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

NAME
----
tep_register_print_function,tep_unregister_print_function -
Registers / Unregisters a helper function.

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

enum *tep_func_arg_type* {
	TEP_FUNC_ARG_VOID,
	TEP_FUNC_ARG_INT,
	TEP_FUNC_ARG_LONG,
	TEP_FUNC_ARG_STRING,
	TEP_FUNC_ARG_PTR,
	TEP_FUNC_ARG_MAX_TYPES
};

typedef unsigned long long (*pass:[*]tep_func_handler*)(struct trace_seq pass:[*]s, unsigned long long pass:[*]args);

int *tep_register_print_function*(struct tep_handle pass:[*]_tep_, tep_func_handler _func_, enum tep_func_arg_type _ret_type_, char pass:[*]_name_, _..._);
int *tep_unregister_print_function*(struct tep_handle pass:[*]_tep_, tep_func_handler _func_, char pass:[*]_name_);
--

DESCRIPTION
-----------
Some events may have helper functions in the print format arguments.
This allows a plugin to dynamically create a way to process one of
these functions.

The _tep_register_print_function()_ registers such helper function. The _tep_
argument is the trace event parser context. The _func_ argument  is a pointer
to the helper function. The _ret_type_ argument is  the return type of the
helper function, value from the _tep_func_arg_type_ enum. The _name_ is the name
of the helper function, as seen in the print format arguments. The _..._ is a
variable list of _tep_func_arg_type_ enums, the _func_ function arguments.
This list must end with _TEP_FUNC_ARG_VOID_. See 'EXAMPLE' section.

The _tep_unregister_print_function()_ unregisters a helper function, previously
registered with _tep_register_print_function()_. The _tep_ argument is the
trace event parser context. The _func_ and _name_ arguments are the same, used
when the helper function was registered.

The _tep_func_handler_ is the type of the helper function. The _s_ argument is
the trace sequence, it can be used to create a custom string.
The _args_  is a list of arguments, defined when the helper function was
registered.

RETURN VALUE
------------
The _tep_register_print_function()_ function returns 0 in case of success.
In case of an error, TEP_ERRNO_... code is returned.

The _tep_unregister_print_function()_ returns 0 in case of success, or -1 in
case of an error.

EXAMPLE
-------
Some events have internal functions calls, that appear in the print format
output. For example "tracefs/events/i915/g4x_wm/format" has:
[source,c]
--
print fmt: "pipe %c, frame=%u, scanline=%u, wm %d/%d/%d, sr %s/%d/%d/%d, hpll %s/%d/%d/%d, fbc %s",
	    ((REC->pipe) + 'A'), REC->frame, REC->scanline, REC->primary,
	    REC->sprite, REC->cursor, yesno(REC->cxsr), REC->sr_plane,
	    REC->sr_cursor, REC->sr_fbc, yesno(REC->hpll), REC->hpll_plane,
	    REC->hpll_cursor, REC->hpll_fbc, yesno(REC->fbc)
--
Notice the call to function _yesno()_ in the print arguments. In the kernel
context, this function has the following implementation:
[source,c]
--
static const char *yesno(int x)
{
	static const char *yes = "yes";
	static const char *no = "no";

	return x ? yes : no;
}
--
The user space event parser has no idea how to handle this _yesno()_ function.
The _tep_register_print_function()_ API can be used to register a user space
helper function, mapped to the kernel's _yesno()_:
[source,c]
--
#include <event-parse.h>
#include <trace-seq.h>
...
struct tep_handle *tep = tep_alloc();
...
static const char *yes_no_helper(int x)
{
	return x ? "yes" : "no";
}
...
	if ( tep_register_print_function(tep,
				    yes_no_helper,
				    TEP_FUNC_ARG_STRING,
				    "yesno",
				    TEP_FUNC_ARG_INT,
				    TEP_FUNC_ARG_VOID) != 0) {
		/* Failed to register yes_no_helper function */
	}

/*
   Now, when the event parser encounters this yesno() function, it will know
   how to handle it.
*/
...
	if (tep_unregister_print_function(tep, yes_no_helper, "yesno") != 0) {
		/* Failed to unregister yes_no_helper function */
	}
--

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* <rostedt@goodmis.org>, author of *libtraceevent*.
*Tzvetomir Stoyanov* <tz.stoyanov@gmail.com>, author of this man page.
--
REPORTING BUGS
--------------
Report bugs to  <linux-trace-devel@vger.kernel.org>

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

RESOURCES
---------
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git