org.freedesktop.LogControl1 - D-Bus interface to query and set logging
configuration
org.freedesktop.LogControl1 is a generic interface that is intended to be used
by any daemon which allows the log level and target to be set over D-Bus. It
is implemented by various daemons that are part of the
systemd(1)
suite.
It is assumed that those settings are global for the whole program, so a fixed
object path is used. The interface should always be available under the path
/org/freedesktop/LogControl1.
The following interface is exposed:
node /org/freedesktop/LogControl1 {
interface org.freedesktop.LogControl1 {
properties:
@org.freedesktop.DBus.Property.EmitsChangedSignal("false")
@org.freedesktop.systemd1.Privileged("true")
readwrite s LogLevel = '...';
@org.freedesktop.DBus.Property.EmitsChangedSignal("false")
@org.freedesktop.systemd1.Privileged("true")
readwrite s LogTarget = '...';
@org.freedesktop.DBus.Property.EmitsChangedSignal("false")
readonly s SyslogIdentifier = '...';
};
interface org.freedesktop.DBus.Peer { ... };
interface org.freedesktop.DBus.Introspectable { ... };
interface org.freedesktop.DBus.Properties { ... };
};
LogLevel describes the
syslog(3)-style log-level, and should be
one of "emerg", "alert", "crit",
"err", "warning", "notice", "info",
"debug", in order of increasing verbosity.
LogTarget describes the log target (mechanism). It should be one of
"console" (log to the console or standard output), "kmsg"
(log to the kernel ring buffer), "journal" (log to the journal
natively, see
systemd-journald.service(8)), "syslog" (log
using the
syslog(3) call).
Those two properties are writable, so they may be set by sufficiently privileged
users.
SyslogIdentifier is a read-only property that shows the "syslog
identifier". It is a short string that identifies the program that is the
source of log messages that is passed to the
syslog(3) call.
journalctl option
-p/
--priority= may be used to filter log
messages by log level, option
-t/
--identifier= may be used to by
the syslog identifier, and filters like "_TRANSPORT=syslog",
"_TRANSPORT=journal", and "_TRANSPORT=kernel" may be used
to filter messages by the mechanism through which they reached
systemd-journald.
systemctl log-level and
systemctl log-target verbs may be used to
query and set the
LogLevel and
LogTarget properties of the
service manager.
systemctl service-log-level and
systemctl
service-log-target may similarly be used for individual services.
(Services must have the
BusName= property set and must implement the
interface described here. See
systemd.service(5) for details about
BusName=.)
Example 1. Create a simple listener on the bus that implements
LogControl1
/* SPDX-License-Identifier: MIT-0 */
/* Implements the LogControl1 interface as per specification:
* https://www.freedesktop.org/software/systemd/man/org.freedesktop.LogControl1.html
*
* Compile with 'cc logcontrol-example.c $(pkg-config --libs --cflags libsystemd)'
*
* To get and set properties via busctl:
*
* $ busctl --user get-property org.freedesktop.Example \
* /org/freedesktop/LogControl1 \
* org.freedesktop.LogControl1 \
* SyslogIdentifier
* s "example"
* $ busctl --user get-property org.freedesktop.Example \
* /org/freedesktop/LogControl1 \
* org.freedesktop.LogControl1 \
* LogTarget
* s "journal"
* $ busctl --user get-property org.freedesktop.Example \
* /org/freedesktop/LogControl1 \
* org.freedesktop.LogControl1 \
* LogLevel
* s "info"
* $ busctl --user set-property org.freedesktop.Example \
* /org/freedesktop/LogControl1 \
* org.freedesktop.LogControl1 \
* LogLevel \
* "s" debug
* $ busctl --user get-property org.freedesktop.Example \
* /org/freedesktop/LogControl1 \
* org.freedesktop.LogControl1 \
* LogLevel
* s "debug"
*/
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <syslog.h>
#include <systemd/sd-bus.h>
#include <systemd/sd-journal.h>
#define _cleanup_(f) __attribute__((cleanup(f)))
#define check(log_level, x) ({ \
int _r = (x); \
errno = _r < 0 ? -_r : 0; \
sd_journal_print((log_level), #x ": %m"); \
if (_r < 0) \
return EXIT_FAILURE; \
})
typedef enum LogTarget {
LOG_TARGET_JOURNAL,
LOG_TARGET_KMSG,
LOG_TARGET_SYSLOG,
LOG_TARGET_CONSOLE,
_LOG_TARGET_MAX,
} LogTarget;
static const char* const log_target_table[_LOG_TARGET_MAX] = {
[LOG_TARGET_JOURNAL] = "journal",
[LOG_TARGET_KMSG] = "kmsg",
[LOG_TARGET_SYSLOG] = "syslog",
[LOG_TARGET_CONSOLE] = "console",
};
static const char* const log_level_table[LOG_DEBUG + 1] = {
[LOG_EMERG] = "emerg",
[LOG_ALERT] = "alert",
[LOG_CRIT] = "crit",
[LOG_ERR] = "err",
[LOG_WARNING] = "warning",
[LOG_NOTICE] = "notice",
[LOG_INFO] = "info",
[LOG_DEBUG] = "debug",
};
typedef struct object {
const char *syslog_identifier;
LogTarget log_target;
int log_level;
} object;
static int property_get(
sd_bus *bus,
const char *path,
const char *interface,
const char *property,
sd_bus_message *reply,
void *userdata,
sd_bus_error *error) {
object *o = userdata;
if (strcmp(property, "LogLevel") == 0)
return sd_bus_message_append(reply, "s", log_level_table[o->log_level]);
if (strcmp(property, "LogTarget") == 0)
return sd_bus_message_append(reply, "s", log_target_table[o->log_target]);
if (strcmp(property, "SyslogIdentifier") == 0)
return sd_bus_message_append(reply, "s", o->syslog_identifier);
return sd_bus_error_setf(error,
SD_BUS_ERROR_UNKNOWN_PROPERTY,
"Unknown property '%s'",
property);
}
static int property_set(
sd_bus *bus,
const char *path,
const char *interface,
const char *property,
sd_bus_message *message,
void *userdata,
sd_bus_error *error) {
object *o = userdata;
const char *value;
int r;
r = sd_bus_message_read(message, "s", &value);
if (r < 0)
return r;
if (strcmp(property, "LogLevel") == 0) {
for (int i = 0; i < LOG_DEBUG + 1; i++)
if (strcmp(value, log_level_table[i]) == 0) {
o->log_level = i;
return 0;
}
return sd_bus_error_setf(error,
SD_BUS_ERROR_INVALID_ARGS,
"Invalid value for LogLevel: '%s'",
value);
}
if (strcmp(property, "LogTarget") == 0) {
for (LogTarget i = 0; i < _LOG_TARGET_MAX; i++)
if (strcmp(value, log_target_table[i]) == 0) {
o->log_target = i;
return 0;
}
return sd_bus_error_setf(error,
SD_BUS_ERROR_INVALID_ARGS,
"Invalid value for LogTarget: '%s'",
value);
}
return sd_bus_error_setf(error,
SD_BUS_ERROR_UNKNOWN_PROPERTY,
"Unknown property '%s'",
property);
}
/* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html
*/
static const sd_bus_vtable vtable[] = {
SD_BUS_VTABLE_START(0),
SD_BUS_WRITABLE_PROPERTY(
"LogLevel", "s",
property_get, property_set,
0,
0),
SD_BUS_WRITABLE_PROPERTY(
"LogTarget", "s",
property_get, property_set,
0,
0),
SD_BUS_PROPERTY(
"SyslogIdentifier", "s",
property_get,
0,
SD_BUS_VTABLE_PROPERTY_CONST),
SD_BUS_VTABLE_END
};
int main(int argc, char **argv) {
/* The bus should be relinquished before the program terminates. The cleanup
* attribute allows us to do it nicely and cleanly whenever we exit the
* block.
*/
_cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
object o = {
.log_level = LOG_INFO,
.log_target = LOG_TARGET_JOURNAL,
.syslog_identifier = "example",
};
/* Acquire a connection to the bus, letting the library work out the details.
* https://www.freedesktop.org/software/systemd/man/sd_bus_default.html
*/
check(o.log_level, sd_bus_default(&bus));
/* Publish an interface on the bus, specifying our well-known object access
* path and public interface name.
* https://www.freedesktop.org/software/systemd/man/sd_bus_add_object.html
* https://dbus.freedesktop.org/doc/dbus-tutorial.html
*/
check(o.log_level, sd_bus_add_object_vtable(bus, NULL,
"/org/freedesktop/LogControl1",
"org.freedesktop.LogControl1",
vtable,
&o));
/* By default the service is assigned an ephemeral name. Also add a fixed
* one, so that clients know whom to call.
* https://www.freedesktop.org/software/systemd/man/sd_bus_request_name.html
*/
check(o.log_level, sd_bus_request_name(bus, "org.freedesktop.Example", 0));
for (;;) {
/* https://www.freedesktop.org/software/systemd/man/sd_bus_wait.html
*/
check(o.log_level, sd_bus_wait(bus, UINT64_MAX));
/* https://www.freedesktop.org/software/systemd/man/sd_bus_process.html
*/
check(o.log_level, sd_bus_process(bus, NULL));
}
/* https://www.freedesktop.org/software/systemd/man/sd_bus_release_name.html
*/
check(o.log_level, sd_bus_release_name(bus, "org.freedesktop.Example"));
return 0;
}
This creates a simple server on the bus. It implements the LogControl1 interface
by providing the required properties and allowing to set the writable ones. It
logs at the configured log level using
sd_journal_print(3).
systemd(1),
journalctl(1),
systemctl(1),
systemd.service(5),
syslog(3)