NAME

       sd_bus_add_match, sd_bus_add_match_async, sd_bus_match_signal, sd_bus_match_signal_async -
       Add a match rule for incoming message dispatching

SYNOPSIS

       #include <systemd/sd-bus.h>

       typedef int (*sd_bus_message_handler_t)(sd_bus_message *m, void *userdata,
                                               sd_bus_error *ret_error);

       int sd_bus_add_match(sd_bus *bus, sd_bus_slot **slot, const char *match,
                            sd_bus_message_handler_t callback, void *userdata);

       int sd_bus_add_match_async(sd_bus *bus, sd_bus_slot **slot, const char *match,
                                  sd_bus_message_handler_t callback,
                                  sd_bus_message_handler_t install_callback, void *userdata);

       int sd_bus_match_signal(sd_bus *bus, sd_bus_slot **slot, const char *sender,
                               const char *path, const char *interface, const char *member,
                               sd_bus_message_handler_t callback, void *userdata);

       int sd_bus_match_signal_async(sd_bus *bus, sd_bus_slot **slot, const char *sender,
                                     const char *path, const char *interface, const char *member,
                                     sd_bus_message_handler_t callback,
                                     sd_bus_message_handler_t install_callback, void *userdata);

DESCRIPTION

       sd_bus_add_match() installs a match rule for messages received on the specified bus
       connection object bus. The syntax of the match rule expression passed in match is
       described in the D-Bus Specification[1]. The specified handler function callback is called
       for each incoming message matching the specified expression, the userdata parameter is
       passed as-is to the callback function. The match is installed synchronously when connected
       to a bus broker, i.e. the call sends a control message requested the match to be added to
       the broker and waits until the broker confirms the match has been installed successfully.

       sd_bus_add_match_async() operates very similarly to sd_bus_add_match(), however it
       installs the match asynchronously, in a non-blocking fashion: a request is sent to the
       broker, but the call does not wait for a response. The install_callback function is called
       when the response is later received, with the response message from the broker as
       parameter. If this function is specified as NULL a default implementation is used that
       terminates the bus connection should installing the match fail.

       sd_bus_match_signal() is very similar to sd_bus_add_match(), but only matches signals, and
       instead of a match expression accepts four parameters: sender (the service name of the
       sender), path (the object path of the emitting object), interface (the interface the
       signal belongs to), member (the signal name), from which the match string is internally
       generated. Optionally, these parameters may be specified as NULL in which case the
       relevant field of incoming signals is not tested.

       sd_bus_match_signal_async() combines the signal matching logic of sd_bus_match_signal()
       with the asynchronous behaviour of sd_bus_add_match_async().

       On success, and if non-NULL, the slot return parameter will be set to a slot object that
       may be used as a reference to the installed match, and may be utilized to remove it again
       at a later time with sd_bus_slot_unref(3). If specified as NULL the lifetime of the match
       is bound to the lifetime of the bus object itself, and the match is generally not removed
       independently. See sd_bus_slot_set_floating(3) for details.

       The message m passed to the callback is only borrowed, that is, the callback should not
       call sd_bus_message_unref(3) on it. If the callback wants to hold on to the message beyond
       the lifetime of the callback, it needs to call sd_bus_message_ref(3) to create a new
       reference.

       If an error occurs during the callback invocation, the callback should return a negative
       error number (optionally, a more precise error may be returned in ret_error, as well). If
       it wants other callbacks that match the same rule to be called, it should return 0.
       Otherwise it should return a positive integer.

       If the bus refers to a direct connection (i.e. not a bus connection, as set with
       sd_bus_set_bus_client(3)) the match is only installed on the client side, and the
       synchronous and asynchronous functions operate the same.

RETURN VALUE

       On success, sd_bus_add_match() and the other calls return 0 or a positive integer. On
       failure, they return a negative errno-style error code.

NOTES

       Functions described here are available as a shared library, which can be compiled against
       and linked to with the libsystemd pkg-config(1) file.

       The code described here uses getenv(3), which is declared to be not multi-thread-safe.
       This means that the code calling the functions described here must not call setenv(3) from
       a parallel thread. It is recommended to only do calls to setenv() from an early phase of
       the program when no other threads have been started.

HISTORY

       sd_bus_add_match() and sd_bus_message_handler_t() were added in version 221.

       sd_bus_add_match_async(), sd_bus_match_signal(), and sd_bus_match_signal_async() were
       added in version 237.

SEE ALSO

       systemd(1), sd-bus(3), sd_bus_slot_unref(3), sd_bus_message_ref(3),
       sd_bus_set_bus_client(3), sd_bus_slot_set_floating(3)

NOTES

        1. D-Bus Specification
           https://dbus.freedesktop.org/doc/dbus-specification.html