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.\" ========================================================================
.\"
.IX Title "nbd_connect_command 3"
.TH nbd_connect_command 3 2024-05-08 libnbd-1.20.1 LIBNBD
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH NAME
nbd_connect_command \- connect to NBD server command
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #include <libnbd.h>
\&
\& int nbd_connect_command (
\&       struct nbd_handle *h, char **argv
\&     );
.Ve
.SH DESCRIPTION
.IX Header "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 \fBnbdkit\fR\|(1) using
the \fI\-s\fR/\fI\-\-single\fR flag, and \fBnbd\-server\fR\|(1) with
port number set to 0.
.PP
To run \fBqemu\-nbd\fR\|(1), use
\&\fBnbd_connect_systemd_socket_activation\fR\|(3) instead.
.SS Subprocess
.IX Subsection "Subprocess"
Libnbd will fork the \f(CW\*(C`argv\*(C'\fR command and pass the NBD socket
to it using file descriptors 0 and 1 (stdin/stdout):
.PP
.Vb 5
\& ┌─────────┬─────────┐    ┌────────────────┐
\& │ program │ libnbd  │    │   NBD server   │
\& │         │         │    │       (argv)   │
\& │         │ socket ╍╍╍╍╍╍╍╍▶ stdin/stdout │
\& └─────────┴─────────┘    └────────────────┘
.Ve
.PP
When the NBD handle is closed the server subprocess
is killed.
.PP
This call returns when the connection has been made.  By default,
this proceeds all the way to transmission phase, but
\&\fBnbd_set_opt_mode\fR\|(3) can be used for manual control over
option negotiation performed before transmission phase.
.SH "RETURN VALUE"
.IX Header "RETURN VALUE"
If the call is successful the function returns \f(CW0\fR.
.SH ERRORS
.IX Header "ERRORS"
On error \f(CW\-1\fR is returned.
.PP
Refer to "ERROR HANDLING" in \fBlibnbd\fR\|(3)
for how to get further details of the error.
.PP
The following parameters must not be NULL: \f(CW\*(C`h\*(C'\fR, \f(CW\*(C`argv\*(C'\fR.
For more information see "Non-NULL parameters" in \fBlibnbd\fR\|(3).
.SH "HANDLE STATE"
.IX Header "HANDLE STATE"
nbd_connect_command
can be called when the handle is in the following state:
.PP
.Vb 8
\& ┌─────────────────────────────────────┬─────────────────────────┐
\& │ Handle created, before connecting   │ ✅ allowed              │
\& │ Connecting                          │ ❌ error                │
\& │ Connecting & handshaking (opt_mode) │ ❌ error                │
\& │ Connected to the server             │ ❌ error                │
\& │ Connection shut down                │ ❌ error                │
\& │ Handle dead                         │ ❌ error                │
\& └─────────────────────────────────────┴─────────────────────────┘
.Ve
.SH VERSION
.IX Header "VERSION"
This function first appeared in libnbd 1.0.
.PP
If you need to test if this function is available at compile time
check if the following macro is defined:
.PP
.Vb 1
\& #define LIBNBD_HAVE_NBD_CONNECT_COMMAND 1
.Ve
.SH EXAMPLE
.IX Header "EXAMPLE"
This example is also available as \fIexamples/connect\-command.c\fR
in the libnbd source code.
.PP
.Vb 3
\& /* 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\en", 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\en", 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\en", nbd_get_error ());
\&     exit (EXIT_FAILURE);
\&   }
\&
\&   /* Read the first sector back. */
\&   if (nbd_pread (nbd, rbuf, sizeof rbuf, 0, 0) == \-1) {
\&     fprintf (stderr, "%s\en", 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\en");
\&     exit (EXIT_FAILURE);
\&   }
\&
\&   exit (EXIT_SUCCESS);
\& }
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbd_aio_connect_command\fR\|(3),
\&\fBnbd_connect_systemd_socket_activation\fR\|(3),
\&\fBnbd_create\fR\|(3),
\&\fBnbd_kill_subprocess\fR\|(3),
\&\fBnbd_set_opt_mode\fR\|(3),
\&\fBlibnbd\fR\|(3),
\&\fBnbdkit\fR\|(1).
.SH AUTHORS
.IX Header "AUTHORS"
Eric Blake
.PP
Richard W.M. Jones
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.SH LICENSE
.IX Header "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.
.PP
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.
.PP
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