This isn't documented. Please see the libguestfs-select and
libguestfs-glib implementations.
+=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
+ struct guestfs_<foo>_args
+
+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:
+
+ total length (header + ret,
+ but not including the length word itself)
+ struct guestfs_message_header
+ struct guestfs_<foo>_ret
+
+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
+ struct guestfs_message_error
+
+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
+ struct guestfs_<foo>_args
+ sequence of chunks for FileIn param #0
+ sequence of chunks for FileIn param #1 etc.
+
+The sequence of chunks is a sequence of C<struct guestfs_chunk>. 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 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
+ struct guestfs_<foo>_ret
+ sequence of chunks for FileOut param #0
+ sequence of chunks for FileOut param #1 etc.
+
=head1 ENVIRONMENT VARIABLES
=over 4
git.annexia.org / libguestfs.git / blobdiff