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 =head1 CONNECTION MANAGEMENT
531 C<guestfs_h> is the opaque type representing a connection handle.
532 Create a handle by calling C<guestfs_create>. Call C<guestfs_close>
533 to free the handle and release all resources used.
535 For information on using multiple handles and threads, see the section
536 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
538 =head2 guestfs_create
540 guestfs_h *guestfs_create (void);
542 Create a connection handle.
544 You have to call C<guestfs_add_drive> on the handle at least once.
546 This function returns a non-NULL pointer to a handle on success or
549 After configuring the handle, you have to call C<guestfs_launch>.
551 You may also want to configure error handling for the handle. See
552 L</ERROR HANDLING> section below.
556 void guestfs_close (guestfs_h *handle);
558 This closes the connection handle and frees up all resources used.
560 =head1 ERROR HANDLING
562 The convention in all functions that return C<int> is that they return
563 C<-1> to indicate an error. You can get additional information on
564 errors by calling C<guestfs_last_error> and/or by setting up an error
565 handler with C<guestfs_set_error_handler>.
567 The default error handler prints the information string to C<stderr>.
569 Out of memory errors are handled differently. The default action is
570 to call L<abort(3)>. If this is undesirable, then you can set a
571 handler using C<guestfs_set_out_of_memory_handler>.
573 =head2 guestfs_last_error
575 const char *guestfs_last_error (guestfs_h *handle);
577 This returns the last error message that happened on C<handle>. If
578 there has not been an error since the handle was created, then this
581 The lifetime of the returned string is until the next error occurs, or
582 C<guestfs_close> is called.
584 The error string is not localized (ie. is always in English), because
585 this makes searching for error messages in search engines give the
586 largest number of results.
588 =head2 guestfs_set_error_handler
590 typedef void (*guestfs_error_handler_cb) (guestfs_h *handle,
593 void guestfs_set_error_handler (guestfs_h *handle,
594 guestfs_error_handler_cb cb,
597 The callback C<cb> will be called if there is an error. The
598 parameters passed to the callback are an opaque data pointer and the
599 error message string.
601 Note that the message string C<msg> is freed as soon as the callback
602 function returns, so if you want to stash it somewhere you must make
605 The default handler prints messages on C<stderr>.
607 If you set C<cb> to C<NULL> then I<no> handler is called.
609 =head2 guestfs_get_error_handler
611 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *handle,
614 Returns the current error handler callback.
616 =head2 guestfs_set_out_of_memory_handler
618 typedef void (*guestfs_abort_cb) (void);
619 int guestfs_set_out_of_memory_handler (guestfs_h *handle,
622 The callback C<cb> will be called if there is an out of memory
623 situation. I<Note this callback must not return>.
625 The default is to call L<abort(3)>.
627 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
630 =head2 guestfs_get_out_of_memory_handler
632 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *handle);
634 This returns the current out of memory handler.
638 Libguestfs needs a kernel and initrd.img, which it finds by looking
639 along an internal path.
641 By default it looks for these in the directory C<$libdir/guestfs>
642 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
644 Use C<guestfs_set_path> or set the environment variable
645 C<LIBGUESTFS_PATH> to change the directories that libguestfs will
646 search in. The value is a colon-separated list of paths. The current
647 directory is I<not> searched unless the path contains an empty element
648 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
649 search the current directory and then C</usr/lib/guestfs>.
651 =head1 HIGH-LEVEL API ACTIONS
655 We guarantee the libguestfs ABI (binary interface), for public,
656 high-level actions as outlined in this section. Although we will
657 deprecate some actions, for example if they get replaced by newer
658 calls, we will keep the old actions forever. This allows you the
659 developer to program in confidence against libguestfs.
669 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
671 Using L</guestfs_available> you can test availability of
672 the following groups of functions. This test queries the
673 appliance to see if the appliance you are currently using
674 supports the functionality.
678 =head2 SINGLE CALLS AT COMPILE TIME
680 If you need to test whether a single libguestfs function is
681 available at compile time, we recommend using build tools
682 such as autoconf or cmake. For example in autotools you could
685 AC_CHECK_LIB([guestfs],[guestfs_create])
686 AC_CHECK_FUNCS([guestfs_dd])
688 which would result in C<HAVE_GUESTFS_DD> being either defined
689 or not defined in your program.
691 =head2 SINGLE CALLS AT RUN TIME
693 Testing at compile time doesn't guarantee that a function really
694 exists in the library. The reason is that you might be dynamically
695 linked against a previous I<libguestfs.so> (dynamic library)
696 which doesn't have the call. This situation unfortunately results
697 in a segmentation fault, which is a shortcoming of the C dynamic
698 linking system itself.
700 You can use L<dlopen(3)> to test if a function is available
701 at run time, as in this example program (note that you still
702 need the compile time check as well):
714 #ifdef HAVE_GUESTFS_DD
718 /* Test if the function guestfs_dd is really available. */
719 dl = dlopen (NULL, RTLD_LAZY);
721 fprintf (stderr, "dlopen: %s\n", dlerror ());
724 has_function = dlsym (dl, "guestfs_dd") != NULL;
728 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
730 printf ("this libguestfs.so has guestfs_dd function\n");
731 /* Now it's safe to call
732 guestfs_dd (g, "foo", "bar");
736 printf ("guestfs_dd function was not found at compile time\n");
740 You may think the above is an awful lot of hassle, and it is.
741 There are other ways outside of the C linking system to ensure
742 that this kind of incompatibility never arises, such as using
745 Requires: libguestfs >= 1.0.80
749 <!-- old anchor for the next section -->
750 <a name="state_machine_and_low_level_event_api"/>
756 Internally, libguestfs is implemented by running an appliance (a
757 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
758 a child process of the main program.
764 | | child process / appliance
765 | | __________________________
767 +-------------------+ RPC | +-----------------+ |
768 | libguestfs <--------------------> guestfsd | |
769 | | | +-----------------+ |
770 \___________________/ | | Linux kernel | |
771 | +--^--------------+ |
772 \_________|________________/
780 The library, linked to the main program, creates the child process and
781 hence the appliance in the L</guestfs_launch> function.
783 Inside the appliance is a Linux kernel and a complete stack of
784 userspace tools (such as LVM and ext2 programs) and a small
785 controlling daemon called C<guestfsd>. The library talks to
786 C<guestfsd> using remote procedure calls (RPC). There is a mostly
787 one-to-one correspondence between libguestfs API calls and RPC calls
788 to the daemon. Lastly the disk image(s) are attached to the qemu
789 process which translates device access by the appliance's Linux kernel
790 into accesses to the image.
792 A common misunderstanding is that the appliance "is" the virtual
793 machine. Although the disk image you are attached to might also be
794 used by some virtual machine, libguestfs doesn't know or care about
795 this. (But you will care if both libguestfs's qemu process and your
796 virtual machine are trying to update the disk image at the same time,
797 since these usually results in massive disk corruption).
801 libguestfs uses a state machine to model the child process:
812 / | \ \ guestfs_launch
823 \______/ <------ \________/
825 The normal transitions are (1) CONFIG (when the handle is created, but
826 there is no child process), (2) LAUNCHING (when the child process is
827 booting up), (3) alternating between READY and BUSY as commands are
828 issued to, and carried out by, the child process.
830 The guest may be killed by C<guestfs_kill_subprocess>, or may die
831 asynchronously at any time (eg. due to some internal error), and that
832 causes the state to transition back to CONFIG.
834 Configuration commands for qemu such as C<guestfs_add_drive> can only
835 be issued when in the CONFIG state.
837 The high-level API offers two calls that go from CONFIG through
838 LAUNCHING to READY. C<guestfs_launch> blocks until the child process
839 is READY to accept commands (or until some failure or timeout).
840 C<guestfs_launch> internally moves the state from CONFIG to LAUNCHING
843 High-level API actions such as C<guestfs_mount> can only be issued
844 when in the READY state. These high-level API calls block waiting for
845 the command to be carried out (ie. the state to transition to BUSY and
846 then back to READY). But using the low-level event API, you get
847 non-blocking versions. (But you can still only carry out one
848 operation per handle at a time - that is a limitation of the
849 communications protocol we use).
851 Finally, the child process sends asynchronous messages back to the
852 main program, such as kernel log messages. Mostly these are ignored
853 by the high-level API, but using the low-level event API you can
854 register to receive these messages.
856 =head2 SETTING CALLBACKS TO HANDLE EVENTS
858 The child process generates events in some situations. Current events
859 include: receiving a log message, the child process exits.
861 Use the C<guestfs_set_*_callback> functions to set a callback for
862 different types of events.
864 Only I<one callback of each type> can be registered for each handle.
865 Calling C<guestfs_set_*_callback> again overwrites the previous
866 callback of that type. Cancel all callbacks of this type by calling
867 this function with C<cb> set to C<NULL>.
869 =head2 guestfs_set_log_message_callback
871 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
873 void guestfs_set_log_message_callback (guestfs_h *handle,
874 guestfs_log_message_cb cb,
877 The callback function C<cb> will be called whenever qemu or the guest
878 writes anything to the console.
880 Use this function to capture kernel messages and similar.
882 Normally there is no log message handler, and log messages are just
885 =head2 guestfs_set_subprocess_quit_callback
887 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
888 void guestfs_set_subprocess_quit_callback (guestfs_h *handle,
889 guestfs_subprocess_quit_cb cb,
892 The callback function C<cb> will be called when the child process
893 quits, either asynchronously or if killed by
894 C<guestfs_kill_subprocess>. (This corresponds to a transition from
895 any state to the CONFIG state).
897 =head2 guestfs_set_launch_done_callback
899 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
900 void guestfs_set_launch_done_callback (guestfs_h *handle,
904 The callback function C<cb> will be called when the child process
905 becomes ready first time after it has been launched. (This
906 corresponds to a transition from LAUNCHING to the READY state).
908 =head1 BLOCK DEVICE NAMING
910 In the kernel there is now quite a profusion of schemata for naming
911 block devices (in this context, by I<block device> I mean a physical
912 or virtual hard drive). The original Linux IDE driver used names
913 starting with C</dev/hd*>. SCSI devices have historically used a
914 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
915 driver became a popular replacement for the old IDE driver
916 (particularly for SATA devices) those devices also used the
917 C</dev/sd*> scheme. Additionally we now have virtual machines with
918 paravirtualized drivers. This has created several different naming
919 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
922 As discussed above, libguestfs uses a qemu appliance running an
923 embedded Linux kernel to access block devices. We can run a variety
924 of appliances based on a variety of Linux kernels.
926 This causes a problem for libguestfs because many API calls use device
927 or partition names. Working scripts and the recipe (example) scripts
928 that we make available over the internet could fail if the naming
931 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
932 scheme>. Internally C</dev/sd*> names are translated, if necessary,
933 to other names as required. For example, under RHEL 5 which uses the
934 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
935 C</dev/hda2> transparently.
937 Note that this I<only> applies to parameters. The
938 C<guestfs_list_devices>, C<guestfs_list_partitions> and similar calls
939 return the true names of the devices and partitions as known to the
942 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
944 Usually this translation is transparent. However in some (very rare)
945 cases you may need to know the exact algorithm. Such cases include
946 where you use C<guestfs_config> to add a mixture of virtio and IDE
947 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
948 and C</dev/vd*> devices.
950 The algorithm is applied only to I<parameters> which are known to be
951 either device or partition names. Return values from functions such
952 as C<guestfs_list_devices> are never changed.
958 Is the string a parameter which is a device or partition name?
962 Does the string begin with C</dev/sd>?
966 Does the named device exist? If so, we use that device.
967 However if I<not> then we continue with this algorithm.
971 Replace initial C</dev/sd> string with C</dev/hd>.
973 For example, change C</dev/sda2> to C</dev/hda2>.
975 If that named device exists, use it. If not, continue.
979 Replace initial C</dev/sd> string with C</dev/vd>.
981 If that named device exists, use it. If not, return an error.
985 =head2 PORTABILITY CONCERNS
987 Although the standard naming scheme and automatic translation is
988 useful for simple programs and guestfish scripts, for larger programs
989 it is best not to rely on this mechanism.
991 Where possible for maximum future portability programs using
992 libguestfs should use these future-proof techniques:
998 Use C<guestfs_list_devices> or C<guestfs_list_partitions> to list
999 actual device names, and then use those names directly.
1001 Since those device names exist by definition, they will never be
1006 Use higher level ways to identify filesystems, such as LVM names,
1007 UUIDs and filesystem labels.
1013 =head2 COMMUNICATION PROTOCOL
1015 Don't rely on using this protocol directly. This section documents
1016 how it currently works, but it may change at any time.
1018 The protocol used to talk between the library and the daemon running
1019 inside the qemu virtual machine is a simple RPC mechanism built on top
1020 of XDR (RFC 1014, RFC 1832, RFC 4506).
1022 The detailed format of structures is in C<src/guestfs_protocol.x>
1023 (note: this file is automatically generated).
1025 There are two broad cases, ordinary functions that don't have any
1026 C<FileIn> and C<FileOut> parameters, which are handled with very
1027 simple request/reply messages. Then there are functions that have any
1028 C<FileIn> or C<FileOut> parameters, which use the same request and
1029 reply messages, but they may also be followed by files sent using a
1032 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1034 For ordinary functions, the request message is:
1036 total length (header + arguments,
1037 but not including the length word itself)
1038 struct guestfs_message_header (encoded as XDR)
1039 struct guestfs_<foo>_args (encoded as XDR)
1041 The total length field allows the daemon to allocate a fixed size
1042 buffer into which it slurps the rest of the message. As a result, the
1043 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1044 4MB), which means the effective size of any request is limited to
1045 somewhere under this size.
1047 Note also that many functions don't take any arguments, in which case
1048 the C<guestfs_I<foo>_args> is completely omitted.
1050 The header contains the procedure number (C<guestfs_proc>) which is
1051 how the receiver knows what type of args structure to expect, or none
1054 The reply message for ordinary functions is:
1056 total length (header + ret,
1057 but not including the length word itself)
1058 struct guestfs_message_header (encoded as XDR)
1059 struct guestfs_<foo>_ret (encoded as XDR)
1061 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1062 for functions that return no formal return values.
1064 As above the total length of the reply is limited to
1065 C<GUESTFS_MESSAGE_MAX>.
1067 In the case of an error, a flag is set in the header, and the reply
1068 message is slightly changed:
1070 total length (header + error,
1071 but not including the length word itself)
1072 struct guestfs_message_header (encoded as XDR)
1073 struct guestfs_message_error (encoded as XDR)
1075 The C<guestfs_message_error> structure contains the error message as a
1078 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1080 A C<FileIn> parameter indicates that we transfer a file I<into> the
1081 guest. The normal request message is sent (see above). However this
1082 is followed by a sequence of file chunks.
1084 total length (header + arguments,
1085 but not including the length word itself,
1086 and not including the chunks)
1087 struct guestfs_message_header (encoded as XDR)
1088 struct guestfs_<foo>_args (encoded as XDR)
1089 sequence of chunks for FileIn param #0
1090 sequence of chunks for FileIn param #1 etc.
1092 The "sequence of chunks" is:
1094 length of chunk (not including length word itself)
1095 struct guestfs_chunk (encoded as XDR)
1097 struct guestfs_chunk (encoded as XDR)
1100 struct guestfs_chunk (with data.data_len == 0)
1102 The final chunk has the C<data_len> field set to zero. Additionally a
1103 flag is set in the final chunk to indicate either successful
1104 completion or early cancellation.
1106 At time of writing there are no functions that have more than one
1107 FileIn parameter. However this is (theoretically) supported, by
1108 sending the sequence of chunks for each FileIn parameter one after
1109 another (from left to right).
1111 Both the library (sender) I<and> the daemon (receiver) may cancel the
1112 transfer. The library does this by sending a chunk with a special
1113 flag set to indicate cancellation. When the daemon sees this, it
1114 cancels the whole RPC, does I<not> send any reply, and goes back to
1115 reading the next request.
1117 The daemon may also cancel. It does this by writing a special word
1118 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1119 during the transfer, and if it gets it, it will cancel the transfer
1120 (it sends a cancel chunk). The special word is chosen so that even if
1121 cancellation happens right at the end of the transfer (after the
1122 library has finished writing and has started listening for the reply),
1123 the "spurious" cancel flag will not be confused with the reply
1126 This protocol allows the transfer of arbitrary sized files (no 32 bit
1127 limit), and also files where the size is not known in advance
1128 (eg. from pipes or sockets). However the chunks are rather small
1129 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1130 daemon need to keep much in memory.
1132 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1134 The protocol for FileOut parameters is exactly the same as for FileIn
1135 parameters, but with the roles of daemon and library reversed.
1137 total length (header + ret,
1138 but not including the length word itself,
1139 and not including the chunks)
1140 struct guestfs_message_header (encoded as XDR)
1141 struct guestfs_<foo>_ret (encoded as XDR)
1142 sequence of chunks for FileOut param #0
1143 sequence of chunks for FileOut param #1 etc.
1145 =head3 INITIAL MESSAGE
1147 Because the underlying channel (QEmu -net channel) doesn't have any
1148 sort of connection control, when the daemon launches it sends an
1149 initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
1150 and daemon is alive. This is what C<guestfs_launch> waits for.
1152 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1154 All high-level libguestfs actions are synchronous. If you want
1155 to use libguestfs asynchronously then you must create a thread.
1157 Only use the handle from a single thread. Either use the handle
1158 exclusively from one thread, or provide your own mutex so that two
1159 threads cannot issue calls on the same handle at the same time.
1161 =head1 QEMU WRAPPERS
1163 If you want to compile your own qemu, run qemu from a non-standard
1164 location, or pass extra arguments to qemu, then you can write a
1165 shell-script wrapper around qemu.
1167 There is one important rule to remember: you I<must C<exec qemu>> as
1168 the last command in the shell script (so that qemu replaces the shell
1169 and becomes the direct child of the libguestfs-using program). If you
1170 don't do this, then the qemu process won't be cleaned up correctly.
1172 Here is an example of a wrapper, where I have built my own copy of
1176 qemudir=/home/rjones/d/qemu
1177 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1179 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1180 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1183 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1185 Note that libguestfs also calls qemu with the -help and -version
1186 options in order to determine features.
1188 =head1 ENVIRONMENT VARIABLES
1192 =item LIBGUESTFS_APPEND
1194 Pass additional options to the guest kernel.
1196 =item LIBGUESTFS_DEBUG
1198 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1199 has the same effect as calling C<guestfs_set_verbose (handle, 1)>.
1201 =item LIBGUESTFS_MEMSIZE
1203 Set the memory allocated to the qemu process, in megabytes. For
1206 LIBGUESTFS_MEMSIZE=700
1208 =item LIBGUESTFS_PATH
1210 Set the path that libguestfs uses to search for kernel and initrd.img.
1211 See the discussion of paths in section PATH above.
1213 =item LIBGUESTFS_QEMU
1215 Set the default qemu binary that libguestfs uses. If not set, then
1216 the qemu which was found at compile time by the configure script is
1219 See also L</QEMU WRAPPERS> above.
1221 =item LIBGUESTFS_TRACE
1223 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1224 has the same effect as calling C<guestfs_set_trace (handle, 1)>.
1228 Location of temporary directory, defaults to C</tmp>.
1230 If libguestfs was compiled to use the supermin appliance then each
1231 handle will require rather a large amount of space in this directory
1232 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1233 configure another directory to use in case C</tmp> is not large
1243 L<http://libguestfs.org/>.
1245 Tools with a similar purpose:
1254 To get a list of bugs against libguestfs use this link:
1256 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1258 To report a new bug against libguestfs use this link:
1260 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1262 When reporting a bug, please check:
1268 That the bug hasn't been reported already.
1272 That you are testing a recent version.
1276 Describe the bug accurately, and give a way to reproduce it.
1280 Run libguestfs-test-tool and paste the B<complete, unedited>
1281 output into the bug report.
1287 Richard W.M. Jones (C<rjones at redhat dot com>)
1291 Copyright (C) 2009 Red Hat Inc.
1292 L<http://libguestfs.org/>
1294 This library is free software; you can redistribute it and/or
1295 modify it under the terms of the GNU Lesser General Public
1296 License as published by the Free Software Foundation; either
1297 version 2 of the License, or (at your option) any later version.
1299 This library is distributed in the hope that it will be useful,
1300 but WITHOUT ANY WARRANTY; without even the implied warranty of
1301 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1302 Lesser General Public License for more details.
1304 You should have received a copy of the GNU Lesser General Public
1305 License along with this library; if not, write to the Free Software
1306 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA