+=head1 CALLS WITH OPTIONAL ARGUMENTS
+
+A recent feature of the API is the introduction of calls which take
+optional arguments. In C these are declared 3 ways. The main way is
+as a call which takes variable arguments (ie. C<...>), as in this
+example:
+
+ int guestfs_add_drive_opts (guestfs_h *g, const char *filename, ...);
+
+Call this with a list of optional arguments, terminated by C<-1>.
+So to call with no optional arguments specified:
+
+ guestfs_add_drive_opts (g, filename, -1);
+
+With a single optional argument:
+
+ guestfs_add_drive_opts (g, filename,
+ GUESTFS_ADD_DRIVE_OPTS_FORMAT, "qcow2",
+ -1);
+
+With two:
+
+ guestfs_add_drive_opts (g, filename,
+ GUESTFS_ADD_DRIVE_OPTS_FORMAT, "qcow2",
+ GUESTFS_ADD_DRIVE_OPTS_READONLY, 1,
+ -1);
+
+and so forth. Don't forget the terminating C<-1> otherwise
+Bad Things will happen!
+
+=head2 USING va_list FOR OPTIONAL ARGUMENTS
+
+The second variant has the same name with the suffix C<_va>, which
+works the same way but takes a C<va_list>. See the C manual for
+details. For the example function, this is declared:
+
+ int guestfs_add_drive_opts_va (guestfs_h *g, const char *filename,
+ va_list args);
+
+=head2 CONSTRUCTING OPTIONAL ARGUMENTS
+
+The third variant is useful where you need to construct these
+calls. You pass in a structure where you fill in the optional
+fields. The structure has a bitmask as the first element which
+you must set to indicate which fields you have filled in. For
+our example function the structure and call are declared:
+
+ struct guestfs_add_drive_opts_argv {
+ uint64_t bitmask;
+ int readonly;
+ const char *format;
+ /* ... */
+ };
+ int guestfs_add_drive_opts_argv (guestfs_h *g, const char *filename,
+ const struct guestfs_add_drive_opts_argv *optargs);
+
+You could call it like this:
+
+ struct guestfs_add_drive_opts_argv optargs = {
+ .bitmask = GUESTFS_ADD_DRIVE_OPTS_READONLY_BITMASK |
+ GUESTFS_ADD_DRIVE_OPTS_FORMAT_BITMASK,
+ .readonly = 1,
+ .format = "qcow2"
+ };
+
+ guestfs_add_drive_opts_argv (g, filename, &optargs);
+
+Notes:
+
+=over 4
+
+=item *
+
+The C<_BITMASK> suffix on each option name when specifying the
+bitmask.
+
+=item *
+
+You do not need to fill in all fields of the structure.
+
+=item *
+
+There must be a one-to-one correspondence between fields of the
+structure that are filled in, and bits set in the bitmask.
+
+=back
+
+=head2 OPTIONAL ARGUMENTS IN OTHER LANGUAGES
+
+In other languages, optional arguments are expressed in the
+way that is natural for that language. We refer you to the
+language-specific documentation for more details on that.
+
+For guestfish, see L<guestfish(1)/OPTIONAL ARGUMENTS>.
+
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