Documentation/libtracefs-snapshot.txt

libtracefs(3)
=============

NAME
----
tracefs_snapshot_snap, tracefs_snapshot_clear, tracefs_snapshot_free -
API to create, clear and read snapshots

SYNOPSIS
--------
[verse]
--
*#include <tracefs.h>*

int *tracefs_snapshot_snap*(struct tracefs_instance pass:[*]instance);
int *tracefs_snapshot_clear*(struct tracefs_instance pass:[*]instance);
int *tracefs_snapshot_free*(struct tracefs_instance pass:[*]instance);
--

DESCRIPTION
-----------
The Linux kernel tracing provides a "snapshot" feature. The kernel has two
ring buffers. One that is written to by the tracing system and another one that
is the "snapshot" buffer. When a snapshot happens, the two buffers are swapped, and
the current snapshot buffer becomes the one being written to, and the buffer
that was being written to becomes the saved snapshot.

Note, the snapshot buffer is allocated the first time it is taken, so it is best
to take a snapshot at the start before one is needed so that it is allocated
and a snapshot is ready, then the snapshot will happen immediately.

The *tracefs_snapshot_snap()* will allocate (if not already allocated) the snapshot
buffer and then take a "snapshot" (swap the main buffer that's being written to with
the allocated snapshot buffer). It will do this to the given _instance_ buffer or
the top instance if _instance_ is NULL.

The *tracefs_snapshot_clear()* will erase the content of the snapshot buffer for
the given _instance_ or the top level instance if _instance_ is NULL.

The *tracefs_snapshot_free()* will free the allocated snapshot for the given _instance_
or the top level instance if _instance_ is NULL. That is, if another call to
*tracefs_snapshot_snap()* is done after this, then it will need to allocate
the snapshot buffer again before it can take a snapshot. This function should
be used to free up the kernel memory used by hte snapshot buffer when no longer in use.


RETURN VALUE
------------
The *tracefs_snapshot_snap()*, *tracefs_snapshot_clear()* and the *tracefs_snapshot_free()*
all return 0 on success and -1 on failure.

EXAMPLE
-------
[source,c]
--
#include <stdlib.h>
#include <stdio.h>
#include <tracefs.h>

static int callback(struct tep_event *event, struct tep_record *record, int cpu, void *data)
{
	static struct trace_seq seq;
	struct tep_handle *tep = event->tep;

	if (!seq.buffer)
		trace_seq_init(&seq);

	trace_seq_reset(&seq);

	tep_print_event(tep, &seq, record, "[%03d] %s-%d %6.1000d\t%s: %s\n",
				TEP_PRINT_CPU,
				TEP_PRINT_COMM,
				TEP_PRINT_PID,
				TEP_PRINT_TIME,
				TEP_PRINT_NAME,
				TEP_PRINT_INFO);
	trace_seq_do_printf(&seq);
	return 0;
}

int main (int argc, char **argv)
{
	struct tracefs_instance *instance;
	struct tep_handle *tep;
	char *line = NULL;
	size_t len = 0;
	int ret;

	instance = tracefs_instance_create("my_snapshots");
	if (!instance) {
		perror("creating instance");
		exit(-1);
	}

	tep = tracefs_local_events(NULL);
	if (!tep) {
		perror("reading event formats");
		goto out;
	}

	/* Make sure the snapshot buffer is allocated */
	ret = tracefs_snapshot_snap(instance);
	if (ret < 0)
		goto out;

	ret = tracefs_event_enable(instance, "sched", NULL);
	if (ret < 0) {
		perror("enabling event");
		goto out;
	}

	for (;;) {
		printf("Hit enter without text to take snapshot!\n");
		printf("Enter any text to display the snapshot\n");
		printf("Enter 'quit' to exit\n");
		getline(&line, &len, stdin);
		ret = tracefs_snapshot_snap(instance);
		if (ret < 0) {
			perror("taking snapshot");
			goto out;
		}
		if (!line)
			break;
		if (strlen(line) < 2)
			continue;
		if (strncmp(line, "quit", 4) == 0)
			break;
		tracefs_iterate_snapshot_events(tep, instance, NULL, 0, callback, NULL);
	}

	free(line);

	tracefs_instance_clear(instance);

 out:
	tracefs_snapshot_free(instance);
	tracefs_event_disable(instance, "sched", NULL);
	tracefs_instance_destroy(instance);
	tracefs_instance_free(instance);

	exit(0);
}
--
FILES
-----
[verse]
--
*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
--------
*tracefs_iterate_snapshot_events*(3)
*libtracefs*(3),
*libtraceevent*(3),
*trace-cmd*(1)

AUTHOR
------
[verse]
--
*Steven Rostedt* <rostedt@goodmis.org>
--
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) 2023 Google, LLC. Free use of this software is granted under
the terms of the GNU Public License (GPL).