=encoding utf8 =head1 NAME guestfs - Library for accessing and modifying virtual machine images =head1 SYNOPSIS #include guestfs_h *g = guestfs_create (); guestfs_add_drive (g, "guest.img"); guestfs_launch (g); guestfs_mount (g, "/dev/sda1", "/"); guestfs_touch (g, "/hello"); guestfs_umount (g, "/"); guestfs_sync (g); guestfs_close (g); cc prog.c -o prog -lguestfs or: cc prog.c -o prog `pkg-config libguestfs --cflags --libs` =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 access filesystems over FUSE. Libguestfs is a library that can be linked with C and C++ management programs (or management programs written in OCaml, Perl, Python, Ruby, Java, PHP, Haskell or C#). 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. Libguestfs is a large API because it can do many things. For a gentle introduction, please read the L section next. =head1 API OVERVIEW 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 in the main section of this manual. =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. By convention we use the single letter C for the name of the handle variable, although of course you can use any name you want. The general structure of all libguestfs-using programs looks like this: guestfs_h *g = guestfs_create (); /* Call guestfs_add_drive additional times if there are * multiple disk images. */ guestfs_add_drive (g, "guest.img"); /* Most manipulation calls won't work until you've launched * the handle 'g'. You have to do this _after_ adding drives * and _before_ other commands. */ guestfs_launch (g); /* Now you can examine what partitions, LVs etc are available. */ char **partitions = guestfs_list_partitions (g); char **logvols = guestfs_lvs (g); /* To access a filesystem in the image, you must mount it. */ guestfs_mount (g, "/dev/sda1", "/"); /* Now you can perform filesystem actions on the guest * disk image. */ guestfs_touch (g, "/hello"); /* You only need to call guestfs_sync if you have made * changes to the guest image. (But if you've made changes * then you *must* sync). See also: guestfs_umount and * guestfs_umount_all calls. */ guestfs_sync (g); /* Close the handle 'g'. */ guestfs_close (g); 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 on error. See section L below for how to handle errors, and consult the documentation for each function call below to see precisely how they return error indications. =head2 DISK IMAGES The image filename (C<"guest.img"> in the example above) could be a disk image from a virtual machine, a L copy of a physical hard disk, an actual block device, or simply an empty file of zeroes that you have created through L. Libguestfs lets you do useful things to all of these. You can add a disk read-only using L, in which case libguestfs won't modify the file. 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. 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 (for the first one you added), C (for the second one you added), etc. Once L has been called you cannot add any more images. You can call L to get a list of the device names, in the order that you added them. See also L below. =head2 MOUNTING 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 L. 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: guestfs_mount (g, "/dev/sda1", "/"); where C means literally the first partition (C<1>) of the first disk image that we added (C). If the disk contains Linux LVM2 logical volumes you could refer to those instead (eg. C). 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 L and L to list possible partitions and LVs, and either try mounting each to see what is mountable, or else examine them with L or L. Libguestfs also has a set of APIs for inspection of disk images (see L below). But you might find it easier to look at higher level programs built on top of libguestfs, in particular L. To mount a disk image read-only, use L. There are several other variations of the C call. =head2 FILESYSTEM ACCESS AND MODIFICATION 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. Specify filenames as full paths, starting with C<"/"> and including the mount point. For example, if you mounted a filesystem at C<"/"> and you want to read the file called C<"etc/passwd"> then you could do: char *data = guestfs_cat (g, "/etc/passwd"); This would return C as a newly allocated buffer containing the full content of that file (with some conditions: see also L below), or C if there was an error. As another example, to create a top-level directory on that filesystem called C<"var"> you would do: guestfs_mkdir (g, "/var"); To create a symlink you could do: guestfs_ln_s (g, "/etc/init.d/portmap", "/etc/rc3.d/S30portmap"); Libguestfs will reject attempts to use relative paths and 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). File writes are affected by the per-handle umask, set by calling L and defaulting to 022. See L. =head2 PARTITIONING Libguestfs contains API calls to read, create and modify partition tables on disk images. In the common case where you want to create a single partition covering the whole disk, you should use the L call: const char *parttype = "mbr"; if (disk_is_larger_than_2TB) parttype = "gpt"; guestfs_part_disk (g, "/dev/sda", parttype); Obviously this effectively wipes anything that was on that disk image before. =head2 LVM2 Libguestfs provides access to a large part of the LVM2 API, such as L and L. 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. =head2 DOWNLOADING Use L 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. L 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. L can be used to download any file, with no limits on content or size (even files larger than 4 GB). To download multiple files, see L and L. =head2 UPLOADING It's often the case that you want to write a file or files to the disk image. To write a small file with fixed content, use L. To create a file of all zeroes, use L (sparse) or L (with all disk blocks allocated). There are a variety of other functions for creating test files, for example L and L. To upload a single file, use L. This call has no limits on file content or size (even files larger than 4 GB). To upload multiple files, see L and L. However the fastest way to upload I is to turn them into a squashfs or CD ISO (see L and L), then attach this using L. If you add the drive in a predictable way (eg. adding it last after all other drives) then you can get the device name from L and mount it directly using L. Note that squashfs images are sometimes non-portable between kernel versions, and they don't support labels or 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 COPYING 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. =over 4 =item B to B Use L to copy a single file, or L to copy directories recursively. =item B to B Use L which efficiently uses L 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) must be at least as large as the source (C). To copy less than the whole source device, use L. =item B to B Use L. See L above. =item B to B Use L. See L above. =back =head2 LISTING FILES L is just designed for humans to read (mainly when using the L-equivalent command C). L is a quick way to get a list of files in a directory from programs, as a flat list of strings. L 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 call on a local filesystem. L and L can be used to recursively list files. =head2 RUNNING COMMANDS Although libguestfs is primarily an API for manipulating files inside guest images, we also provide some limited facilities for running commands inside guests. There are many limitations to this: =over 4 =item * The kernel version that the command runs under will be different from what it expects. =item * If the command needs to communicate with daemons, then most likely they won't be running. =item * The command will be running in limited memory. =item * The network may not be available unless you enable it (see L). =item * Only supports Linux guests (not Windows, BSD, etc). =item * Architecture limitations (eg. won't work for a PPC guest on an X86 host). =item * For SELinux guests, you may need to enable SELinux and load policy first. See L in this manpage. =back The two main API calls to run commands are L and L (there are also variations). The difference is that L runs commands using the shell, so any shell globs, redirections, etc will work. =head2 CONFIGURATION FILES 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. The main Augeas calls are bound through the C APIs. We don't document Augeas itself here because there is excellent documentation on the L website. If you don't want to use Augeas (you fool!) then try calling L to get the file as a list of lines which you can iterate over. =head2 SELINUX We support SELinux guests. To ensure that labeling happens correctly in SELinux guests, you need to enable SELinux and load the guest's policy: =over 4 =item 1. Before launching, do: guestfs_set_selinux (g, 1); =item 2. After mounting the guest's filesystem(s), load the policy. This is best done by running the L command in the guest itself: guestfs_sh (g, "/usr/sbin/load_policy"); (Older versions of C require you to specify the name of the policy file). =item 3. Optionally, set the security context for the API. The correct security context to use can only be known by inspecting the guest. As an example: guestfs_setcon (g, "unconfined_u:unconfined_r:unconfined_t:s0"); =back This will work for running commands and editing existing files. When new files are created, you may need to label them explicitly, for example by running the external command C. =head2 UMASK Certain calls are affected by the current file mode creation mask (the "umask"). In particular ones which create files or directories, such as L, L or L. This affects either the default mode that the file is created with or modifies the mode that you supply. The default umask is C<022>, so files are created with modes such as C<0644> and directories with C<0755>. There are two ways to avoid being affected by umask. Either set umask to 0 (call C early after launching). Or call L after creating each file or directory. For more information about umask, see L. =head2 ENCRYPTED DISKS Libguestfs allows you to access Linux guests which have been encrypted using whole disk encryption that conforms to the Linux Unified Key Setup (LUKS) standard. This includes nearly all whole disk encryption systems used by modern Linux guests. Use L to identify LUKS-encrypted block devices (it returns the string C). Then open these devices by calling L. Obviously you will require the passphrase! Opening a LUKS device creates a new device mapper device called C (where C is the string you supply to L). Reads and writes to this mapper device are decrypted from and encrypted to the underlying block device respectively. LVM volume groups on the device can be made visible by calling L followed by L. The logical volume(s) can now be mounted in the usual way. Use the reverse process to close a LUKS device. Unmount any logical volumes on it, deactivate the volume groups by caling C. Then close the mapper device by calling L on the C device (I the underlying encrypted block device). =head2 INSPECTION Libguestfs has APIs for inspecting an unknown disk image to find out if it contains operating systems. (These APIs used to be in a separate Perl-only library called L but since version 1.5.3 the most frequently used part of this library has been rewritten in C and moved into the core code). Add all disks belonging to the unknown virtual machine and call L in the usual way. Then call L. This function uses other libguestfs calls and certain heuristics, and returns a list of operating systems that were found. An empty list means none were found. A single element is the root filesystem of the operating system. For dual- or multi-boot guests, multiple roots can be returned, each one corresponding to a separate operating system. (Multi-boot virtual machines are extremely rare in the world of virtualization, but since this scenario can happen, we have built libguestfs to deal with it.) For each root, you can then call various C functions to get additional details about that operating system. For example, call L to return the string C or C for Windows and Linux-based operating systems respectively. Un*x-like and Linux-based operating systems usually consist of several filesystems which are mounted at boot time (for example, a separate boot partition mounted on C). The inspection rules are able to detect how filesystems correspond to mount points. Call C to get this mapping. It might return a hash table like this example: /boot => /dev/sda1 / => /dev/vg_guest/lv_root /usr => /dev/vg_guest/lv_usr The caller can then make calls to L to mount the filesystems as suggested. Be careful to mount filesystems in the right order (eg. C before C). Sorting the keys of the hash by length, shortest first, should work. Inspection currently only works for some common operating systems. Contributors are welcome to send patches for other operating systems that we currently cannot detect. Encrypted disks must be opened before inspection. See L for more details. The L function just ignores any encrypted devices. A note on the implementation: The call L performs inspection and caches the results in the guest handle. Subsequent calls to C return this cached information, but I re-read the disks. If you change the content of the guest disks, you can redo inspection by calling L again. =head2 SPECIAL CONSIDERATIONS FOR WINDOWS GUESTS Libguestfs can mount NTFS partitions. It does this using the L 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. When the filesystem is mounted in libguestfs, that directory might be referred to as C. 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 and L). 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 L. Libguestfs also provides some help for decoding Windows Registry "hive" files, through the library C which is part of the libguestfs project although ships as a separate tarball. You have to locate and download the hive file(s) yourself, and then pass them to C functions. See also the programs L, L, L and L 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 is C<$g-Emount($path)> in Perl, C in Python, and C 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 You can use the I header file from C++ programs. The C++ API is identical to the C API. C++ classes and exceptions are not used. =item B The C# bindings are highly experimental. Please read the warnings at the top of C. =item B This is the only language binding that is working but incomplete. Only calls which return simple integers have been bound in Haskell, and we are looking for help to complete this binding. =item B Full documentation is contained in the Javadoc which is distributed with libguestfs. =item B For documentation see the file C. =item B For documentation see L. =item B For documentation see C supplied with libguestfs sources or in the php-libguestfs package for your distribution. The PHP binding only works correctly on 64 bit machines. =item B For documentation do: $ python >>> import guestfs >>> help (guestfs) =item B Use the Guestfs module. There is no Ruby-specific documentation, but you can find examples written in Ruby in the libguestfs source. =item B For documentation see L. =back =head2 LIBGUESTFS GOTCHAS L: "A feature of a system [...] that works in the way it is documented but is counterintuitive and almost invites mistakes." Since we developed libguestfs and the associated tools, there are several things we would have designed differently, but are now stuck with for backwards compatibility or other reasons. If there is ever a libguestfs 2.0 release, you can expect these to change. Beware of them. =over 4 =item Autosync / forgetting to sync. When modifying a filesystem from C or another language, you B unmount all filesystems and call L explicitly before you close the libguestfs handle. You can also call: guestfs_set_autosync (g, 1); to have the unmount/sync done automatically for you when the handle 'g' is closed. (This feature is called "autosync", L q.v.) If you forget to do this, then it is entirely possible that your changes won't be written out, or will be partially written, or (very rarely) that you'll get disk corruption. Note that in L autosync is the default. So quick and dirty guestfish scripts that forget to sync will work just fine, which can make this very puzzling if you are trying to debug a problem. =item Mount option C<-o sync> should not be the default. If you use L, then C<-o sync,noatime> are added implicitly. However C<-o sync> does not add any reliability benefit, but does have a very large performance impact. The work around is to use L and set the mount options that you actually want to use. =item Read-only should be the default. In L, I<--ro> should be the default, and you should have to specify I<--rw> if you want to make changes to the image. This would reduce the potential to corrupt live VM images. Note that many filesystems change the disk when you just mount and unmount, even if you didn't perform any writes. You need to use L to guarantee that the disk is not changed. =item guestfish command line is hard to use. C doesn't do what people expect (open C for examination). It tries to run a guestfish command C which doesn't exist, so it fails. In earlier versions of guestfish the error message was also unintuitive, but we have corrected this since. Like the Bourne shell, we should have used C to run commands. =item guestfish megabyte modifiers don't work right on all commands In recent guestfish you can use C<1M> to mean 1 megabyte (and similarly for other modifiers). What guestfish actually does is to multiply the number part by the modifier part and pass the result to the C API. However this doesn't work for a few APIs which aren't expecting bytes, but are already expecting some other unit (eg. megabytes). The most common is L. The guestfish command: lvcreate LV VG 100M does not do what you might expect. Instead because L is already expecting megabytes, this tries to create a 100 I (100 megabytes * megabytes) logical volume. The error message you get from this is also a little obscure. This could be fixed in the generator by specially marking parameters and return values which take bytes or other units. =item Library should return errno with error messages. It would be a nice-to-have to be able to get the original value of 'errno' from inside the appliance along error paths (where set). Currently L goes through hoops to try to reverse the error message string into an errno, see the function error() in fuse/guestmount.c. In libguestfs 1.5.4, the protocol was changed so that the Linux errno is sent back from the daemon. =back =head2 PROTOCOL LIMITS Internally libguestfs uses a message-based protocol to pass API calls and their responses to and from a small "appliance" (see L for plenty more detail about this). The maximum message size used by the protocol is slightly less than 4 MB. For some API calls you may need to be aware of this limit. The API calls which may be affected are individually documented, with a link back to this section of the documentation. A simple call such as L returns its result (the file data) in a simple string. Because this string is at some point internally encoded as a message, the maximum size that it can return is slightly under 4 MB. If the requested file is larger than this then you will get an error. In order to transfer large files into and out of the guest filesystem, you need to use particular calls that support this. The sections L and L document how to do this. You might also consider mounting the disk image using our FUSE filesystem support (L). =head2 KEYS AND PASSPHRASES Certain libguestfs calls take a parameter that contains sensitive key material, passed in as a C string. In the future we would hope to change the libguestfs implementation so that keys are L-ed into physical RAM, and thus can never end up in swap. However this is I done at the moment, because of the complexity of such an implementation. Therefore you should be aware that any key parameter you pass to libguestfs might end up being written out to the swap partition. If this is a concern, scrub the swap partition or don't use libguestfs on encrypted devices. =head1 CONNECTION MANAGEMENT =head2 guestfs_h * C is the opaque type representing a connection handle. Create a handle by calling L. Call L to free the handle and release all resources used. For information on using multiple handles and threads, see the section L below. =head2 guestfs_create guestfs_h *guestfs_create (void); Create a connection handle. You have to call L 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 L. You may also want to configure error handling for the handle. See L section below. =head2 guestfs_close void guestfs_close (guestfs_h *g); This closes the connection handle and frees up all resources used. =head1 ERROR HANDLING The convention in all functions that return C is that they return C<-1> to indicate an error. You can get additional information on errors by calling L and/or by setting up an error handler with L. The default error handler prints the information string to C. Out of memory errors are handled differently. The default action is to call L. If this is undesirable, then you can set a handler using L. =head2 guestfs_last_error const char *guestfs_last_error (guestfs_h *g); This returns the last error message that happened on C. If there has not been an error since the handle was created, then this returns C. The lifetime of the returned string is until the next error occurs, or L 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 *g, void *opaque, const char *msg); void guestfs_set_error_handler (guestfs_h *g, guestfs_error_handler_cb cb, void *opaque); The callback C 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 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. If you set C to C then I handler is called. =head2 guestfs_get_error_handler guestfs_error_handler_cb guestfs_get_error_handler (guestfs_h *g, void **opaque_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 *g, guestfs_abort_cb); The callback C will be called if there is an out of memory situation. I. The default is to call L. You cannot set C to C. 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 *g); 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 or C). Use L or set the environment variable L to change the directories that libguestfs will search in. The value is a colon-separated list of paths. The current directory is I searched unless the path contains an empty element or C<.>. For example C would search the current directory and then C. =head1 HIGH-LEVEL API ACTIONS =head2 ABI GUARANTEE We guarantee the libguestfs ABI (binary interface), for public, high-level actions as outlined in this section. Although we will deprecate some actions, for example if they get replaced by newer calls, we will keep the old actions forever. This allows you the developer to program in confidence against the libguestfs API. @ACTIONS@ =head1 STRUCTURES @STRUCTS@ =head1 AVAILABILITY =head2 GROUPS OF FUNCTIONALITY IN THE APPLIANCE Using L 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 GUESTFISH supported COMMAND In L there is a handy interactive command C which prints out the available groups and whether they are supported by this build of libguestfs. Note however that you have to do C first. =head2 SINGLE CALLS AT COMPILE TIME Since version 1.5.8, Cguestfs.hE> defines symbols for each C API function, such as: #define LIBGUESTFS_HAVE_DD 1 if L is available. Before version 1.5.8, if you needed to test whether a single libguestfs function is available at compile time, we recommended 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 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 (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 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 #include #include #include #include main () { #ifdef LIBGUESTFS_HAVE_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 (EXIT_FAILURE); } 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 =end html =head1 ARCHITECTURE Internally, libguestfs is implemented by running an appliance (a special type of small virtual machine) using L. 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 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 L. The library talks to L 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: | guestfs_create | | ____V_____ / \ | CONFIG | \__________/ ^ ^ ^ \ / | \ \ guestfs_launch / | _\__V______ / | / \ / | | LAUNCHING | / | \___________/ / | / / | guestfs_launch / | / ______ / __|____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 L, 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 L can only be issued when in the CONFIG state. The API offers one call that goes from CONFIG through LAUNCHING to READY. L blocks until the child process is READY to accept commands (or until some failure or timeout). L internally moves the state from CONFIG to LAUNCHING while it is running. API actions such as L can only be issued when in the READY state. These API calls block waiting for the command to be carried out (ie. the state to transition to BUSY and then back to READY). There are no non-blocking versions, and no way to issue more than one command per handle at the same time. Finally, the child process sends asynchronous messages back to the main program, such as kernel log messages. You can register a callback to receive these messages. =head2 SETTING CALLBACKS TO HANDLE EVENTS The child process generates events in some situations. Current events include: receiving a log message, the child process exits. Use the C functions to set a callback for different types of events. Only I can be registered for each handle. Calling C again overwrites the previous callback of that type. Cancel all callbacks of this type by calling this function with C set to C. =head2 guestfs_set_log_message_callback typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque, char *buf, int len); void guestfs_set_log_message_callback (guestfs_h *g, guestfs_log_message_cb cb, void *opaque); The callback function C 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 typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque); void guestfs_set_subprocess_quit_callback (guestfs_h *g, guestfs_subprocess_quit_cb cb, void *opaque); The callback function C will be called when the child process quits, either asynchronously or if killed by L. (This corresponds to a transition from any state to the CONFIG state). =head2 guestfs_set_launch_done_callback typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque); void guestfs_set_launch_done_callback (guestfs_h *g, guestfs_launch_done_cb cb, void *opaque); The callback function C 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). =head2 guestfs_set_close_callback typedef void (*guestfs_close_cb) (guestfs_h *g, void *opaque); void guestfs_set_close_callback (guestfs_h *g, guestfs_close_cb cb, void *opaque); The callback function C will be called while the handle is being closed (synchronously from L). Note that libguestfs installs an L handler to try to clean up handles that are open when the program exits. This means that this callback might be called indirectly from L, which can cause unexpected problems in higher-level languages (eg. if your HLL interpreter has already been cleaned up by the time this is called, and if your callback then jumps into some HLL function). =head2 guestfs_set_progress_callback typedef void (*guestfs_progress_cb) (guestfs_h *g, void *opaque, int proc_nr, int serial, uint64_t position, uint64_t total); void guestfs_set_progress_callback (guestfs_h *g, guestfs_progress_cb cb, void *opaque); Some long-running operations can generate progress messages. If this callback is registered, then it will be called each time a progress message is generated (usually two seconds after the operation started, and three times per second thereafter until it completes, although the frequency may change in future versions). The callback receives two numbers: C and C. The units of C are not defined, although for some operations C may relate in some way to the amount of data to be transferred (eg. in bytes or megabytes), and C may be the portion which has been transferred. The only defined and stable parts of the API are: =over 4 =item * The callback can display to the user some type of progress bar or indicator which shows the ratio of C:C. =item * 0 E= C E= C =item * If any progress notification is sent during a call, then a final progress notification is always sent when C = C. This is to simplify caller code, so callers can easily set the progress indicator to "100%" at the end of the operation, without requiring special code to detect this case. =back The callback also receives the procedure number and serial number of the call. These are only useful for debugging protocol issues, and the callback can normally ignore them. The callback may want to print these numbers in error messages or debugging messages. =head1 PRIVATE DATA AREA You can attach named pieces of private data to the libguestfs handle, and fetch them by name for the lifetime of the handle. This is called the private data area and is only available from the C API. To attach a named piece of data, use the following call: void guestfs_set_private (guestfs_h *g, const char *key, void *data); C is the name to associate with this data, and C is an arbitrary pointer (which can be C). Any previous item with the same name is overwritten. You can use any C you want, but names beginning with an underscore character are reserved for internal libguestfs purposes (for implementing language bindings). It is recommended to prefix the name with some unique string to avoid collisions with other users. To retrieve the pointer, use: void *guestfs_get_private (guestfs_h *g, const char *key); This function returns C if either no data is found associated with C, or if the user previously set the C's C pointer to C. Libguestfs does not try to look at or interpret the C pointer in any way. As far as libguestfs is concerned, it need not be a valid pointer at all. In particular, libguestfs does I try to free the data when the handle is closed. If the data must be freed, then the caller must either free it before calling L or must set up a close callback to do it (see L, and note that only one callback can be registered for a handle). The private data area is implemented using a hash table, and should be reasonably efficient for moderate numbers of keys. =head1 BLOCK DEVICE NAMING In the kernel there is now quite a profusion of schemata for naming block devices (in this context, by I I mean a physical or virtual hard drive). The original Linux IDE driver used names starting with C. SCSI devices have historically used a different naming scheme, C. When the Linux kernel I driver became a popular replacement for the old IDE driver (particularly for SATA devices) those devices also used the C scheme. Additionally we now have virtual machines with paravirtualized drivers. This has created several different naming systems, such as C for virtio disks and C for Xen PV disks. As discussed above, libguestfs uses a qemu appliance running an embedded Linux kernel to access block devices. We can run a variety of appliances based on a variety of Linux kernels. This causes a problem for libguestfs because many API calls use device or partition names. Working scripts and the recipe (example) scripts that we make available over the internet could fail if the naming scheme changes. Therefore libguestfs defines C as the I. Internally C names are translated, if necessary, to other names as required. For example, under RHEL 5 which uses the C scheme, any device parameter C is translated to C transparently. Note that this I applies to parameters. The L, L and similar calls return the true names of the devices and partitions as known to the appliance. =head2 ALGORITHM FOR BLOCK DEVICE NAME TRANSLATION Usually this translation is transparent. However in some (very rare) cases you may need to know the exact algorithm. Such cases include where you use L to add a mixture of virtio and IDE devices to the qemu-based appliance, so have a mixture of C and C devices. The algorithm is applied only to I which are known to be either device or partition names. Return values from functions such as L are never changed. =over 4 =item * Is the string a parameter which is a device or partition name? =item * Does the string begin with C? =item * Does the named device exist? If so, we use that device. However if I then we continue with this algorithm. =item * Replace initial C string with C. For example, change C to C. If that named device exists, use it. If not, continue. =item * Replace initial C string with C. If that named device exists, use it. If not, return an error. =back =head2 PORTABILITY CONCERNS Although the standard naming scheme and automatic translation is useful for simple programs and guestfish scripts, for larger programs it is best not to rely on this mechanism. Where possible for maximum future portability programs using libguestfs should use these future-proof techniques: =over 4 =item * Use L or L to list actual device names, and then use those names directly. Since those device names exist by definition, they will never be translated. =item * Use higher level ways to identify filesystems, such as LVM names, UUIDs and filesystem labels. =back =head1 INTERNALS =head2 COMMUNICATION PROTOCOL 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 C (note: this file is automatically generated). There are two broad cases, ordinary functions that don't have any C and C parameters, which are handled with very simple request/reply messages. Then there are functions that have any C or C parameters, which use the same request and reply messages, but they may also be followed by files sent using a chunked encoding. =head3 ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS) 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__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 C 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 C_args> is completely omitted. The header contains the procedure number (C) 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__ret (encoded as XDR) As above the C_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 C. 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 C structure contains the error message as a string. =head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS A C parameter indicates that we transfer a file I 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__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 C 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) I 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 I send any reply, and goes back to reading the next request. The daemon may also cancel. It does this by writing a special word C 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 (C), so that neither the library nor the daemon need to keep much in memory. =head3 FUNCTIONS THAT HAVE FILEOUT PARAMETERS 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__ret (encoded as XDR) sequence of chunks for FileOut param #0 sequence of chunks for FileOut param #1 etc. =head3 INITIAL MESSAGE When the daemon launches it sends an initial word (C) which indicates that the guest and daemon is alive. This is what L waits for. =head3 PROGRESS NOTIFICATION MESSAGES The daemon may send progress notification messages at any time. These are distinguished by the normal length word being replaced by C, followed by a fixed size progress message. The library turns them into progress callbacks (see C) if there is a callback registered, or discards them if not. The daemon self-limits the frequency of progress messages it sends (see C). Not all calls generate progress messages. =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. See the graphical program guestfs-browser for one possible architecture for multithreaded programs using libvirt and libguestfs. =head1 QEMU WRAPPERS If you want to compile your own qemu, run qemu from a non-standard location, or pass extra arguments to qemu, then you can write a shell-script wrapper around qemu. There is one important rule to remember: you I> as the last command in the shell script (so that qemu replaces the shell and becomes the direct child of the libguestfs-using program). If you don't do this, then the qemu process won't be cleaned up correctly. Here is an example of a wrapper, where I have built my own copy of qemu from source: #!/bin/sh - qemudir=/home/rjones/d/qemu exec $qemudir/x86_64-softmmu/qemu-system-x86_64 -L $qemudir/pc-bios "$@" Save this script as C (or wherever), C, and then use it by setting the LIBGUESTFS_QEMU environment variable. For example: LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish Note that libguestfs also calls qemu with the -help and -version options in order to determine features. =head1 LIBGUESTFS VERSION NUMBERS Since April 2010, libguestfs has started to make separate development and stable releases, along with corresponding branches in our git repository. These separate releases can be identified by version number: even numbers for stable: 1.2.x, 1.4.x, ... .-------- odd numbers for development: 1.3.x, 1.5.x, ... | v 1 . 3 . 5 ^ ^ | | | `-------- sub-version | `------ always '1' because we don't change the ABI Thus "1.3.5" is the 5th update to the development branch "1.3". As time passes we cherry pick fixes from the development branch and backport those into the stable branch, the effect being that the stable branch should get more stable and less buggy over time. So the stable releases are ideal for people who don't need new features but would just like the software to work. Our criteria for backporting changes are: =over 4 =item * Documentation changes which don't affect any code are backported unless the documentation refers to a future feature which is not in stable. =item * Bug fixes which are not controversial, fix obvious problems, and have been well tested are backported. =item * Simple rearrangements of code which shouldn't affect how it works get backported. This is so that the code in the two branches doesn't get too far out of step, allowing us to backport future fixes more easily. =item * We I backport new features, new APIs, new tools etc, except in one exceptional case: the new feature is required in order to implement an important bug fix. =back A new stable branch starts when we think the new features in development are substantial and compelling enough over the current stable branch to warrant it. When that happens we create new stable and development versions 1.N.0 and 1.(N+1).0 [N is even]. The new dot-oh release won't necessarily be so stable at this point, but by backporting fixes from development, that branch will stabilize over time. =head1 ENVIRONMENT VARIABLES =over 4 =item LIBGUESTFS_APPEND Pass additional options to the guest kernel. =item LIBGUESTFS_DEBUG Set C to enable verbose messages. This has the same effect as calling C. =item LIBGUESTFS_MEMSIZE Set the memory allocated to the qemu process, in megabytes. For example: LIBGUESTFS_MEMSIZE=700 =item LIBGUESTFS_PATH Set the path that libguestfs uses to search for kernel and initrd.img. See the discussion of paths in section PATH above. =item LIBGUESTFS_QEMU Set the default qemu binary that libguestfs uses. If not set, then the qemu which was found at compile time by the configure script is used. See also L above. =item LIBGUESTFS_TRACE Set C to enable command traces. This has the same effect as calling C. =item TMPDIR Location of temporary directory, defaults to C. If libguestfs was compiled to use the supermin appliance then each handle will require rather a large amount of space in this directory for short periods of time (~ 80 MB). You can use C<$TMPDIR> to configure another directory to use in case C is not large enough. =back =head1 SEE ALSO L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L, L. Tools with a similar purpose: L, L, L, L, L. =head1 BUGS To get a list of bugs against libguestfs use this link: L To report a new bug against libguestfs use this link: L When reporting a bug, please check: =over 4 =item * That the bug hasn't been reported already. =item * That you are testing a recent version. =item * Describe the bug accurately, and give a way to reproduce it. =item * Run libguestfs-test-tool and paste the B output into the bug report. =back =head1 AUTHORS Richard W.M. Jones (C) =head1 COPYRIGHT Copyright (C) 2009-2010 Red Hat Inc. L 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