5 guestfs - Library for accessing and modifying virtual machine images
11 guestfs_h *g = guestfs_create ();
12 guestfs_add_drive (g, "guest.img");
14 guestfs_mount (g, "/dev/sda1", "/");
15 guestfs_touch (g, "/hello");
16 guestfs_umount (g, "/");
20 cc prog.c -o prog -lguestfs
22 cc prog.c -o prog `pkg-config libguestfs --cflags --libs`
26 Libguestfs is a library for accessing and modifying guest disk images.
27 Amongst the things this is good for: making batch configuration
28 changes to guests, getting disk used/free statistics (see also:
29 virt-df), migrating between virtualization systems (see also:
30 virt-p2v), performing partial backups, performing partial guest
31 clones, cloning guests and changing registry/UUID/hostname info, and
34 Libguestfs uses Linux kernel and qemu code, and can access any type of
35 guest filesystem that Linux and qemu can, including but not limited
36 to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition
37 schemes, qcow, qcow2, vmdk.
39 Libguestfs provides ways to enumerate guest storage (eg. partitions,
40 LVs, what filesystem is in each LV, etc.). It can also run commands
41 in the context of the guest. Also you can access filesystems over
44 Libguestfs is a library that can be linked with C and C++ management
45 programs (or management programs written in OCaml, Perl, Python, Ruby,
46 Java, Haskell or C#). You can also use it from shell scripts or the
49 You don't need to be root to use libguestfs, although obviously you do
50 need enough permissions to access the disk images.
52 Libguestfs is a large API because it can do many things. For a gentle
53 introduction, please read the L</API OVERVIEW> section next.
57 This section provides a gentler overview of the libguestfs API. We
58 also try to group API calls together, where that may not be obvious
59 from reading about the individual calls in the main section of this
64 Before you can use libguestfs calls, you have to create a handle.
65 Then you must add at least one disk image to the handle, followed by
66 launching the handle, then performing whatever operations you want,
67 and finally closing the handle. By convention we use the single
68 letter C<g> for the name of the handle variable, although of course
69 you can use any name you want.
71 The general structure of all libguestfs-using programs looks like
74 guestfs_h *g = guestfs_create ();
76 /* Call guestfs_add_drive additional times if there are
77 * multiple disk images.
79 guestfs_add_drive (g, "guest.img");
81 /* Most manipulation calls won't work until you've launched
82 * the handle 'g'. You have to do this _after_ adding drives
83 * and _before_ other commands.
87 /* Now you can examine what partitions, LVs etc are available.
89 char **partitions = guestfs_list_partitions (g);
90 char **logvols = guestfs_lvs (g);
92 /* To access a filesystem in the image, you must mount it.
94 guestfs_mount (g, "/dev/sda1", "/");
96 /* Now you can perform filesystem actions on the guest
99 guestfs_touch (g, "/hello");
101 /* You only need to call guestfs_sync if you have made
102 * changes to the guest image. (But if you've made changes
103 * then you *must* sync). See also: guestfs_umount and
104 * guestfs_umount_all calls.
108 /* Close the handle 'g'. */
111 The code above doesn't include any error checking. In real code you
112 should check return values carefully for errors. In general all
113 functions that return integers return C<-1> on error, and all
114 functions that return pointers return C<NULL> on error. See section
115 L</ERROR HANDLING> below for how to handle errors, and consult the
116 documentation for each function call below to see precisely how they
117 return error indications.
121 The image filename (C<"guest.img"> in the example above) could be a
122 disk image from a virtual machine, a L<dd(1)> copy of a physical hard
123 disk, an actual block device, or simply an empty file of zeroes that
124 you have created through L<posix_fallocate(3)>. Libguestfs lets you
125 do useful things to all of these.
127 You can add a disk read-only using L</guestfs_add_drive_ro>, in which
128 case libguestfs won't modify the file.
130 Be extremely cautious if the disk image is in use, eg. if it is being
131 used by a virtual machine. Adding it read-write will almost certainly
132 cause disk corruption, but adding it read-only is safe.
134 You must add at least one disk image, and you may add multiple disk
135 images. In the API, the disk images are usually referred to as
136 C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
139 Once L</guestfs_launch> has been called you cannot add any more images.
140 You can call L</guestfs_list_devices> to get a list of the device
141 names, in the order that you added them. See also L</BLOCK DEVICE
146 Before you can read or write files, create directories and so on in a
147 disk image that contains filesystems, you have to mount those
148 filesystems using L</guestfs_mount>. If you already know that a disk
149 image contains (for example) one partition with a filesystem on that
150 partition, then you can mount it directly:
152 guestfs_mount (g, "/dev/sda1", "/");
154 where C</dev/sda1> means literally the first partition (C<1>) of the
155 first disk image that we added (C</dev/sda>). If the disk contains
156 Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).
158 If you are given a disk image and you don't know what it contains then
159 you have to find out. Libguestfs can do that too: use
160 L</guestfs_list_partitions> and L</guestfs_lvs> to list possible
161 partitions and LVs, and either try mounting each to see what is
162 mountable, or else examine them with L</guestfs_vfs_type> or
163 L</guestfs_file>. But you might find it easier to look at higher level
164 programs built on top of libguestfs, in particular
165 L<virt-inspector(1)>.
167 To mount a disk image read-only, use L</guestfs_mount_ro>. There are
168 several other variations of the C<guestfs_mount_*> call.
170 =head2 FILESYSTEM ACCESS AND MODIFICATION
172 The majority of the libguestfs API consists of fairly low-level calls
173 for accessing and modifying the files, directories, symlinks etc on
174 mounted filesystems. There are over a hundred such calls which you
175 can find listed in detail below in this man page, and we don't even
176 pretend to cover them all in this overview.
178 Specify filenames as full paths, starting with C<"/"> and including
181 For example, if you mounted a filesystem at C<"/"> and you want to
182 read the file called C<"etc/passwd"> then you could do:
184 char *data = guestfs_cat (g, "/etc/passwd");
186 This would return C<data> as a newly allocated buffer containing the
187 full content of that file (with some conditions: see also
188 L</DOWNLOADING> below), or C<NULL> if there was an error.
190 As another example, to create a top-level directory on that filesystem
191 called C<"var"> you would do:
193 guestfs_mkdir (g, "/var");
195 To create a symlink you could do:
197 guestfs_ln_s (g, "/etc/init.d/portmap",
198 "/etc/rc3.d/S30portmap");
200 Libguestfs will reject attempts to use relative paths and there is no
201 concept of a current working directory.
203 Libguestfs can return errors in many situations: for example if the
204 filesystem isn't writable, or if a file or directory that you
205 requested doesn't exist. If you are using the C API (documented here)
206 you have to check for those error conditions after each call. (Other
207 language bindings turn these errors into exceptions).
209 File writes are affected by the per-handle umask, set by calling
210 L</guestfs_umask> and defaulting to 022. See L</UMASK>.
214 Libguestfs contains API calls to read, create and modify partition
215 tables on disk images.
217 In the common case where you want to create a single partition
218 covering the whole disk, you should use the L</guestfs_part_disk>
221 const char *parttype = "mbr";
222 if (disk_is_larger_than_2TB)
224 guestfs_part_disk (g, "/dev/sda", parttype);
226 Obviously this effectively wipes anything that was on that disk image
231 Libguestfs provides access to a large part of the LVM2 API, such as
232 L</guestfs_lvcreate> and L</guestfs_vgremove>. It won't make much sense
233 unless you familiarize yourself with the concepts of physical volumes,
234 volume groups and logical volumes.
236 This author strongly recommends reading the LVM HOWTO, online at
237 L<http://tldp.org/HOWTO/LVM-HOWTO/>.
241 Use L</guestfs_cat> to download small, text only files. This call
242 is limited to files which are less than 2 MB and which cannot contain
243 any ASCII NUL (C<\0>) characters. However it has a very simple
246 L</guestfs_read_file> can be used to read files which contain
247 arbitrary 8 bit data, since it returns a (pointer, size) pair.
248 However it is still limited to "small" files, less than 2 MB.
250 L</guestfs_download> can be used to download any file, with no
251 limits on content or size (even files larger than 4 GB).
253 To download multiple files, see L</guestfs_tar_out> and
258 It's often the case that you want to write a file or files to the disk
261 For small, single files, use L</guestfs_write_file>. This call
262 currently contains a bug which limits the call to plain text files
263 (not containing ASCII NUL characters).
265 To upload a single file, use L</guestfs_upload>. This call has no
266 limits on file content or size (even files larger than 4 GB).
268 To upload multiple files, see L</guestfs_tar_in> and L</guestfs_tgz_in>.
270 However the fastest way to upload I<large numbers of arbitrary files>
271 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
272 L<mkisofs(8)>), then attach this using L</guestfs_add_drive_ro>. If
273 you add the drive in a predictable way (eg. adding it last after all
274 other drives) then you can get the device name from
275 L</guestfs_list_devices> and mount it directly using
276 L</guestfs_mount_ro>. Note that squashfs images are sometimes
277 non-portable between kernel versions, and they don't support labels or
278 UUIDs. If you want to pre-build an image or you need to mount it
279 using a label or UUID, use an ISO image instead.
283 There are various different commands for copying between files and
284 devices and in and out of the guest filesystem. These are summarised
289 =item B<file> to B<file>
291 Use L</guestfs_cp> to copy a single file, or
292 L</guestfs_cp_a> to copy directories recursively.
294 =item B<file or device> to B<file or device>
296 Use L</guestfs_dd> which efficiently uses L<dd(1)>
297 to copy between files and devices in the guest.
299 Example: duplicate the contents of an LV:
301 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
303 The destination (C</dev/VG/Copy>) must be at least as large as the
304 source (C</dev/VG/Original>). To copy less than the whole
305 source device, use L</guestfs_copy_size>.
307 =item B<file on the host> to B<file or device>
309 Use L</guestfs_upload>. See L</UPLOADING> above.
311 =item B<file or device> to B<file on the host>
313 Use L</guestfs_download>. See L</DOWNLOADING> above.
319 L</guestfs_ll> is just designed for humans to read (mainly when using
320 the L<guestfish(1)>-equivalent command C<ll>).
322 L</guestfs_ls> is a quick way to get a list of files in a directory
323 from programs, as a flat list of strings.
325 L</guestfs_readdir> is a programmatic way to get a list of files in a
326 directory, plus additional information about each one. It is more
327 equivalent to using the L<readdir(3)> call on a local filesystem.
329 L</guestfs_find> and L</guestfs_find0> can be used to recursively list
332 =head2 RUNNING COMMANDS
334 Although libguestfs is a primarily an API for manipulating files
335 inside guest images, we also provide some limited facilities for
336 running commands inside guests.
338 There are many limitations to this:
344 The kernel version that the command runs under will be different
345 from what it expects.
349 If the command needs to communicate with daemons, then most likely
350 they won't be running.
354 The command will be running in limited memory.
358 Only supports Linux guests (not Windows, BSD, etc).
362 Architecture limitations (eg. won't work for a PPC guest on
367 For SELinux guests, you may need to enable SELinux and load policy
368 first. See L</SELINUX> in this manpage.
372 The two main API calls to run commands are L</guestfs_command> and
373 L</guestfs_sh> (there are also variations).
375 The difference is that L</guestfs_sh> runs commands using the shell, so
376 any shell globs, redirections, etc will work.
378 =head2 CONFIGURATION FILES
380 To read and write configuration files in Linux guest filesystems, we
381 strongly recommend using Augeas. For example, Augeas understands how
382 to read and write, say, a Linux shadow password file or X.org
383 configuration file, and so avoids you having to write that code.
385 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
386 don't document Augeas itself here because there is excellent
387 documentation on the L<http://augeas.net/> website.
389 If you don't want to use Augeas (you fool!) then try calling
390 L</guestfs_read_lines> to get the file as a list of lines which
391 you can iterate over.
395 We support SELinux guests. To ensure that labeling happens correctly
396 in SELinux guests, you need to enable SELinux and load the guest's
403 Before launching, do:
405 guestfs_set_selinux (g, 1);
409 After mounting the guest's filesystem(s), load the policy. This
410 is best done by running the L<load_policy(8)> command in the
413 guestfs_sh (g, "/usr/sbin/load_policy");
415 (Older versions of C<load_policy> require you to specify the
416 name of the policy file).
420 Optionally, set the security context for the API. The correct
421 security context to use can only be known by inspecting the
422 guest. As an example:
424 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
428 This will work for running commands and editing existing files.
430 When new files are created, you may need to label them explicitly,
431 for example by running the external command
432 C<restorecon pathname>.
436 Certain calls are affected by the current file mode creation mask (the
437 "umask"). In particular ones which create files or directories, such
438 as L</guestfs_touch>, L</guestfs_mknod> or L</guestfs_mkdir>. This
439 affects either the default mode that the file is created with or
440 modifies the mode that you supply.
442 The default umask is C<022>, so files are created with modes such as
443 C<0644> and directories with C<0755>.
445 There are two ways to avoid being affected by umask. Either set umask
446 to 0 (call C<guestfs_umask (g, 0)> early after launching). Or call
447 L</guestfs_chmod> after creating each file or directory.
449 For more information about umask, see L<umask(2)>.
451 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
453 Libguestfs can mount NTFS partitions. It does this using the
454 L<http://www.ntfs-3g.org/> driver.
456 DOS and Windows still use drive letters, and the filesystems are
457 always treated as case insensitive by Windows itself, and therefore
458 you might find a Windows configuration file referring to a path like
459 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
460 that directory might be referred to as C</WINDOWS/System32>.
462 Drive letter mappings are outside the scope of libguestfs. You have
463 to use libguestfs to read the appropriate Windows Registry and
464 configuration files, to determine yourself how drives are mapped (see
465 also L<virt-inspector(1)>).
467 Replacing backslash characters with forward slash characters is also
468 outside the scope of libguestfs, but something that you can easily do.
470 Where we can help is in resolving the case insensitivity of paths.
471 For this, call L</guestfs_case_sensitive_path>.
473 Libguestfs also provides some help for decoding Windows Registry
474 "hive" files, through the library C<hivex> which is part of the
475 libguestfs project although ships as a separate tarball. You have to
476 locate and download the hive file(s) yourself, and then pass them to
477 C<hivex> functions. See also the programs L<hivexml(1)>,
478 L<hivexsh(1)>, L<hivexregedit(1)> and L<virt-win-reg(1)> for more help
481 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
483 Although we don't want to discourage you from using the C API, we will
484 mention here that the same API is also available in other languages.
486 The API is broadly identical in all supported languages. This means
487 that the C call C<guestfs_mount(g,path)> is
488 C<$g-E<gt>mount($path)> in Perl, C<g.mount(path)> in Python,
489 and C<Guestfs.mount g path> in OCaml. In other words, a
490 straightforward, predictable isomorphism between each language.
492 Error messages are automatically transformed
493 into exceptions if the language supports it.
495 We don't try to "object orientify" parts of the API in OO languages,
496 although contributors are welcome to write higher level APIs above
497 what we provide in their favourite languages if they wish.
503 You can use the I<guestfs.h> header file from C++ programs. The C++
504 API is identical to the C API. C++ classes and exceptions are not
509 The C# bindings are highly experimental. Please read the warnings
510 at the top of C<csharp/Libguestfs.cs>.
514 This is the only language binding that is working but incomplete.
515 Only calls which return simple integers have been bound in Haskell,
516 and we are looking for help to complete this binding.
520 Full documentation is contained in the Javadoc which is distributed
525 For documentation see the file C<guestfs.mli>.
529 For documentation see L<Sys::Guestfs(3)>.
533 For documentation do:
541 Use the Guestfs module. There is no Ruby-specific documentation, but
542 you can find examples written in Ruby in the libguestfs source.
544 =item B<shell scripts>
546 For documentation see L<guestfish(1)>.
550 =head2 LIBGUESTFS GOTCHAS
552 L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
553 system [...] that works in the way it is documented but is
554 counterintuitive and almost invites mistakes."
556 Since we developed libguestfs and the associated tools, there are
557 several things we would have designed differently, but are now stuck
558 with for backwards compatibility or other reasons. If there is ever a
559 libguestfs 2.0 release, you can expect these to change. Beware of
564 =item Autosync / forgetting to sync.
566 When modifying a filesystem from C or another language, you B<must>
567 unmount all filesystems and call L</guestfs_sync> explicitly before
568 you close the libguestfs handle. You can also call:
570 guestfs_set_autosync (g, 1);
572 to have the unmount/sync done automatically for you when the handle 'g'
573 is closed. (This feature is called "autosync", L</guestfs_set_autosync>
576 If you forget to do this, then it is entirely possible that your
577 changes won't be written out, or will be partially written, or (very
578 rarely) that you'll get disk corruption.
580 Note that in L<guestfish(3)> autosync is the default. So quick and
581 dirty guestfish scripts that forget to sync will work just fine, which
582 can make this very puzzling if you are trying to debug a problem.
584 =item Mount option C<-o sync> should not be the default.
586 If you use L</guestfs_mount>, then C<-o sync,noatime> are added
587 implicitly. However C<-o sync> does not add any reliability benefit,
588 but does have a very large performance impact.
590 The work around is to use L</guestfs_mount_options> and set the mount
591 options that you actually want to use.
593 =item Read-only should be the default.
595 In L<guestfish(3)>, I<--ro> should be the default, and you should
596 have to specify I<--rw> if you want to make changes to the image.
598 This would reduce the potential to corrupt live VM images.
600 Note that many filesystems change the disk when you just mount and
601 unmount, even if you didn't perform any writes. You need to use
602 L</guestfs_add_drive_ro> to guarantee that the disk is not changed.
604 =item guestfish command line is hard to use.
606 C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
607 for examination). It tries to run a guestfish command C<disk.img>
608 which doesn't exist, so it fails. In earlier versions of guestfish
609 the error message was also unintuitive, but we have corrected this
610 since. Like the Bourne shell, we should have used C<guestfish -c
611 command> to run commands.
615 =head2 PROTOCOL LIMITS
617 Internally libguestfs uses a message-based protocol to pass API calls
618 and their responses to and from a small "appliance" (see L</INTERNALS>
619 for plenty more detail about this). The maximum message size used by
620 the protocol is slightly less than 4 MB. For some API calls you may
621 need to be aware of this limit. The API calls which may be affected
622 are individually documented, with a link back to this section of the
625 A simple call such as L</guestfs_cat> returns its result (the file
626 data) in a simple string. Because this string is at some point
627 internally encoded as a message, the maximum size that it can return
628 is slightly under 4 MB. If the requested file is larger than this
629 then you will get an error.
631 In order to transfer large files into and out of the guest filesystem,
632 you need to use particular calls that support this. The sections
633 L</UPLOADING> and L</DOWNLOADING> document how to do this.
635 You might also consider mounting the disk image using our FUSE
636 filesystem support (L<guestmount(1)>).
638 =head1 CONNECTION MANAGEMENT
642 C<guestfs_h> is the opaque type representing a connection handle.
643 Create a handle by calling L</guestfs_create>. Call L</guestfs_close>
644 to free the handle and release all resources used.
646 For information on using multiple handles and threads, see the section
647 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
649 =head2 guestfs_create
651 guestfs_h *guestfs_create (void);
653 Create a connection handle.
655 You have to call L</guestfs_add_drive> on the handle at least once.
657 This function returns a non-NULL pointer to a handle on success or
660 After configuring the handle, you have to call L</guestfs_launch>.
662 You may also want to configure error handling for the handle. See
663 L</ERROR HANDLING> section below.
667 void guestfs_close (guestfs_h *g);
669 This closes the connection handle and frees up all resources used.
671 =head1 ERROR HANDLING
673 The convention in all functions that return C<int> is that they return
674 C<-1> to indicate an error. You can get additional information on
675 errors by calling L</guestfs_last_error> and/or by setting up an error
676 handler with L</guestfs_set_error_handler>.
678 The default error handler prints the information string to C<stderr>.
680 Out of memory errors are handled differently. The default action is
681 to call L<abort(3)>. If this is undesirable, then you can set a
682 handler using L</guestfs_set_out_of_memory_handler>.
684 =head2 guestfs_last_error
686 const char *guestfs_last_error (guestfs_h *g);
688 This returns the last error message that happened on C<g>. If
689 there has not been an error since the handle was created, then this
692 The lifetime of the returned string is until the next error occurs, or
693 L</guestfs_close> is called.
695 The error string is not localized (ie. is always in English), because
696 this makes searching for error messages in search engines give the
697 largest number of results.
699 =head2 guestfs_set_error_handler
701 typedef void (*guestfs_error_handler_cb) (guestfs_h *g,
704 void guestfs_set_error_handler (guestfs_h *g,
705 guestfs_error_handler_cb cb,
708 The callback C<cb> will be called if there is an error. The
709 parameters passed to the callback are an opaque data pointer and the
710 error message string.
712 Note that the message string C<msg> is freed as soon as the callback
713 function returns, so if you want to stash it somewhere you must make
716 The default handler prints messages on C<stderr>.
718 If you set C<cb> to C<NULL> then I<no> handler is called.
720 =head2 guestfs_get_error_handler
722 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g,
725 Returns the current error handler callback.
727 =head2 guestfs_set_out_of_memory_handler
729 typedef void (*guestfs_abort_cb) (void);
730 int guestfs_set_out_of_memory_handler (guestfs_h *g,
733 The callback C<cb> will be called if there is an out of memory
734 situation. I<Note this callback must not return>.
736 The default is to call L<abort(3)>.
738 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
741 =head2 guestfs_get_out_of_memory_handler
743 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *g);
745 This returns the current out of memory handler.
749 Libguestfs needs a kernel and initrd.img, which it finds by looking
750 along an internal path.
752 By default it looks for these in the directory C<$libdir/guestfs>
753 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
755 Use L</guestfs_set_path> or set the environment variable
756 L</LIBGUESTFS_PATH> to change the directories that libguestfs will
757 search in. The value is a colon-separated list of paths. The current
758 directory is I<not> searched unless the path contains an empty element
759 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
760 search the current directory and then C</usr/lib/guestfs>.
762 =head1 HIGH-LEVEL API ACTIONS
766 We guarantee the libguestfs ABI (binary interface), for public,
767 high-level actions as outlined in this section. Although we will
768 deprecate some actions, for example if they get replaced by newer
769 calls, we will keep the old actions forever. This allows you the
770 developer to program in confidence against the libguestfs API.
780 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
782 Using L</guestfs_available> you can test availability of
783 the following groups of functions. This test queries the
784 appliance to see if the appliance you are currently using
785 supports the functionality.
789 =head2 SINGLE CALLS AT COMPILE TIME
791 If you need to test whether a single libguestfs function is
792 available at compile time, we recommend using build tools
793 such as autoconf or cmake. For example in autotools you could
796 AC_CHECK_LIB([guestfs],[guestfs_create])
797 AC_CHECK_FUNCS([guestfs_dd])
799 which would result in C<HAVE_GUESTFS_DD> being either defined
800 or not defined in your program.
802 =head2 SINGLE CALLS AT RUN TIME
804 Testing at compile time doesn't guarantee that a function really
805 exists in the library. The reason is that you might be dynamically
806 linked against a previous I<libguestfs.so> (dynamic library)
807 which doesn't have the call. This situation unfortunately results
808 in a segmentation fault, which is a shortcoming of the C dynamic
809 linking system itself.
811 You can use L<dlopen(3)> to test if a function is available
812 at run time, as in this example program (note that you still
813 need the compile time check as well):
825 #ifdef HAVE_GUESTFS_DD
829 /* Test if the function guestfs_dd is really available. */
830 dl = dlopen (NULL, RTLD_LAZY);
832 fprintf (stderr, "dlopen: %s\n", dlerror ());
835 has_function = dlsym (dl, "guestfs_dd") != NULL;
839 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
841 printf ("this libguestfs.so has guestfs_dd function\n");
842 /* Now it's safe to call
843 guestfs_dd (g, "foo", "bar");
847 printf ("guestfs_dd function was not found at compile time\n");
851 You may think the above is an awful lot of hassle, and it is.
852 There are other ways outside of the C linking system to ensure
853 that this kind of incompatibility never arises, such as using
856 Requires: libguestfs >= 1.0.80
860 <!-- old anchor for the next section -->
861 <a name="state_machine_and_low_level_event_api"/>
867 Internally, libguestfs is implemented by running an appliance (a
868 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
869 a child process of the main program.
875 | | child process / appliance
876 | | __________________________
878 +-------------------+ RPC | +-----------------+ |
879 | libguestfs <--------------------> guestfsd | |
880 | | | +-----------------+ |
881 \___________________/ | | Linux kernel | |
882 | +--^--------------+ |
883 \_________|________________/
891 The library, linked to the main program, creates the child process and
892 hence the appliance in the L</guestfs_launch> function.
894 Inside the appliance is a Linux kernel and a complete stack of
895 userspace tools (such as LVM and ext2 programs) and a small
896 controlling daemon called L</guestfsd>. The library talks to
897 L</guestfsd> using remote procedure calls (RPC). There is a mostly
898 one-to-one correspondence between libguestfs API calls and RPC calls
899 to the daemon. Lastly the disk image(s) are attached to the qemu
900 process which translates device access by the appliance's Linux kernel
901 into accesses to the image.
903 A common misunderstanding is that the appliance "is" the virtual
904 machine. Although the disk image you are attached to might also be
905 used by some virtual machine, libguestfs doesn't know or care about
906 this. (But you will care if both libguestfs's qemu process and your
907 virtual machine are trying to update the disk image at the same time,
908 since these usually results in massive disk corruption).
912 libguestfs uses a state machine to model the child process:
923 / | \ \ guestfs_launch
934 \______/ <------ \________/
936 The normal transitions are (1) CONFIG (when the handle is created, but
937 there is no child process), (2) LAUNCHING (when the child process is
938 booting up), (3) alternating between READY and BUSY as commands are
939 issued to, and carried out by, the child process.
941 The guest may be killed by L</guestfs_kill_subprocess>, or may die
942 asynchronously at any time (eg. due to some internal error), and that
943 causes the state to transition back to CONFIG.
945 Configuration commands for qemu such as L</guestfs_add_drive> can only
946 be issued when in the CONFIG state.
948 The high-level API offers two calls that go from CONFIG through
949 LAUNCHING to READY. L</guestfs_launch> blocks until the child process
950 is READY to accept commands (or until some failure or timeout).
951 L</guestfs_launch> internally moves the state from CONFIG to LAUNCHING
954 High-level API actions such as L</guestfs_mount> can only be issued
955 when in the READY state. These high-level API calls block waiting for
956 the command to be carried out (ie. the state to transition to BUSY and
957 then back to READY). But using the low-level event API, you get
958 non-blocking versions. (But you can still only carry out one
959 operation per handle at a time - that is a limitation of the
960 communications protocol we use).
962 Finally, the child process sends asynchronous messages back to the
963 main program, such as kernel log messages. Mostly these are ignored
964 by the high-level API, but using the low-level event API you can
965 register to receive these messages.
967 =head2 SETTING CALLBACKS TO HANDLE EVENTS
969 The child process generates events in some situations. Current events
970 include: receiving a log message, the child process exits.
972 Use the C<guestfs_set_*_callback> functions to set a callback for
973 different types of events.
975 Only I<one callback of each type> can be registered for each handle.
976 Calling C<guestfs_set_*_callback> again overwrites the previous
977 callback of that type. Cancel all callbacks of this type by calling
978 this function with C<cb> set to C<NULL>.
980 =head2 guestfs_set_log_message_callback
982 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
984 void guestfs_set_log_message_callback (guestfs_h *g,
985 guestfs_log_message_cb cb,
988 The callback function C<cb> will be called whenever qemu or the guest
989 writes anything to the console.
991 Use this function to capture kernel messages and similar.
993 Normally there is no log message handler, and log messages are just
996 =head2 guestfs_set_subprocess_quit_callback
998 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
999 void guestfs_set_subprocess_quit_callback (guestfs_h *g,
1000 guestfs_subprocess_quit_cb cb,
1003 The callback function C<cb> will be called when the child process
1004 quits, either asynchronously or if killed by
1005 L</guestfs_kill_subprocess>. (This corresponds to a transition from
1006 any state to the CONFIG state).
1008 =head2 guestfs_set_launch_done_callback
1010 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
1011 void guestfs_set_launch_done_callback (guestfs_h *g,
1012 guestfs_ready_cb cb,
1015 The callback function C<cb> will be called when the child process
1016 becomes ready first time after it has been launched. (This
1017 corresponds to a transition from LAUNCHING to the READY state).
1019 =head1 BLOCK DEVICE NAMING
1021 In the kernel there is now quite a profusion of schemata for naming
1022 block devices (in this context, by I<block device> I mean a physical
1023 or virtual hard drive). The original Linux IDE driver used names
1024 starting with C</dev/hd*>. SCSI devices have historically used a
1025 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
1026 driver became a popular replacement for the old IDE driver
1027 (particularly for SATA devices) those devices also used the
1028 C</dev/sd*> scheme. Additionally we now have virtual machines with
1029 paravirtualized drivers. This has created several different naming
1030 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
1033 As discussed above, libguestfs uses a qemu appliance running an
1034 embedded Linux kernel to access block devices. We can run a variety
1035 of appliances based on a variety of Linux kernels.
1037 This causes a problem for libguestfs because many API calls use device
1038 or partition names. Working scripts and the recipe (example) scripts
1039 that we make available over the internet could fail if the naming
1042 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
1043 scheme>. Internally C</dev/sd*> names are translated, if necessary,
1044 to other names as required. For example, under RHEL 5 which uses the
1045 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
1046 C</dev/hda2> transparently.
1048 Note that this I<only> applies to parameters. The
1049 L</guestfs_list_devices>, L</guestfs_list_partitions> and similar calls
1050 return the true names of the devices and partitions as known to the
1053 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
1055 Usually this translation is transparent. However in some (very rare)
1056 cases you may need to know the exact algorithm. Such cases include
1057 where you use L</guestfs_config> to add a mixture of virtio and IDE
1058 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
1059 and C</dev/vd*> devices.
1061 The algorithm is applied only to I<parameters> which are known to be
1062 either device or partition names. Return values from functions such
1063 as L</guestfs_list_devices> are never changed.
1069 Is the string a parameter which is a device or partition name?
1073 Does the string begin with C</dev/sd>?
1077 Does the named device exist? If so, we use that device.
1078 However if I<not> then we continue with this algorithm.
1082 Replace initial C</dev/sd> string with C</dev/hd>.
1084 For example, change C</dev/sda2> to C</dev/hda2>.
1086 If that named device exists, use it. If not, continue.
1090 Replace initial C</dev/sd> string with C</dev/vd>.
1092 If that named device exists, use it. If not, return an error.
1096 =head2 PORTABILITY CONCERNS
1098 Although the standard naming scheme and automatic translation is
1099 useful for simple programs and guestfish scripts, for larger programs
1100 it is best not to rely on this mechanism.
1102 Where possible for maximum future portability programs using
1103 libguestfs should use these future-proof techniques:
1109 Use L</guestfs_list_devices> or L</guestfs_list_partitions> to list
1110 actual device names, and then use those names directly.
1112 Since those device names exist by definition, they will never be
1117 Use higher level ways to identify filesystems, such as LVM names,
1118 UUIDs and filesystem labels.
1124 =head2 COMMUNICATION PROTOCOL
1126 Don't rely on using this protocol directly. This section documents
1127 how it currently works, but it may change at any time.
1129 The protocol used to talk between the library and the daemon running
1130 inside the qemu virtual machine is a simple RPC mechanism built on top
1131 of XDR (RFC 1014, RFC 1832, RFC 4506).
1133 The detailed format of structures is in C<src/guestfs_protocol.x>
1134 (note: this file is automatically generated).
1136 There are two broad cases, ordinary functions that don't have any
1137 C<FileIn> and C<FileOut> parameters, which are handled with very
1138 simple request/reply messages. Then there are functions that have any
1139 C<FileIn> or C<FileOut> parameters, which use the same request and
1140 reply messages, but they may also be followed by files sent using a
1143 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1145 For ordinary functions, the request message is:
1147 total length (header + arguments,
1148 but not including the length word itself)
1149 struct guestfs_message_header (encoded as XDR)
1150 struct guestfs_<foo>_args (encoded as XDR)
1152 The total length field allows the daemon to allocate a fixed size
1153 buffer into which it slurps the rest of the message. As a result, the
1154 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1155 4MB), which means the effective size of any request is limited to
1156 somewhere under this size.
1158 Note also that many functions don't take any arguments, in which case
1159 the C<guestfs_I<foo>_args> is completely omitted.
1161 The header contains the procedure number (C<guestfs_proc>) which is
1162 how the receiver knows what type of args structure to expect, or none
1165 The reply message for ordinary functions is:
1167 total length (header + ret,
1168 but not including the length word itself)
1169 struct guestfs_message_header (encoded as XDR)
1170 struct guestfs_<foo>_ret (encoded as XDR)
1172 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1173 for functions that return no formal return values.
1175 As above the total length of the reply is limited to
1176 C<GUESTFS_MESSAGE_MAX>.
1178 In the case of an error, a flag is set in the header, and the reply
1179 message is slightly changed:
1181 total length (header + error,
1182 but not including the length word itself)
1183 struct guestfs_message_header (encoded as XDR)
1184 struct guestfs_message_error (encoded as XDR)
1186 The C<guestfs_message_error> structure contains the error message as a
1189 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1191 A C<FileIn> parameter indicates that we transfer a file I<into> the
1192 guest. The normal request message is sent (see above). However this
1193 is followed by a sequence of file chunks.
1195 total length (header + arguments,
1196 but not including the length word itself,
1197 and not including the chunks)
1198 struct guestfs_message_header (encoded as XDR)
1199 struct guestfs_<foo>_args (encoded as XDR)
1200 sequence of chunks for FileIn param #0
1201 sequence of chunks for FileIn param #1 etc.
1203 The "sequence of chunks" is:
1205 length of chunk (not including length word itself)
1206 struct guestfs_chunk (encoded as XDR)
1208 struct guestfs_chunk (encoded as XDR)
1211 struct guestfs_chunk (with data.data_len == 0)
1213 The final chunk has the C<data_len> field set to zero. Additionally a
1214 flag is set in the final chunk to indicate either successful
1215 completion or early cancellation.
1217 At time of writing there are no functions that have more than one
1218 FileIn parameter. However this is (theoretically) supported, by
1219 sending the sequence of chunks for each FileIn parameter one after
1220 another (from left to right).
1222 Both the library (sender) I<and> the daemon (receiver) may cancel the
1223 transfer. The library does this by sending a chunk with a special
1224 flag set to indicate cancellation. When the daemon sees this, it
1225 cancels the whole RPC, does I<not> send any reply, and goes back to
1226 reading the next request.
1228 The daemon may also cancel. It does this by writing a special word
1229 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1230 during the transfer, and if it gets it, it will cancel the transfer
1231 (it sends a cancel chunk). The special word is chosen so that even if
1232 cancellation happens right at the end of the transfer (after the
1233 library has finished writing and has started listening for the reply),
1234 the "spurious" cancel flag will not be confused with the reply
1237 This protocol allows the transfer of arbitrary sized files (no 32 bit
1238 limit), and also files where the size is not known in advance
1239 (eg. from pipes or sockets). However the chunks are rather small
1240 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1241 daemon need to keep much in memory.
1243 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1245 The protocol for FileOut parameters is exactly the same as for FileIn
1246 parameters, but with the roles of daemon and library reversed.
1248 total length (header + ret,
1249 but not including the length word itself,
1250 and not including the chunks)
1251 struct guestfs_message_header (encoded as XDR)
1252 struct guestfs_<foo>_ret (encoded as XDR)
1253 sequence of chunks for FileOut param #0
1254 sequence of chunks for FileOut param #1 etc.
1256 =head3 INITIAL MESSAGE
1258 Because the underlying channel (QEmu -net channel) doesn't have any
1259 sort of connection control, when the daemon launches it sends an
1260 initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
1261 and daemon is alive. This is what L</guestfs_launch> waits for.
1263 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1265 All high-level libguestfs actions are synchronous. If you want
1266 to use libguestfs asynchronously then you must create a thread.
1268 Only use the handle from a single thread. Either use the handle
1269 exclusively from one thread, or provide your own mutex so that two
1270 threads cannot issue calls on the same handle at the same time.
1272 =head1 QEMU WRAPPERS
1274 If you want to compile your own qemu, run qemu from a non-standard
1275 location, or pass extra arguments to qemu, then you can write a
1276 shell-script wrapper around qemu.
1278 There is one important rule to remember: you I<must C<exec qemu>> as
1279 the last command in the shell script (so that qemu replaces the shell
1280 and becomes the direct child of the libguestfs-using program). If you
1281 don't do this, then the qemu process won't be cleaned up correctly.
1283 Here is an example of a wrapper, where I have built my own copy of
1287 qemudir=/home/rjones/d/qemu
1288 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1290 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1291 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1294 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1296 Note that libguestfs also calls qemu with the -help and -version
1297 options in order to determine features.
1299 =head1 LIBGUESTFS VERSION NUMBERS
1301 Since April 2010, libguestfs has started to make separate development
1302 and stable releases, along with corresponding branches in our git
1303 repository. These separate releases can be identified by version
1306 even numbers for stable: 1.2.x, 1.4.x, ...
1307 .-------- odd numbers for development: 1.3.x, 1.5.x, ...
1313 | `-------- sub-version
1315 `------ always '1' because we don't change the ABI
1317 Thus "1.3.5" is the 5th update to the development branch "1.3".
1319 As time passes we cherry pick fixes from the development branch and
1320 backport those into the stable branch, the effect being that the
1321 stable branch should get more stable and less buggy over time. So the
1322 stable releases are ideal for people who don't need new features but
1323 would just like the software to work.
1325 Our criteria for backporting changes are:
1331 Documentation changes which don't affect any code are
1332 backported unless the documentation refers to a future feature
1333 which is not in stable.
1337 Bug fixes which are not controversial, fix obvious problems, and
1338 have been well tested are backported.
1342 Simple rearrangements of code which shouldn't affect how it works get
1343 backported. This is so that the code in the two branches doesn't get
1344 too far out of step, allowing us to backport future fixes more easily.
1348 We I<don't> backport new features, new APIs, new tools etc, except in
1349 one exceptional case: the new feature is required in order to
1350 implement an important bug fix.
1354 A new stable branch starts when we think the new features in
1355 development are substantial and compelling enough over the current
1356 stable branch to warrant it. When that happens we create new stable
1357 and development versions 1.N.0 and 1.(N+1).0 [N is even]. The new
1358 dot-oh release won't necessarily be so stable at this point, but by
1359 backporting fixes from development, that branch will stabilize over
1362 =head1 ENVIRONMENT VARIABLES
1366 =item LIBGUESTFS_APPEND
1368 Pass additional options to the guest kernel.
1370 =item LIBGUESTFS_DEBUG
1372 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1373 has the same effect as calling C<guestfs_set_verbose (g, 1)>.
1375 =item LIBGUESTFS_MEMSIZE
1377 Set the memory allocated to the qemu process, in megabytes. For
1380 LIBGUESTFS_MEMSIZE=700
1382 =item LIBGUESTFS_PATH
1384 Set the path that libguestfs uses to search for kernel and initrd.img.
1385 See the discussion of paths in section PATH above.
1387 =item LIBGUESTFS_QEMU
1389 Set the default qemu binary that libguestfs uses. If not set, then
1390 the qemu which was found at compile time by the configure script is
1393 See also L</QEMU WRAPPERS> above.
1395 =item LIBGUESTFS_TRACE
1397 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1398 has the same effect as calling C<guestfs_set_trace (g, 1)>.
1402 Location of temporary directory, defaults to C</tmp>.
1404 If libguestfs was compiled to use the supermin appliance then each
1405 handle will require rather a large amount of space in this directory
1406 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1407 configure another directory to use in case C</tmp> is not large
1419 L<virt-inspector(1)>,
1420 L<virt-list-filesystems(1)>,
1421 L<virt-list-partitions(1)>,
1430 L<http://libguestfs.org/>.
1432 Tools with a similar purpose:
1441 To get a list of bugs against libguestfs use this link:
1443 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1445 To report a new bug against libguestfs use this link:
1447 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1449 When reporting a bug, please check:
1455 That the bug hasn't been reported already.
1459 That you are testing a recent version.
1463 Describe the bug accurately, and give a way to reproduce it.
1467 Run libguestfs-test-tool and paste the B<complete, unedited>
1468 output into the bug report.
1474 Richard W.M. Jones (C<rjones at redhat dot com>)
1478 Copyright (C) 2009-2010 Red Hat Inc.
1479 L<http://libguestfs.org/>
1481 This library is free software; you can redistribute it and/or
1482 modify it under the terms of the GNU Lesser General Public
1483 License as published by the Free Software Foundation; either
1484 version 2 of the License, or (at your option) any later version.
1486 This library is distributed in the hope that it will be useful,
1487 but WITHOUT ANY WARRANTY; without even the implied warranty of
1488 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1489 Lesser General Public License for more details.
1491 You should have received a copy of the GNU Lesser General Public
1492 License along with this library; if not, write to the Free Software
1493 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA