1libtraceevent(3)
  2================
  3
  4NAME
  5----
  6tep_register_print_function,tep_unregister_print_function -
  7Registers / Unregisters a helper function.
  8
  9SYNOPSIS
 10--------
 11[verse]
 12--
 13*#include <event-parse.h>*
 14
 15enum *tep_func_arg_type* {
 16	TEP_FUNC_ARG_VOID,
 17	TEP_FUNC_ARG_INT,
 18	TEP_FUNC_ARG_LONG,
 19	TEP_FUNC_ARG_STRING,
 20	TEP_FUNC_ARG_PTR,
 21	TEP_FUNC_ARG_MAX_TYPES
 22};
 23
 24typedef unsigned long long (*pass:[*]tep_func_handler*)(struct trace_seq pass:[*]s, unsigned long long pass:[*]args);
 25
 26int *tep_register_print_function*(struct tep_handle pass:[*]_tep_, tep_func_handler _func_, enum tep_func_arg_type _ret_type_, char pass:[*]_name_, _..._);
 27int *tep_unregister_print_function*(struct tep_handle pass:[*]_tep_, tep_func_handler _func_, char pass:[*]_name_);
 28--
 29
 30DESCRIPTION
 31-----------
 32Some events may have helper functions in the print format arguments.
 33This allows a plugin to dynamically create a way to process one of
 34these functions.
 35
 36The _tep_register_print_function()_ registers such helper function. The _tep_
 37argument is the trace event parser context. The _func_ argument  is a pointer
 38to the helper function. The _ret_type_ argument is  the return type of the
 39helper function, value from the _tep_func_arg_type_ enum. The _name_ is the name
 40of the helper function, as seen in the print format arguments. The _..._ is a
 41variable list of _tep_func_arg_type_ enums, the _func_ function arguments.
 42This list must end with _TEP_FUNC_ARG_VOID_. See 'EXAMPLE' section.
 43
 44The _tep_unregister_print_function()_ unregisters a helper function, previously
 45registered with _tep_register_print_function()_. The _tep_ argument is the
 46trace event parser context. The _func_ and _name_ arguments are the same, used
 47when the helper function was registered.
 48
 49The _tep_func_handler_ is the type of the helper function. The _s_ argument is
 50the trace sequence, it can be used to create a custom string.
 51The _args_  is a list of arguments, defined when the helper function was
 52registered.
 53
 54RETURN VALUE
 55------------
 56The _tep_register_print_function()_ function returns 0 in case of success.
 57In case of an error, TEP_ERRNO_... code is returned.
 58
 59The _tep_unregister_print_function()_ returns 0 in case of success, or -1 in
 60case of an error.
 61
 62EXAMPLE
 63-------
 64Some events have internal functions calls, that appear in the print format
 65output. For example "tracefs/events/i915/g4x_wm/format" has:
 66[source,c]
 67--
 68print fmt: "pipe %c, frame=%u, scanline=%u, wm %d/%d/%d, sr %s/%d/%d/%d, hpll %s/%d/%d/%d, fbc %s",
 69	    ((REC->pipe) + 'A'), REC->frame, REC->scanline, REC->primary,
 70	    REC->sprite, REC->cursor, yesno(REC->cxsr), REC->sr_plane,
 71	    REC->sr_cursor, REC->sr_fbc, yesno(REC->hpll), REC->hpll_plane,
 72	    REC->hpll_cursor, REC->hpll_fbc, yesno(REC->fbc)
 73--
 74Notice the call to function _yesno()_ in the print arguments. In the kernel
 75context, this function has the following implementation:
 76[source,c]
 77--
 78static const char *yesno(int x)
 79{
 80	static const char *yes = "yes";
 81	static const char *no = "no";
 82
 83	return x ? yes : no;
 84}
 85--
 86The user space event parser has no idea how to handle this _yesno()_ function.
 87The _tep_register_print_function()_ API can be used to register a user space
 88helper function, mapped to the kernel's _yesno()_:
 89[source,c]
 90--
 91#include <event-parse.h>
 92#include <trace-seq.h>
 93...
 94struct tep_handle *tep = tep_alloc();
 95...
 96static const char *yes_no_helper(int x)
 97{
 98	return x ? "yes" : "no";
 99}
100...
101	if ( tep_register_print_function(tep,
102				    yes_no_helper,
103				    TEP_FUNC_ARG_STRING,
104				    "yesno",
105				    TEP_FUNC_ARG_INT,
106				    TEP_FUNC_ARG_VOID) != 0) {
107		/* Failed to register yes_no_helper function */
108	}
109
110/*
111   Now, when the event parser encounters this yesno() function, it will know
112   how to handle it.
113*/
114...
115	if (tep_unregister_print_function(tep, yes_no_helper, "yesno") != 0) {
116		/* Failed to unregister yes_no_helper function */
117	}
118--
119
120FILES
121-----
122[verse]
123--
124*event-parse.h*
125	Header file to include in order to have access to the library APIs.
126*trace-seq.h*
127	Header file to include in order to have access to trace sequences
128	related APIs. Trace sequences are used to allow a function to call
129	several other functions to create a string of data to use.
130*-ltraceevent*
131	Linker switch to add when building a program that uses the library.
132--
133
134SEE ALSO
135--------
136_libtraceevent(3)_, _trace-cmd(1)_
137
138AUTHOR
139------
140[verse]
141--
142*Steven Rostedt* <rostedt@goodmis.org>, author of *libtraceevent*.
143*Tzvetomir Stoyanov* <tz.stoyanov@gmail.com>, author of this man page.
144--
145REPORTING BUGS
146--------------
147Report bugs to  <linux-trace-devel@vger.kernel.org>
148
149LICENSE
150-------
151libtraceevent is Free Software licensed under the GNU LGPL 2.1
152
153RESOURCES
154---------
155https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git