=head1 NAME

nbd_connect_command - connect to NBD server command

=head1 SYNOPSIS

 #include <libnbd.h>

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

=head1 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 L<nbdkit(1)> using
the I<-s>/I<--single> flag, and L<nbd-server(1)> with
port number set to 0.

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

=head2 Subprocess

Libnbd will fork the C<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.


This call returns when the connection has been made.  By default,
this proceeds all the way to transmission phase, but
L<nbd_set_opt_mode(3)> can be used for manual control over
option negotiation performed before transmission phase.

=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>, C<argv>.
For more information see L<libnbd(3)/Non-NULL parameters>.

=head1 HANDLE STATE

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

=head1 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

=head1 EXAMPLE

This example is also available as F<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);
 }

=head1 SEE ALSO

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

=head1 AUTHORS

Eric Blake

Richard W.M. Jones

=head1 COPYRIGHT

Copyright Red Hat
