HOWTO, online at
234 L<http://tldp.org/HOWTO/LVM-HOWTO/>.
238 Use C<guestfs_cat> to download small, text only files. This call
239 is limited to files which are less than 2 MB and which cannot contain
240 any ASCII NUL (C<\0>) characters. However it has a very simple
243 C<guestfs_read_file> can be used to read files which contain
244 arbitrary 8 bit data, since it returns a (pointer, size) pair.
245 However it is still limited to "small" files, less than 2 MB.
247 C<guestfs_download> can be used to download any file, with no
248 limits on content or size (even files larger than 4 GB).
250 To download multiple files, see C<guestfs_tar_out> and
255 It's often the case that you want to write a file or files to the disk
258 For small, single files, use C<guestfs_write_file>. This call
259 currently contains a bug which limits the call to plain text files
260 (not containing ASCII NUL characters).
262 To upload a single file, use C<guestfs_upload>. This call has no
263 limits on file content or size (even files larger than 4 GB).
265 To upload multiple files, see C<guestfs_tar_in> and C<guestfs_tgz_in>.
267 However the fastest way to upload I<large numbers of arbitrary files>
268 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
269 L<mkisofs(8)>), then attach this using C<guestfs_add_drive_ro>. If
270 you add the drive in a predictable way (eg. adding it last after all
271 other drives) then you can get the device name from
272 C<guestfs_list_devices> and mount it directly using
273 C<guestfs_mount_ro>. Note that squashfs images are sometimes
274 non-portable between kernel versions, and they don't support labels or
275 UUIDs. If you want to pre-build an image or you need to mount it
276 using a label or UUID, use an ISO image instead.
280 There are various different commands for copying between files and
281 devices and in and out of the guest filesystem. These are summarised
286 =item B<file> to B<file>
288 Use L</guestfs_cp> to copy a single file, or
289 L</guestfs_cp_a> to copy directories recursively.
291 =item B<file or device> to B<file or device>
293 Use L</guestfs_dd> which efficiently uses L<dd(1)>
294 to copy between files and devices in the guest.
296 Example: duplicate the contents of an LV:
298 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
300 The destination (C</dev/VG/Copy>) must be at least as large as the
301 source (C</dev/VG/Original>).
303 =item B<file on the host> to B<file or device>
305 Use L</guestfs_upload>. See L</UPLOADING> above.
307 =item B<file or device> to B<file on the host>
309 Use L</guestfs_download>. See L</DOWNLOADING> above.
315 C<guestfs_ll> is just designed for humans to read (mainly when using
316 the L<guestfish(1)>-equivalent command C<ll>).
318 C<guestfs_ls> is a quick way to get a list of files in a directory
319 from programs, as a flat list of strings.
321 C<guestfs_readdir> is a programmatic way to get a list of files in a
322 directory, plus additional information about each one. It is more
323 equivalent to using the L<readdir(3)> call on a local filesystem.
325 C<guestfs_find> can be used to recursively list files.
327 =head2 RUNNING COMMANDS
329 Although libguestfs is a primarily an API for manipulating files
330 inside guest images, we also provide some limited facilities for
331 running commands inside guests.
333 There are many limitations to this:
339 The kernel version that the command runs under will be different
340 from what it expects.
344 If the command needs to communicate with daemons, then most likely
345 they won't be running.
349 The command will be running in limited memory.
353 Only supports Linux guests (not Windows, BSD, etc).
357 Architecture limitations (eg. won't work for a PPC guest on
362 For SELinux guests, you may need to enable SELinux and load policy
363 first. See L</SELINUX> in this manpage.
367 The two main API calls to run commands are C<guestfs_command> and
368 C<guestfs_sh> (there are also variations).
370 The difference is that C<guestfs_sh> runs commands using the shell, so
371 any shell globs, redirections, etc will work.
373 =head2 CONFIGURATION FILES
375 To read and write configuration files in Linux guest filesystems, we
376 strongly recommend using Augeas. For example, Augeas understands how
377 to read and write, say, a Linux shadow password file or X.org
378 configuration file, and so avoids you having to write that code.
380 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
381 don't document Augeas itself here because there is excellent
382 documentation on the L<http://augeas.net/> website.
384 If you don't want to use Augeas (you fool!) then try calling
385 C<guestfs_read_lines> to get the file as a list of lines which
386 you can iterate over.
390 We support SELinux guests. To ensure that labeling happens correctly
391 in SELinux guests, you need to enable SELinux and load the guest's
398 Before launching, do:
400 guestfs_set_selinux (g, 1);
404 After mounting the guest's filesystem(s), load the policy. This
405 is best done by running the L<load_policy(8)> command in the
408 guestfs_sh (g, "/usr/sbin/load_policy");
410 (Older versions of C<load_policy> require you to specify the
411 name of the policy file).
415 Optionally, set the security context for the API. The correct
416 security context to use can only be known by inspecting the
417 guest. As an example:
419 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
423 This will work for running commands and editing existing files.
425 When new files are created, you may need to label them explicitly,
426 for example by running the external command
427 C<restorecon pathname>.
429 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
431 Libguestfs can mount NTFS partitions. It does this using the
432 L<http://www.ntfs-3g.org/> driver.
434 DOS and Windows still use drive letters, and the filesystems are
435 always treated as case insensitive by Windows itself, and therefore
436 you might find a Windows configuration file referring to a path like
437 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
438 that directory might be referred to as C</WINDOWS/System32>.
440 Drive letter mappings are outside the scope of libguestfs. You have
441 to use libguestfs to read the appropriate Windows Registry and
442 configuration files, to determine yourself how drives are mapped (see
443 also L<virt-inspector(1)>).
445 Replacing backslash characters with forward slash characters is also
446 outside the scope of libguestfs, but something that you can easily do.
448 Where we can help is in resolving the case insensitivity of paths.
449 For this, call C<guestfs_case_sensitive_path>.
451 Libguestfs also provides some help for decoding Windows Registry
452 "hive" files, through the library C<libhivex> which is part of
453 libguestfs. You have to locate and download the hive file(s)
454 yourself, and then pass them to C<libhivex> functions. See also the
455 programs L<hivexml(1)>, L<hivexget(1)> and L<virt-win-reg(1)> for more
458 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
460 Although we don't want to discourage you from using the C API, we will
461 mention here that the same API is also available in other languages.
463 The API is broadly identical in all supported languages. This means
464 that the C call C<guestfs_mount(handle,path)> is
465 C<$handle-E<gt>mount($path)> in Perl, C<handle.mount(path)> in Python,
466 and C<Guestfs.mount handle path> in OCaml. In other words, a
467 straightforward, predictable isomorphism between each language.
469 Error messages are automatically transformed
470 into exceptions if the language supports it.
472 We don't try to "object orientify" parts of the API in OO languages,
473 although contributors are welcome to write higher level APIs above
474 what we provide in their favourite languages if they wish.
480 You can use the I<guestfs.h> header file from C++ programs. The C++
481 API is identical to the C API. C++ classes and exceptions are
486 The C# bindings are highly experimental. Please read the warnings
487 at the top of C<csharp/Libguestfs.cs>.
491 This is the only language binding that working but incomplete. Only
492 calls which return simple integers have been bound in Haskell, and we
493 are looking for help to complete this binding.
497 Full documentation is contained in the Javadoc which is distributed
502 For documentation see the file C<guestfs.mli>.
506 For documentation see L<Sys::Guestfs(3)>.
510 For documentation do:
518 Use the Guestfs module. There is no Ruby-specific documentation, but
519 you can find examples written in Ruby in the libguestfs source.
521 =item B<shell scripts>
523 For documentation see L<guestfish(1)>.
527 =head2 LIBGUESTFS GOTCHAS
529 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
530 system [...] that works in the way it is documented but is
531 counterintuitive and almost invites mistakes."
533 Since we developed libguestfs and the associated tools, there are
534 several things we would have designed differently, but are now stuck
535 with for backwards compatibility or other reasons. If there is ever a
536 libguestfs 2.0 release, you can expect these to change. Beware of
541 =item Autosync / forgetting to sync.
543 When modifying a filesystem from C or another language, you B<must>
544 unmount all filesystems and call L</guestfs_sync> explicitly before
545 you close the libguestfs handle. You can also call:
547 guestfs_set_autosync (handle, 1);
549 to have the unmount/sync done automatically for you when the handle is
550 closed. (This feature is called "autosync", L</guestfs_set_autosync>
553 If you forget to do this, then it is entirely possible that your
554 changes won't be written out, or will be partially written, or (very
555 rarely) that you'll get disk corruption.
557 Note that in L<guestfish(3)> I<autosync is the default>. So quick and
558 dirty guestfish scripts that forget to sync will work just fine, which
559 can make this extra-puzzling if you are trying to debug a problem.
561 =item Read-only should be the default.
563 In L<guestfish(3)>, I<--ro> should be the default, and you should
564 have to specify I<--rw> if you want to make changes to the image.
566 This would reduce the potential to corrupt live VM images.
568 Note that many filesystems change the disk when you just mount and
569 unmount, even if you didn't perform any writes. You need to use
570 C<guestfs_add_drive_ro> to guarantee that the disk is not changed.
572 =item guestfish command line is hard to use.
574 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
575 for examination). It tries to run a guestfish command C<disk.img>
576 which doesn't exist, so it fails, and it fails with a strange and
577 unintuitive error message. Like the Bourne shell, we should have used
578 C<guestfish -c command> to run commands.
582 =head1 CONNECTION MANAGEMENT
586 C<guestfs_h> is the opaque type representing a connection handle.
587 Create a handle by calling C<guestfs_create>. Call C<guestfs_close>
588 to free the handle and release all resources used.
590 For information on using multiple handles and threads, see the section
591 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
593 =head2 guestfs_create
595 guestfs_h *guestfs_create (void);
597 Create a connection handle.
599 You have to call C<guestfs_add_drive> on the handle at least once.
601 This function returns a non-NULL pointer to a handle on success or
604 After configuring the handle, you have to call C<guestfs_launch>.
606 You may also want to configure error handling for the handle. See
607 L</ERROR HANDLING> section below.
611 void guestfs_close (guestfs_h *handle);
613 This closes the connection handle and frees up all resources used.
615 =head1 ERROR HANDLING
617 The convention in all functions that return C<int> is that they return
618 C<-1> to indicate an error. You can get additional information on
619 errors by calling C<guestfs_last_error> and/or by setting up an error
620 handler with C<guestfs_set_error_handler>.
622 The default error handler prints the information string to C<stderr>.
624 Out of memory errors are handled differently. The default action is
625 to call L<abort(3)>. If this is undesirable, then you can set a
626 handler using C<guestfs_set_out_of_memory_handler>.
628 =head2 guestfs_last_error
630 const char *guestfs_last_error (guestfs_h *handle);
632 This returns the last error message that happened on C<handle>. If
633 there has not been an error since the handle was created, then this
636 The lifetime of the returned string is until the next error occurs, or
637 C<guestfs_close> is called.
639 The error string is not localized (ie. is always in English), because
640 this makes searching for error messages in search engines give the
641 largest number of results.
643 =head2 guestfs_set_error_handler
645 typedef void (*guestfs_error_handler_cb) (guestfs_h *handle,
648 void guestfs_set_error_handler (guestfs_h *handle,
649 guestfs_error_handler_cb cb,
652 The callback C<cb> will be called if there is an error. The
653 parameters passed to the callback are an opaque data pointer and the
654 error message string.
656 Note that the message string C<msg> is freed as soon as the callback
657 function returns, so if you want to stash it somewhere you must make
660 The default handler prints messages on C<stderr>.
662 If you set C<cb> to C<NULL> then I<no> handler is called.
664 =head2 guestfs_get_error_handler
666 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *handle,
669 Returns the current error handler callback.
671 =head2 guestfs_set_out_of_memory_handler
673 typedef void (*guestfs_abort_cb) (void);
674 int guestfs_set_out_of_memory_handler (guestfs_h *handle,
677 The callback C<cb> will be called if there is an out of memory
678 situation. I<Note this callback must not return>.
680 The default is to call L<abort(3)>.
682 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
685 =head2 guestfs_get_out_of_memory_handler
687 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *handle);
689 This returns the current out of memory handler.
693 Libguestfs needs a kernel and initrd.img, which it finds by looking
694 along an internal path.
696 By default it looks for these in the directory C<$libdir/guestfs>
697 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
699 Use C<guestfs_set_path> or set the environment variable
700 C<LIBGUESTFS_PATH> to change the directories that libguestfs will
701 search in. The value is a colon-separated list of paths. The current
702 directory is I<not> searched unless the path contains an empty element
703 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
704 search the current directory and then C</usr/lib/guestfs>.
706 =head1 HIGH-LEVEL API ACTIONS
710 We guarantee the libguestfs ABI (binary interface), for public,
711 high-level actions as outlined in this section. Although we will
712 deprecate some actions, for example if they get replaced by newer
713 calls, we will keep the old actions forever. This allows you the
714 developer to program in confidence against libguestfs.
724 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
726 Using L</guestfs_available> you can test availability of
727 the following groups of functions. This test queries the
728 appliance to see if the appliance you are currently using
729 supports the functionality.
733 =head2 SINGLE CALLS AT COMPILE TIME
735 If you need to test whether a single libguestfs function is
736 available at compile time, we recommend using build tools
737 such as autoconf or cmake. For example in autotools you could
740 AC_CHECK_LIB([guestfs],[guestfs_create])
741 AC_CHECK_FUNCS([guestfs_dd])
743 which would result in C<HAVE_GUESTFS_DD> being either defined
744 or not defined in your program.
746 =head2 SINGLE CALLS AT RUN TIME
748 Testing at compile time doesn't guarantee that a function really
749 exists in the library. The reason is that you might be dynamically
750 linked against a previous I<libguestfs.so> (dynamic library)
751 which doesn't have the call. This situation unfortunately results
752 in a segmentation fault, which is a shortcoming of the C dynamic
753 linking system itself.
755 You can use L<dlopen(3)> to test if a function is available
756 at run time, as in this example program (note that you still
757 need the compile time check as well):
769 #ifdef HAVE_GUESTFS_DD
773 /* Test if the function guestfs_dd is really available. */
774 dl = dlopen (NULL, RTLD_LAZY);
776 fprintf (stderr, "dlopen: %s\n", dlerror ());
779 has_function = dlsym (dl, "guestfs_dd") != NULL;
783 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
785 printf ("this libguestfs.so has guestfs_dd function\n");
786 /* Now it's safe to call
787 guestfs_dd (g, "foo", "bar");
791 printf ("guestfs_dd function was not found at compile time\n");
795 You may think the above is an awful lot of hassle, and it is.
796 There are other ways outside of the C linking system to ensure
797 that this kind of incompatibility never arises, such as using
800 Requires: libguestfs >= 1.0.80
804 <!-- old anchor for the next section -->
805 <a name="state_machine_and_low_level_event_api"/>
811 Internally, libguestfs is implemented by running an appliance (a
812 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
813 a child process of the main program.
819 | | child process / appliance
820 | | __________________________
822 +-------------------+ RPC | +-----------------+ |
823 | libguestfs <--------------------> guestfsd | |
824 | | | +-----------------+ |
825 \___________________/ | | Linux kernel | |
826 | +--^--------------+ |
827 \_________|________________/
835 The library, linked to the main program, creates the child process and
836 hence the appliance in the L</guestfs_launch> function.
838 Inside the appliance is a Linux kernel and a complete stack of
839 userspace tools (such as LVM and ext2 programs) and a small
840 controlling daemon called C<guestfsd>. The library talks to
841 C<guestfsd> using remote procedure calls (RPC). There is a mostly
842 one-to-one correspondence between libguestfs API calls and RPC calls
843 to the daemon. Lastly the disk image(s) are attached to the qemu
844 process which translates device access by the appliance's Linux kernel
845 into accesses to the image.
847 A common misunderstanding is that the appliance "is" the virtual
848 machine. Although the disk image you are attached to might also be
849 used by some virtual machine, libguestfs doesn't know or care about
850 this. (But you will care if both libguestfs's qemu process and your
851 virtual machine are trying to update the disk image at the same time,
852 since these usually results in massive disk corruption).
856 libguestfs uses a state machine to model the child process:
867 / | \ \ guestfs_launch
878 \______/ <------ \________/
880 The normal transitions are (1) CONFIG (when the handle is created, but
881 there is no child process), (2) LAUNCHING (when the child process is
882 booting up), (3) alternating between READY and BUSY as commands are
883 issued to, and carried out by, the child process.
885 The guest may be killed by C<guestfs_kill_subprocess>, or may die
886 asynchronously at any time (eg. due to some internal error), and that
887 causes the state to transition back to CONFIG.
889 Configuration commands for qemu such as C<guestfs_add_drive> can only
890 be issued when in the CONFIG state.
892 The high-level API offers two calls that go from CONFIG through
893 LAUNCHING to READY. C<guestfs_launch> blocks until the child process
894 is READY to accept commands (or until some failure or timeout).
895 C<guestfs_launch> internally moves the state from CONFIG to LAUNCHING
898 High-level API actions such as C<guestfs_mount> can only be issued
899 when in the READY state. These high-level API calls block waiting for
900 the command to be carried out (ie. the state to transition to BUSY and
901 then back to READY). But using the low-level event API, you get
902 non-blocking versions. (But you can still only carry out one
903 operation per handle at a time - that is a limitation of the
904 communications protocol we use).
906 Finally, the child process sends asynchronous messages back to the
907 main program, such as kernel log messages. Mostly these are ignored
908 by the high-level API, but using the low-level event API you can
909 register to receive these messages.
911 =head2 SETTING CALLBACKS TO HANDLE EVENTS
913 The child process generates events in some situations. Current events
914 include: receiving a log message, the child process exits.
916 Use the C<guestfs_set_*_callback> functions to set a callback for
917 different types of events.
919 Only I<one callback of each type> can be registered for each handle.
920 Calling C<guestfs_set_*_callback> again overwrites the previous
921 callback of that type. Cancel all callbacks of this type by calling
922 this function with C<cb> set to C<NULL>.
924 =head2 guestfs_set_log_message_callback
926 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
928 void guestfs_set_log_message_callback (guestfs_h *handle,
929 guestfs_log_message_cb cb,
932 The callback function C<cb> will be called whenever qemu or the guest
933 writes anything to the console.
935 Use this function to capture kernel messages and similar.
937 Normally there is no log message handler, and log messages are just
940 =head2 guestfs_set_subprocess_quit_callback
942 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
943 void guestfs_set_subprocess_quit_callback (guestfs_h *handle,
944 guestfs_subprocess_quit_cb cb,
947 The callback function C<cb> will be called when the child process
948 quits, either asynchronously or if killed by
949 C<guestfs_kill_subprocess>. (This corresponds to a transition from
950 any state to the CONFIG state).
952 =head2 guestfs_set_launch_done_callback
954 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
955 void guestfs_set_launch_done_callback (guestfs_h *handle,
959 The callback function C<cb> will be called when the child process
960 becomes ready first time after it has been launched. (This
961 corresponds to a transition from LAUNCHING to the READY state).
963 =head1 BLOCK DEVICE NAMING
965 In the kernel there is now quite a profusion of schemata for naming
966 block devices (in this context, by I<block device> I mean a physical
967 or virtual hard drive). The original Linux IDE driver used names
968 starting with C</dev/hd*>. SCSI devices have historically used a
969 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
970 driver became a popular replacement for the old IDE driver
971 (particularly for SATA devices) those devices also used the
972 C</dev/sd*> scheme. Additionally we now have virtual machines with
973 paravirtualized drivers. This has created several different naming
974 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
977 As discussed above, libguestfs uses a qemu appliance running an
978 embedded Linux kernel to access block devices. We can run a variety
979 of appliances based on a variety of Linux kernels.
981 This causes a problem for libguestfs because many API calls use device
982 or partition names. Working scripts and the recipe (example) scripts
983 that we make available over the internet could fail if the naming
986 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
987 scheme>. Internally C</dev/sd*> names are translated, if necessary,
988 to other names as required. For example, under RHEL 5 which uses the
989 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
990 C</dev/hda2> transparently.
992 Note that this I<only> applies to parameters. The
993 C<guestfs_list_devices>, C<guestfs_list_partitions> and similar calls
994 return the true names of the devices and partitions as known to the
997 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
999 Usually this translation is transparent. However in some (very rare)
1000 cases you may need to know the exact algorithm. Such cases include
1001 where you use C<guestfs_config> to add a mixture of virtio and IDE
1002 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
1003 and C</dev/vd*> devices.
1005 The algorithm is applied only to I<parameters> which are known to be
1006 either device or partition names. Return values from functions such
1007 as C<guestfs_list_devices> are never changed.
1013 Is the string a parameter which is a device or partition name?
1017 Does the string begin with C</dev/sd>?
1021 Does the named device exist? If so, we use that device.
1022 However if I<not> then we continue with this algorithm.
1026 Replace initial C</dev/sd> string with C</dev/hd>.
1028 For example, change C</dev/sda2> to C</dev/hda2>.
1030 If that named device exists, use it. If not, continue.
1034 Replace initial C</dev/sd> string with C</dev/vd>.
1036 If that named device exists, use it. If not, return an error.
1040 =head2 PORTABILITY CONCERNS
1042 Although the standard naming scheme and automatic translation is
1043 useful for simple programs and guestfish scripts, for larger programs
1044 it is best not to rely on this mechanism.
1046 Where possible for maximum future portability programs using
1047 libguestfs should use these future-proof techniques:
1053 Use C<guestfs_list_devices> or C<guestfs_list_partitions> to list
1054 actual device names, and then use those names directly.
1056 Since those device names exist by definition, they will never be
1061 Use higher level ways to identify filesystems, such as LVM names,
1062 UUIDs and filesystem labels.
1068 =head2 COMMUNICATION PROTOCOL
1070 Don't rely on using this protocol directly. This section documents
1071 how it currently works, but it may change at any time.
1073 The protocol used to talk between the library and the daemon running
1074 inside the qemu virtual machine is a simple RPC mechanism built on top
1075 of XDR (RFC 1014, RFC 1832, RFC 4506).
1077 The detailed format of structures is in C<src/guestfs_protocol.x>
1078 (note: this file is automatically generated).
1080 There are two broad cases, ordinary functions that don't have any
1081 C<FileIn> and C<FileOut> parameters, which are handled with very
1082 simple request/reply messages. Then there are functions that have any
1083 C<FileIn> or C<FileOut> parameters, which use the same request and
1084 reply messages, but they may also be followed by files sent using a
1087 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1089 For ordinary functions, the request message is:
1091 total length (header + arguments,
1092 but not including the length word itself)
1093 struct guestfs_message_header (encoded as XDR)
1094 struct guestfs_<foo>_args (encoded as XDR)
1096 The total length field allows the daemon to allocate a fixed size
1097 buffer into which it slurps the rest of the message. As a result, the
1098 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1099 4MB), which means the effective size of any request is limited to
1100 somewhere under this size.
1102 Note also that many functions don't take any arguments, in which case
1103 the C<guestfs_I<foo>_args> is completely omitted.
1105 The header contains the procedure number (C<guestfs_proc>) which is
1106 how the receiver knows what type of args structure to expect, or none
1109 The reply message for ordinary functions is:
1111 total length (header + ret,
1112 but not including the length word itself)
1113 struct guestfs_message_header (encoded as XDR)
1114 struct guestfs_<foo>_ret (encoded as XDR)
1116 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1117 for functions that return no formal return values.
1119 As above the total length of the reply is limited to
1120 C<GUESTFS_MESSAGE_MAX>.
1122 In the case of an error, a flag is set in the header, and the reply
1123 message is slightly changed:
1125 total length (header + error,
1126 but not including the length word itself)
1127 struct guestfs_message_header (encoded as XDR)
1128 struct guestfs_message_error (encoded as XDR)
1130 The C<guestfs_message_error> structure contains the error message as a
1133 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1135 A C<FileIn> parameter indicates that we transfer a file I<into> the
1136 guest. The normal request message is sent (see above). However this
1137 is followed by a sequence of file chunks.
1139 total length (header + arguments,
1140 but not including the length word itself,
1141 and not including the chunks)
1142 struct guestfs_message_header (encoded as XDR)
1143 struct guestfs_<foo>_args (encoded as XDR)
1144 sequence of chunks for FileIn param #0
1145 sequence of chunks for FileIn param #1 etc.
1147 The "sequence of chunks" is:
1149 length of chunk (not including length word itself)
1150 struct guestfs_chunk (encoded as XDR)
1152 struct guestfs_chunk (encoded as XDR)
1155 struct guestfs_chunk (with data.data_len == 0)
1157 The final chunk has the C<data_len> field set to zero. Additionally a
1158 flag is set in the final chunk to indicate either successful
1159 completion or early cancellation.
1161 At time of writing there are no functions that have more than one
1162 FileIn parameter. However this is (theoretically) supported, by
1163 sending the sequence of chunks for each FileIn parameter one after
1164 another (from left to right).
1166 Both the library (sender) I<and> the daemon (receiver) may cancel the
1167 transfer. The library does this by sending a chunk with a special
1168 flag set to indicate cancellation. When the daemon sees this, it
1169 cancels the whole RPC, does I<not> send any reply, and goes back to
1170 reading the next request.
1172 The daemon may also cancel. It does this by writing a special word
1173 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1174 during the transfer, and if it gets it, it will cancel the transfer
1175 (it sends a cancel chunk). The special word is chosen so that even if
1176 cancellation happens right at the end of the transfer (after the
1177 library has finished writing and has started listening for the reply),
1178 the "spurious" cancel flag will not be confused with the reply
1181 This protocol allows the transfer of arbitrary sized files (no 32 bit
1182 limit), and also files where the size is not known in advance
1183 (eg. from pipes or sockets). However the chunks are rather small
1184 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1185 daemon need to keep much in memory.
1187 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1189 The protocol for FileOut parameters is exactly the same as for FileIn
1190 parameters, but with the roles of daemon and library reversed.
1192 total length (header + ret,
1193 but not including the length word itself,
1194 and not including the chunks)
1195 struct guestfs_message_header (encoded as XDR)
1196 struct guestfs_<foo>_ret (encoded as XDR)
1197 sequence of chunks for FileOut param #0
1198 sequence of chunks for FileOut param #1 etc.
1200 =head3 INITIAL MESSAGE
1202 Because the underlying channel (QEmu -net channel) doesn't have any
1203 sort of connection control, when the daemon launches it sends an
1204 initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
1205 and daemon is alive. This is what C<guestfs_launch> waits for.
1207 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1209 All high-level libguestfs actions are synchronous. If you want
1210 to use libguestfs asynchronously then you must create a thread.
1212 Only use the handle from a single thread. Either use the handle
1213 exclusively from one thread, or provide your own mutex so that two
1214 threads cannot issue calls on the same handle at the same time.
1216 =head1 QEMU WRAPPERS
1218 If you want to compile your own qemu, run qemu from a non-standard
1219 location, or pass extra arguments to qemu, then you can write a
1220 shell-script wrapper around qemu.
1222 There is one important rule to remember: you I<must C<exec qemu>> as
1223 the last command in the shell script (so that qemu replaces the shell
1224 and becomes the direct child of the libguestfs-using program). If you
1225 don't do this, then the qemu process won't be cleaned up correctly.
1227 Here is an example of a wrapper, where I have built my own copy of
1231 qemudir=/home/rjones/d/qemu
1232 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1234 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1235 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1238 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1240 Note that libguestfs also calls qemu with the -help and -version
1241 options in order to determine features.
1243 =head1 ENVIRONMENT VARIABLES
1247 =item LIBGUESTFS_APPEND
1249 Pass additional options to the guest kernel.
1251 =item LIBGUESTFS_DEBUG
1253 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1254 has the same effect as calling C<guestfs_set_verbose (handle, 1)>.
1256 =item LIBGUESTFS_MEMSIZE
1258 Set the memory allocated to the qemu process, in megabytes. For
1261 LIBGUESTFS_MEMSIZE=700
1263 =item LIBGUESTFS_PATH
1265 Set the path that libguestfs uses to search for kernel and initrd.img.
1266 See the discussion of paths in section PATH above.
1268 =item LIBGUESTFS_QEMU
1270 Set the default qemu binary that libguestfs uses. If not set, then
1271 the qemu which was found at compile time by the configure script is
1274 See also L</QEMU WRAPPERS> above.
1276 =item LIBGUESTFS_TRACE
1278 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1279 has the same effect as calling C<guestfs_set_trace (handle, 1)>.
1283 Location of temporary directory, defaults to C</tmp>.
1285 If libguestfs was compiled to use the supermin appliance then each
1286 handle will require rather a large amount of space in this directory
1287 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1288 configure another directory to use in case C</tmp> is not large
1298 L<http://libguestfs.org/>.
1300 Tools with a similar purpose:
1309 To get a list of bugs against libguestfs use this link:
1311 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1313 To report a new bug against libguestfs use this link:
1315 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1317 When reporting a bug, please check:
1323 That the bug hasn't been reported already.
1327 That you are testing a recent version.
1331 Describe the bug accurately, and give a way to reproduce it.
1335 Run libguestfs-test-tool and paste the B<complete, unedited>
1336 output into the bug report.
1342 Richard W.M. Jones (C<rjones at redhat dot com>)
1346 Copyright (C) 2009 Red Hat Inc.
1347 L<http://libguestfs.org/>
1349 This library is free software; you can redistribute it and/or
1350 modify it under the terms of the GNU Lesser General Public
1351 License as published by the Free Software Foundation; either
1352 version 2 of the License, or (at your option) any later version.
1354 This library is distributed in the hope that it will be useful,
1355 but WITHOUT ANY WARRANTY; without even the implied warranty of
1356 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1357 Lesser General Public License for more details.
1359 You should have received a copy of the GNU Lesser General Public
1360 License along with this library; if not, write to the Free Software
1361 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA