---
name: "sd_event_source_set_userdata"
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_event_source_set_userdata"
---

# sd\_event\_source\_set\_userdata(3)

Set or retrieve user data pointer of event sources

## NAME

sd\_event\_source\_set\_userdata, sd\_event\_source\_get\_userdata - Set or retrieve user data pointer of event sources

## SYNOPSIS

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

**void\* sd\_event\_source\_set\_userdata(sd\_event\_source \****source***, void \****userdata***);**

**void\* sd\_event\_source\_get\_userdata(sd\_event\_source \****source***);**

## DESCRIPTION

**sd\_event\_source\_set\_userdata()** may be used to set an arbitrary user data pointer for the event source object specified as *source*. The user data pointer is usually specified when creating an event source object with calls such as [**sd\_event\_add\_io**(3)](/sd_event_add_io/3.md) or [**sd\_event\_add\_time**(3)](/sd_event_add_time/3.md), and may be updated with this call. The user data pointer is also passed to all handler callback functions associated with the event source. The *userdata* parameter specifies the new user data pointer to set, the function returns the previous user data pointer. Note that **NULL** is a valid user data pointer.

**sd\_event\_source\_get\_userdata()** may be used to query the current user data pointer assigned to the event source object *source*.

## RETURN VALUE

On success, **sd\_event\_source\_set\_userdata()** and **sd\_event\_source\_get\_userdata()** return the previously set user data pointer. On failure, they return **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.

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\_event\_source\_set\_userdata()** and **sd\_event\_source\_get\_userdata()** were added in version 229.

## SEE ALSO

[**sd-event**(3)](/sd-event/3.md), [**sd\_event\_add\_io**(3)](/sd_event_add_io/3.md), [**sd\_event\_add\_time**(3)](/sd_event_add_time/3.md), [**sd\_event\_add\_signal**(3)](/sd_event_add_signal/3.md), [**sd\_event\_add\_child**(3)](/sd_event_add_child/3.md), [**sd\_event\_add\_inotify**(3)](/sd_event_add_inotify/3.md), [**sd\_event\_add\_defer**(3)](/sd_event_add_defer/3.md), [**sd\_event\_source\_set\_description**(3)](/sd_event_source_set_description/3.md)