[libguestfs.git] / guestfs.pod

=encoding utf8

=head1 NAME

guestfs - Library for accessing and modifying virtual machine images

=head1 SYNOPSIS

 #include <guestfs.h>
 
 guestfs_h *handle = guestfs_create ();
 guestfs_add_drive (handle, "guest.img");
 guestfs_launch (handle);
 guestfs_wait_ready (handle);
 guestfs_mount (handle, "/dev/sda1", "/");
 guestfs_touch (handle, "/hello");
 guestfs_sync (handle);
 guestfs_close (handle);

=head1 DESCRIPTION

Libguestfs is a library for accessing and modifying guest disk images.
Amongst the things this is good for: making batch configuration
changes to guests, getting disk used/free statistics (see also:
virt-df), migrating between virtualization systems (see also:
virt-p2v), performing partial backups, performing partial guest
clones, cloning guests and changing registry/UUID/hostname info, and
much else besides.

Libguestfs uses Linux kernel and qemu code, and can access any type of
guest filesystem that Linux and qemu can, including but not limited
to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition
schemes, qcow, qcow2, vmdk.

Libguestfs provides ways to enumerate guest storage (eg. partitions,
LVs, what filesystem is in each LV, etc.).  It can also run commands
in the context of the guest.  Also you can mount guest filesystems on
the host (requires root privs and NFS).

Libguestfs is a library that can be linked with C and C++ management
programs (or management programs written in other languages, if people
contribute the language bindings).  You can also use it from shell
scripts or the command line.

You don't need to be root to use libguestfs, although obviously you do
need enough permissions to access the disk images.

=head1 CONNECTION MANAGEMENT

If you are using the high-level API, then you should call the
functions in the following order:

 guestfs_h *handle = guestfs_create ();
 
 guestfs_add_drive (handle, "guest.img");
 /* call guestfs_add_drive additional times if the guest has
  * multiple disks
  */
 
 guestfs_launch (handle);
 guestfs_wait_ready (handle);

 /* now you can examine what partitions, LVs etc are available
  * you have to mount / at least
  */ 
 guestfs_mount (handle, "/dev/sda1", "/");

 /* now you can perform actions on the guest disk image */
 guestfs_touch (handle, "/hello");
 
 /* you only need to call guestfs_sync if you have made
  * changes to the guest image
  */
 guestfs_sync (handle);
 
 guestfs_close (handle);

C<guestfs_wait_ready> and all of the actions including C<guestfs_sync>
are blocking calls.  You can use the low-level event API to do
non-blocking operations instead.

All functions that return integers, return C<-1> on error.  See
section ERROR HANDLING below for how to handle errors.

=head2 guestfs_h *

C<guestfs_h> is the opaque type representing a connection handle.
Create a handle by calling C<guestfs_create>.  Call C<guestfs_close>
to free the handle and release all resources used.

Handles and operations on handles are not thread safe.  However you
can use a separate handle for each thread (but not on the same disk
image).

=head2 guestfs_create

 guestfs_h *guestfs_create (void);

Create a connection handle.

You have to call C<guestfs_add_drive> on the handle at least once.
See CONFIGURATION MANAGEMENT section below.

This function returns a non-NULL pointer to a handle on success or
NULL on error.

After configuring the handle, you have to call C<guestfs_launch> and
C<guestfs_wait_ready>.

You may also want to configure error handling for the handle.  See
ERROR HANDLING section below.

=head2 guestfs_close

 void guestfs_close (guestfs_h *handle);

This closes the connection handle and frees up all resources used.

=head2 guestfs_launch, guestfs_wait_ready

 int guestfs_launch (guestfs_h *handle);
 int guestfs_wait_ready (guestfs_h *handle);

Internally libguestfs is implemented by running a virtual machine
using L<qemu(1)>.  These calls are necessary in order to boot the
virtual machine.  More discussion of this is available in the section
STATE MACHINE AND LOW-LEVEL EVENT API below.

You should call these two functions after configuring the handle
(eg. adding drives) but before performing any actions.

=head1 CONFIGURATION MANAGEMENT

The configuration functions allow you to configure which drive images
will be examined or modified, and set other aspects of the L<qemu(1)>
virtual machine that we will be running.  You need to call only
C<guestfs_add_drive> at least once for each guest image that you want
to examine.

=head2 guestfs_add_drive

 int guestfs_add_drive (guestfs_h *handle, const char *filename);

This function adds a virtual machine disk image C<filename> to the
guest.  The first time you call this function, the disk appears as IDE
disk 0 (C</dev/sda>) in the guest, the second time as C</dev/sdb>, and
so on.

You don't necessarily need to be root when using libguestfs.  However
you obviously do need sufficient permissions to access the filename
for whatever operations you want to perform (ie. read access if you
just want to read the image or write access if you want to modify the
image).

This is equivalent to the qemu parameter C<-drive file=filename>.

=head2 guestfs_add_cdrom

 int guestfs_add_cdrom (guestfs_h *handle, const char *filename);

This function adds a virtual CD-ROM disk image to the guest.

This is equivalent to the qemu parameter C<-cdrom filename>.

=head2 guestfs_config

 int guestfs_config (guestfs_h *handle,
                     const char *qemu_param, const char *qemu_value);

This can be used to add arbitrary qemu command line parameters
of the form C<-param value>.  Actually it's not quite arbitrary - we
prevent you from setting some parameters which would interfere with
parameters that we use.

The first character of C<qemu_param> string must be a C<-> (dash).

C<qemu_value> can be NULL.

=head1 ERROR HANDLING

The convention in all functions that return C<int> is that they return
C<-1> to indicate an error.  You can get additional information on
errors by calling C<guestfs_set_error_handler>.  The default error
handler prints the information string to C<stderr>.

Out of memory errors are handled differently.  The default action is
to call L<abort(3)>.  If this is undesirable, then you can set a
handler using C<guestfs_set_out_of_memory_handler>.

=head2 guestfs_set_error_handler

 typedef void (*guestfs_error_handler_cb) (void *data, const char *msg);
 void guestfs_set_error_handler (guestfs_h *handle,
                                 guestfs_error_handler_cb cb,
                                 void *data);

The callback C<cb> will be called if there is an error.  The
parameters passed to the callback are an opaque data pointer and the
error message string.

The default handler prints messages on C<stderr>.

If you set C<cb> to C<NULL> then I<no> handler is called and the error
message is completely discarded.

=head2 guestfs_get_error_handler

 guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *handle);

Returns the current error handler callback.

=head2 guestfs_set_out_of_memory_handler 

 typedef void (*guestfs_abort_cb) (void);
 int guestfs_set_out_of_memory_handler (guestfs_h *handle,
                                        guestfs_abort_cb);

The callback C<cb> will be called if there is an out of memory
situation.  I<Note this callback must not return>.

The default is to call L<abort(3)>.

You cannot set C<cb> to C<NULL>.  You can't ignore out of memory
situations.

=head2 guestfs_get_out_of_memory_handler

 guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *handle);

This returns the current out of memory handler.

=head1 VERBOSE MESSAGES

=head2 guestfs_set_verbose

 void guestfs_set_verbose (guestfs_h *handle, int verbose);

If C<verbose> is true, this turns on verbose messages (to C<stderr>).

Verbose messages are disabled unless the environment variable
C<LIBGUESTFS_DEBUG> is defined and set to C<1>.

=head2 guestfs_get_verbose

 int guestfs_get_verbose (guestfs_h *handle);

This returns the verbose messages flag.

=head1 HIGH-LEVEL API ACTIONS

=head2 guestfs_sync

 int guestfs_sync (guestfs_h *handle);

This syncs the disk, so that any writes are flushed through to the
underlying disk image.

You should always call this if you have modified a disk image, before
calling C<guestfs_close>.

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Documentation will be auto-generated from here, including for
guestfs_sync.

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 do_[action] ([parameters])
 {
   guestfs_set_reply_callback (handle, [action]_cb, data);
   guestfs_nb_[action] (handle, [parameters ...]);

   guestfs_main_loop_run (); /* --> blocks, then calls my_cb */
 }

 [action]_cb (guestfs_h *handle, void *data)
 {
   retval = guestfs_nb_[action]_r (handle);
   /* ... */
   guestfs_main_loop_quit ();
   return retval;
 }

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=head1 STATE MACHINE AND LOW-LEVEL EVENT API

Internally, libguestfs is implemented by running a virtual machine
using L<qemu(1)>.  QEmu runs as a child process of the main program,
and most of this discussion won't make sense unless you understand
that the complexity is dealing with the (asynchronous) actions of the
child process.

                            child process
  ___________________       _________________________
 /                   \     /                         \
 | main program      |     | qemu +-----------------+|
 |                   |     |      | Linux kernel    ||
 +-------------------+     |      +-----------------+|
 | libguestfs     <-------------->| guestfsd        ||
 |                   |     |      +-----------------+|
 \___________________/     \_________________________/

The diagram above shows libguestfs communicating with the guestfsd
daemon running inside the qemu child process.  There are several
points of failure here: qemu can fail to start, the virtual machine
inside qemu can fail to boot, guestfsd can fail to start or not
establish communication, any component can start successfully but fail
asynchronously later, and so on.

=head2 STATE MACHINE

libguestfs uses a state machine to model the child process:

                         |
                    guestfs_create
                         |
                         |
                     ____V_____
                    /          \
                    |  CONFIG  |
                    \__________/
                     ^ ^   ^  \
                    /  |    \  \ guestfs_launch
                   /   |    _\__V______
                  /    |   /           \
                 /     |   | LAUNCHING |
                /      |   \___________/
               /       |       /
              /        |  guestfs_wait_ready
             /         |     /
    ______  /        __|____V
   /      \ ------> /        \
   | BUSY |         | READY  |
   \______/ <------ \________/

The normal transitions are (1) CONFIG (when the handle is created, but
there is no child process), (2) LAUNCHING (when the child process is
booting up), (3) alternating between READY and BUSY as commands are
issued to, and carried out by, the child process.

The guest may be killed by C<guestfs_kill_subprocess>, or may die
asynchronously at any time (eg. due to some internal error), and that
causes the state to transition back to CONFIG.

Configuration commands for qemu such as C<guestfs_add_drive> can only
be issued when in the CONFIG state.

The high-level API offers two calls that go from CONFIG through
LAUNCHING to READY.  C<guestfs_launch> is a non-blocking call that
starts up the child process, immediately moving from CONFIG to
LAUNCHING.  C<guestfs_wait_ready> blocks until the child process is
READY to accept commands (or until some failure or timeout).  The
low-level event API described below provides a non-blocking way to
replace C<guestfs_wait_ready>.

High-level API actions such as C<guestfs_mount> can only be issued
when in the READY state.  These high-level API calls block waiting for
the command to be carried out (ie. the state to transition to BUSY and
then back to READY).  But using the low-level event API, you get
non-blocking versions.  (But you can still only carry out one
operation per handle at a time - that is a limitation of the
communications protocol we use).

Finally, the child process sends asynchronous messages back to the
main program, such as kernel log messages.  Mostly these are ignored
by the high-level API, but using the low-level event API you can
register to receive these messages.

=head2 SETTING CALLBACKS TO HANDLE EVENTS

The child process generates events in some situations.  Current events
include: receiving a reply message after some action, receiving a log
message, the child process exits, &c.

Use the C<guestfs_set_*_callback> functions to set a callback for
different types of events.

Only I<one callback of each type> can be registered for each handle.
Calling C<guestfs_set_*_callback> again overwrites the previous
callback of that type.  Cancel all callbacks of this type by calling
this function with C<cb> set to C<NULL>.

=head2 NON-BLOCKING ACTIONS

C<guestfs_set_reply_callback> is the most interesting callback to
play with, since it allows you to perform actions without blocking.

For example:

 do_it ()
 {
   start_call ();
   guestfs_main_loop_run (); /* --> blocks, then calls my_cb */
 }

 start_call ()
 {
   guestfs_set_reply_callback (handle, my_cb, data);
   guestfs_nb_[action] (handle, [other parameters ...]);
   /* returns immediately */
 }
 
 my_cb (guestfs_h *handle, void *data)
 {
   retval = guestfs_nb_[action]_r (handle);
   /* ... */
 }

There are C<guestfs_nb_*> and C<guestfs_nb_*_r> functions
corresponding to (very nearly) every C<guestfs_*> action in the
high-level API.

=head2 guestfs_set_reply_callback

 void guestfs_set_reply_callback (guestfs_h *handle,
                                  guestfs_reply_cb cb,
                                  void *opaque);

The callback function C<cb> will be called whenever a reply is
received from the child process.  (This corresponds to a transition
from the BUSY state to the READY state).

Note (I<important!>) that high-level API calls overwrite this
callback.

=head2 guestfs_set_log_message_callback

 void guestfs_set_log_message_callback (guestfs_h *handle,
                                        guestfs_log_message_cb cb,
                                        void *opaque);

The callback function C<cb> will be called whenever qemu or the guest
writes anything to the console.

Use this function to capture kernel messages and similar.

Normally there is no log message handler, and log messages are just
discarded.

=head2 guestfs_set_subprocess_quit_callback

 void guestfs_set_subprocess_quit_callback (guestfs_h *handle,
                                            guestfs_subprocess_quit_cb cb,
                                            void *opaque);

The callback function C<cb> will be called when the child process
quits, either asynchronously or if killed by
C<guestfs_kill_subprocess>.  (This corresponds to a transition from
any state to the CONFIG state).

=head2 guestfs_set_launch_done_callback

 void guestfs_set_launch_done_callback (guestfs_h *handle,
                                        guestfs_ready_cb cb,
                                        void *opaque);

The callback function C<cb> will be called when the child process
becomes ready first time after it has been launched.  (This
corresponds to a transition from LAUNCHING to the READY state).

You can use this instead of C<guestfs_wait_ready> to implement a
non-blocking wait for the child process to finish booting up.

=head2 EVENT MAIN LOOP

To use the low-level event API, you have to provide an event "main
loop".  You can write your own, but if you don't want to write one,
two are provided for you:

=over 4

=item libguestfs-poll

A simple main loop that is implemented using L<poll(2)>.

This is the default main loop unless you call C<guestfs_set_main_loop>
or C<guestfs_glib_set_main_loop>.

=item libguestfs-glib

An implementation which can be used with GLib and GTK+ programs.  You
can use this to write graphical (GTK+) programs which use libguestfs
without hanging during long or slow operations.

=back

=head2 guestfs_set_main_loop

 void guestfs_set_main_loop (guestfs_main_loop *);

This call sets the current main loop to the list of callbacks
contained in the C<guestfs_main_loop> structure.

Only one main loop implementation can be used by libguestfs, so
calling this replaces the previous one.  (So this is something that
has to be done by the main program, but only the main program "knows"
that it is a GTK+ program or whatever).

You should call this early in the main program, certainly before
calling C<guestfs_create>.

=head2 guestfs_glib_set_main_loop

 void guestfs_glib_set_main_loop (GMainLoop *);

This helper calls C<guestfs_set_main_loop> with the correct callbacks
for integrating with the GLib main loop.

The libguestfs-glib main loop is contained in a separate library, so
that libguestfs doesn't depend on the whole of GLib:

 #include <glib.h>
 #include <guestfs-glib.h>

 main ()
 {
   GMainLoop *loop =
     g_main_loop_new (g_main_context_default (), 1);
   ...
   guestfs_glib_set_main_loop (loop);
   ...
   g_main_loop_run (loop);
 }

To use this main loop you must link with C<-lguestfs-glib>.  (See also
the GLib and GTK+ documentation).

=head2 guestfs_main_loop_run

 void guestfs_main_loop_run (void);

This calls the main loop.

For some types of main loop you may want or prefer to call another
function, eg. C<g_main_loop_run>, or the main loop may already be
invoked by another part of your program.  In those cases, ignore this
call.

=head2 guestfs_main_loop_quit

 void guestfs_main_loop_quit (void);

This instructs the main loop to quit.  In other words,
C<guestfs_main_loop_run> will return.

For some types of main loop you may want or prefer to call another
function, eg. C<g_main_loop_quit>.  In those cases, ignore this call.

=head2 WRITING A CUSTOM MAIN LOOP

This isn't documented.  Please see the libguestfs-poll and libguestfs-glib
implementations.

=head1 SEE ALSO

L<qemu(1)>





=head1 AUTHORS

Richard W.M. Jones (C<rjones at redhat dot com>)

=head1 COPYRIGHT

Copyright (C) 2009 Red Hat Inc.
L<http://et.redhat.com/~rjones/libguestfs>

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA