SD_EVENT_SOURCE_SET_PREPARE(3)                     sd_event_source_set_prepare

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_time(3). Preparation callback functions are supported for
       all event source types with the exception of those created with
       sd_event_add_exit(3). 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). Preparation callbacks of disabled
       event sources (see sd_event_source_set_enabled(3)) 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).

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.

HISTORY
       sd_event_source_set_prepare() and sd_event_handler_t() were added in
       version 229.

SEE ALSO
       sd-event(3), sd_event_add_io(3), sd_event_add_time(3),
       sd_event_add_signal(3), sd_event_add_child(3), sd_event_add_inotify(3),
       sd_event_add_defer(3), sd_event_source_set_enabled(3),
       sd_event_source_set_priority(3), sd_event_source_set_userdata(3)

systemd 255                                     SD_EVENT_SOURCE_SET_PREPARE(3)