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LIBTRACEFS(3) libtracefs Manual LIBTRACEFS(3)


tracefs_synth_alloc, tracefs_synth_add_match_field, tracefs_synth_add_compare_field, tracefs_synth_add_start_field, tracefs_synth_add_end_field, tracefs_synth_append_start_filter, tracefs_synth_append_end_filter, tracefs_synth_free, - Creation of a synthetic event descriptor


#include <tracefs.h>
struct tracefs_synth *tracefs_synth_alloc(struct tep_handle *tep,

const char *name,
const char *start_system,
const char *start_event,
const char *end_system,
const char *end_event,
const char *start_match_field,
const char *end_match_field,
const char *match_name); int tracefs_synth_add_match_field(struct tracefs_synth *synth,
const char *start_match_field,
const char *end_match_field,
const char *name); int tracefs_synth_add_compare_field(struct tracefs_synth *synth,
const char *start_compare_field,
const char *end_compare_field,
enum tracefs_synth_calc calc,
const char *name); int tracefs_synth_add_start_field(struct tracefs_synth *synth,
const char *start_field,
const char *name); int tracefs_synth_add_end_field(struct tracefs_synth *synth,
const char *end_field,
const char *name); int tracefs_synth_append_start_filter(struct tracefs_synth *synth,
struct tracefs_filter type,
const char *field,
enum tracefs_synth_compare compare,
const char *val); int tracefs_synth_append_end_filter(struct tracefs_synth *synth,
struct tracefs_filter type,
const char *field,
enum tracefs_synth_compare compare,
const char *val); void tracefs_synth_free(struct tracefs_synth *synth);


Synthetic events are dynamic events that are created by matching two other events which triggers a synthetic event. One event is the starting event which some field is recorded, and when the second event is executed, if it has a field (or fields) that matches the starting event’s field (or fields) then it will trigger the synthetic event. The field values other than the matching fields may be passed from the starting event to the end event to perform calculations on, or to simply pass as a parameter to the synthetic event.

One common use case is to set "sched_waking" as the starting event. This event is triggered when a process is awoken. Then set "sched_switch" as the ending event. This event is triggered when a new task is scheduled on the CPU. By setting the "common_pid" of both events as the matching fields, the time between the two events is considered the wake up latency of that process. Use TRACEFS_TIMESTAMP as a field for both events to calculate the delta in nanoseconds, or use *TRACEFS_TIMESTAMP_USECS" as the compare fields for both events to calculate the delta in microseconds. This is used as the example below.

tracefs_synth_alloc() allocates and initializes a synthetic event. It does not create the synthetic event, but supplies the minimal information to do so. See tracefs_synth_create(3) for how to create the synthetic event in the system. It requires a tep handler that can be created by tracefs_local_events(3) for more information. The name holds the name of the synthetic event that will be created. The start_system is the name of the system for the starting event. It may be NULL and the first event with the name of start_event will be chosen. The end_system is the name of the system for theh ending event. It may be NULL and the first event with the name of end_event will be chosen as the ending event. If match_name is given, then this will be the field of the created synthetic event that holds the matching keys of the starting event’s start_match_field and the ending event’s end_match_field. If match_name is NULL, then it will not be recorded in the created synthetic event.

tracefs_synth_add_match_field() will add a second key to match between the starting event and the ending event. If name is given, then the content of the matching field will be saved by this name in the synthetic event. The start_match_field is the field of the starting event to mach with the ending event’s end_match_field.

tracefs_synth_add_compare_field() is used to compare the start_compare_field of the starting event with the end_compare_field of the ending event. The name must be given so that the result will be saved by the synthetic event. It makes no sense to not pass this to the synthetic event after doing the work of the compared fields, as it serves no other purpose. The calc parameter can be one of:

TRACEFS_SYNTH_DELTA_END - calculate the difference between the content in the end_compare_field from the content of the start_compare_field.

name = end_compare_field - start_compare_field

TRACEFS_SYNTH_DELTA_START - calculate the difference between the content in the start_compare_field from the content of the end_compare_field.

name = start_compare_field - end_compare_field

TRACEFS_SYNTH_ADD - Add the content of the start_compare_field to the content of the end_compare_field.

name = start_compare_field + end_compare_field

tracefs_synth_add_start_field() - Records the start_field of the starting event as name in the synthetic event. If name is NULL, then the name used will be the same as start_field.

tracefs_synth_add_end_field() - Records the end_field of the ending event as name in the synthetic event. If name is NULL, then the name used will be the same as end_field.

tracefs_synth_append_start_filter() creates a filter or appends to it for the starting event. Depending on type, it will build a string of tokens for parenthesis or logic statements, or it may add a comparison of field to val based on compare.

If type is: TRACEFS_FILTER_COMPARE - See below TRACEFS_FILTER_AND - Append "&&" to the filter TRACEFS_FILTER_OR - Append "||" to the filter TRACEFS_FILTER_NOT - Append "!" to the filter TRACEFS_FILTER_OPEN_PAREN - Append "(" to the filter TRACEFS_FILTER_CLOSE_PAREN - Append ")" to the filter

field, compare, and val are ignored unless type is equal to TRACEFS_FILTER_COMPARE, then _compare will be used for the following:

TRACEFS_COMPARE_EQ - field == val

TRACEFS_COMPARE_NE - field != val

TRACEFS_COMPARE_GT - field > val

TRACEFS_COMPARE_GE - field >= val

TRACEFS_COMPARE_LT - field < val

TRACEFS_COMPARE_LE - field <= val

TRACEFS_COMPARE_RE - field ~ "val" : where field is a string.

TRACEFS_COMPARE_AND - field & val : where field is a flags field.

tracefs_synth_append_end_filter() is the same as tracefs_synth_append_start_filter but filters on the ending event.

tracefs_synth_free() frees the allocated descriptor returned by tracefs_synth_alloc().


tracefs_synth_alloc() returns an allocated struct tracefs_synth descriptor on success or NULL on error.

All other functions that return an integer returns zero on success or -1 on error.


The following errors are for all the above calls:

EPERM Not run as root user when required.

EINVAL Either a parameter is not valid (NULL when it should not be) or a field that is not compatible for calculations.

ENODEV An event or one of its fields is not found.

EBADE The fields of the start and end events are not compatible for either matching or comparing.

ENOMEM not enough memory is available.

And more errors may have happened from the system calls to the system.


#include <stdlib.h>
#include <tracefs.h>
#define start_event "sched_waking"
#define start_field "pid"
#define end_event "sched_switch"
#define end_field "next_pid"
#define match_name "pid"
static struct tracefs_synth *synth;
static void make_event(void)

struct tep_handle *tep;
/* Load all events from the system */
tep = tracefs_local_events(NULL);
/* Initialize the synthetic event */
synth = tracefs_synth_alloc(tep, "wakeup_lat",
NULL, start_event,
NULL, end_event,
start_field, end_field,
/* The tep is no longer needed */
/* Save the "prio" field as "prio" from the start event */
tracefs_synth_add_start_field(synth, "prio", NULL);
/* Save the "next_comm" as "comm" from the end event */
tracefs_synth_add_end_field(synth, "next_comm", "comm");
/* Save the "prev_prio" as "prev_prio" from the end event */
tracefs_synth_add_end_field(synth, "prev_prio", NULL);
* Take a microsecond time difference between end and start
* and record as "delta"
tracefs_synth_add_compare_field(synth, TRACEFS_TIMESTAMP_USECS,
/* Only record if start event "prio" is less than 100 */
tracefs_synth_append_start_filter(synth, TRACEFS_FILTER_COMPARE,
"prio", TRACEFS_COMPARE_LT, "100");
* Only record if end event "next_prio" is less than 50
* or the previous task's prio was not greater than or equal to 100.
* next_prio < 50 || !(prev_prio >= 100)
tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_COMPARE,
"next_prio", TRACEFS_COMPARE_LT, "50");
tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_OR, NULL, 0, NULL);
tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_NOT, NULL, 0, NULL);
tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_OPEN_PAREN, NULL, 0, NULL);
tracefs_synth_append_end_filter(synth, TRACEFS_FILTER_COMPARE,
"prev_prio", TRACEFS_COMPARE_GE, "100");
* Note, the above only added: "next_prio < 50 || !(prev_prio >= 100"
* That's because, when the synth is executed, the remaining close parenthesis
* will be added. That is, the string will end up being:
* "next_prio < 50 || !(prev_prio >= 100)" when one of tracefs_sync_create()
* or tracefs_sync_echo_cmd() is run.
*/ } /* Display how to create the synthetic event */ static void show_event(void) {
struct trace_seq s;
tracefs_synth_echo_cmd(&s, synth);
trace_seq_destroy(&s); } int main (int argc, char **argv) {
if (argc > 1) {
if (!strcmp(argv[1], "create")) {
/* Create the synthetic event */
} else if (!strcmp(argv[1], "delete")) {
/* Delete the synthetic event */
} else {
printf("usage: %s [create|delete]\n", argv[0]);
} else
return 0; }



Header file to include in order to have access to the library APIs. -ltracefs
Linker switch to add when building a program that uses the library.


tracefs_synth_create(3), tracefs_synth_destroy(3), tracfes_synth_echo_cmd(3), libtracefs(3), libtraceevent(3), trace-cmd(1), tracefs_hist_alloc(3), tracefs_hist_alloc_2d(3), tracefs_hist_alloc_nd(3), tracefs_hist_free(3), tracefs_hist_add_key(3), tracefs_hist_add_value(3), tracefs_hist_add_name(3), tracefs_hist_start(3), tracefs_hist_destory(3), tracefs_hist_add_sort_key(3), tracefs_hist_sort_key_direction(3), tracefs_synth_create(3), tracefs_synth_destroy(3), tracefs_synth_complete(3), tracefs_synth_trace(3), tracefs_synth_snapshot(3), tracefs_synth_save(3), tracefs_synth_echo_cmd(3), tracefs_synth_get_start_hist(3), tracefs_synth_get_name(3), tracefs_synth_raw_fmt(3), tracefs_synth_show_event(3), tracefs_synth_show_start_hist(3), tracefs_synth_show_end_hist(3), tracefs_synth_get_event(3),


Steven Rostedt <[1]>
Tzvetomir Stoyanov <[2]>
sameeruddin shaik <[3]>


Report bugs to <[4]>


libtracefs is Free Software licensed under the GNU LGPL 2.1



Copyright (C) 2020 VMware, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL).


06/15/2023 libtracefs