From 5de49dc0d82a86032eb51e2cb9e43813e2480594 Mon Sep 17 00:00:00 2001 From: Richard Jones Date: Wed, 1 Apr 2009 20:29:03 +0100 Subject: [PATCH] Documentation of low-level API. --- README | 10 +- guestfs.pod | 355 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 360 insertions(+), 5 deletions(-) create mode 100644 guestfs.pod diff --git a/README b/README index 369295c..05bb350 100644 --- a/README +++ b/README @@ -1,4 +1,4 @@ -libguestfs is a library for accessing and modifying guest disk images. +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: @@ -6,22 +6,22 @@ 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 +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, +Libguestfs provides ways to enumerate guest storage (eg. partitions, LVs, what filesystem is in each LV, etc.). It can also run commands in the context of the guest. Also you can mount guest filesystems on the host (requires root privs and NFS). -libguestfs is a library that can be linked with C and C++ management +Libguestfs is a library that can be linked with C and C++ management programs (or management programs written in other languages, if people contribute the language bindings). You can also use it from shell scripts or the command line. -libguestfs was written by Richard W.M. Jones (rjones@redhat.com). +Libguestfs was written by Richard W.M. Jones (rjones@redhat.com). For discussion please use the fedora-virt mailing list: https://www.redhat.com/mailman/listinfo/fedora-virt diff --git a/guestfs.pod b/guestfs.pod new file mode 100644 index 0000000..63bd14c --- /dev/null +++ b/guestfs.pod @@ -0,0 +1,355 @@ +=encoding utf8 + +=head1 NAME + +guestfs - Library for accessing and modifying virtual machine images + +=head1 SYNOPSIS + + 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_file (handle, "/hello"); + guestfs_sync (handle); + guestfs_close (handle); + +=head1 DESCRIPTION + +Libguestfs is a library for accessing and modifying guest disk images. +Amongst the things this is good for: making batch configuration +changes to guests, getting disk used/free statistics (see also: +virt-df), migrating between virtualization systems (see also: +virt-p2v), performing partial backups, performing partial guest +clones, cloning guests and changing registry/UUID/hostname info, and +much else besides. + +Libguestfs uses Linux kernel and qemu code, and can access any type of +guest filesystem that Linux and qemu can, including but not limited +to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition +schemes, qcow, qcow2, vmdk. + +Libguestfs provides ways to enumerate guest storage (eg. partitions, +LVs, what filesystem is in each LV, etc.). It can also run commands +in the context of the guest. Also you can mount guest filesystems on +the host (requires root privs and NFS). + +Libguestfs is a library that can be linked with C and C++ management +programs (or management programs written in other languages, if people +contribute the language bindings). You can also use it from shell +scripts or the command line. + +=head1 CONNECTION MANAGEMENT + + + + + + +=head1 CONFIGURATION MANAGEMENT + + + + + + +=head1 HIGH-LEVEL API + + + + + + + + + +=head1 ERROR HANDLING + + + + + + +=head1 STATE MACHINE AND LOW-LEVEL EVENT API + +Internally, libguestfs is implemented by running a virtual machine +using L. 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. + +=head2 STATE MACHINE + +libguestfs uses a state machine to model the child process: + + | + guestfs_create + | + | + ____V_____ + / \ + | CONFIG | + \__________/ + ^ ^ ^ \ + / | \ \ guestfs_launch + / | _\__V______ + / | / \ + / | | LAUNCHING | + / | \___________/ + / | / + / | guestfs_wait_ready + / | / + ______ / __|____V + / \ ------> / \ + | BUSY | | READY | + \______/ <------ \________/ + +The normal transitions are (1) CONFIG (when the handle is created, but +there is no child process), (2) LAUNCHING (when the child process is +booting up), (3) alternating between READY and BUSY as commands are +issued to, and carried out by, the child process. + +The guest may be killed by C, or may die +asynchronously at any time (eg. due to some internal error), and that +causes the state to transition back to CONFIG. + +Configuration commands for qemu such as C can only +be issued when in the CONFIG state. + +The high-level API offers two calls that go from CONFIG through +LAUNCHING to READY. C is a non-blocking call that +starts up the child process, immediately moving from CONFIG to +LAUNCHING. C blocks until the child process is +READY to accept commands (or until some failure or timeout). The +low-level event API described below provides a non-blocking way to +replace C. + +High-level API actions such as C can only be issued +when in the READY state. These high-level API calls block waiting for +the command to be carried out (ie. the state to transition to BUSY and +then back to READY). But using the low-level event API, you get +non-blocking versions. (But you can still only carry out one +operation per handle at a time - that is a limitation of the +communications protocol we use). + +Finally, the child process sends asynchronous messages back to the +main program, such as kernel log messages. Mostly these are ignored +by the high-level API, but using the low-level event API you can +register to receive these messages. + +=head2 SETTING CALLBACKS TO HANDLE EVENTS + +The child process generates events in some situations. Current events +include: receiving a reply message after some action, receiving a log +message, the child process exits, &c. + +Use the C 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 NON-BLOCKING ACTIONS + +C is the most interesting callback to +play with, since it allows you to perform actions without blocking. + +For example: + + do_it () + { + start_call (); + guestfs_main_loop_run (); /* --> blocks, then calls my_cb */ + } + + start_call () + { + guestfs_set_reply_callback (handle, my_cb, data); + guestfs_nb_[action] (handle, [other parameters ...]); + /* returns immediately */ + } + + my_cb (guestfs_h handle, void *data) + { + retval = guestfs_nb_[action]_r (handle); + /* ... */ + } + +There are C and C functions +corresponding to (very nearly) every C action in the +high-level API. + +=head2 guestfs_set_reply_callback + + void guestfs_set_reply_callback (guestfs_handle h, + guestfs_reply_cb cb, + void *opaque); + +The callback function C will be called whenever a reply is +received from the child process. (This corresponds to a transition +from the BUSY state to the READY state). + +Note (I) that high-level API calls overwrite this +callback. + +=head2 guestfs_set_log_message_callback + + void guestfs_set_log_message_callback (guestfs_handle h, + 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 + + void guestfs_set_subprocess_quit_callback (guestfs_handle h, + 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 +C. (This corresponds to a transition from +any state to the CONFIG state). + +=head2 guestfs_set_ready_callback + + void guestfs_set_ready_callback (guestfs_handle h, + guestfs_ready_cb cb, + void *opaque); + +The callback function C will be called when the child process +becomes ready. (This corresponds to a transition from I +LAUNCHING I BUSY to the READY state). + +You can use this instead of C to implement a +non-blocking wait for the child process to finish booting up. + +=head2 EVENT MAIN LOOP + +To use the low-level event API, you have to provide an event "main +loop". You can write your own, but if you don't want to write one, +two are provided for you: + +=over 4 + +=item libguestfs-poll + +A simple main loop that is implemented using L. + +This is the default main loop unless you call C +or C. + +=item libguestfs-glib + +An implementation which can be used with GLib and GTK+ programs. You +can use this to write graphical (GTK+) programs which use libguestfs +without hanging during long or slow operations. + +=back + +=head2 guestfs_set_main_loop + + void guestfs_set_main_loop (guestfs_main_loop *); + +This call sets the current main loop to the list of callbacks +contained in the C structure. + +Only one main loop implementation can be used by libguestfs, so +calling this replaces the previous one. (So this is something that +has to be done by the main program, but only the main program "knows" +that it is a GTK+ program or whatever). + +You should call this early in the main program, certainly before +calling C. + +=head2 guestfs_glib_set_main_loop + + void guestfs_glib_set_main_loop (GMainLoop *); + +This helper calls C with the correct callbacks +for integrating with the GLib main loop. + +The libguestfs-glib main loop is contained in a separate library, so +that libguestfs doesn't depend on the whole of GLib: + + #include + #include + + main () + { + GMainLoop *loop = + g_main_loop_new (g_main_context_default (), 1); + ... + guestfs_glib_set_main_loop (loop); + ... + g_main_loop_run (loop); + } + +To use this main loop you must link with C<-lguestfs-glib>. (See also +the GLib and GTK+ documentation). + +=head2 guestfs_main_loop_run + + void guestfs_main_loop_run (void); + +This calls the main loop. + +For some types of main loop you may want or prefer to call another +function, eg. C, or the main loop may already be +invoked by another part of your program. In those cases, ignore this +call. + +=head2 guestfs_main_loop_quit + + void guestfs_main_loop_quit (void); + +This instructs the main loop to quit. In other words, +C will return. + +For some types of main loop you may want or prefer to call another +function, eg. C. In those cases, ignore this call. + +=head2 WRITING A CUSTOM MAIN LOOP + +This isn't documented. Please see the libguestfs-poll and libguestfs-glib +implementations. + +=head1 SEE ALSO + +L + + + + + +=head1 AUTHORS + +Richard W.M. Jones (C) + +=head1 COPYRIGHT + +Copyright (C) 2009 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 -- 1.8.3.1