nbd_connect_command - Man Page

 #include <libnbd.h>

 int nbd_connect_command (
       struct nbd_handle *h, char **argv
     );

Run the command as a subprocess and connect to it over stdin/stdout.  This is for use with NBD servers which can behave like inetd clients, such as nbdkit(1) using the -s/--single flag, and nbd-server(1) with port number set to 0.

To run qemu-nbd(1), use nbd_connect_systemd_socket_activation(3) instead.

 ┌─────────┬─────────┐    ┌────────────────┐
 │ program │ libnbd  │    │   NBD server   │
 │         │         │    │       (argv)   │
 │         │ socket ╍╍╍╍╍╍╍╍▶ stdin/stdout │
 └─────────┴─────────┘    └────────────────┘

If the call is successful the function returns 0.

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, argv. For more information see "Non-NULL parameters" in libnbd(3).

nbd_connect_command 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                │
 └─────────────────────────────────────┴─────────────────────────┘

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_COMMAND 1

This example is also available as examples/connect-command.c in the libnbd source code.

 /* This example shows how to run an NBD server
  * (nbdkit) as a subprocess of libnbd.
  */

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

 #include <libnbd.h>

 int
 main (int argc, char *argv[])
 {
   struct nbd_handle *nbd;
   char wbuf[512], rbuf[512];
   size_t i;

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

   /* Run nbdkit as a subprocess. */
   char *args[] = {
     "nbdkit",

     /* You must use ‘-s’ (which tells nbdkit to serve
      * a single connection on stdin/stdout).
      */
     "-s",

     /* It is recommended to use ‘--exit-with-parent’
      * to ensure nbdkit is always cleaned up even
      * if the main program crashes.
      */
     "--exit-with-parent",

     /* Use this to enable nbdkit debugging. */
     "-v",

     /* The nbdkit plugin name - this is a RAM disk. */
     "memory", "size=1M",

     /* Use NULL to terminate the arg list. */
     NULL
   };
   if (nbd_connect_command (nbd, args) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }

   /* Write some random data to the first sector. */
   for (i = 0; i < sizeof wbuf; ++i)
     wbuf[i] = i % 13;
   if (nbd_pwrite (nbd, wbuf, sizeof wbuf, 0, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }

   /* Read the first sector back. */
   if (nbd_pread (nbd, rbuf, sizeof rbuf, 0, 0) == -1) {
     fprintf (stderr, "%s\n", nbd_get_error ());
     exit (EXIT_FAILURE);
   }

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

   /* What was read must be exactly the same as what
    * was written.
    */
   if (memcmp (rbuf, wbuf, sizeof rbuf) != 0) {
     fprintf (stderr, "FAILED: "
              "read data did not match written data\n");
     exit (EXIT_FAILURE);
   }

   exit (EXIT_SUCCESS);
 }

nbd_aio_connect_command(3), nbd_connect_systemd_socket_activation(3), nbd_create(3), nbd_get_subprocess_pid(3), nbd_kill_subprocess(3), nbd_set_opt_mode(3), libnbd(3), nbdkit(1).

Eric Blake

Richard W.M. Jones

Copyright Red Hat

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(3), libnbd-release-notes-1.16(1), NBD(3), nbd_aio_connect_command(3), nbd_connect_socket(3), nbd_connect_systemd_socket_activation(3), nbd_create(3), nbdfuse(1), nbd_get_subprocess_pid(3), nbd_kill_subprocess(3), nbdublk(1).