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.\" ========================================================================
.\"
.IX Title "nbd_connect_systemd_socket_activation 3"
.TH nbd_connect_systemd_socket_activation 3 2024-05-31 libnbd-1.20.0 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_systemd_socket_activation \- connect using systemd socket
activation
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #include <libnbd.h>
\&
\& int nbd_connect_systemd_socket_activation (
\&       struct nbd_handle *h, char **argv
\&     );
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
Run the command as a subprocess and connect to it using
systemd socket activation.
.PP
This is especially useful for running \fBqemu\-nbd\fR\|(1) as
a subprocess of libnbd, for example to use it to open
qcow2 files.
.PP
To run nbdkit as a subprocess, this function can be used,
or \fBnbd_connect_command\fR\|(3).
.PP
To run \fBnbd\-server\fR\|(1) as a subprocess, this function
cannot be used, you must use \fBnbd_connect_command\fR\|(3).
.SS "Socket activation"
.IX Subsection "Socket activation"
Libnbd will fork the \f(CW\*(C`argv\*(C'\fR command and pass an NBD
socket to it using special \f(CW\*(C`LISTEN_*\*(C'\fR environment
variables (as defined by the systemd socket activation
protocol).
.PP
.Vb 5
\& ┌─────────┬─────────┐    ┌───────────────┐
\& │ program │ libnbd  │    │  qemu\-nbd or  │
\& │         │         │    │  other server │
\& │         │ socket ╍╍╍╍╍╍╍╍▶             │
\& └─────────┴─────────┘    └───────────────┘
.Ve
.PP
When the NBD handle is closed the server subprocess
is killed.
.PP
\fISocket name\fR
.IX Subsection "Socket name"
.PP
The socket activation protocol lets you optionally give
the socket a name.  If used, the name is passed to the
NBD server using the \f(CW\*(C`LISTEN_FDNAMES\*(C'\fR environment
variable.  To provide a socket name, call
\&\fBnbd_set_socket_activation_name\fR\|(3) before calling
the connect function.
.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_systemd_socket_activation
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 1.2.
.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_SYSTEMD_SOCKET_ACTIVATION 1
.Ve
.SH EXAMPLE
.IX Header "EXAMPLE"
This example is also available as \fIexamples/open\-qcow2.c\fR
in the libnbd source code.
.PP
.Vb 3
\& /* This example shows how to use qemu\-nbd
\&  * to open a local qcow2 file.
\&  */
\&
\& #include <stdio.h>
\& #include <stdlib.h>
\& #include <string.h>
\&
\& #include <libnbd.h>
\&
\& int
\& main (int argc, const char *argv[])
\& {
\&   const char *filename;
\&   struct nbd_handle *nbd;
\&   char buf[512];
\&   FILE *fp;
\&
\&   if (argc != 2) {
\&     fprintf (stderr, "open\-qcow2 file.qcow2\en");
\&     exit (EXIT_FAILURE);
\&   }
\&   filename = argv[1];
\&
\&   /* Create the libnbd handle. */
\&   nbd = nbd_create ();
\&   if (nbd == NULL) {
\&     fprintf (stderr, "%s\en", nbd_get_error ());
\&     exit (EXIT_FAILURE);
\&   }
\&
\&   /* Run qemu\-nbd as a subprocess using
\&    * systemd socket activation.
\&    */
\&   char *args[] = {
\&     "qemu\-nbd", "\-f", "qcow2",
\&     (char *)filename,
\&     NULL
\&   };
\&   if (nbd_connect_systemd_socket_activation (nbd,
\&                                              args) == \-1) {
\&     fprintf (stderr, "%s\en", nbd_get_error ());
\&     exit (EXIT_FAILURE);
\&   }
\&
\&   /* Read the first sector and print it. */
\&   if (nbd_pread (nbd, buf, sizeof buf, 0, 0) == \-1) {
\&     fprintf (stderr, "%s\en", nbd_get_error ());
\&     exit (EXIT_FAILURE);
\&   }
\&
\&   fp = popen ("hexdump \-C", "w");
\&   if (fp == NULL) {
\&     perror ("popen: hexdump");
\&     exit (EXIT_FAILURE);
\&   }
\&   fwrite (buf, sizeof buf, 1, fp);
\&   pclose (fp);
\&
\&   /* Close the libnbd handle. */
\&   nbd_close (nbd);
\&
\&   exit (EXIT_SUCCESS);
\& }
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbd_aio_connect_systemd_socket_activation\fR\|(3),
\&\fBnbd_connect_command\fR\|(3),
\&\fBnbd_create\fR\|(3),
\&\fBnbd_get_socket_activation_name\fR\|(3),
\&\fBnbd_kill_subprocess\fR\|(3),
\&\fBnbd_set_opt_mode\fR\|(3),
\&\fBnbd_set_socket_activation_name\fR\|(3),
\&\fBlibnbd\fR\|(3),
\&\fBqemu\-nbd\fR\|(1),
http://0pointer.de/blog/projects/socket\-activation.html.
.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