NAME

       nbd_connect_command - connect to NBD server command

SYNOPSIS

        #include <libnbd.h>

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

DESCRIPTION

       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.

   Subprocess
       Libnbd will fork the "argv" command and pass the NBD socket to it using file descriptors 0
       and 1 (stdin/stdout):

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

       When the NBD handle is closed the server subprocess is killed.

RETURN VALUE

       If the call is successful the function returns 0.

ERRORS

       On error "-1" is returned.

       Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error.

HANDLE STATE

       The handle must be newly created, otherwise this call will return an 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_COMMAND 1

EXAMPLE

       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);
        }

SEE ALSO

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

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2019-2021 Red Hat Inc.

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