guestfs - Library for accessing and modifying virtual machine images
#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);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 access filesystems over FTP.
Libguestfs is a library that can be linked with C and C++ management programs (or management programs written in OCaml, Perl or Python). 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.
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);guestfs_wait_ready and all of the actions including 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 -1 on error. See section ERROR HANDLING below for how to handle errors.
guestfs_h is the opaque type representing a connection handle. Create a handle by calling guestfs_create. Call guestfs_close to free the handle and release all resources used.
For information on using multiple handles and threads, see the section MULTIPLE HANDLES AND MULTIPLE THREADS below.
guestfs_h *guestfs_create (void);Create a connection handle.
You have to call guestfs_add_drive on the handle at least once.
This function returns a non-NULL pointer to a handle on success or NULL on error.
After configuring the handle, you have to call guestfs_launch and guestfs_wait_ready.
You may also want to configure error handling for the handle. See ERROR HANDLING section below.
void guestfs_close (guestfs_h *handle);This closes the connection handle and frees up all resources used.
The convention in all functions that return int is that they return -1 to indicate an error. You can get additional information on errors by calling guestfs_last_error and/or by setting up an error handler with guestfs_set_error_handler.
The default error handler prints the information string to stderr.
Out of memory errors are handled differently. The default action is to call abort(3). If this is undesirable, then you can set a handler using guestfs_set_out_of_memory_handler.
const char *guestfs_last_error (guestfs_h *handle);This returns the last error message that happened on handle. If there has not been an error since the handle was created, then this returns NULL.
The lifetime of the returned string is until the next error occurs, or guestfs_close is called.
The error string is not localized (ie. is always in English), because this makes searching for error messages in search engines give the largest number of results.
typedef void (*guestfs_error_handler_cb) (guestfs_h *handle,
void *data,
const char *msg);
void guestfs_set_error_handler (guestfs_h *handle,
guestfs_error_handler_cb cb,
void *data);The callback 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.
Note that the message string msg is freed as soon as the callback function returns, so if you want to stash it somewhere you must make your own copy.
The default handler prints messages on stderr.
If you set cb to NULL then no handler is called.
guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *handle,
void **data_rtn);Returns the current error handler callback.
typedef void (*guestfs_abort_cb) (void);
int guestfs_set_out_of_memory_handler (guestfs_h *handle,
guestfs_abort_cb);The callback cb will be called if there is an out of memory situation. Note this callback must not return.
The default is to call abort(3).
You cannot set cb to NULL. You can't ignore out of memory situations.
guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *handle);This returns the current out of memory handler.
Libguestfs needs a kernel and initrd.img, which it finds by looking along an internal path.
By default it looks for these in the directory $libdir/guestfs (eg. /usr/local/lib/guestfs or /usr/lib64/guestfs).
Use guestfs_set_path or set the environment variable LIBGUESTFS_PATH to change the directories that libguestfs will search in. The value is a colon-separated list of paths. The current directory is not searched unless the path contains an empty element or .. For example LIBGUESTFS_PATH=:/usr/lib/guestfs would search the current directory and then /usr/lib/guestfs.
@ACTIONS@
@STRUCTS@
Internally, libguestfs is implemented by running a virtual machine using 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.
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 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 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. guestfs_launch is a non-blocking call that starts up the child process, immediately moving from CONFIG to LAUNCHING. 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 guestfs_wait_ready.
High-level API actions such as 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.
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 guestfs_set_*_callback functions to set a callback for different types of events.
Only one callback of each type can be registered for each handle. Calling guestfs_set_*_callback again overwrites the previous callback of that type. Cancel all callbacks of this type by calling this function with cb set to NULL.
XXX This section was documented in previous versions but never implemented in a way which matched the documentation. For now I have removed the documentation, pending a working implementation. See also src/guestfs-actions.c in the source.
typedef void (*guestfs_send_cb) (guestfs_h *g, void *opaque);
void guestfs_set_send_callback (guestfs_h *handle,
guestfs_send_cb cb,
void *opaque);The callback function cb will be called whenever a message which is queued for sending, has been sent.
typedef void (*guestfs_reply_cb) (guestfs_h *g, void *opaque, XDR *xdr);
void guestfs_set_reply_callback (guestfs_h *handle,
guestfs_reply_cb cb,
void *opaque);The callback function 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 that the xdr that you get in the callback is in XDR_DECODE mode, and you need to consume it before you return from the callback function (since it gets destroyed after).
typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
char *buf, int len);
void guestfs_set_log_message_callback (guestfs_h *handle,
guestfs_log_message_cb cb,
void *opaque);The callback function 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.
typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
void guestfs_set_subprocess_quit_callback (guestfs_h *handle,
guestfs_subprocess_quit_cb cb,
void *opaque);The callback function cb will be called when the child process quits, either asynchronously or if killed by guestfs_kill_subprocess. (This corresponds to a transition from any state to the CONFIG state).
typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
void guestfs_set_launch_done_callback (guestfs_h *handle,
guestfs_ready_cb cb,
void *opaque);The callback function 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 guestfs_wait_ready to implement a non-blocking wait for the child process to finish booting up.
To use the low-level event API and/or to use handles from multiple threads, you have to provide an event "main loop". You can write your own, but if you don't want to write one, two types are provided for you:
- libguestfs-select
-
A simple main loop that is implemented using select(2).
This is the default main loop for new guestfs handles, unless you call
guestfs_set_main_loopafter a handle is created. - 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.
The support for multiple handles and multiple threads is modelled after glib (although doesn't require glib, if you use the select-based main loop).
http://library.gnome.org/devel/glib/unstable/glib-The-Main-Event-Loop.html
You will need to create one main loop for each thread that wants to use libguestfs. Each guestfs handle should be confined to one thread. If you try to pass guestfs handles between threads, you will get undefined results.
If you only want to use guestfs handles from one thread in your program, but your program has other threads doing other things, then you don't need to do anything special.
In the single thread case, there is a single select-based main loop created for you. All guestfs handles will use this main loop to execute high level API actions.
In the multiple threads case, you will need to create a main loop for each thread that wants to use libguestfs.
To create main loops for other threads, use guestfs_create_main_loop or guestfs_glib_create_main_loop.
Then you will need to attach each handle to the thread-specific main loop by calling:
handle = guestfs_create ();
guestfs_set_main_loop (handle, main_loop_of_current_thread);void guestfs_set_main_loop (guestfs_h *handle,
guestfs_main_loop *main_loop);Sets the main loop used by high level API actions for this handle. By default, the select-based main loop is used (see guestfs_get_default_main_loop).
You only need to use this in multi-threaded programs, where multiple threads want to use libguestfs. Create a main loop for each thread, then call this function.
You cannot pass guestfs handles between threads.
guestfs_main_loop *guestfs_get_main_loop (guestfs_h *handle);Return the main loop used by handle.
guestfs_main_loop *guestfs_get_default_main_loop (void);Return the default select-based main loop.
guestfs_main_loop *guestfs_create_main_loop (void);This creates a select-based main loop. You should create one main loop for each additional thread that needs to use libguestfs.
void guestfs_free_main_loop (guestfs_main_loop *);Free the select-based main loop which was previously allocated with guestfs_create_main_loop.
This isn't documented. Please see the libguestfs-select and libguestfs-glib implementations.
Don't rely on using this protocol directly. This section documents how it currently works, but it may change at any time.
The protocol used to talk between the library and the daemon running inside the qemu virtual machine is a simple RPC mechanism built on top of XDR (RFC 1014, RFC 1832, RFC 4506).
The detailed format of structures is in src/guestfs_protocol.x (note: this file is automatically generated).
There are two broad cases, ordinary functions that don't have any FileIn and FileOut parameters, which are handled with very simple request/reply messages. Then there are functions that have any FileIn or FileOut parameters, which use the same request and reply messages, but they may also be followed by files sent using a chunked encoding.
For ordinary functions, the request message is:
total length (header + arguments,
but not including the length word itself)
struct guestfs_message_header (encoded as XDR)
struct guestfs_<foo>_args (encoded as XDR)The total length field allows the daemon to allocate a fixed size buffer into which it slurps the rest of the message. As a result, the total length is limited to GUESTFS_MESSAGE_MAX bytes (currently 4MB), which means the effective size of any request is limited to somewhere under this size.
Note also that many functions don't take any arguments, in which case the guestfs_foo_args is completely omitted.
The header contains the procedure number (guestfs_proc) which is how the receiver knows what type of args structure to expect, or none at all.
The reply message for ordinary functions is:
total length (header + ret,
but not including the length word itself)
struct guestfs_message_header (encoded as XDR)
struct guestfs_<foo>_ret (encoded as XDR)As above the guestfs_foo_ret structure may be completely omitted for functions that return no formal return values.
As above the total length of the reply is limited to GUESTFS_MESSAGE_MAX.
In the case of an error, a flag is set in the header, and the reply message is slightly changed:
total length (header + error,
but not including the length word itself)
struct guestfs_message_header (encoded as XDR)
struct guestfs_message_error (encoded as XDR)The guestfs_message_error structure contains the error message as a string.
A FileIn parameter indicates that we transfer a file into the guest. The normal request message is sent (see above). However this is followed by a sequence of file chunks.
total length (header + arguments,
but not including the length word itself,
and not including the chunks)
struct guestfs_message_header (encoded as XDR)
struct guestfs_<foo>_args (encoded as XDR)
sequence of chunks for FileIn param #0
sequence of chunks for FileIn param #1 etc.The "sequence of chunks" is:
length of chunk (not including length word itself)
struct guestfs_chunk (encoded as XDR)
length of chunk
struct guestfs_chunk (encoded as XDR)
...
length of chunk
struct guestfs_chunk (with data.data_len == 0)The final chunk has the data_len field set to zero. Additionally a flag is set in the final chunk to indicate either successful completion or early cancellation.
At time of writing there are no functions that have more than one FileIn parameter. However this is (theoretically) supported, by sending the sequence of chunks for each FileIn parameter one after another (from left to right).
Both the library (sender) and the daemon (receiver) may cancel the transfer. The library does this by sending a chunk with a special flag set to indicate cancellation. When the daemon sees this, it cancels the whole RPC, does not send any reply, and goes back to reading the next request.
The daemon may also cancel. It does this by writing a special word GUESTFS_CANCEL_FLAG to the socket. The library listens for this during the transfer, and if it gets it, it will cancel the transfer (it sends a cancel chunk). The special word is chosen so that even if cancellation happens right at the end of the transfer (after the library has finished writing and has started listening for the reply), the "spurious" cancel flag will not be confused with the reply message.
This protocol allows the transfer of arbitrary sized files (no 32 bit limit), and also files where the size is not known in advance (eg. from pipes or sockets). However the chunks are rather small (GUESTFS_MAX_CHUNK_SIZE), so that neither the library nor the daemon need to keep much in memory.
The protocol for FileOut parameters is exactly the same as for FileIn parameters, but with the roles of daemon and library reversed.
total length (header + ret,
but not including the length word itself,
and not including the chunks)
struct guestfs_message_header (encoded as XDR)
struct guestfs_<foo>_ret (encoded as XDR)
sequence of chunks for FileOut param #0
sequence of chunks for FileOut param #1 etc.Because the underlying channel (QEmu -net channel) doesn't have any sort of connection control, when the daemon launches it sends an initial word (GUESTFS_LAUNCH_FLAG) which indicates that the guest and daemon is alive. This is what guestfs_wait_ready waits for.
- LIBGUESTFS_DEBUG
-
Set
LIBGUESTFS_DEBUG=1to enable verbose messages. This has the same effect as callingguestfs_set_verbose (handle, 1). - LIBGUESTFS_PATH
-
Set the path that libguestfs uses to search for kernel and initrd.img. See the discussion of paths in section PATH above.
guestfish(1), qemu(1), febootstrap(1), http://et.redhat.com/~rjones/libguestfs.
Richard W.M. Jones (rjones at redhat dot com)
Copyright (C) 2009 Red Hat Inc. 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