org.freedesktop.LogControl1(5) — Linux manual page

NAME | INTRODUCTION | DESCRIPTION | TOOLS | EXAMPLE | SEE ALSO | COLOPHON

ORG.FRE...GCONTROL1(5) org.freedesktop.LogControl1ORG.FRE...GCONTROL1(5)

NAME         top

       org.freedesktop.LogControl1 - D-Bus interface to query and set
       logging configuration

INTRODUCTION         top

       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.

DESCRIPTION         top

       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 { ... };
           };

   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.

TOOLS         top

       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         top

       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;
                   setlogmask(LOG_UPTO(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",
             };

             /* https://man7.org/linux/man-pages/man3/setlogmask.3.html
              * Programs using syslog() instead of sd_journal can use this API to cut logs
              * emission at the source.
              */
             setlogmask(LOG_UPTO(o.log_level));

             /* 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).

SEE ALSO         top

       systemd(1), journalctl(1), systemctl(1), systemd.service(5),
       syslog(3)

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have
       a bug report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
       This page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-22.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

systemd 255                                       ORG.FRE...GCONTROL1(5)

Pages that refer to this page: systemctl(1)systemd(1)systemd.directives(7)systemd.index(7)systemd-hostnamed.service(8)systemd-importd.service(8)systemd-localed.service(8)systemd-logind.service(8)systemd-machined.service(8)systemd-resolved.service(8)systemd-timedated.service(8)