.\" $OpenBSD: event_set_log_callback.3,v 1.2 2023/05/01 07:04:38 jsg Exp $
.\" Copyright (c) 2023 Ted Bullock <tbullock@comlore.com>
.\"
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.Dd $Mdocdate: May 1 2023 $
.Dt EVENT_SET_LOG_CALLBACK 3
.Os
.Sh NAME
.Nm event_set_log_callback
.Nd set callback for diagnostics
.Sh SYNOPSIS
.In event.h
.Ft typedef void
.Fo (*event_log_cb)
.Fa "int sev"
.Fa "const char *msg"
.Fc
.Ft void
.Fo event_set_log_callback
.Fa "event_log_cb cb"
.Fc
.Sh DESCRIPTION
The event library uses an optional callback function configured using
.Fn event_set_log_callback
to report error and diagnostic messages from many functions.
By default the library does not report diagnostics.
After executing the callback to report an error the event library invokes
.Xr exit 3 ;
this happens even if no callback is configured.
.Pp
.Fa cb
is a function pointer to a callback with the following parameters:
.Bl -tag -width 4n
.It Fa sev :
represents the severity of the message and may be one of:
.Bl -tag -width "_EVENT_LOG_DEBUG"
.It Dv _EVENT_LOG_DEBUG
Messages for debugging purposes.
These message are suppressed by the library unless it is compiled with
.Dv USE_DEBUG .
.It Dv _EVENT_LOG_MSG
Messages providing information.
.It Dv _EVENT_LOG_WARN
Messages indicating non-fatal issues.
.It Dv _EVENT_LOG_ERR
Messages indicating fatal issues.
The library terminates the program by calling
.Xr exit 3
after reporting the message.
.El
.It Fa msg :
an ASCII string containing the message.
.El
.Pp
Default behavior is restored and callbacks are prevented if
.Fa cb
is
.Dv NULL .
.Sh EXAMPLES
The following C program illustrates use of
.Fn event_set_log_callback .
The callback function
.Fn cb
includes logic to identify the severity levels of diagnostic messages.
.Bd -literal -offset indent
#include <event.h>
#include <stdio.h>
#include <stdlib.h>

void
cb(int sev, const char *msg)
{
	switch (sev) {
	case _EVENT_LOG_DEBUG:
		printf("DEBUG: %s\en", msg);
		break;
	case _EVENT_LOG_MSG:
		printf("INFO: %s\en", msg);
		break;
	case _EVENT_LOG_WARN:
		printf("WARNING: %s\en", msg);
		break;
	case _EVENT_LOG_ERR:
		printf("ERROR: %s\en", msg);
		break;
	}
}

int
main(int argc, char *argv[])
{
	struct event_base *eb;
	/* Redirect diagnostic messages to `cb` callback */
	event_set_log_callback(cb);
	/* Report the kernel notification method */
	setenv("EVENT_SHOW_METHOD", "", 1);
	/* Initialize library; failures won't return */
	eb = event_base_new();
	/* Disable diagnostic messages */
	event_set_log_callback(NULL);

	/* Do something with the event library here */

	/* Deallocate memory */
	event_base_free(eb);
	return 0;
}
.Ed
.Sh SEE ALSO
.Xr event_base_new 3 ,
.Xr exit 3
.Sh HISTORY
.Fn event_set_log_callback
first appeared in libevent-1.0c and has been available since
.Ox 3.8 .
.Sh AUTHORS
The event library was written by
.An -nosplit
.An Niels Provos .
.Pp
.Fn event_set_log_callback
and the diagnostic reporting system was written by
.An Nick Mathewson .
.Pp
This manual page was written by
.An Ted Bullock Aq Mt tbullock@comlore.com .