# tracefs_synth_echo_cmd

## 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.
    
               #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,
                                                   match_name);
    
                       /* The tep is no longer needed */
                       tep_free(tep);
    
                       /* 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,
                                                       TRACEFS_TIMESTAMP_USECS,
                                                       TRACEFS_SYNTH_DELTA_END, "delta");
    
                       /* 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;
    
                       trace_seq_init(&s);
    
                       tracefs_synth_echo_cmd(&s, synth);
                       trace_seq_terminate(&s);
                       trace_seq_do_printf(&s);
                       trace_seq_destroy(&s);
               }
    
               int main (int argc, char **argv)
               {
                       make_event();
    
                       if (argc > 1) {
                               if (!strcmp(argv[1], "create")) {
                                       /* Create the synthetic event */
                                       tracefs_synth_create(synth);
                               } else if (!strcmp(argv[1], "delete")) {
                                       /* Delete the synthetic event */
                                       tracefs_synth_destroy(synth);
                               } else {
                                       printf("usage: %s [create|delete]\n", argv[0]);
                                       exit(-1);
                               }
                       } else
                               show_event();
    
                       tracefs_synth_free(synth);
    
                       return 0;
               }
    


## 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.
    


## 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).
    


## NOTES



            1. rostedt@goodmis.org
               mailto:rostedt@goodmis.org
    
            2. tz.stoyanov@gmail.com
               mailto:tz.stoyanov@gmail.com
    
            3. sameeruddin.shaik8@gmail.com
               mailto:sameeruddin.shaik8@gmail.com
    
            4. linux-trace-devel@vger.kernel.org
               mailto:linux-trace-devel@vger.kernel.org
    


* * *