Description¶

sd_bus_track_add_name() adds a peer to track to a bus peer tracking object. The first argument should refer to a bus peer tracking object created with sd_bus_track_new(3), the second name should refer to a D-Bus peer name to track, either in unique or well-known service format. If the name is not tracked yet it will be added to the list of names to track. If it already is being tracked and non-recursive mode is enabled, no operation is executed by this call. If recursive mode is enabled a per-name counter is increased by one each time this call is invoked, and sd_bus_track_remove_name() has to be called as many times as sd_bus_track_add_name() was invoked before in order to stop tracking of the name. Use sd_bus_track_set_recursive(3) to switch from the default non-recursive mode to recursive mode, or back. Note that the specified name is tracked as it is, well-known names are not resolved to unique names by this call. Note that multiple bus peer tracking objects may track the same name.

sd_bus_track_remove_name() undoes the effect of sd_bus_track_add_name() and removes a bus peer name from the list of peers to watch. Depending on whether non-recursive or recursive mode is enabled for the bus peer tracking object this call will either remove the name fully from the tracking object, or will simply decrement the per-name counter by one, removing the name only when the counter reaches zero (see above). Note that a bus peer disconnecting from the bus will implicitly remove its names fully from the bus peer tracking object, regardless of the current per-name counter.

sd_bus_track_add_sender() and sd_bus_track_remove_sender() are similar to sd_bus_track_add_name() and sd_bus_track_remove_name() but take a bus message as argument. The sender of this bus message is determined and added to/removed from the bus peer tracking object. As messages always originate from unique names, and never from well-known names this means that this call will effectively only add unique names to the bus peer tracking object.

sd_bus_track_count() returns the number of names currently being tracked by the specified bus peer tracking object. Note that this function always returns the actual number of names tracked, and hence if sd_bus_track_add_name() has been invoked multiple times for the same name it is only counted as one, regardless if recursive mode is used or not.

sd_bus_track_count_name() returns the current per-name counter for the specified name. If non-recursive mode is used this returns either 1 or 0, depending on whether the specified name has been added to the tracking object before, or not. If recursive mode has been enabled, values larger than 1 may be returned too, in case sd_bus_track_add_name() has been called multiple times for the same name.

sd_bus_track_count_sender() is similar to sd_bus_track_count_name(), but takes a bus message object and returns the per-name counter matching the sender of the message.

sd_bus_track_contains() may be used to determine whether the specified name has been added at least once to the specified bus peer tracking object.

sd_bus_track_first() and sd_bus_track_next() may be used to enumerate all names currently being tracked by the passed bus peer tracking object. sd_bus_track_first() returns the first entry in the object, and resets an internally maintained read index. Each subsequent invocation of sd_bus_track_next() returns the next name contained in the bus object. If the end is reached NULL is returned. If no names have been added to the object yet sd_bus_track_first() will return NULL immediately. The order in which names are returned is undefined; in particular which name is considered the first returned is not defined. If recursive mode is enabled and the same name has been added multiple times to the bus peer tracking object it is only returned once by this enumeration. If new names are added to or existing names removed from the bus peer tracking object while it is being enumerated the enumeration ends on the next invocation of sd_bus_track_next() as NULL is returned.

Return Value¶

On success, sd_bus_track_add_name() and sd_bus_track_add_sender() return 0 if the specified name has already been added to the bus peer tracking object before and positive if it hasn't. On failure, they return a negative errno-style error code.

sd_bus_track_remove_name() and sd_bus_track_remove_sender() return positive if the specified name was previously tracked by the bus peer tracking object and has now been removed. In non-recursive mode, 0 is returned if the specified name was not being tracked yet. In recursive mode -EUNATCH is returned in this case. On failure, they return a negative errno-style error code.

sd_bus_track_count() returns the number of names currently being tracked, or 0 on failure.

sd_bus_track_count_name() and sd_bus_track_count_sender() return the current per-name counter for the specified name or the sender of the specified message. Zero is returned for names that are not being tracked yet, a positive value for names added at least once. Larger values than 1 are only returned in recursive mode. On failure, a negative errno-style error code is returned.

sd_bus_track_contains() returns the passed name if it exists in the bus peer tracking object. On failure, and if the name has not been added yet NULL is returned.

sd_bus_track_first() and sd_bus_track_next() return the first/next name contained in the bus peer tracking object, and NULL if the end of the enumeration is reached and on error.

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.

See Also¶

systemd(1), sd-bus(3), sd_bus_track_new(3)