--- name: "sd_bus_add_match" section: "3" platform: "linux" package: "systemd-libs" version: "262.1-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/sd_bus_add_match" --- # sd\_bus\_add\_match(3) Add a match rule for incoming message dispatching ## 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 ``` **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)](/sd_bus_slot_unref/3.md). 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)](/sd_bus_slot_set_floating/3.md) for details. The message *m* passed to the callback is only borrowed, that is, the callback should not call [**sd\_bus\_message\_unref**(3)](/sd_bus_message_unref/3.md) 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)](/sd_bus_message_ref/3.md) 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)](/sd_bus_set_bus_client/3.md)) 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)](/pkg-config/1.md) file. All functions listed here are thread-agnostic and only a single thread may operate on a given object at any given time. Different threads may access the same object at different times. Multiple independent objects may be used from different threads in parallel. The code described here uses [**getenv**(3)](/getenv/3.md), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call [**setenv**(3)](/setenv/3.md) 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)](/systemd/1.md), [**sd-bus**(3)](/sd-bus/3.md), [**sd\_bus\_slot\_unref**(3)](/sd_bus_slot_unref/3.md), [**sd\_bus\_message\_ref**(3)](/sd_bus_message_ref/3.md), [**sd\_bus\_set\_bus\_client**(3)](/sd_bus_set_bus_client/3.md), [**sd\_bus\_slot\_set\_floating**(3)](/sd_bus_slot_set_floating/3.md) ## NOTES - 1\. D-Bus Specification https://dbus.freedesktop.org/doc/dbus-specification.html