sd_bus_add_match

systemd
Julian Orth ju.orth@gmail.com
sd_bus_add_match 3
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

    #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. The specified handler function callback is called for eaching 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 similar to `sd_bus_match_signal()`, 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()` is 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 cannot be removed independently.

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

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

## See Also

systemd 1, sd-bus 3, sd_bus_slot_unref 3, sd_bus_message_ref 3, sd_bus_set_bus_client 3