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)