xref: /aosp_15_r20/external/libtraceevent/Documentation/libtraceevent-reg_event_handler.txt (revision 436bf2bcd5202612ffffe471bbcc1f277cc8d28e)

libtraceevent(3)
================

NAME
----
tep_register_event_handler, tep_unregister_event_handler -  Register /
unregisters a callback function to parse an event information.

SYNOPSIS
--------
[verse]
--
*#include <event-parse.h>*

enum *tep_reg_handler* {
	_TEP_REGISTER_SUCCESS_,
	_TEP_REGISTER_SUCCESS_OVERWRITE_,
};

int *tep_register_event_handler*(struct tep_handle pass:[*]_tep_, int _id_, const char pass:[*]_sys_name_, const char pass:[*]_event_name_, tep_event_handler_func _func_, void pass:[*]_context_);
int *tep_unregister_event_handler*(struct tep_handle pass:[*]tep, int id, const char pass:[*]sys_name, const char pass:[*]event_name, tep_event_handler_func func, void pass:[*]_context_);

typedef int (*pass:[*]tep_event_handler_func*)(struct trace_seq pass:[*]s, struct tep_record pass:[*]record, struct tep_event pass:[*]event, void pass:[*]context);
--

DESCRIPTION
-----------
The *tep_register_event_handler()* function registers a handler function,
which is going to be called to parse the information for a given event.
The _tep_ argument is the trace event parser context. The _id_ argument is
the id of the event. The _sys_name_ argument is the name of the system,
the event belongs to. The _event_name_ argument is the name of the event.
If _id_ is >= 0, it is used to find the event, otherwise _sys_name_ and
_event_name_ are used. The _func_ is a pointer to the function, which is going
to be called to parse the event information. The _context_ argument is a pointer
to the context data, which will be passed to the _func_. If a handler function
for the same event is already registered, it will be overridden with the new
one. This mechanism allows a developer to override the parsing of a given event.
If for some reason the default print format is not sufficient, the developer
can register a function for an event to be used to parse the data instead.

The *tep_unregister_event_handler()* function unregisters the handler function,
previously registered with *tep_register_event_handler()*. The _tep_ argument
is the trace event parser context. The _id_, _sys_name_, _event_name_, _func_,
and _context_ are the same arguments, as when the callback function _func_ was
registered.

The _tep_event_handler_func_ is the type of the custom event handler
function. The _s_ argument is the trace sequence, it can be used to create a
custom string, describing the event. A _record_  to get the event from is passed
as input parameter and also the _event_ - the handle to the record's event. The
_context_ is custom context, set when the custom event handler is registered.

RETURN VALUE
------------
The *tep_register_event_handler()* function returns _TEP_REGISTER_SUCCESS_
if the new handler is registered successfully or
_TEP_REGISTER_SUCCESS_OVERWRITE_ if an existing handler is overwritten.
If there is not  enough memory to complete the registration,
TEP_ERRNO__MEM_ALLOC_FAILED is returned.

The *tep_unregister_event_handler()* function returns 0 if _func_ was removed
successful or, -1 if the event was not found.

The _tep_event_handler_func_ should return -1 in case of an error,
or 0 otherwise.

EXAMPLE
-------
[source,c]
--
#include <event-parse.h>
#include <trace-seq.h>
...
struct tep_handle *tep = tep_alloc();
...
int timer_expire_handler(struct trace_seq *s, struct tep_record *record,
			 struct tep_event *event, void *context)
{
	trace_seq_printf(s, "hrtimer=");

	if (tep_print_num_field(s, "0x%llx", event, "timer", record, 0) == -1)
		tep_print_num_field(s, "0x%llx", event, "hrtimer", record, 1);

	trace_seq_printf(s, " now=");

	tep_print_num_field(s, "%llu", event, "now", record, 1);

	tep_print_func_field(s, " function=%s", event, "function", record, 0);

	return 0;
}
...
	int ret;

	ret = tep_register_event_handler(tep, -1, "timer", "hrtimer_expire_entry",
					 timer_expire_handler, NULL);
	if (ret < 0) {
		char buf[32];

		tep_strerror(tep, ret, buf, 32)
		printf("Failed to register handler for hrtimer_expire_entry: %s\n", buf);
	} else {
		switch (ret) {
		case TEP_REGISTER_SUCCESS:
			printf ("Registered handler for hrtimer_expire_entry\n");
			break;
		case TEP_REGISTER_SUCCESS_OVERWRITE:
			printf ("Overwrote handler for hrtimer_expire_entry\n");
			break;
		}
	}
...
	ret = tep_unregister_event_handler(tep, -1, "timer", "hrtimer_expire_entry",
					   timer_expire_handler, NULL);
	if ( ret )
		printf ("Failed to unregister handler for hrtimer_expire_entry\n");

--

FILES
-----
[verse]
--
*event-parse.h*
	Header file to include in order to have access to the library APIs.
*trace-seq.h*
	Header file to include in order to have access to trace sequences
	related APIs. Trace sequences are used to allow a function to call
	several other functions to create a string of data to use.
*-ltraceevent*
	Linker switch to add when building a program that uses the library.
--

SEE ALSO
--------
*libtraceevent*(3), *trace-cmd*(1)

AUTHOR
------
[verse]
--
*Steven Rostedt* <[email protected]>, author of *libtraceevent*.
*Tzvetomir Stoyanov* <[email protected]>, author of this man page.
--
REPORTING BUGS
--------------
Report bugs to  <[email protected]>

LICENSE
-------
libtraceevent is Free Software licensed under the GNU LGPL 2.1

RESOURCES
---------
https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/