---
name: "nbd_connect_unix"
section: "3"
platform: "linux"
package: "libnbd"
version: "1.24.4-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/nbd_connect_unix"
---

# nbd\_connect\_unix(3)

connect to NBD server over a Unix domain socket

## NAME

nbd\_connect\_unix - connect to NBD server over a Unix domain socket

## SYNOPSIS

```
 #include <libnbd.h>
 int nbd_connect_unix (
       struct nbd_handle *h, const char *unixsocket
     );
```

## DESCRIPTION

Connect (synchronously) over the named Unix domain socket (`"unixsocket"`) to an NBD server running on the same machine.

This call returns when the connection has been made.  By default, this proceeds all the way to transmission phase, but [**nbd\_set\_opt\_mode**(3)](/nbd_set_opt_mode/3.md) can be used for manual control over option negotiation performed before transmission phase.

## RETURN VALUE

If the call is successful the function returns `0`.

## ERRORS

On error `-1` is returned.

Refer to "ERROR HANDLING" in [**libnbd**(3)](/libnbd/3.md) for how to get further details of the error.

The following parameters must not be NULL: `"h"`, `"unixsocket"`. For more information see "Non-NULL parameters" in [**libnbd**(3)](/libnbd/3.md).

## HANDLE STATE

nbd\_connect\_unix 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                │
 └─────────────────────────────────────┴─────────────────────────┘
```

## VERSION

This function first appeared in libnbd 1.0.

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

```
 #define LIBNBD_HAVE_NBD_CONNECT_UNIX 1
```

## EXAMPLE

This example is also available as *examples/fetch-first-sector.c* in the libnbd source code.

```
 /* This example shows how to connect to an NBD server
  * and fetch and print the first sector (usually the
  * boot sector or partition table or filesystem
  * superblock).
  *
  * You can test it with nbdkit like this:
  *
  * nbdkit -U - floppy . \
  *   --run './fetch-first-sector $unixsocket'
  *
  * The nbdkit floppy plugin creates an MBR disk so the
  * first sector is the partition table.
  */
 #include <stdio.h>
 #include <stdlib.h>
 #include <libnbd.h>
 int
 main (int argc, char *argv[])
 {
   struct nbd_handle *nbd;
   char buf[512];
   FILE *pp;
   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);
   }
   /* Connect to the NBD server over a
    * Unix domain socket.
    */
   if (nbd_connect_unix (nbd, argv[1]) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   /* Read the first sector synchronously. */
   if (nbd_pread (nbd, buf, sizeof buf, 0, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }
   /* Close the libnbd handle. */
   nbd_close (nbd);
   /* Print the first sector. */
   pp = popen ("hexdump -C", "w");
   if (pp == NULL) {
     perror ("popen: hexdump");
     exit (EXIT_FAILURE);
   }
   fwrite (buf, sizeof buf, 1, pp);
   pclose (pp);
   exit (EXIT_SUCCESS);
 }
```

## SEE ALSO

[**nbd\_aio\_connect\_unix**(3)](/nbd_aio_connect_unix/3.md), [**nbd\_create**(3)](/nbd_create/3.md), [**nbd\_set\_opt\_mode**(3)](/nbd_set_opt_mode/3.md), [**libnbd**(3)](/libnbd/3.md).

## AUTHORS

Eric Blake

Richard W.M. Jones

## COPYRIGHT

Copyright Red Hat

## 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