1LIBTRACEEVENT(3) libtraceevent Manual LIBTRACEEVENT(3)
2
6 tep_register_print_function, tep_unregister_print_function - Registers
7 / Unregisters a helper function.
8
10 #include <event-parse.h>
11 enum tep_func_arg_type {
13 TEP_FUNC_ARG_VOID,
14 TEP_FUNC_ARG_INT,
15 TEP_FUNC_ARG_LONG,
16 TEP_FUNC_ARG_STRING,
17 TEP_FUNC_ARG_PTR,
18 TEP_FUNC_ARG_MAX_TYPES
19 };
20 typedef unsigned long long (*tep_func_handler)(struct trace_seq *s, unsigned long long *args);
22 int tep_register_print_function(struct tep_handle *tep, tep_func_handler func, enum tep_func_arg_type ret_type, char *name, ...);
24 int tep_unregister_print_function(struct tep_handle *tep, tep_func_handler func, char *name);
25
27 Some events may have helper functions in the print format arguments.
28 This allows a plugin to dynamically create a way to process one of
29 these functions.
30 The tep_register_print_function() registers such helper function. The
32 tep argument is the trace event parser context. The func argument is a
33 pointer to the helper function. The ret_type argument is the return
34 type of the helper function, value from the tep_func_arg_type enum. The
35 name is the name of the helper function, as seen in the print format
36 arguments. The ... is a variable list of tep_func_arg_type enums, the
37 func function arguments. This list must end with TEP_FUNC_ARG_VOID. See
38 EXAMPLE section.
39 The tep_unregister_print_function() unregisters a helper function,
41 previously registered with tep_register_print_function(). The tep
42 argument is the trace event parser context. The func and name arguments
43 are the same, used when the helper function was registered.
44 The tep_func_handler is the type of the helper function. The s argument
46 is the trace sequence, it can be used to create a custom string. The
47 args is a list of arguments, defined when the helper function was
48 registered.
49
51 The tep_register_print_function() function returns 0 in case of
52 success. In case of an error, TEP_ERRNO_... code is returned.
53 The tep_unregister_print_function() returns 0 in case of success, or -1
55 in case of an error.
56
58 Some events have internal functions calls, that appear in the print
59 format output. For example "tracefs/events/i915/g4x_wm/format" has:
60 print fmt: "pipe %c, frame=%u, scanline=%u, wm %d/%d/%d, sr %s/%d/%d/%d, hpll %s/%d/%d/%d, fbc %s",
62 ((REC->pipe) + 'A'), REC->frame, REC->scanline, REC->primary,
63 REC->sprite, REC->cursor, yesno(REC->cxsr), REC->sr_plane,
64 REC->sr_cursor, REC->sr_fbc, yesno(REC->hpll), REC->hpll_plane,
65 REC->hpll_cursor, REC->hpll_fbc, yesno(REC->fbc)
66 Notice the call to function yesno() in the print arguments. In the
68 kernel context, this function has the following implementation:
69 static const char *yesno(int x)
71 {
72 static const char *yes = "yes";
73 static const char *no = "no";
74 return x ? yes : no;
76 }
77 The user space event parser has no idea how to handle this yesno()
79 function. The tep_register_print_function() API can be used to register
80 a user space helper function, mapped to the kernel’s yesno():
81 #include <event-parse.h>
83 #include <trace-seq.h>
84 ...
85 struct tep_handle *tep = tep_alloc();
86 ...
87 static const char *yes_no_helper(int x)
88 {
89 return x ? "yes" : "no";
90 }
91 ...
92 if ( tep_register_print_function(tep,
93 yes_no_helper,
94 TEP_FUNC_ARG_STRING,
95 "yesno",
96 TEP_FUNC_ARG_INT,
97 TEP_FUNC_ARG_VOID) != 0) {
98 /* Failed to register yes_no_helper function */
99 }
100 /*
102 Now, when the event parser encounters this yesno() function, it will know
103 how to handle it.
104 */
105 ...
106 if (tep_unregister_print_function(tep, yes_no_helper, "yesno") != 0) {
107 /* Failed to unregister yes_no_helper function */
108 }
109
111 event-parse.h
112 Header file to include in order to have access to the library APIs.
113 trace-seq.h
114 Header file to include in order to have access to trace sequences
115 related APIs. Trace sequences are used to allow a function to call
116 several other functions to create a string of data to use.
117 -ltraceevent
118 Linker switch to add when building a program that uses the library.
119
121 libtraceevent(3), trace-cmd(1)
122
124 Steven Rostedt <rostedt@goodmis.org[1]>, author of libtraceevent.
125 Tzvetomir Stoyanov <tz.stoyanov@gmail.com[2]>, author of this man page.
126
128 Report bugs to <linux-trace-devel@vger.kernel.org[3]>
129
131 libtraceevent is Free Software licensed under the GNU LGPL 2.1
132
134 https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/
135
137 1. rostedt@goodmis.org
138 mailto:rostedt@goodmis.org
139 2. tz.stoyanov@gmail.com
141 mailto:tz.stoyanov@gmail.com
142 3. linux-trace-devel@vger.kernel.org
144 mailto:linux-trace-devel@vger.kernel.org
145libtraceevent 1.5.3 04/15/2022 LIBTRACEEVENT(3)