# libtracefs(3) Manual Page

## NAME

tracefs_synth_echo_cmd, tracefs_synth_get_start_hist, tracefs_synth_get_name, tracefs_synth_raw_fmt, tracefs_synth_show_event, tracefs_synth_show_start_hist, tracefs_synth_show_end_hist, tracefs_synth_get_event - Retrieve data of synthetic events.

## SYNOPSIS

    #include <tracefs.h>

    int tracefs_synth_echo_cmd(struct trace_seq *seq, struct tracefs_synth *synth);
    struct tracefs_hist *tracefs_synth_get_start_hist(struct tracefs_synth *synth);

    const char *tracefs_synth_get_name(struct tracefs_synth *synth);
    int tracefs_synth_raw_fmt(struct trace_seq *seq, struct tracefs_synth *synth);
    const char *tracefs_synth_show_event(struct tracefs_synth *synth);
    const char *tracefs_synth_show_start_hist(struct tracefs_synth *synth);
    const char *tracefs_synth_show_end_hist(struct tracefs_synth *synth);
    struct tep_event *tracefs_synth_get_event(struct tep_handle *tep, struct tracefs_synth *synth);

## DESCRIPTION

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.

See tracefs_synth_alloc(3) for allocation of synthetic events, and tracefs_synth_create() for creating the synthetic event on the system.

tracefs_synth_echo_cmd() acts like tracefs_synth_create(), but instead of creating the synthetic event in the system, it will write the echo commands to manually create it in the seq given.

tracefs_synth_get_start_hist() returns a struct tracefs_hist descriptor describing the histogram used to create the synthetic event.

    enum tracefs_synth_handler {
            TRACEFS_SYNTH_HANDLE_MATCH,
            TRACEFS_SYNTH_HANDLE_MAX,
            TRACEFS_SYNTH_HANDLE_CHANGE,
    };

tracefs_synth_get_name() returns the name of the synthetic event or NULL on error. The returned string belongs to the synth event object and is freed with the event by tracefs_synth_free().

tracefs_synth_raw_fmt() writes the raw format strings (dynamic event and histograms) of the synthetic event in the seq given.

tracefs_synth_show_event() returns the format of the dynamic event used by the synthetic event or NULL on error. The returned string belongs to the synth event object and is freed with the event by tracefs_synth_free().

tracefs_synth_show_start_hist() returns the format of the start histogram used by the synthetic event or NULL on error. The returned string belongs to the synth event object and is freed with the event by tracefs_synth_free().

tracefs_synth_show_end_hist() returns the format of the end histogram used by the synthetic event or NULL on error. The returned string belongs to the synth event object and is freed with the event by tracefs_synth_free().

The tracefs_synth_get_event() function returns a tep event, describing the given synthetic event. The API detects any newly created or removed dynamic events. The returned pointer to tep event is controlled by @tep and must not be freed.

## RETURN VALUE

tracefs_synth_get_name(), tracefs_synth_show_event(), tracefs_synth_show_start_hist() and tracefs_synth_show_end_hist() return a string owned by the synth event object.

The tracefs_synth_get_event() function returns a pointer to a tep event or NULL in case of an error or if the requested synthetic event is missing. The returned pointer to tep event is controlled by @tep and must not be freed.

All other functions return zero on success or -1 on error.

## ERRORS

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.

## EXAMPLE

See tracefs_sql(3) for a more indepth use of some of this code.

## FILES

    tracefs.h
            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.

## SEE ALSO

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_alloc(3), tracefs_synth_add_match_field(3), tracefs_synth_add_compare_field(3), tracefs_synth_add_start_field(3), tracefs_synth_add_end_field(3), tracefs_synth_append_start_filter(3), tracefs_synth_append_end_filter(3), tracefs_synth_free(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),

## AUTHOR

    Steven Rostedt <rostedt@goodmis.org>
    Tzvetomir Stoyanov <tz.stoyanov@gmail.com>
    sameeruddin shaik <sameeruddin.shaik8@gmail.com>

## REPORTING BUGS

Report bugs to <linux-trace-devel@vger.kernel.org>

## LICENSE

libtracefs is Free Software licensed under the GNU LGPL 2.1

## RESOURCES

https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/

## COPYING

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