---
name: "sd_bus_slot_get_userdata"
section: "3"
platform: "linux"
package: "libsystemd-dev"
version: "257.13-1~deb13u1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/sd_bus_slot_set_userdata"
---

# sd\_bus\_slot\_get\_userdata(3)

Set and query the value in the "userdata" field

Part of [sd\_bus\_slot\_set\_userdata(3)](/sd_bus_slot_set_userdata/3.md).

## NAME

sd\_bus\_slot\_set\_userdata, sd\_bus\_slot\_get\_userdata - Set and query the value in the "userdata" field

## SYNOPSIS

```
#include <systemd/sd-bus.h>
```

**void\* sd\_bus\_slot\_set\_userdata(sd\_bus\_slot\*** *slot***, void\*** *userdata***);**

**void\* sd\_bus\_slot\_get\_userdata(sd\_bus\_slot\*** *slot***);**

## DESCRIPTION

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)](/sd_bus_add_filter/3.md) or an asynchronous function call is made using [**sd\_bus\_call\_async**(3)](/sd_bus_call_async/3.md), 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*.

## RETURN VALUE

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**.

## 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.

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\_slot\_set\_userdata()** and **sd\_bus\_slot\_get\_userdata()** were added in version 240.

## SEE ALSO

[**systemd**(1)](/systemd/1.md), [**sd-bus**(3)](/sd-bus/3.md), [**sd\_bus\_slot\_set\_destroy\_callback**(3)](/sd_bus_slot_set_destroy_callback/3.md), [**sd\_bus\_add\_match**(3)](/sd_bus_add_match/3.md), [**sd\_bus\_slot\_get\_current\_userdata**(3)](/sd_bus_slot_get_current_userdata/3.md)