xref: /aosp_15_r20/external/libtracefs/Documentation/libtracefs-events-file.txt (revision 287e80b3a36113050663245e7f2c00d274188f18)

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

NAME
----
tracefs_event_get_file, tracefs_event_file_read, tracefs_event_file_write, tracefs_event_file_append,
tracefs_event_file_clear, tracefs_event_file_exists - Work with trace event files.

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

char pass:[*]*tracefs_event_get_file*(struct tracefs_instance pass:[*]_instance_, const char pass:[*]_system_, const char pass:[*]_event_,
			     const char pass:[*]_file_);
char pass:[*]*tracefs_event_file_read*(struct tracefs_instance pass:[*]_instance_, const char pass:[*]_system_, const char pass:[*]_event_,
			      const char pass:[*]_file_, int pass:[*]_psize_);
int *tracefs_event_file_write*(struct tracefs_instance pass:[*]_instance_, const char pass:[*]_system_, const char pass:[*]_event_,
			     const char pass:[*]_file_, const char pass:[*]_str_);
int *tracefs_event_file_append*(struct tracefs_instance pass:[*]_instance_, const char pass:[*]_system_, const char pass:[*]_event_,
			      const char pass:[*]_file_, const char pass:[*]_str_);
int *tracefs_event_file_clear*(struct tracefs_instance pass:[*]_instance_, const char pass:[*]_system_, const char pass:[*]_event_,
			     const char pass:[*]_file_);
bool *tracefs_event_file_exists*(struct tracefs_instance pass:[*]_instance_, const char pass:[*]_system_, const char pass:[*]_event_,
			       const char pass:[*]_file_)

--

DESCRIPTION
-----------
These are functions for accessing tracefs event specific files.
These functions act similar to the tracefs instance file functions
but are easier to get to if the system and events are known before hand.

The *tracefs_event_get_file()* returns the full path of the _file_ for
the given _system_ and _event_ that is within the given _instance_.
If _instance_ is NULL, then the file for the _event_ for the top level
instance is returned. Note, there is no check to see if the file actually
exists or even if the system and event exist. It only creates the path
name for such an event if it did exist. This acts similar to the
*tracefs_instance_get_file*(3), but is to be used to get to event files
if the _system_ and _event_ are already known.

The *tracefs_event_file_read()* reads the content for the _event_ _file_
for the given _instance_ or the top level instance if _instance_ is
NULL. The content of the file is returned and _psize_ is set to the amount
of data that was read. The returned content must be freed with *free*(3).
This acts similar to the *tracefs_instance_file_read*(3), but is
to be used to read the event file if the _system_ and _event_ are already
known.

The *tracefs_event_file_write()* writes _str_ to the _event_ _file_.
It will truncate anything that is already in that file.
This acts similar to the *tracefs_instance_file_write*(3), but is
to be used to read the event file if the _system_ and _event_ are already
known.

The *tracefs_event_file_append()* appends _str_ to the _event_ _file_.
It will not clear out the file as it writes _sting_.
This acts similar to the *tracefs_instance_file_append*(3), but is
to be used to read the event file if the _system_ and _event_ are already
known.

The *tracefs_event_file_clear()* clears the content of the _event_ _file_.
This acts similar to the *tracefs_instance_file_clear*(3), but is
to be used to read the event file if the _system_ and _event_ are already
known.

The *tracefs_event_file_exists()* returns true if the _event_ _file_
exists, and false otherwise. This acts similar to the *tracefs_instance_file_exists*(3),
but is to be used to read the event file if the _system_ and _event_ are already
known.

RETURN VALUE
------------
*tracefs_event_get_file()* returns the path of the given _system_/_event_ _file_ on
success and NULL on error. The return value must be freed with *tracefs_put_tracing_file*(3).

*tracefs_event_file_read()* reads the content of the _system_/_event_ _file_ or
NULL on error. The return pointer must be freed with *free*(3).

*tracefs_event_file_write()* and *tracefs_event_file_append()* returns the number of
bytes written to the _system_/_event_ _file_ or negative on error.

*tracefs_event_file_clear()* returns zero on success and -1 on error.

*tracefs_event_file_exists()* returns true if the _system_/_event_ _file_ exists for
the given _instance_ (or top level if _instance_ is NULL) or false otherwise.

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

int main(int argc, char **argv)
{
	char *system;
	char *event;
	char *file;
	char *cmd = NULL;
	char *buf;
	char *str;
	char ch = 'r';
	int size;

	if (argc < 4) {
		printf("usage: %s sytem event file [(-a|-w) write | -c]\n"
		       "   reads the system/event file or writes if [write is supplied]\n",
		       argv[0]);
		exit(0);
	}

	system = argv[1];
	event = argv[2];
	file = argv[3];
	if (argc > 4)
		cmd = argv[4];

	if (!tracefs_event_file_exists(NULL, system, event, file)) {
		fprintf(stderr, "File %s/%s/%s does not exist\n",
				system, event, file);
		exit(-1);
	}

	if (cmd) {
		if (cmd[0] != '-')
			ch = cmd[0];
		else
			ch = cmd[1];
		if (!ch)
			ch = 'c';
	}

	switch (ch) {
	case 'r':
		buf = tracefs_event_file_read(NULL, system, event, file, &size);
		if (buf)
			printf("%s", buf);
		else
			fprintf(stderr, "Failed to read %s/%s/%s\n",
				system, event, file);
		free(buf);
		break;
	case 'w':
	case 'a':
		if (argc < 6) {
			fprintf(stderr, "%s command requires something to write\n",
				ch == 'w' ? "write" : "append");
			exit(-1);
		}
		if (ch == 'w')
			size = tracefs_event_file_write(NULL, system, event, file, argv[5]);
		else
			size = tracefs_event_file_append(NULL, system, event, file, argv[5]);
		if (size < 0) {
			fprintf(stderr, "Failed to write '%s' to %s/%s/%s\n",
					argv[5], system, event, file);
			exit(-1);
		}
		break;
	case 'c':
		if (tracefs_event_file_clear(NULL, system, event, file) < 0) {
			fprintf(stderr, "Failed to clear %s/%s/%s\n",
				system, event, file);
			exit(-1);
		}
		break;
	default:
		fprintf(stderr, "Unknown command '%c'\n", ch);
		exit(-1);
	}
	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
--------
*libtracefs*(3),
*libtraceevent*(3),
*trace-cmd*(1)

AUTHOR
------
[verse]
--
*Steven Rostedt* <[email protected]>
--
REPORTING BUGS
--------------
Report bugs to  <[email protected]>

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) 2022 Google, Inc. Free use of this software is granted under
the terms of the GNU Public License (GPL).