---
name: "sd_event_source_set_prepare"
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_prepare"
---

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

Set a preparation callback for event sources

## NAME

sd\_event\_source\_set\_prepare - Set a preparation callback for event sources

## SYNOPSIS

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

**int sd\_event\_source\_set\_prepare(sd\_event\_source \****source***, sd\_event\_handler\_t** *callback***);**

**typedef int (\*sd\_event\_handler\_t)(sd\_event\_source \****s***, void \****userdata***);**

## DESCRIPTION

**sd\_event\_source\_set\_prepare()** may be used to set a preparation callback for the event source object specified as *source*. The callback function specified as *callback* will be invoked immediately before the event loop goes to sleep to wait for incoming events. It is invoked with the user data pointer passed when the event source was created. The event source will be disabled if the callback function returns a negative error code. The callback function may be used to reconfigure the precise events to wait for. If the *callback* parameter is passed as **NULL** the callback function is reset.

Event source objects have no preparation callback associated when they are first created with calls such as [**sd\_event\_add\_io**(3)](/sd_event_add_io/3.md), [**sd\_event\_add\_time**(3)](/sd_event_add_time/3.md). Preparation callback functions are supported for all event source types with the exception of those created with [**sd\_event\_add\_exit**(3)](/sd_event_add_exit/3.md). Preparation callback functions are dispatched in the order indicated by the event source's priority field, as set with [**sd\_event\_source\_set\_priority**(3)](/sd_event_source_set_priority/3.md). Preparation callbacks of disabled event sources (see [**sd\_event\_source\_set\_enabled**(3)](/sd_event_source_set_enabled/3.md)) are not invoked.

## RETURN VALUE

On success, **sd\_event\_source\_set\_prepare()** returns a non-negative integer. On failure, it returns a negative errno-style error code.

### Errors

Returned errors may indicate the following problems:

**-EINVAL**

*source* is not a valid pointer to an sd\_event\_source object.

**-ESTALE**

The event loop is already terminated.

**-ENOMEM**

Not enough memory.

**-ECHILD**

The event loop has been created in a different process, library or module instance.

**-EDOM**

The specified event source has been created with [**sd\_event\_add\_exit**(3)](/sd_event_add_exit/3.md).

## 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\_prepare()** and **sd\_event\_handler\_t()** 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\_enabled**(3)](/sd_event_source_set_enabled/3.md), [**sd\_event\_source\_set\_priority**(3)](/sd_event_source_set_priority/3.md), [**sd\_event\_source\_set\_userdata**(3)](/sd_event_source_set_userdata/3.md)