=head1 NAME

nbd_set_opt_mode - control option mode, for pausing during option
negotiation

=head1 SYNOPSIS

 #include <libnbd.h>

 int nbd_set_opt_mode (
       struct nbd_handle *h, bool enable
     );

=head1 DESCRIPTION

Set this flag to true in order to request
that a connection command
C<nbd_connect_*> will pause for negotiation options rather than
proceeding all the way to the ready state, when communicating with a
newstyle server.  This setting has no effect when connecting to an
oldstyle server.

Note that libnbd defaults to attempting C<NBD_OPT_STARTTLS>,
C<NBD_OPT_EXTENDED_HEADERS>, and C<NBD_OPT_STRUCTURED_REPLY>
before letting you control remaining negotiation steps; if you
need control over these steps as well, first set L<nbd_set_tls(3)>
to C<LIBNBD_TLS_DISABLE>, and L<nbd_set_request_extended_headers(3)>
or L<nbd_set_request_structured_replies(3)> to false, before
starting the connection attempt.

When option mode is enabled, you have fine-grained control over which
options are negotiated, compared to the default of the server
negotiating everything on your behalf using settings made before
starting the connection.  To leave the mode and proceed on to the
ready state, you must use L<nbd_opt_go(3)> successfully; a failed
L<nbd_opt_go(3)> returns to the negotiating state to allow a change of
export name before trying again.  You may also use L<nbd_opt_abort(3)>
or L<nbd_shutdown(3)> to end the connection without finishing
negotiation.

=head1 RETURN VALUE

If the call is successful the function returns C<0>.

=head1 ERRORS

On error C<-1> is returned.

Refer to L<libnbd(3)/ERROR HANDLING>
for how to get further details of the error.

The following parameters must not be NULL: C<h>.
For more information see L<libnbd(3)/Non-NULL parameters>.

=head1 HANDLE STATE

nbd_set_opt_mode
can be called when the handle is in the following state:

 ┌─────────────────────────────────────┬─────────────────────────┐
 │ Handle created, before connecting   │ ✅ allowed              │
 │ Connecting                          │ ❌ error                │
 │ Connecting & handshaking (opt_mode) │ ❌ error                │
 │ Connected to the server             │ ❌ error                │
 │ Connection shut down                │ ❌ error                │
 │ Handle dead                         │ ❌ error                │
 └─────────────────────────────────────┴─────────────────────────┘

=head1 VERSION

This function first appeared in libnbd 1.4.

If you need to test if this function is available at compile time
check if the following macro is defined:

 #define LIBNBD_HAVE_NBD_SET_OPT_MODE 1

=head1 EXAMPLE

This example is also available as F<examples/list-exports.c>
in the libnbd source code.

 /* This example shows how to list NBD exports.
  *
  * To test this with qemu-nbd:
  *   $ qemu-nbd -x "hello" -t -k /tmp/sock disk.img
  *   $ ./run examples/list-exports /tmp/sock
  *   [0] hello
  *   Which export to connect to (-1 to quit)? 0
  *   Connecting to hello ...
  *   /tmp/sock: hello: size = 2048 bytes
  *
  * To test this with nbdkit (requires 1.22):
  *   $ nbdkit -U /tmp/sock sh - <<\EOF
  *   case $1 in
  *     list_exports) echo NAMES; echo foo; echo foobar ;;
  *     open) echo "$3" ;;
  *     get_size) echo "$2" | wc -c ;;
  *     pread) echo "$2" | dd bs=1 skip=$4 count=$3 ;;
  *     *) exit 2 ;;
  *   esac
  *   EOF
  *   $ ./run examples/list-exports /tmp/sock
  *   [0] foo
  *   [1] foobar
  *   Which export to connect to (-1 to quit)? 1
  *   Connecting to foobar ...
  *   /tmp/sock: foobar: size = 7 bytes
  */

 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
 #include <inttypes.h>
 #include <errno.h>

 #include <libnbd.h>

 struct export_list {
   int i;
   char **names;
 };

 /* Callback function for nbd_opt_list */
 static int
 list_one (void *opaque, const char *name,
           const char *description)
 {
   struct export_list *l = opaque;
   char **names;

   printf ("[%d] %s\n", l->i, name);
   if (*description)
     printf ("  (%s)\n", description);
   names = realloc (l->names,
                    (l->i + 1) * sizeof *names);
   if (!names) {
     perror ("realloc");
     exit (EXIT_FAILURE);
   }
   names[l->i] = strdup (name);
   if (!names[l->i]) {
     perror ("strdup");
     exit (EXIT_FAILURE);
   }
   l->names = names;
   l->i++;
   return 0;
 }

 int
 main (int argc, char *argv[])
 {
   struct nbd_handle *nbd;
   int i;
   const char *name;
   int64_t size;
   struct export_list list = { 0 };

   if (argc != 2) {
     fprintf (stderr, "%s socket\n", argv[0]);
     exit (EXIT_FAILURE);
   }

   /* Create the libnbd handle. */
   nbd = nbd_create ();
   if (nbd == NULL) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }

   /* Set opt mode. */
   nbd_set_opt_mode (nbd, true);

   /* Connect to the NBD server over a
    * Unix domain socket.  If we did not
    * end up in option mode, then a
    * listing is not possible.
    */
   if (nbd_connect_unix (nbd, argv[1]) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   if (!nbd_aio_is_negotiating (nbd)) {
     fprintf (stderr, "Server does not support "
              "listing exports.\n");
     exit (EXIT_FAILURE);
   }

   /* Print the export list. */
   if (nbd_opt_list (nbd,
                     (nbd_list_callback) {
                       .callback = list_one,
                       .user_data = &list, }) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }

   /* Display the list of exports. */
   printf ("Which export to connect to? ");
   if (scanf ("%d", &i) != 1) exit (EXIT_FAILURE);
   if (i == -1) {
     if (nbd_opt_abort (nbd) == -1) {
       fprintf (stderr, "%s\n", nbd_get_error ());
       exit (EXIT_FAILURE);
     }
     nbd_close (nbd);
     exit (EXIT_SUCCESS);
   }
   if (i < 0 || i >= list.i) {
     fprintf (stderr, "index %d out of range", i);
     exit (EXIT_FAILURE);
   }
   name = list.names[i];
   printf ("Connecting to %s ...\n", name);

   /* Resume connecting to the chosen export. */
   if (nbd_set_export_name (nbd, name) == -1 ||
       nbd_opt_go (nbd) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   if (!nbd_aio_is_ready (nbd)) {
     fprintf (stderr, "server closed early\n");
     exit (EXIT_FAILURE);
   }

   /* Read the size in bytes and print it. */
   size = nbd_get_size (nbd);
   if (size == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   printf ("%s: %s: size = %" PRIi64 " bytes\n",
           argv[1], name, size);

   /* Close the libnbd handle. */
   nbd_close (nbd);

   for (i = 0; i < list.i; i++)
     free (list.names[i]);
   free (list.names);

   exit (EXIT_SUCCESS);
 }

=head1 SEE ALSO

L<nbd_aio_connect(3)>,
L<nbd_aio_is_negotiating(3)>,
L<nbd_create(3)>,
L<nbd_get_opt_mode(3)>,
L<nbd_opt_abort(3)>,
L<nbd_opt_go(3)>,
L<nbd_opt_info(3)>,
L<nbd_opt_list(3)>,
L<nbd_opt_list_meta_context(3)>,
L<nbd_opt_set_meta_context(3)>,
L<nbd_opt_starttls(3)>,
L<nbd_opt_structured_reply(3)>,
L<nbd_set_request_extended_headers(3)>,
L<nbd_set_request_structured_replies(3)>,
L<nbd_set_tls(3)>,
L<nbd_shutdown(3)>,
L<libnbd(3)>.

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
