Bug: Segfault in Perl bindings.

[libguestfs.git] / guestfs.pod

diff --git a/guestfs.pod b/guestfs.pod
index 63bd14c..9cef777 100644 (file)
--- a/guestfs.pod
+++ b/guestfs.pod
@@ -6,12 +6,14 @@ guestfs - Library for accessing and modifying virtual machine images
 
 =head1 SYNOPSIS
 
 
 =head1 SYNOPSIS
 

- guestfs_h handle = guestfs_create ();
+ #include <guestfs.h>
+ 
+ guestfs_h *handle = guestfs_create ();

  guestfs_add_drive (handle, "guest.img");
  guestfs_launch (handle);
  guestfs_wait_ready (handle);
  guestfs_mount (handle, "/dev/sda1", "/");
  guestfs_add_drive (handle, "guest.img");
  guestfs_launch (handle);
  guestfs_wait_ready (handle);
  guestfs_mount (handle, "/dev/sda1", "/");

- guestfs_touch_file (handle, "/hello");
+ guestfs_touch (handle, "/hello");

  guestfs_sync (handle);
  guestfs_close (handle);
 
  guestfs_sync (handle);
  guestfs_close (handle);
 


@@ -32,44 +34,182 @@ 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
 
 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).
+in the context of the guest.  Also you can access filesystems over FTP.

 
 Libguestfs is a library that can be linked with C and C++ management
 
 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.
+programs (or management programs written in OCaml, Perl, Python, Ruby or Java).
+You can also use it from shell scripts or the command line.

 
 

-=head1 CONNECTION MANAGEMENT
+You don't need to be root to use libguestfs, although obviously you do
+need enough permissions to access the disk images.

 
 

+=head1 CONNECTION MANAGEMENT

 
 

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

 
 

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

 
 

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

 
 

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

 
 

-=head1 CONFIGURATION MANAGEMENT
+C<guestfs_wait_ready> and all of the actions including C<guestfs_sync>
+are blocking calls.  You can use the low-level event API to do
+non-blocking operations instead.

 
 

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

 
 

+=head2 guestfs_h *

 
 

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

 
 

+For information on using multiple handles and threads, see the section
+MULTIPLE HANDLES AND MULTIPLE THREADS below.

 
 

+=head2 guestfs_create

 
 

-=head1 HIGH-LEVEL API
+ 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> and
+C<guestfs_wait_ready>.

 
 

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

 
 

+=head2 guestfs_close

 
 

+ void guestfs_close (guestfs_h *handle);

 
 

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

 
 =head1 ERROR HANDLING
 
 
 =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
+
+@ACTIONS@
+
+=head1 STRUCTURES
+
+@STRUCTS@

 
 =head1 STATE MACHINE AND LOW-LEVEL EVENT API
 
 
 =head1 STATE MACHINE AND LOW-LEVEL EVENT API
 


@@ -79,6 +219,23 @@ 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.
 
 that the complexity is dealing with the (asynchronous) actions of the
 child process.
 

+                            child process
+  ___________________       _________________________
+ /                   \     /                         \
+ | main program      |     | qemu +-----------------+|
+ |                   |     |      | Linux kernel    ||
+ +-------------------+     |      +-----------------+|
+ | libguestfs     <-------------->| guestfsd        ||
+ |                   |     |      +-----------------+|
+ \___________________/     \_________________________/
+
+The diagram above shows libguestfs communicating with the guestfsd
+daemon running inside the qemu child process.  There are several
+points of failure here: qemu can fail to start, the virtual machine
+inside qemu can fail to boot, guestfsd can fail to start or not
+establish communication, any component can start successfully but fail
+asynchronously later, and so on.
+

 =head2 STATE MACHINE
 
 libguestfs uses a state machine to model the child process:
 =head2 STATE MACHINE
 
 libguestfs uses a state machine to model the child process:


@@ -154,37 +311,26 @@ this function with C<cb> set to C<NULL>.
 
 =head2 NON-BLOCKING ACTIONS
 
 
 =head2 NON-BLOCKING ACTIONS
 

-C<guestfs_set_reply_callback> is the most interesting callback to
-play with, since it allows you to perform actions without blocking.
+XXX This section was documented in previous versions but never
+implemented in a way which matched the documentation.  For now I have
+removed the documentation, pending a working implementation.  See also
+C<src/guestfs-actions.c> in the source.

 
 

-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);
-   /* ... */
- }
+=head2 guestfs_set_send_callback
+
+ typedef void (*guestfs_send_cb) (guestfs_h *g, void *opaque);
+ void guestfs_set_send_callback (guestfs_h *handle,
+                                 guestfs_send_cb cb,
+                                 void *opaque);

 
 

-There are C<guestfs_nb_*> and C<guestfs_nb_*_r> functions
-corresponding to (very nearly) every C<guestfs_*> action in the
-high-level API.
+The callback function C<cb> will be called whenever a message
+which is queued for sending, has been sent.

 
 =head2 guestfs_set_reply_callback
 
 
 =head2 guestfs_set_reply_callback
 

- void guestfs_set_reply_callback (guestfs_handle h,
+ typedef void (*guestfs_reply_cb) (guestfs_h *g, void *opaque, XDR *xdr);
+ void guestfs_set_reply_callback (guestfs_h *handle,

                                   guestfs_reply_cb cb,
                                   void *opaque);
 
                                   guestfs_reply_cb cb,
                                   void *opaque);
 


@@ -192,12 +338,15 @@ The callback function C<cb> will be called whenever a reply is
 received from the child process.  (This corresponds to a transition
 from the BUSY state to the READY state).
 
 received from the child process.  (This corresponds to a transition
 from the BUSY state to the READY state).
 

-Note (I<important!>) that high-level API calls overwrite this
-callback.
+Note that the C<xdr> that you get in the callback is in C<XDR_DECODE>
+mode, and you need to consume it before you return from the callback
+function (since it gets destroyed after).

 
 =head2 guestfs_set_log_message_callback
 
 
 =head2 guestfs_set_log_message_callback
 

- void guestfs_set_log_message_callback (guestfs_handle h,
+ typedef void (*guestfs_log_message_cb) (guestfs_h *g, void *opaque,
+                                         char *buf, int len);
+ void guestfs_set_log_message_callback (guestfs_h *handle,

                                         guestfs_log_message_cb cb,
                                         void *opaque);
 
                                         guestfs_log_message_cb cb,
                                         void *opaque);
 


@@ -211,7 +360,8 @@ discarded.
 
 =head2 guestfs_set_subprocess_quit_callback
 
 
 =head2 guestfs_set_subprocess_quit_callback
 

- void guestfs_set_subprocess_quit_callback (guestfs_handle h,
+ typedef void (*guestfs_subprocess_quit_cb) (guestfs_h *g, void *opaque);
+ void guestfs_set_subprocess_quit_callback (guestfs_h *handle,

                                             guestfs_subprocess_quit_cb cb,
                                             void *opaque);
 
                                             guestfs_subprocess_quit_cb cb,
                                             void *opaque);
 


@@ -220,33 +370,35 @@ quits, either asynchronously or if killed by
 C<guestfs_kill_subprocess>.  (This corresponds to a transition from
 any state to the CONFIG state).
 
 C<guestfs_kill_subprocess>.  (This corresponds to a transition from
 any state to the CONFIG state).
 

-=head2 guestfs_set_ready_callback
+=head2 guestfs_set_launch_done_callback

 
 

- void guestfs_set_ready_callback (guestfs_handle h,
-                                  guestfs_ready_cb cb,
-                                  void *opaque);
+ typedef void (*guestfs_launch_done_cb) (guestfs_h *g, void *opaque);
+ void guestfs_set_launch_done_callback (guestfs_h *handle,
+                                        guestfs_ready_cb cb,
+                                        void *opaque);

 
 The callback function C<cb> will be called when the child process
 
 The callback function C<cb> will be called when the child process

-becomes ready.  (This corresponds to a transition from I<either>
-LAUNCHING I<or> BUSY to the READY state).
+becomes ready first time after it has been launched.  (This
+corresponds to a transition from LAUNCHING to the READY state).

 
 You can use this instead of C<guestfs_wait_ready> to implement a
 non-blocking wait for the child process to finish booting up.
 
 =head2 EVENT MAIN LOOP
 
 
 You can use this instead of C<guestfs_wait_ready> to implement a
 non-blocking wait for the child process to finish booting up.
 
 =head2 EVENT MAIN LOOP
 

-To use the low-level event API, you have to provide an event "main
-loop".  You can write your own, but if you don't want to write one,
-two are provided for you:
+To use the low-level event API and/or to use handles from multiple
+threads, you have to provide an event "main loop".  You can write your
+own, but if you don't want to write one, two types are provided for
+you:

 
 =over 4
 
 
 =over 4
 

-=item libguestfs-poll
+=item libguestfs-select

 
 

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

 
 

-This is the default main loop unless you call C<guestfs_set_main_loop>
-or C<guestfs_glib_set_main_loop>.
+This is the default main loop for new guestfs handles, unless you
+call C<guestfs_set_main_loop> after a handle is created.

 
 =item libguestfs-glib
 
 
 =item libguestfs-glib
 


@@ -256,80 +408,284 @@ without hanging during long or slow operations.
 
 =back
 
 
 =back
 

-=head2 guestfs_set_main_loop
+=head2 MULTIPLE HANDLES AND MULTIPLE THREADS

 
 

- void guestfs_set_main_loop (guestfs_main_loop *);
+The support for multiple handles and multiple threads is modelled
+after glib (although doesn't require glib, if you use the select-based
+main loop).

 
 

-This call sets the current main loop to the list of callbacks
-contained in the C<guestfs_main_loop> structure.
+L<http://library.gnome.org/devel/glib/unstable/glib-The-Main-Event-Loop.html>

 
 

-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 will need to create one main loop for each thread that wants to
+use libguestfs.  Each guestfs handle should be confined to one thread.
+If you try to pass guestfs handles between threads, you will get
+undefined results.

 
 

-You should call this early in the main program, certainly before
-calling C<guestfs_create>.
+If you only want to use guestfs handles from one thread in your
+program, but your program has other threads doing other things, then
+you don't need to do anything special.

 
 

-=head2 guestfs_glib_set_main_loop
+=head2 SINGLE THREAD CASE

 
 

- void guestfs_glib_set_main_loop (GMainLoop *);
+In the single thread case, there is a single select-based main loop
+created for you.  All guestfs handles will use this main loop to
+execute high level API actions.

 
 

-This helper calls C<guestfs_set_main_loop> with the correct callbacks
-for integrating with the GLib main loop.
+=head2 MULTIPLE THREADS CASE

 
 

-The libguestfs-glib main loop is contained in a separate library, so
-that libguestfs doesn't depend on the whole of GLib:
+In the multiple threads case, you will need to create a main loop for
+each thread that wants to use libguestfs.

 
 

- #include <glib.h>
- #include <guestfs-glib.h>
+To create main loops for other threads, use
+C<guestfs_create_main_loop> or C<guestfs_glib_create_main_loop>.

 
 

- main ()
- {
-   GMainLoop *loop =
-     g_main_loop_new (g_main_context_default (), 1);
-   ...
-   guestfs_glib_set_main_loop (loop);
-   ...
-   g_main_loop_run (loop);
- }
+Then you will need to attach each handle to the thread-specific main
+loop by calling:
+
+ handle = guestfs_create ();
+ guestfs_set_main_loop (handle, main_loop_of_current_thread);
+
+=head2 guestfs_set_main_loop
+
+ void guestfs_set_main_loop (guestfs_h *handle,
+                             guestfs_main_loop *main_loop);
+
+Sets the main loop used by high level API actions for this handle.  By
+default, the select-based main loop is used (see
+C<guestfs_get_default_main_loop>).

 
 

-To use this main loop you must link with C<-lguestfs-glib>.  (See also
-the GLib and GTK+ documentation).
+You only need to use this in multi-threaded programs, where multiple
+threads want to use libguestfs.  Create a main loop for each thread,
+then call this function.

 
 

-=head2 guestfs_main_loop_run
+You cannot pass guestfs handles between threads.

 
 

- void guestfs_main_loop_run (void);
+=head2 guestfs_get_main_loop

 
 

-This calls the main loop.
+ guestfs_main_loop *guestfs_get_main_loop (guestfs_h *handle);

 
 

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

 
 

-=head2 guestfs_main_loop_quit
+=head2 guestfs_get_default_main_loop

 
 

- void guestfs_main_loop_quit (void);
+ guestfs_main_loop *guestfs_get_default_main_loop (void);

 
 

-This instructs the main loop to quit.  In other words,
-C<guestfs_main_loop_run> will return.
+Return the default select-based main loop.

 
 

-For some types of main loop you may want or prefer to call another
-function, eg. C<g_main_loop_quit>.  In those cases, ignore this call.
+=head2 guestfs_create_main_loop
+
+ guestfs_main_loop *guestfs_create_main_loop (void);
+
+This creates a select-based main loop.  You should create one main
+loop for each additional thread that needs to use libguestfs.
+
+=head2 guestfs_free_main_loop
+
+ void guestfs_free_main_loop (guestfs_main_loop *);
+
+Free the select-based main loop which was previously allocated with
+C<guestfs_create_main_loop>.

 
 =head2 WRITING A CUSTOM MAIN LOOP
 
 
 =head2 WRITING A CUSTOM MAIN LOOP
 

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

 
 

-=head1 SEE ALSO
+=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<src/guestfs_protocol.x>
+(note: this file is automatically generated).
+
+There are two broad cases, ordinary functions that don't have any
+C<FileIn> and C<FileOut> parameters, which are handled with very
+simple request/reply messages.  Then there are functions that have any
+C<FileIn> or C<FileOut> 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_<foo>_args (encoded as XDR)
+
+The total length field allows the daemon to allocate a fixed size
+buffer into which it slurps the rest of the message.  As a result, the
+total length is limited to C<GUESTFS_MESSAGE_MAX> bytes (currently
+4MB), which means the effective size of any request is limited to
+somewhere under this size.
+
+Note also that many functions don't take any arguments, in which case
+the C<guestfs_I<foo>_args> is completely omitted.
+
+The header contains the procedure number (C<guestfs_proc>) which is
+how the receiver knows what type of args structure to expect, or none
+at all.
+
+The reply message for ordinary functions is:

 
 

-L<qemu(1)>
+ total length (header + ret,
+      but not including the length word itself)
+ struct guestfs_message_header (encoded as XDR)
+ struct guestfs_<foo>_ret (encoded as XDR)

 
 

+As above the C<guestfs_I<foo>_ret> structure may be completely omitted
+for functions that return no formal return values.

 
 

+As above the total length of the reply is limited to
+C<GUESTFS_MESSAGE_MAX>.

 
 

+In the case of an error, a flag is set in the header, and the reply
+message is slightly changed:
+
+ total length (header + error,
+      but not including the length word itself)
+ struct guestfs_message_header (encoded as XDR)
+ struct guestfs_message_error (encoded as XDR)
+
+The C<guestfs_message_error> structure contains the error message as a
+string.
+
+=head3 FUNCTIONS THAT HAVE FILEIN PARAMETERS
+
+A C<FileIn> parameter indicates that we transfer a file I<into> the
+guest.  The normal request message is sent (see above).  However this
+is followed by a sequence of file chunks.
+
+ total length (header + arguments,
+      but not including the length word itself,
+      and not including the chunks)
+ struct guestfs_message_header (encoded as XDR)
+ struct guestfs_<foo>_args (encoded as XDR)
+ sequence of chunks for FileIn param #0
+ sequence of chunks for FileIn param #1 etc.
+
+The "sequence of chunks" is:
+
+ length of chunk (not including length word itself)
+ struct guestfs_chunk (encoded as XDR)
+ length of chunk
+ struct guestfs_chunk (encoded as XDR)
+   ...
+ length of chunk
+ struct guestfs_chunk (with data.data_len == 0)
+
+The final chunk has the C<data_len> field set to zero.  Additionally a
+flag is set in the final chunk to indicate either successful
+completion or early cancellation.
+
+At time of writing there are no functions that have more than one
+FileIn parameter.  However this is (theoretically) supported, by
+sending the sequence of chunks for each FileIn parameter one after
+another (from left to right).
+
+Both the library (sender) I<and> the daemon (receiver) may cancel the
+transfer.  The library does this by sending a chunk with a special
+flag set to indicate cancellation.  When the daemon sees this, it
+cancels the whole RPC, does I<not> send any reply, and goes back to
+reading the next request.
+
+The daemon may also cancel.  It does this by writing a special word
+C<GUESTFS_CANCEL_FLAG> to the socket.  The library listens for this
+during the transfer, and if it gets it, it will cancel the transfer
+(it sends a cancel chunk).  The special word is chosen so that even if
+cancellation happens right at the end of the transfer (after the
+library has finished writing and has started listening for the reply),
+the "spurious" cancel flag will not be confused with the reply
+message.
+
+This protocol allows the transfer of arbitrary sized files (no 32 bit
+limit), and also files where the size is not known in advance
+(eg. from pipes or sockets).  However the chunks are rather small
+(C<GUESTFS_MAX_CHUNK_SIZE>), 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_<foo>_ret (encoded as XDR)
+ sequence of chunks for FileOut param #0
+ sequence of chunks for FileOut param #1 etc.
+
+=head3 INITIAL MESSAGE
+
+Because the underlying channel (QEmu -net channel) doesn't have any
+sort of connection control, when the daemon launches it sends an
+initial word (C<GUESTFS_LAUNCH_FLAG>) which indicates that the guest
+and daemon is alive.  This is what C<guestfs_wait_ready> waits for.
+
+=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<must C<exec qemu>> 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</tmp/qemu.wrapper> (or wherever), C<chmod +x>,
+and then use it by setting the LIBGUESTFS_QEMU environment variable.
+For example:
+
+ LIBGUESTFS_QEMU=/tmp/qemu.wrapper guestfish
+
+=head1 ENVIRONMENT VARIABLES
+
+=over 4
+
+=item LIBGUESTFS_DEBUG
+
+Set C<LIBGUESTFS_DEBUG=1> to enable verbose messages.  This
+has the same effect as calling C<guestfs_set_verbose (handle, 1)>.
+
+=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<QEMU WRAPPERS> above.
+
+=back
+
+=head1 SEE ALSO

 
 

+L<guestfish(1)>,
+L<qemu(1)>,
+L<febootstrap(1)>,
+L<http://et.redhat.com/~rjones/libguestfs>.

 
 =head1 AUTHORS
 
 
 =head1 AUTHORS