=head1 SYNOPSIS
#include <guestfs.h>
-
+
guestfs_h *handle = guestfs_create ();
guestfs_add_drive (handle, "guest.img");
guestfs_launch (handle);
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
+Libguestfs is a large API because it can do many things. For a gentle
+introduction, please read the L</API OVERVIEW> section next.
+
+=head1 API OVERVIEW
-If you are using the high-level API, then you should call the
-functions in the following order:
+This section provides a gentler overview of the libguestfs API. We
+also try to group API calls together, where that may not be obvious
+from reading about the individual calls below.
- guestfs_h *handle = guestfs_create ();
+=head2 HANDLES
+
+Before you can use libguestfs calls, you have to create a handle.
+Then you must add at least one disk image to the handle, followed by
+launching the handle, then performing whatever operations you want,
+and finally closing the handle. So the general structure of all
+libguestfs-using programs looks like this:
+ guestfs_h *handle = guestfs_create ();
+
+ /* Call guestfs_add_drive additional times if there are
+ * multiple disk images.
+ */
guestfs_add_drive (handle, "guest.img");
- /* call guestfs_add_drive additional times if the guest has
- * multiple disks
+
+ /* Most manipulation calls won't work until you've launched
+ * the handle. You have to do this _after_ adding drives
+ * and _before_ other commands.
*/
-
guestfs_launch (handle);
-
- /* now you can examine what partitions, LVs etc are available
- * you have to mount / at least
+
+ /* Now you can examine what partitions, LVs etc are available.
+ */
+ char **partitions = guestfs_list_partitions (handle);
+ char **logvols = guestfs_lvs (handle);
+
+ /* To access a filesystem in the image, you must mount it.
*/
guestfs_mount (handle, "/dev/sda1", "/");
-
- /* now you can perform actions on the guest disk image */
+
+ /* Now you can perform filesystem 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
+
+ /* You only need to call guestfs_sync if you have made
+ * changes to the guest image.
*/
guestfs_sync (handle);
-
+
+ /* Close the handle. */
guestfs_close (handle);
-C<guestfs_launch> 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 L</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.
-
-For information on using multiple handles and threads, see the section
-L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
-
-=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.
-
-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>.
-
-You may also want to configure error handling for the handle. See
-L</ERROR HANDLING> section below.
-
-=head2 guestfs_close
-
- void guestfs_close (guestfs_h *handle);
+The code above doesn't include any error checking. In real code you
+should check return values carefully for errors. In general all
+functions that return integers return C<-1> on error, and all
+functions that return pointers return C<NULL> on error. See section
+L</ERROR HANDLING> below for how to handle errors, and consult the
+documentation for each function call below to see precisely how they
+return error indications.
-This closes the connection handle and frees up all resources used.
+=head2 DISK IMAGES
-=head1 ERROR HANDLING
+The image filename (C<"guest.img"> in the example above) could be a
+disk image from a virtual machine, a L<dd(1)> copy of a physical hard
+disk, an actual block device, or simply an empty file of zeroes that
+you have created through L<posix_fallocate(3)>. Libguestfs lets you
+do useful things to all of these.
-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_last_error> and/or by setting up an error
-handler with 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>.
+You can add a disk read-only using C<guestfs_add_drive_ro>, in which
+case libguestfs won't modify the file.
-=head2 guestfs_last_error
+Be extremely cautious if the disk image is in use, eg. if it is being
+used by a virtual machine. Adding it read-write will almost certainly
+cause disk corruption, but adding it read-only is safe.
- const char *guestfs_last_error (guestfs_h *handle);
+You must add at least one disk image, and you may add multiple disk
+images. In the API, the disk images are usually referred to as
+C</dev/sda> (for the first one you added), C</dev/sdb> (for the second
+one you added), etc.
-This returns the last error message that happened on C<handle>. If
-there has not been an error since the handle was created, then this
-returns C<NULL>.
+Once C<guestfs_launch> has been called you cannot add any more images.
+You can call C<guestfs_list_devices> to get a list of the device
+names, in the order that you added them. See also L</BLOCK DEVICE
+NAMING> below.
-The lifetime of the returned string is until the next error occurs, or
-C<guestfs_close> is called.
+=head2 MOUNTING
-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.
+Before you can read or write files, create directories and so on in a
+disk image that contains filesystems, you have to mount those
+filesystems using C<guestfs_mount>. If you already know that a disk
+image contains (for example) one partition with a filesystem on that
+partition, then you can mount it directly:
-=head2 guestfs_set_error_handler
+ guestfs_mount (handle, "/dev/sda1", "/");
- 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);
+where C</dev/sda1> means literally the first partition (C<1>) of the
+first disk image that we added (C</dev/sda>). If the disk contains
+Linux LVM2 logical volumes you could refer to those instead (eg. C</dev/VG/LV>).
-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.
+If you are given a disk image and you don't know what it contains then
+you have to find out. Libguestfs can do that too: use
+C<guestfs_list_partitions> and C<guestfs_lvs> to list possible
+partitions and LVs, and either try mounting each to see what is
+mountable, or else examine them with C<guestfs_file>. But you might
+find it easier to look at higher level programs built on top of
+libguestfs, in particular L<virt-inspector(1)>.
-Note that the message string C<msg> is freed as soon as the callback
-function returns, so if you want to stash it somewhere you must make
-your own copy.
+To mount a disk image read-only, use C<guestfs_mount_ro>. There are
+several other variations of the C<guestfs_mount_*> call.
-The default handler prints messages on C<stderr>.
+=head2 FILESYSTEM ACCESS AND MODIFICATION
-If you set C<cb> to C<NULL> then I<no> handler is called.
+The majority of the libguestfs API consists of fairly low-level calls
+for accessing and modifying the files, directories, symlinks etc on
+mounted filesystems. There are over a hundred such calls which you
+can find listed in detail below in this man page, and we don't even
+pretend to cover them all in this overview.
-=head2 guestfs_get_error_handler
+Specify filenames as full paths including the mount point.
- guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *handle,
- void **data_rtn);
+For example, if you mounted a filesystem at C<"/"> and you want to
+read the file called C<"etc/passwd"> then you could do:
-Returns the current error handler callback.
+ char *data = guestfs_cat (handle, "/etc/passwd");
-=head2 guestfs_set_out_of_memory_handler
+This would return C<data> as a newly allocated buffer containing the
+full content of that file (with some conditions: see also
+L</DOWNLOADING> below), or C<NULL> if there was an error.
- typedef void (*guestfs_abort_cb) (void);
- int guestfs_set_out_of_memory_handler (guestfs_h *handle,
- guestfs_abort_cb);
+As another example, to create a top-level directory on that filesystem
+called C<"var"> you would do:
-The callback C<cb> will be called if there is an out of memory
-situation. I<Note this callback must not return>.
+ guestfs_mkdir (handle, "/var");
-The default is to call L<abort(3)>.
+To create a symlink you could do:
-You cannot set C<cb> to C<NULL>. You can't ignore out of memory
-situations.
+ guestfs_ln_s (handle, "/etc/init.d/portmap",
+ "/etc/rc3.d/S30portmap");
-=head2 guestfs_get_out_of_memory_handler
+Libguestfs will reject attempts to use relative paths. There is no
+concept of a current working directory. Libguestfs can return errors
+in many situations: for example if the filesystem isn't writable, or
+if a file or directory that you requested doesn't exist. If you are
+using the C API (documented here) you have to check for those error
+conditions after each call. (Other language bindings turn these
+errors into exceptions).
- guestfs_abort_fn guestfs_get_out_of_memory_handler (guestfs_h *handle);
+File writes are affected by the per-handle umask, set by calling
+C<guestfs_umask> and defaulting to 022.
-This returns the current out of memory handler.
+=head2 PARTITIONING
-=head1 PATH
+Libguestfs contains API calls to read, create and modify partition
+tables on disk images.
-Libguestfs needs a kernel and initrd.img, which it finds by looking
-along an internal path.
+In the common case where you want to create a single partition
+covering the whole disk, you should use the C<guestfs_part_disk>
+call:
-By default it looks for these in the directory C<$libdir/guestfs>
-(eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
+ const char *parttype = "mbr";
+ if (disk_is_larger_than_2TB)
+ parttype = "gpt";
+ guestfs_part_disk (g, "/dev/sda", parttype);
-Use C<guestfs_set_path> or set the environment variable
-C<LIBGUESTFS_PATH> to change the directories that libguestfs will
-search in. The value is a colon-separated list of paths. The current
-directory is I<not> searched unless the path contains an empty element
-or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
-search the current directory and then C</usr/lib/guestfs>.
+Obviously this effectively wipes anything that was on that disk image
+before.
-=head1 API OVERVIEW
+In general MBR partitions are both unnecessarily complicated and
+depend on archaic details, namely the Cylinder-Head-Sector (CHS)
+geometry of the disk. C<guestfs_sfdiskM> can be used to
+create more complex arrangements where the relative sizes are
+expressed in megabytes instead of cylinders, which is a small win.
+C<guestfs_sfdiskM> will choose the nearest cylinder to approximate the
+requested size. There's a lot of crazy stuff to do with IDE and
+virtio disks having different, incompatible CHS geometries, that you
+probably don't want to know about.
-This section provides additional documentation for groups of API
-calls, which may not be obvious from reading about the individual
-calls below.
+My advice: make a single partition to cover the whole disk, then use
+LVM on top.
=head2 LVM2
-Libguestfs provides access to a large part of the LVM2 API. It won't
-make much sense unless you familiarize yourself with the concepts of
-physical volumes, volume groups and logical volumes.
+Libguestfs provides access to a large part of the LVM2 API, such as
+C<guestfs_lvcreate> and C<guestfs_vgremove>. It won't make much sense
+unless you familiarize yourself with the concepts of physical volumes,
+volume groups and logical volumes.
This author strongly recommends reading the LVM HOWTO, online at
L<http://tldp.org/HOWTO/LVM-HOWTO/>.
-=head2 PARTITIONING
-
-To create MBR-style (ie. normal PC) partitions use one of the
-C<guestfs_sfdisk*> variants. These calls use the external
-L<sfdisk(8)> command.
-
-The simplest call is:
+=head2 DOWNLOADING
- char *lines[] = { ",", NULL };
- guestfs_sfdiskM (g, "/dev/sda", lines);
+Use C<guestfs_cat> to download small, text only files. This call
+is limited to files which are less than 2 MB and which cannot contain
+any ASCII NUL (C<\0>) characters. However it has a very simple
+to use API.
-This will create a single partition on C</dev/sda> called
-C</dev/sda1> covering the whole disk.
+C<guestfs_read_file> can be used to read files which contain
+arbitrary 8 bit data, since it returns a (pointer, size) pair.
+However it is still limited to "small" files, less than 2 MB.
-In general MBR partitions are both unnecessarily complicated and
-depend on archaic details, namely the Cylinder-Head-Sector (CHS)
-geometry of the disk. C<guestfs_sfdiskM> allows you to specify sizes
-in megabytes instead of cylinders, which is a small win.
-C<guestfs_sfdiskM> will choose the nearest cylinder to approximate the
-requested size. There's a lot of crazy stuff to do with IDE and
-virtio disks having different, incompatible CHS geometries, that you
-probably don't want to know about. My advice: make a single partition
-to cover the whole disk, then use LVM on top.
+C<guestfs_download> can be used to download any file, with no
+limits on content or size (even files larger than 4 GB).
-In future we aim to provide access to libparted.
+To download multiple files, see C<guestfs_tar_out> and
+C<guestfs_tgz_out>.
=head2 UPLOADING
-For small, single files, use C<guestfs_write_file>. In some versions
-of libguestfs there was a bug which limited this call to text files
+It's often the case that you want to write a file or files to the disk
+image.
+
+For small, single files, use C<guestfs_write_file>. This call
+currently contains a bug which limits the call to plain text files
(not containing ASCII NUL characters).
To upload a single file, use C<guestfs_upload>. This call has no
UUIDs. If you want to pre-build an image or you need to mount it
using a label or UUID, use an ISO image instead.
-=head2 DOWNLOADING
+=head2 COPYING
-Use C<guestfs_cat> to download small, text only files. This call
-is limited to files which are less than 2 MB and which cannot contain
-any ASCII NUL (C<\0>) characters. However it has a very simple
-to use API.
+There are various different commands for copying between files and
+devices and in and out of the guest filesystem. These are summarised
+in the table below.
-C<guestfs_read_file> can be used to read files which contain
-arbitrary 8 bit data, since it returns a (pointer, size) pair.
-However it is still limited to "small" files, less than 2 MB.
+=over 4
-C<guestfs_download> can be used to download any file, with no
-limits on content or size (even files larger than 4 GB).
+=item B<file> to B<file>
-To download multiple files, see C<guestfs_tar_out> and
-C<guestfs_tgz_out>.
+Use L</guestfs_cp> to copy a single file, or
+L</guestfs_cp_a> to copy directories recursively.
+
+=item B<file or device> to B<file or device>
+
+Use L</guestfs_dd> which efficiently uses L<dd(1)>
+to copy between files and devices in the guest.
+
+Example: duplicate the contents of an LV:
+
+ guestfs_dd (g, "/dev/VG/Original", "/dev/VG/Copy");
+
+The destination (C</dev/VG/Copy>) must be at least as large as the
+source (C</dev/VG/Original>).
+
+=item B<file on the host> to B<file or device>
+
+Use L</guestfs_upload>. See L</UPLOADING> above.
+
+=item B<file or device> to B<file on the host>
+
+Use L</guestfs_download>. See L</DOWNLOADING> above.
+
+=back
+
+=head2 LISTING FILES
+
+C<guestfs_ll> is just designed for humans to read (mainly when using
+the L<guestfish(1)>-equivalent command C<ll>).
+
+C<guestfs_ls> is a quick way to get a list of files in a directory
+from programs, as a flat list of strings.
+
+C<guestfs_readdir> is a programmatic way to get a list of files in a
+directory, plus additional information about each one. It is more
+equivalent to using the L<readdir(3)> call on a local filesystem.
+
+C<guestfs_find> can be used to recursively list files.
=head2 RUNNING COMMANDS
The difference is that C<guestfs_sh> runs commands using the shell, so
any shell globs, redirections, etc will work.
-=head2 LISTING FILES
-
-C<guestfs_ll> is just designed for humans to read (mainly when using
-the L<guestfish(1)>-equivalent command C<ll>).
+=head2 CONFIGURATION FILES
-C<guestfs_ls> is a quick way to get a list of files in a directory
-from programs.
+To read and write configuration files in Linux guest filesystems, we
+strongly recommend using Augeas. For example, Augeas understands how
+to read and write, say, a Linux shadow password file or X.org
+configuration file, and so avoids you having to write that code.
-C<guestfs_readdir> is a programmatic way to get a list of files in a
-directory, plus additional information about each one.
+The main Augeas calls are bound through the C<guestfs_aug_*> APIs. We
+don't document Augeas itself here because there is excellent
+documentation on the L<http://augeas.net/> website.
-C<guestfs_find> can be used to recursively list files.
+If you don't want to use Augeas (you fool!) then try calling
+C<guestfs_read_lines> to get the file as a list of lines which
+you can iterate over.
=head2 SELINUX
for example by running the external command
C<restorecon pathname>.
+=head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS
+
+Libguestfs can mount NTFS partitions. It does this using the
+L<http://www.ntfs-3g.org/> driver.
+
+DOS and Windows still use drive letters, and the filesystems are
+always treated as case insensitive by Windows itself, and therefore
+you might find a Windows configuration file referring to a path like
+C<c:\windows\system32>. When the filesystem is mounted in libguestfs,
+that directory might be referred to as C</WINDOWS/System32>.
+
+Drive letter mappings are outside the scope of libguestfs. You have
+to use libguestfs to read the appropriate Windows Registry and
+configuration files, to determine yourself how drives are mapped (see
+also L<virt-inspector(1)>).
+
+Replacing backslash characters with forward slash characters is also
+outside the scope of libguestfs, but something that you can easily do.
+
+Where we can help is in resolving the case insensitivity of paths.
+For this, call C<guestfs_case_sensitive_path>.
+
+Libguestfs also provides some help for decoding Windows Registry
+"hive" files, through the library C<libhivex> which is part of
+libguestfs. You have to locate and download the hive file(s)
+yourself, and then pass them to C<libhivex> functions. See also the
+programs L<hivexml(1)>, L<hivexget(1)> and L<virt-win-reg(1)> for more
+help on this issue.
+
+=head2 USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES
+
+Although we don't want to discourage you from using the C API, we will
+mention here that the same API is also available in other languages.
+
+The API is broadly identical in all supported languages. This means
+that the C call C<guestfs_mount(handle,path)> is
+C<$handle-E<gt>mount($path)> in Perl, C<handle.mount(path)> in Python,
+and C<Guestfs.mount handle path> in OCaml. In other words, a
+straightforward, predictable isomorphism between each language.
+
+Error messages are automatically transformed
+into exceptions if the language supports it.
+
+We don't try to "object orientify" parts of the API in OO languages,
+although contributors are welcome to write higher level APIs above
+what we provide in their favourite languages if they wish.
+
+=over 4
+
+=item B<C++>
+
+You can use the I<guestfs.h> header file from C++ programs. The C++
+API is identical to the C API. C++ classes and exceptions are
+not implemented.
+
+=item B<Haskell>
+
+This is the only language binding that is incomplete. Only calls
+which return simple integers have been bound in Haskell, and we are
+looking for help to complete this binding.
+
+=item B<Java>
+
+Full documentation is contained in the Javadoc which is distributed
+with libguestfs.
+
+=item B<OCaml>
+
+For documentation see the file C<guestfs.mli>.
+
+=item B<Perl>
+
+For documentation see L<Sys::Guestfs(3)>.
+
+=item B<Python>
+
+For documentation do:
+
+ $ python
+ >>> import guestfs
+ >>> help (guestfs)
+
+=item B<Ruby>
+
+Use the Guestfs module. There is no Ruby-specific documentation, but
+you can find examples written in Ruby in the libguestfs source.
+
+=item B<shell scripts>
+
+For documentation see L<guestfish(1)>.
+
+=back
+
+=head1 CONNECTION MANAGEMENT
+
+=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.
+
+For information on using multiple handles and threads, see the section
+L</MULTIPLE HANDLES AND MULTIPLE THREADS> below.
+
+=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.
+
+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>.
+
+You may also want to configure error handling for the handle. See
+L</ERROR HANDLING> section below.
+
+=head2 guestfs_close
+
+ void guestfs_close (guestfs_h *handle);
+
+This closes the connection handle and frees up all resources used.
+
+=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_last_error> and/or by setting up an error
+handler with 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_last_error
+
+ const char *guestfs_last_error (guestfs_h *handle);
+
+This returns the last error message that happened on C<handle>. If
+there has not been an error since the handle was created, then this
+returns C<NULL>.
+
+The lifetime of the returned string is until the next error occurs, or
+C<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.
+
+=head2 guestfs_set_error_handler
+
+ 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 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.
+
+Note that the message string C<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 C<stderr>.
+
+If you set C<cb> to C<NULL> then I<no> handler is called.
+
+=head2 guestfs_get_error_handler
+
+ guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *handle,
+ void **data_rtn);
+
+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 PATH
+
+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 C<$libdir/guestfs>
+(eg. C</usr/local/lib/guestfs> or C</usr/lib64/guestfs>).
+
+Use C<guestfs_set_path> or set the environment variable
+C<LIBGUESTFS_PATH> to change the directories that libguestfs will
+search in. The value is a colon-separated list of paths. The current
+directory is I<not> searched unless the path contains an empty element
+or C<.>. For example C<LIBGUESTFS_PATH=:/usr/lib/guestfs> would
+search the current directory and then C</usr/lib/guestfs>.
+
=head1 HIGH-LEVEL API ACTIONS
=head2 ABI GUARANTEE
@STRUCTS@
-=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
+=head1 AVAILABILITY
+
+=head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE
+
+Using L</guestfs_available> you can test availability of
+the following groups of functions. This test queries the
+appliance to see if the appliance you are currently using
+supports the functionality.
+
+@AVAILABILITY@
+
+=head2 SINGLE CALLS AT COMPILE TIME
+
+If you need to test whether a single libguestfs function is
+available at compile time, we recommend using build tools
+such as autoconf or cmake. For example in autotools you could
+use:
+
+ AC_CHECK_LIB([guestfs],[guestfs_create])
+ AC_CHECK_FUNCS([guestfs_dd])
+
+which would result in C<HAVE_GUESTFS_DD> being either defined
+or not defined in your program.
+
+=head2 SINGLE CALLS AT RUN TIME
+
+Testing at compile time doesn't guarantee that a function really
+exists in the library. The reason is that you might be dynamically
+linked against a previous I<libguestfs.so> (dynamic library)
+which doesn't have the call. This situation unfortunately results
+in a segmentation fault, which is a shortcoming of the C dynamic
+linking system itself.
+
+You can use L<dlopen(3)> to test if a function is available
+at run time, as in this example program (note that you still
+need the compile time check as well):
+
+ #include <config.h>
+
+ #include <stdio.h>
+ #include <stdlib.h>
+ #include <unistd.h>
+ #include <dlfcn.h>
+ #include <guestfs.h>
+
+ main ()
+ {
+ #ifdef HAVE_GUESTFS_DD
+ void *dl;
+ int has_function;
+
+ /* Test if the function guestfs_dd is really available. */
+ dl = dlopen (NULL, RTLD_LAZY);
+ if (!dl) {
+ fprintf (stderr, "dlopen: %s\n", dlerror ());
+ exit (1);
+ }
+ has_function = dlsym (dl, "guestfs_dd") != NULL;
+ dlclose (dl);
+
+ if (!has_function)
+ printf ("this libguestfs.so does NOT have guestfs_dd function\n");
+ else {
+ printf ("this libguestfs.so has guestfs_dd function\n");
+ /* Now it's safe to call
+ guestfs_dd (g, "foo", "bar");
+ */
+ }
+ #else
+ printf ("guestfs_dd function was not found at compile time\n");
+ #endif
+ }
+
+You may think the above is an awful lot of hassle, and it is.
+There are other ways outside of the C linking system to ensure
+that this kind of incompatibility never arises, such as using
+package versioning:
+
+ Requires: libguestfs >= 1.0.80
+
+=begin html
+
+<!-- old anchor for the next section -->
+<a name="state_machine_and_low_level_event_api"/>
+
+=end html
+
+=head1 ARCHITECTURE
+
+Internally, libguestfs is implemented by running an appliance (a
+special type of small virtual machine) using L<qemu(1)>. Qemu runs as
+a child process of the main program.
+
+ ___________________
+ / \
+ | main program |
+ | |
+ | | child process / appliance
+ | | __________________________
+ | | / qemu \
+ +-------------------+ RPC | +-----------------+ |
+ | libguestfs <--------------------> guestfsd | |
+ | | | +-----------------+ |
+ \___________________/ | | Linux kernel | |
+ | +--^--------------+ |
+ \_________|________________/
+ |
+ _______v______
+ / \
+ | Device or |
+ | disk image |
+ \______________/
+
+The library, linked to the main program, creates the child process and
+hence the appliance in the L</guestfs_launch> function.
+
+Inside the appliance is a Linux kernel and a complete stack of
+userspace tools (such as LVM and ext2 programs) and a small
+controlling daemon called C<guestfsd>. The library talks to
+C<guestfsd> using remote procedure calls (RPC). There is a mostly
+one-to-one correspondence between libguestfs API calls and RPC calls
+to the daemon. Lastly the disk image(s) are attached to the qemu
+process which translates device access by the appliance's Linux kernel
+into accesses to the image.
+
+A common misunderstanding is that the appliance "is" the virtual
+machine. Although the disk image you are attached to might also be
+used by some virtual machine, libguestfs doesn't know or care about
+this. (But you will care if both libguestfs's qemu process and your
+virtual machine are trying to update the disk image at the same time,
+since these usually results in massive disk corruption).
+
+=head1 STATE MACHINE
libguestfs uses a state machine to model the child process:
initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
and daemon is alive. This is what C<guestfs_launch> waits for.
+=head1 MULTIPLE HANDLES AND MULTIPLE THREADS
+
+All high-level libguestfs actions are synchronous. If you want
+to use libguestfs asynchronously then you must create a thread.
+
+Only use the handle from a single thread. Either use the handle
+exclusively from one thread, or provide your own mutex so that two
+threads cannot issue calls on the same handle at the same time.
+
=head1 QEMU WRAPPERS
If you want to compile your own qemu, run qemu from a non-standard
git.annexia.org / libguestfs.git / blobdiff