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.\" ========================================================================
.\"
.IX Title "nbd_connect_systemd_socket_activation 3"
.TH nbd_connect_systemd_socket_activation 3 "2021-02-09" "libnbd-1.6.1" "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_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.  To run nbdkit as a subprocess it is usually
better to 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 \s-1NBD\s0
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 \s-1NBD\s0 handle is closed the server subprocess
is killed.
.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\*(C`\-1\*(C'\fR is returned.
.PP
Refer to \*(L"\s-1ERROR HANDLING\*(R"\s0 in \fBlibnbd\fR\|(3)
for how to get further details of the error.
.SH "HANDLE STATE"
.IX Header "HANDLE STATE"
The handle must be
newly created,
otherwise this call will return an error.
.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, 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_connect_command\fR\|(3),
\&\fBnbd_create\fR\|(3),
\&\fBnbd_kill_subprocess\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 (C) 2019\-2020 Red Hat Inc.
.SH "LICENSE"
.IX Header "LICENSE"
This library is free software; you can redistribute it and/or
modify it under the terms of the \s-1GNU\s0 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 \s-1WITHOUT ANY WARRANTY\s0; without even the implied warranty of
\&\s-1MERCHANTABILITY\s0 or \s-1FITNESS FOR A PARTICULAR PURPOSE.\s0  See the \s-1GNU\s0
Lesser General Public License for more details.
.PP
You should have received a copy of the \s-1GNU\s0 Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, \s-1MA 02110\-1301 USA\s0