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, Java
40 or Haskell). You can also use it from shell scripts or the command line.
42 You don't need to be root to use libguestfs, although obviously you do
43 need enough permissions to access the disk images.
45 Libguestfs is a large API because it can do many things. For a gentle
46 introduction, please read the L</API OVERVIEW> section next.
50 This section provides a gentler overview of the libguestfs API. We
51 also try to group API calls together, where that may not be obvious
52 from reading about the individual calls below.
56 Before you can use libguestfs calls, you have to create a handle.
57 Then you must add at least one disk image to the handle, followed by
58 launching the handle, then performing whatever operations you want,
59 and finally closing the handle. So the general structure of all
60 libguestfs-using programs looks like this:
62 guestfs_h *handle = guestfs_create ();
64 /* Call guestfs_add_drive additional times if there are
65 * multiple disk images.
67 guestfs_add_drive (handle, "guest.img");
69 /* Most manipulation calls won't work until you've launched
70 * the handle. You have to do this _after_ adding drives
71 * and _before_ other commands.
73 guestfs_launch (handle);
75 /* Now you can examine what partitions, LVs etc are available.
77 char **partitions = guestfs_list_partitions (handle);
78 char **logvols = guestfs_lvs (handle);
80 /* To access a filesystem in the image, you must mount it.
82 guestfs_mount (handle, "/dev/sda1", "/");
84 /* Now you can perform filesystem actions on the guest
87 guestfs_touch (handle, "/hello");
89 /* You only need to call guestfs_sync if you have made
90 * changes to the guest image.
92 guestfs_sync (handle);
94 /* Close the handle. */
95 guestfs_close (handle);
97 The code above doesn't include any error checking. In real code you
98 should check return values carefully for errors. In general all
99 functions that return integers return C<-1> on error, and all
100 functions that return pointers return C<NULL> on error. See section
101 L</ERROR HANDLING> below for how to handle errors, and consult the
102 documentation for each function call below to see precisely how they
103 return error indications.
107 The image filename (C<"guest.img"> in the example above) could be a
108 disk image from a virtual machine, a L<dd(1)> copy of a physical hard
109 disk, an actual block device, or simply an empty file of zeroes that
110 you have created through L<posix_fallocate(3)>. Libguestfs lets you
111 do useful things to all of these.
113 You can add a disk read-only using C<guestfs_add_drive_ro>, in which
114 case libguestfs won't modify the file.
116 Be extremely cautious if the disk image is in use, eg. if it is being
117 used by a virtual machine. Adding it read-write will almost certainly
118 cause disk corruption, but adding it read-only is safe.
120 You must add at least one disk image, and you may add multiple disk
121 images. In the API, the disk images are usually referred to as
122 C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
125 Once C<guestfs_launch> has been called you cannot add any more images.
126 You can call C<guestfs_list_devices> to get a list of the device
127 names, in the order that you added them. See also L</BLOCK DEVICE
132 Before you can read or write files, create directories and so on in a
133 disk image that contains filesystems, you have to mount those
134 filesystems using C<guestfs_mount>. If you already know that a disk
135 image contains (for example) one partition with a filesystem on that
136 partition, then you can mount it directly:
138 guestfs_mount (handle, "/dev/sda1", "/");
140 where C</dev/sda1> means literally the first partition (C<1>) of the
141 first disk image that we added (C</dev/sda>). If the disk contains
142 Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).
144 If you are given a disk image and you don't know what it contains then
145 you have to find out. Libguestfs can do that too: use
146 C<guestfs_list_partitions> and C<guestfs_lvs> to list possible
147 partitions and LVs, and either try mounting each to see what is
148 mountable, or else examine them with C<guestfs_file>. But you might
149 find it easier to look at higher level programs built on top of
150 libguestfs, in particular L<virt-inspector(1)>.
152 To mount a disk image read-only, use C<guestfs_mount_ro>. There are
153 several other variations of the C<guestfs_mount_*> call.
155 =head2 FILESYSTEM ACCESS AND MODIFICATION
157 The majority of the libguestfs API consists of fairly low-level calls
158 for accessing and modifying the files, directories, symlinks etc on
159 mounted filesystems. There are over a hundred such calls which you
160 can find listed in detail below in this man page, and we don't even
161 pretend to cover them all in this overview.
163 Specify filenames as full paths including the mount point.
165 For example, if you mounted a filesystem at C<"/"> and you want to
166 read the file called C<"etc/passwd"> then you could do:
168 char *data = guestfs_cat (handle, "/etc/passwd");
170 This would return C<data> as a newly allocated buffer containing the
171 full content of that file (with some conditions: see also
172 L</DOWNLOADING> below), or C<NULL> if there was an error.
174 As another example, to create a top-level directory on that filesystem
175 called C<"var"> you would do:
177 guestfs_mkdir (handle, "/var");
179 To create a symlink you could do:
181 guestfs_ln_s (handle, "/etc/init.d/portmap",
182 "/etc/rc3.d/S30portmap");
184 Libguestfs will reject attempts to use relative paths. There is no
185 concept of a current working directory. Libguestfs can return errors
186 in many situations: for example if the filesystem isn't writable, or
187 if a file or directory that you requested doesn't exist. If you are
188 using the C API (documented here) you have to check for those error
189 conditions after each call. (Other language bindings turn these
190 errors into exceptions).
192 File writes are affected by the per-handle umask, set by calling
193 C<guestfs_umask> and defaulting to 022.
197 Libguestfs contains API calls to read, create and modify partition
198 tables on disk images.
200 In the common case where you want to create a single partition
201 covering the whole disk, you should use the C<guestfs_part_disk>
204 const char *parttype = "mbr";
205 if (disk_is_larger_than_2TB)
207 guestfs_part_disk (g, "/dev/sda", parttype);
209 Obviously this effectively wipes anything that was on that disk image
212 In general MBR partitions are both unnecessarily complicated and
213 depend on archaic details, namely the Cylinder-Head-Sector (CHS)
214 geometry of the disk. C<guestfs_sfdiskM> can be used to
215 create more complex arrangements where the relative sizes are
216 expressed in megabytes instead of cylinders, which is a small win.
217 C<guestfs_sfdiskM> will choose the nearest cylinder to approximate the
218 requested size. There's a lot of crazy stuff to do with IDE and
219 virtio disks having different, incompatible CHS geometries, that you
220 probably don't want to know about.
222 My advice: make a single partition to cover the whole disk, then use
227 Libguestfs provides access to a large part of the LVM2 API, such as
228 C<guestfs_lvcreate> and C<guestfs_vgremove>. It won't make much sense
229 unless you familiarize yourself with the concepts of physical volumes,
230 volume groups and logical volumes.
232 This author strongly recommends reading the LVM HOWTO, online at
233 L<http://tldp.org/HOWTO/LVM-HOWTO/>.
237 Use C<guestfs_cat> to download small, text only files. This call
238 is limited to files which are less than 2 MB and which cannot contain
239 any ASCII NUL (C<\0>) characters. However it has a very simple
242 C<guestfs_read_file> can be used to read files which contain
243 arbitrary 8 bit data, since it returns a (pointer, size) pair.
244 However it is still limited to "small" files, less than 2 MB.
246 C<guestfs_download> can be used to download any file, with no
247 limits on content or size (even files larger than 4 GB).
249 To download multiple files, see C<guestfs_tar_out> and
254 It's often the case that you want to write a file or files to the disk
257 For small, single files, use C<guestfs_write_file>. This call
258 currently contains a bug which limits the call to plain text files
259 (not containing ASCII NUL characters).
261 To upload a single file, use C<guestfs_upload>. This call has no
262 limits on file content or size (even files larger than 4 GB).
264 To upload multiple files, see C<guestfs_tar_in> and C<guestfs_tgz_in>.
266 However the fastest way to upload I<large numbers of arbitrary files>
267 is to turn them into a squashfs or CD ISO (see L<mksquashfs(8)> and
268 L<mkisofs(8)>), then attach this using C<guestfs_add_drive_ro>. If
269 you add the drive in a predictable way (eg. adding it last after all
270 other drives) then you can get the device name from
271 C<guestfs_list_devices> and mount it directly using
272 C<guestfs_mount_ro>. Note that squashfs images are sometimes
273 non-portable between kernel versions, and they don't support labels or
274 UUIDs. If you want to pre-build an image or you need to mount it
275 using a label or UUID, use an ISO image instead.
279 There are various different commands for copying between files and
280 devices and in and out of the guest filesystem. These are summarised
285 =item B<file> to B<file>
287 Use L</guestfs_cp> to copy a single file, or
288 L</guestfs_cp_a> to copy directories recursively.
290 =item B<file or device> to B<file or device>
292 Use L</guestfs_dd> which efficiently uses L<dd(1)>
293 to copy between files and devices in the guest.
295 Example: duplicate the contents of an LV:
297 guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
299 The destination (C</dev/VG/Copy>) must be at least as large as the
300 source (C</dev/VG/Original>).
302 =item B<file on the host> to B<file or device>
304 Use L</guestfs_upload>. See L</UPLOADING> above.
306 =item B<file or device> to B<file on the host>
308 Use L</guestfs_download>. See L</DOWNLOADING> above.
314 C<guestfs_ll> is just designed for humans to read (mainly when using
315 the L<guestfish(1)>-equivalent command C<ll>).
317 C<guestfs_ls> is a quick way to get a list of files in a directory
318 from programs, as a flat list of strings.
320 C<guestfs_readdir> is a programmatic way to get a list of files in a
321 directory, plus additional information about each one. It is more
322 equivalent to using the L<readdir(3)> call on a local filesystem.
324 C<guestfs_find> can be used to recursively list files.
326 =head2 RUNNING COMMANDS
328 Although libguestfs is a primarily an API for manipulating files
329 inside guest images, we also provide some limited facilities for
330 running commands inside guests.
332 There are many limitations to this:
338 The kernel version that the command runs under will be different
339 from what it expects.
343 If the command needs to communicate with daemons, then most likely
344 they won't be running.
348 The command will be running in limited memory.
352 Only supports Linux guests (not Windows, BSD, etc).
356 Architecture limitations (eg. won't work for a PPC guest on
361 For SELinux guests, you may need to enable SELinux and load policy
362 first. See L</SELINUX> in this manpage.
366 The two main API calls to run commands are C<guestfs_command> and
367 C<guestfs_sh> (there are also variations).
369 The difference is that C<guestfs_sh> runs commands using the shell, so
370 any shell globs, redirections, etc will work.
372 =head2 CONFIGURATION FILES
374 To read and write configuration files in Linux guest filesystems, we
375 strongly recommend using Augeas. For example, Augeas understands how
376 to read and write, say, a Linux shadow password file or X.org
377 configuration file, and so avoids you having to write that code.
379 The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
380 don't document Augeas itself here because there is excellent
381 documentation on the L<http://augeas.net/> website.
383 If you don't want to use Augeas (you fool!) then try calling
384 C<guestfs_read_lines> to get the file as a list of lines which
385 you can iterate over.
389 We support SELinux guests. To ensure that labeling happens correctly
390 in SELinux guests, you need to enable SELinux and load the guest's
397 Before launching, do:
399 guestfs_set_selinux (g, 1);
403 After mounting the guest's filesystem(s), load the policy. This
404 is best done by running the L<load_policy(8)> command in the
407 guestfs_sh (g, "/usr/sbin/load_policy");
409 (Older versions of C<load_policy> require you to specify the
410 name of the policy file).
414 Optionally, set the security context for the API. The correct
415 security context to use can only be known by inspecting the
416 guest. As an example:
418 guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0");
422 This will work for running commands and editing existing files.
424 When new files are created, you may need to label them explicitly,
425 for example by running the external command
426 C<restorecon pathname>.
428 =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
430 Libguestfs can mount NTFS partitions. It does this using the
431 L<http://www.ntfs-3g.org/> driver.
433 DOS and Windows still use drive letters, and the filesystems are
434 always treated as case insensitive by Windows itself, and therefore
435 you might find a Windows configuration file referring to a path like
436 C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
437 that directory might be referred to as C</WINDOWS/System32>.
439 Drive letter mappings are outside the scope of libguestfs. You have
440 to use libguestfs to read the appropriate Windows Registry and
441 configuration files, to determine yourself how drives are mapped (see
442 also L<virt-inspector(1)>).
444 Replacing backslash characters with forward slash characters is also
445 outside the scope of libguestfs, but something that you can easily do.
447 Where we can help is in resolving the case insensitivity of paths.
448 For this, call C<guestfs_case_sensitive_path>.
450 Libguestfs also provides some help for decoding Windows Registry
451 "hive" files, through the library C<libhivex> which is part of
452 libguestfs. You have to locate and download the hive file(s)
453 yourself, and then pass them to C<libhivex> functions. See also the
454 programs L<hivexml(1)>, L<hivexget(1)> and L<virt-win-reg(1)> for more
457 =head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
459 Although we don't want to discourage you from using the C API, we will
460 mention here that the same API is also available in other languages.
462 The API is broadly identical in all supported languages. This means
463 that the C call C<guestfs_mount(handle,path)> is
464 C<$handle-E<gt>mount($path)> in Perl, C<handle.mount(path)> in Python,
465 and C<Guestfs.mount handle path> in OCaml. In other words, a
466 straightforward, predictable isomorphism between each language.
468 Error messages are automatically transformed
469 into exceptions if the language supports it.
471 We don't try to "object orientify" parts of the API in OO languages,
472 although contributors are welcome to write higher level APIs above
473 what we provide in their favourite languages if they wish.
479 You can use the I<guestfs.h> header file from C++ programs. The C++
480 API is identical to the C API. C++ classes and exceptions are
485 This is the only language binding that is incomplete. Only calls
486 which return simple integers have been bound in Haskell, and we are
487 looking for help to complete this binding.
491 Full documentation is contained in the Javadoc which is distributed
496 For documentation see the file C<guestfs.mli>.
500 For documentation see L<Sys::Guestfs(3)>.
504 For documentation do:
512 Use the Guestfs module. There is no Ruby-specific documentation, but
513 you can find examples written in Ruby in the libguestfs source.
515 =item B<shell scripts>
517 For documentation see L<guestfish(1)>.
521 =head1 CONNECTION MANAGEMENT
525 C<guestfs_h> is the opaque type representing a connection handle.
526 Create a handle by calling C<guestfs_create>. Call C<guestfs_close>
527 to free the handle and release all resources used.
529 For information on using multiple handles and threads, see the section
530 L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
532 =head2 guestfs_create
534 guestfs_h *guestfs_create (void);
536 Create a connection handle.
538 You have to call C<guestfs_add_drive> on the handle at least once.
540 This function returns a non-NULL pointer to a handle on success or
543 After configuring the handle, you have to call C<guestfs_launch>.
545 You may also want to configure error handling for the handle. See
546 L</ERROR HANDLING> section below.
550 void guestfs_close (guestfs_h *handle);
552 This closes the connection handle and frees up all resources used.
554 =head1 ERROR HANDLING
556 The convention in all functions that return C<int> is that they return
557 C<-1> to indicate an error. You can get additional information on
558 errors by calling C<guestfs_last_error> and/or by setting up an error
559 handler with C<guestfs_set_error_handler>.
561 The default error handler prints the information string to C<stderr>.
563 Out of memory errors are handled differently. The default action is
564 to call L<abort(3)>. If this is undesirable, then you can set a
565 handler using C<guestfs_set_out_of_memory_handler>.
567 =head2 guestfs_last_error
569 const char *guestfs_last_error (guestfs_h *handle);
571 This returns the last error message that happened on C<handle>. If
572 there has not been an error since the handle was created, then this
575 The lifetime of the returned string is until the next error occurs, or
576 C<guestfs_close> is called.
578 The error string is not localized (ie. is always in English), because
579 this makes searching for error messages in search engines give the
580 largest number of results.
582 =head2 guestfs_set_error_handler
584 typedef void (*guestfs_error_handler_cb) (guestfs_h *handle,
587 void guestfs_set_error_handler (guestfs_h *handle,
588 guestfs_error_handler_cb cb,
591 The callback C<cb> will be called if there is an error. The
592 parameters passed to the callback are an opaque data pointer and the
593 error message string.
595 Note that the message string C<msg> is freed as soon as the callback
596 function returns, so if you want to stash it somewhere you must make
599 The default handler prints messages on C<stderr>.
601 If you set C<cb> to C<NULL> then I<no> handler is called.
603 =head2 guestfs_get_error_handler
605 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *handle,
608 Returns the current error handler callback.
610 =head2 guestfs_set_out_of_memory_handler
612 typedef void (*guestfs_abort_cb) (void);
613 int guestfs_set_out_of_memory_handler (guestfs_h *handle,
616 The callback C<cb> will be called if there is an out of memory
617 situation. I<Note this callback must not return>.
619 The default is to call L<abort(3)>.
621 You cannot set C<cb> to C<NULL>. You can't ignore out of memory
624 =head2 guestfs_get_out_of_memory_handler
626 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *handle);
628 This returns the current out of memory handler.
632 Libguestfs needs a kernel and initrd.img, which it finds by looking
633 along an internal path.
635 By default it looks for these in the directory C<$libdir/guestfs>
636 (eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
638 Use C<guestfs_set_path> or set the environment variable
639 C<LIBGUESTFS_PATH> to change the directories that libguestfs will
640 search in. The value is a colon-separated list of paths. The current
641 directory is I<not> searched unless the path contains an empty element
642 or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
643 search the current directory and then C</usr/lib/guestfs>.
645 =head1 HIGH-LEVEL API ACTIONS
649 We guarantee the libguestfs ABI (binary interface), for public,
650 high-level actions as outlined in this section. Although we will
651 deprecate some actions, for example if they get replaced by newer
652 calls, we will keep the old actions forever. This allows you the
653 developer to program in confidence against libguestfs.
663 =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
665 Using L</guestfs_available> you can test availability of
666 the following groups of functions. This test queries the
667 appliance to see if the appliance you are currently using
668 supports the functionality.
672 =head2 SINGLE CALLS AT COMPILE TIME
674 If you need to test whether a single libguestfs function is
675 available at compile time, we recommend using build tools
676 such as autoconf or cmake. For example in autotools you could
679 AC_CHECK_LIB([guestfs],[guestfs_create])
680 AC_CHECK_FUNCS([guestfs_dd])
682 which would result in C<HAVE_GUESTFS_DD> being either defined
683 or not defined in your program.
685 =head2 SINGLE CALLS AT RUN TIME
687 Testing at compile time doesn't guarantee that a function really
688 exists in the library. The reason is that you might be dynamically
689 linked against a previous I<libguestfs.so> (dynamic library)
690 which doesn't have the call. This situation unfortunately results
691 in a segmentation fault, which is a shortcoming of the C dynamic
692 linking system itself.
694 You can use L<dlopen(3)> to test if a function is available
695 at run time, as in this example program (note that you still
696 need the compile time check as well):
708 #ifdef HAVE_GUESTFS_DD
712 /* Test if the function guestfs_dd is really available. */
713 dl = dlopen (NULL, RTLD_LAZY);
715 fprintf (stderr, "dlopen: %s\n", dlerror ());
718 has_function = dlsym (dl, "guestfs_dd") != NULL;
722 printf ("this libguestfs.so does NOT have guestfs_dd function\n");
724 printf ("this libguestfs.so has guestfs_dd function\n");
725 /* Now it's safe to call
726 guestfs_dd (g, "foo", "bar");
730 printf ("guestfs_dd function was not found at compile time\n");
734 You may think the above is an awful lot of hassle, and it is.
735 There are other ways outside of the C linking system to ensure
736 that this kind of incompatibility never arises, such as using
739 Requires: libguestfs >= 1.0.80
743 <!-- old anchor for the next section -->
744 <a name="state_machine_and_low_level_event_api"/>
750 Internally, libguestfs is implemented by running an appliance (a
751 special type of small virtual machine) using L<qemu(1)>. Qemu runs as
752 a child process of the main program.
758 | | child process / appliance
759 | | __________________________
761 +-------------------+ RPC | +-----------------+ |
762 | libguestfs <--------------------> guestfsd | |
763 | | | +-----------------+ |
764 \___________________/ | | Linux kernel | |
765 | +--^--------------+ |
766 \_________|________________/
774 The library, linked to the main program, creates the child process and
775 hence the appliance in the L</guestfs_launch> function.
777 Inside the appliance is a Linux kernel and a complete stack of
778 userspace tools (such as LVM and ext2 programs) and a small
779 controlling daemon called C<guestfsd>. The library talks to
780 C<guestfsd> using remote procedure calls (RPC). There is a mostly
781 one-to-one correspondence between libguestfs API calls and RPC calls
782 to the daemon. Lastly the disk image(s) are attached to the qemu
783 process which translates device access by the appliance's Linux kernel
784 into accesses to the image.
786 A common misunderstanding is that the appliance "is" the virtual
787 machine. Although the disk image you are attached to might also be
788 used by some virtual machine, libguestfs doesn't know or care about
789 this. (But you will care if both libguestfs's qemu process and your
790 virtual machine are trying to update the disk image at the same time,
791 since these usually results in massive disk corruption).
795 libguestfs uses a state machine to model the child process:
806 / | \ \ guestfs_launch
817 \______/ <------ \________/
819 The normal transitions are (1) CONFIG (when the handle is created, but
820 there is no child process), (2) LAUNCHING (when the child process is
821 booting up), (3) alternating between READY and BUSY as commands are
822 issued to, and carried out by, the child process.
824 The guest may be killed by C<guestfs_kill_subprocess>, or may die
825 asynchronously at any time (eg. due to some internal error), and that
826 causes the state to transition back to CONFIG.
828 Configuration commands for qemu such as C<guestfs_add_drive> can only
829 be issued when in the CONFIG state.
831 The high-level API offers two calls that go from CONFIG through
832 LAUNCHING to READY. C<guestfs_launch> blocks until the child process
833 is READY to accept commands (or until some failure or timeout).
834 C<guestfs_launch> internally moves the state from CONFIG to LAUNCHING
837 High-level API actions such as C<guestfs_mount> can only be issued
838 when in the READY state. These high-level API calls block waiting for
839 the command to be carried out (ie. the state to transition to BUSY and
840 then back to READY). But using the low-level event API, you get
841 non-blocking versions. (But you can still only carry out one
842 operation per handle at a time - that is a limitation of the
843 communications protocol we use).
845 Finally, the child process sends asynchronous messages back to the
846 main program, such as kernel log messages. Mostly these are ignored
847 by the high-level API, but using the low-level event API you can
848 register to receive these messages.
850 =head2 SETTING CALLBACKS TO HANDLE EVENTS
852 The child process generates events in some situations. Current events
853 include: receiving a log message, the child process exits.
855 Use the C<guestfs_set_*_callback> functions to set a callback for
856 different types of events.
858 Only I<one callback of each type> can be registered for each handle.
859 Calling C<guestfs_set_*_callback> again overwrites the previous
860 callback of that type. Cancel all callbacks of this type by calling
861 this function with C<cb> set to C<NULL>.
863 =head2 guestfs_set_log_message_callback
865 typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
867 void guestfs_set_log_message_callback (guestfs_h *handle,
868 guestfs_log_message_cb cb,
871 The callback function C<cb> will be called whenever qemu or the guest
872 writes anything to the console.
874 Use this function to capture kernel messages and similar.
876 Normally there is no log message handler, and log messages are just
879 =head2 guestfs_set_subprocess_quit_callback
881 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
882 void guestfs_set_subprocess_quit_callback (guestfs_h *handle,
883 guestfs_subprocess_quit_cb cb,
886 The callback function C<cb> will be called when the child process
887 quits, either asynchronously or if killed by
888 C<guestfs_kill_subprocess>. (This corresponds to a transition from
889 any state to the CONFIG state).
891 =head2 guestfs_set_launch_done_callback
893 typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
894 void guestfs_set_launch_done_callback (guestfs_h *handle,
898 The callback function C<cb> will be called when the child process
899 becomes ready first time after it has been launched. (This
900 corresponds to a transition from LAUNCHING to the READY state).
902 =head1 BLOCK DEVICE NAMING
904 In the kernel there is now quite a profusion of schemata for naming
905 block devices (in this context, by I<block device> I mean a physical
906 or virtual hard drive). The original Linux IDE driver used names
907 starting with C</dev/hd*>. SCSI devices have historically used a
908 different naming scheme, C</dev/sd*>. When the Linux kernel I<libata>
909 driver became a popular replacement for the old IDE driver
910 (particularly for SATA devices) those devices also used the
911 C</dev/sd*> scheme. Additionally we now have virtual machines with
912 paravirtualized drivers. This has created several different naming
913 systems, such as C</dev/vd*> for virtio disks and C</dev/xvd*> for Xen
916 As discussed above, libguestfs uses a qemu appliance running an
917 embedded Linux kernel to access block devices. We can run a variety
918 of appliances based on a variety of Linux kernels.
920 This causes a problem for libguestfs because many API calls use device
921 or partition names. Working scripts and the recipe (example) scripts
922 that we make available over the internet could fail if the naming
925 Therefore libguestfs defines C</dev/sd*> as the I<standard naming
926 scheme>. Internally C</dev/sd*> names are translated, if necessary,
927 to other names as required. For example, under RHEL 5 which uses the
928 C</dev/hd*> scheme, any device parameter C</dev/sda2> is translated to
929 C</dev/hda2> transparently.
931 Note that this I<only> applies to parameters. The
932 C<guestfs_list_devices>, C<guestfs_list_partitions> and similar calls
933 return the true names of the devices and partitions as known to the
936 =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION
938 Usually this translation is transparent. However in some (very rare)
939 cases you may need to know the exact algorithm. Such cases include
940 where you use C<guestfs_config> to add a mixture of virtio and IDE
941 devices to the qemu-based appliance, so have a mixture of C</dev/sd*>
942 and C</dev/vd*> devices.
944 The algorithm is applied only to I<parameters> which are known to be
945 either device or partition names. Return values from functions such
946 as C<guestfs_list_devices> are never changed.
952 Is the string a parameter which is a device or partition name?
956 Does the string begin with C</dev/sd>?
960 Does the named device exist? If so, we use that device.
961 However if I<not> then we continue with this algorithm.
965 Replace initial C</dev/sd> string with C</dev/hd>.
967 For example, change C</dev/sda2> to C</dev/hda2>.
969 If that named device exists, use it. If not, continue.
973 Replace initial C</dev/sd> string with C</dev/vd>.
975 If that named device exists, use it. If not, return an error.
979 =head2 PORTABILITY CONCERNS
981 Although the standard naming scheme and automatic translation is
982 useful for simple programs and guestfish scripts, for larger programs
983 it is best not to rely on this mechanism.
985 Where possible for maximum future portability programs using
986 libguestfs should use these future-proof techniques:
992 Use C<guestfs_list_devices> or C<guestfs_list_partitions> to list
993 actual device names, and then use those names directly.
995 Since those device names exist by definition, they will never be
1000 Use higher level ways to identify filesystems, such as LVM names,
1001 UUIDs and filesystem labels.
1007 =head2 COMMUNICATION PROTOCOL
1009 Don't rely on using this protocol directly. This section documents
1010 how it currently works, but it may change at any time.
1012 The protocol used to talk between the library and the daemon running
1013 inside the qemu virtual machine is a simple RPC mechanism built on top
1014 of XDR (RFC 1014, RFC 1832, RFC 4506).
1016 The detailed format of structures is in C<src/guestfs_protocol.x>
1017 (note: this file is automatically generated).
1019 There are two broad cases, ordinary functions that don't have any
1020 C<FileIn> and C<FileOut> parameters, which are handled with very
1021 simple request/reply messages. Then there are functions that have any
1022 C<FileIn> or C<FileOut> parameters, which use the same request and
1023 reply messages, but they may also be followed by files sent using a
1026 =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
1028 For ordinary functions, the request message is:
1030 total length (header + arguments,
1031 but not including the length word itself)
1032 struct guestfs_message_header (encoded as XDR)
1033 struct guestfs_<foo>_args (encoded as XDR)
1035 The total length field allows the daemon to allocate a fixed size
1036 buffer into which it slurps the rest of the message. As a result, the
1037 total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
1038 4MB), which means the effective size of any request is limited to
1039 somewhere under this size.
1041 Note also that many functions don't take any arguments, in which case
1042 the C<guestfs_I<foo>_args> is completely omitted.
1044 The header contains the procedure number (C<guestfs_proc>) which is
1045 how the receiver knows what type of args structure to expect, or none
1048 The reply message for ordinary functions is:
1050 total length (header + ret,
1051 but not including the length word itself)
1052 struct guestfs_message_header (encoded as XDR)
1053 struct guestfs_<foo>_ret (encoded as XDR)
1055 As above the C<guestfs_I<foo>_ret> structure may be completely omitted
1056 for functions that return no formal return values.
1058 As above the total length of the reply is limited to
1059 C<GUESTFS_MESSAGE_MAX>.
1061 In the case of an error, a flag is set in the header, and the reply
1062 message is slightly changed:
1064 total length (header + error,
1065 but not including the length word itself)
1066 struct guestfs_message_header (encoded as XDR)
1067 struct guestfs_message_error (encoded as XDR)
1069 The C<guestfs_message_error> structure contains the error message as a
1072 =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
1074 A C<FileIn> parameter indicates that we transfer a file I<into> the
1075 guest. The normal request message is sent (see above). However this
1076 is followed by a sequence of file chunks.
1078 total length (header + arguments,
1079 but not including the length word itself,
1080 and not including the chunks)
1081 struct guestfs_message_header (encoded as XDR)
1082 struct guestfs_<foo>_args (encoded as XDR)
1083 sequence of chunks for FileIn param #0
1084 sequence of chunks for FileIn param #1 etc.
1086 The "sequence of chunks" is:
1088 length of chunk (not including length word itself)
1089 struct guestfs_chunk (encoded as XDR)
1091 struct guestfs_chunk (encoded as XDR)
1094 struct guestfs_chunk (with data.data_len == 0)
1096 The final chunk has the C<data_len> field set to zero. Additionally a
1097 flag is set in the final chunk to indicate either successful
1098 completion or early cancellation.
1100 At time of writing there are no functions that have more than one
1101 FileIn parameter. However this is (theoretically) supported, by
1102 sending the sequence of chunks for each FileIn parameter one after
1103 another (from left to right).
1105 Both the library (sender) I<and> the daemon (receiver) may cancel the
1106 transfer. The library does this by sending a chunk with a special
1107 flag set to indicate cancellation. When the daemon sees this, it
1108 cancels the whole RPC, does I<not> send any reply, and goes back to
1109 reading the next request.
1111 The daemon may also cancel. It does this by writing a special word
1112 C<GUESTFS_CANCEL_FLAG> to the socket. The library listens for this
1113 during the transfer, and if it gets it, it will cancel the transfer
1114 (it sends a cancel chunk). The special word is chosen so that even if
1115 cancellation happens right at the end of the transfer (after the
1116 library has finished writing and has started listening for the reply),
1117 the "spurious" cancel flag will not be confused with the reply
1120 This protocol allows the transfer of arbitrary sized files (no 32 bit
1121 limit), and also files where the size is not known in advance
1122 (eg. from pipes or sockets). However the chunks are rather small
1123 (C<GUESTFS_MAX_CHUNK_SIZE>), so that neither the library nor the
1124 daemon need to keep much in memory.
1126 =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS
1128 The protocol for FileOut parameters is exactly the same as for FileIn
1129 parameters, but with the roles of daemon and library reversed.
1131 total length (header + ret,
1132 but not including the length word itself,
1133 and not including the chunks)
1134 struct guestfs_message_header (encoded as XDR)
1135 struct guestfs_<foo>_ret (encoded as XDR)
1136 sequence of chunks for FileOut param #0
1137 sequence of chunks for FileOut param #1 etc.
1139 =head3 INITIAL MESSAGE
1141 Because the underlying channel (QEmu -net channel) doesn't have any
1142 sort of connection control, when the daemon launches it sends an
1143 initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
1144 and daemon is alive. This is what C<guestfs_launch> waits for.
1146 =head1 MULTIPLE HANDLES AND MULTIPLE THREADS
1148 All high-level libguestfs actions are synchronous. If you want
1149 to use libguestfs asynchronously then you must create a thread.
1151 Only use the handle from a single thread. Either use the handle
1152 exclusively from one thread, or provide your own mutex so that two
1153 threads cannot issue calls on the same handle at the same time.
1155 =head1 QEMU WRAPPERS
1157 If you want to compile your own qemu, run qemu from a non-standard
1158 location, or pass extra arguments to qemu, then you can write a
1159 shell-script wrapper around qemu.
1161 There is one important rule to remember: you I<must C<exec qemu>> as
1162 the last command in the shell script (so that qemu replaces the shell
1163 and becomes the direct child of the libguestfs-using program). If you
1164 don't do this, then the qemu process won't be cleaned up correctly.
1166 Here is an example of a wrapper, where I have built my own copy of
1170 qemudir=/home/rjones/d/qemu
1171 exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@"
1173 Save this script as C</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
1174 and then use it by setting the LIBGUESTFS_QEMU environment variable.
1177 LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
1179 Note that libguestfs also calls qemu with the -help and -version
1180 options in order to determine features.
1182 =head1 ENVIRONMENT VARIABLES
1186 =item LIBGUESTFS_APPEND
1188 Pass additional options to the guest kernel.
1190 =item LIBGUESTFS_DEBUG
1192 Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages. This
1193 has the same effect as calling C<guestfs_set_verbose (handle, 1)>.
1195 =item LIBGUESTFS_MEMSIZE
1197 Set the memory allocated to the qemu process, in megabytes. For
1200 LIBGUESTFS_MEMSIZE=700
1202 =item LIBGUESTFS_PATH
1204 Set the path that libguestfs uses to search for kernel and initrd.img.
1205 See the discussion of paths in section PATH above.
1207 =item LIBGUESTFS_QEMU
1209 Set the default qemu binary that libguestfs uses. If not set, then
1210 the qemu which was found at compile time by the configure script is
1213 See also L</QEMU WRAPPERS> above.
1215 =item LIBGUESTFS_TRACE
1217 Set C<LIBGUESTFS_TRACE=1> to enable command traces. This
1218 has the same effect as calling C<guestfs_set_trace (handle, 1)>.
1222 Location of temporary directory, defaults to C</tmp>.
1224 If libguestfs was compiled to use the supermin appliance then each
1225 handle will require rather a large amount of space in this directory
1226 for short periods of time (~ 80 MB). You can use C<$TMPDIR> to
1227 configure another directory to use in case C</tmp> is not large
1237 L<http://libguestfs.org/>.
1239 Tools with a similar purpose:
1248 To get a list of bugs against libguestfs use this link:
1250 L<https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools>
1252 To report a new bug against libguestfs use this link:
1254 L<https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools>
1256 When reporting a bug, please check:
1262 That the bug hasn't been reported already.
1266 That you are testing a recent version.
1270 Describe the bug accurately, and give a way to reproduce it.
1274 Run libguestfs-test-tool and paste the B<complete, unedited>
1275 output into the bug report.
1281 Richard W.M. Jones (C<rjones at redhat dot com>)
1285 Copyright (C) 2009 Red Hat Inc.
1286 L<http://libguestfs.org/>
1288 This library is free software; you can redistribute it and/or
1289 modify it under the terms of the GNU Lesser General Public
1290 License as published by the Free Software Foundation; either
1291 version 2 of the License, or (at your option) any later version.
1293 This library is distributed in the hope that it will be useful,
1294 but WITHOUT ANY WARRANTY; without even the implied warranty of
1295 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
1296 Lesser General Public License for more details.
1298 You should have received a copy of the GNU Lesser General Public
1299 License along with this library; if not, write to the Free Software
1300 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA