sd_bus_slot_set_userdata, sd_bus_slot_get_userdata - Set and query the value in the "userdata" field
void* sd_bus_slot_set_userdata(sd_bus_slot* slot, void* userdata);
void* sd_bus_slot_get_userdata(sd_bus_slot* slot);
The userdata pointer allows data to be passed between the point where a callback is registered, for example when a filter is added using sd_bus_add_filter(3) or an asynchronous function call is made using sd_bus_call_async(3), and the point where the callback is called, without having any global state. The pointer has type void* and is not used by the sd-bus functions in any way, except to pass to the callback function.
Usually, the userdata field is set when the slot object is initially registered. sd_bus_slot_set_userdata() may be used to change it later for the bus slot object slot. Previous value of the field is returned. The argument and returned value may be NULL. It will be passed as the userdata argument to the callback function attached to the slot.
sd_bus_slot_set_userdata() gets the value of the userdata field in the bus slot object slot.
On success, these functions return the value of the userdata field before the function call. If the slot object is NULL, NULL will be returned to signify an error, but this is not distinguishable from the userdata field value being NULL.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.