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=head1 NAME
nbd_connect_systemd_socket_activation - connect using systemd socket
activation
=head1 SYNOPSIS
#include <libnbd.h>
int nbd_connect_systemd_socket_activation (
struct nbd_handle *h, char **argv
);
=head1 DESCRIPTION
Run the command as a subprocess and connect to it using
systemd socket activation.
This is especially useful for running L<qemu-nbd(1)> as
a subprocess of libnbd, for example to use it to open
qcow2 files.
To run nbdkit as a subprocess, this function can be used,
or L<nbd_connect_command(3)>.
To run L<nbd-server(1)> as a subprocess, this function
cannot be used, you must use L<nbd_connect_command(3)>.
=head2 Socket activation
Libnbd will fork the C<argv> command and pass an NBD
socket to it using special C<LISTEN_*> environment
variables (as defined by the systemd socket activation
protocol).
┌─────────┬─────────┐ ┌───────────────┐
│ program │ libnbd │ │ qemu-nbd or │
│ │ │ │ other server │
│ │ socket ╍╍╍╍╍╍╍╍▶ │
└─────────┴─────────┘ └───────────────┘
When the NBD handle is closed the server subprocess
is killed.
=head3 Socket name
The socket activation protocol lets you optionally give
the socket a name. If used, the name is passed to the
NBD server using the C<LISTEN_FDNAMES> environment
variable. To provide a socket name, call
L<nbd_set_socket_activation_name(3)> before calling
the connect function.
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_systemd_socket_activation
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 1.2.
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_SYSTEMD_SOCKET_ACTIVATION 1
=head1 EXAMPLE
This example is also available as F<examples/open-qcow2.c>
in the libnbd source code.
/* 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\n");
exit (EXIT_FAILURE);
}
filename = argv[1];
/* Create the libnbd handle. */
nbd = nbd_create ();
if (nbd == NULL) {
fprintf (stderr, "%s\n", 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\n", 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\n", 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);
}
=head1 SEE ALSO
L<nbd_aio_connect_systemd_socket_activation(3)>,
L<nbd_connect_command(3)>,
L<nbd_create(3)>,
L<nbd_get_socket_activation_name(3)>,
L<nbd_get_subprocess_pid(3)>,
L<nbd_kill_subprocess(3)>,
L<nbd_set_opt_mode(3)>,
L<nbd_set_socket_activation_name(3)>,
L<libnbd(3)>,
L<qemu-nbd(1)>,
L<http://0pointer.de/blog/projects/socket-activation.html>.
=head1 AUTHORS
Eric Blake
Richard W.M. Jones
=head1 COPYRIGHT
Copyright Red Hat
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