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nbd_opt_go - end negotiation and move on to using an export
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Copyright
Copyright Red Hat
Description
Request that the server finish negotiation and move on to serving the export previously specified by the
most recent nbd_set_export_name(3) or nbd_connect_uri(3). This can only be used if nbd_set_opt_mode(3)
enabled option mode.
By default, libnbd will automatically request all meta contexts registered by nbd_add_meta_context(3) as
part of this call; but this can be suppressed with nbd_set_request_meta_context(3), particularly if
nbd_opt_set_meta_context(3) was used earlier in the negotiation sequence.
If this fails, the server may still be in negotiation, where it is possible to attempt another option
such as a different export name; although older servers will instead have killed the connection.
Errors
On error -1 is returned.
Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.
The following parameters must not be NULL: "h". For more information see "Non-NULL parameters" in
libnbd(3).
Example
This example is also available as 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);
}
Handle State
nbd_opt_go can be called when the handle is in the following state:
┌─────────────────────────────────────┬─────────────────────────┐
│ Handle created, before connecting │ ❌ error │
│ Connecting │ ❌ error │
│ Connecting & handshaking (opt_mode) │ ✅ allowed │
│ Connected to the server │ ❌ error │
│ Connection shut down │ ❌ error │
│ Handle dead │ ❌ error │
└─────────────────────────────────────┴─────────────────────────┘
License
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
libnbd-1.22.2 2025-06-16 nbd_opt_go(3)
Name
nbd_opt_go - end negotiation and move on to using an export
Return Value
If the call is successful the function returns 0.
See Also
nbd_add_meta_context(3), nbd_aio_opt_go(3), nbd_connect_uri(3), nbd_create(3), nbd_opt_abort(3), nbd_opt_info(3), nbd_opt_set_meta_context(3), nbd_set_export_name(3), nbd_set_opt_mode(3), nbd_set_request_meta_context(3), libnbd(3).
Synopsis
#include <libnbd.h>
int nbd_opt_go (
struct nbd_handle *h
);
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_OPT_GO 1
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