'\" t
.TH "SD_EVENT_SOURCE_SET_PREPARE" "3" "" "systemd 255" "sd_event_source_set_prepare"
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.SH "NAME"
sd_event_source_set_prepare \- Set a preparation callback for event sources
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <systemd/sd\-event\&.h>
.fi
.ft
.HP \w'int\ sd_event_source_set_prepare('u
.BI "int sd_event_source_set_prepare(sd_event_source\ *" "source" ", sd_event_handler_t\ " "callback" ");"
.HP \w'typedef\ int\ (*sd_event_handler_t)('u
.BI "typedef int (*sd_event_handler_t)(sd_event_source\ *" "s" ", void\ *" "userdata" ");"
.SH "DESCRIPTION"
.PP
\fBsd_event_source_set_prepare()\fR
may be used to set a preparation callback for the event source object specified as
\fIsource\fR\&. The callback function specified as
\fIcallback\fR
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
\fIcallback\fR
parameter is passed as
\fBNULL\fR
the callback function is reset\&.
.PP
Event source objects have no preparation callback associated when they are first created with calls such as
\fBsd_event_add_io\fR(3),
\fBsd_event_add_time\fR(3)\&. Preparation callback functions are supported for all event source types with the exception of those created with
\fBsd_event_add_exit\fR(3)\&. Preparation callback functions are dispatched in the order indicated by the event source\*(Aqs priority field, as set with
\fBsd_event_source_set_priority\fR(3)\&. Preparation callbacks of disabled event sources (see
\fBsd_event_source_set_enabled\fR(3)) are not invoked\&.
.SH "RETURN VALUE"
.PP
On success,
\fBsd_event_source_set_prepare()\fR
returns a non\-negative integer\&. On failure, it returns a negative errno\-style error code\&.
.SS "Errors"
.PP
Returned errors may indicate the following problems:
.PP
\fB\-EINVAL\fR
.RS 4
\fIsource\fR
is not a valid pointer to an
sd_event_source
object\&.
.RE
.PP
\fB\-ESTALE\fR
.RS 4
The event loop is already terminated\&.
.RE
.PP
\fB\-ENOMEM\fR
.RS 4
Not enough memory\&.
.RE
.PP
\fB\-ECHILD\fR
.RS 4
The event loop has been created in a different process, library or module instance\&.
.RE
.PP
\fB\-EDOM\fR
.RS 4
The specified event source has been created with
\fBsd_event_add_exit\fR(3)\&.
.RE
.SH "NOTES"
.PP
Functions described here are available as a shared library, which can be compiled against and linked to with the
\fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
file\&.
.PP
The code described here uses
\fBgetenv\fR(3), which is declared to be not multi\-thread\-safe\&. This means that the code calling the functions described here must not call
\fBsetenv\fR(3)
from a parallel thread\&. It is recommended to only do calls to
\fBsetenv()\fR
from an early phase of the program when no other threads have been started\&.
.SH "HISTORY"
.PP
\fBsd_event_source_set_prepare()\fR
and
\fBsd_event_handler_t()\fR
were added in version 229\&.
.SH "SEE ALSO"
.PP
\fBsd-event\fR(3),
\fBsd_event_add_io\fR(3),
\fBsd_event_add_time\fR(3),
\fBsd_event_add_signal\fR(3),
\fBsd_event_add_child\fR(3),
\fBsd_event_add_inotify\fR(3),
\fBsd_event_add_defer\fR(3),
\fBsd_event_source_set_enabled\fR(3),
\fBsd_event_source_set_priority\fR(3),
\fBsd_event_source_set_userdata\fR(3)