xref: /aosp_15_r20/external/trace-cmd/Documentation/libtracecmd/libtracecmd-files.txt (revision 58e6ee5f017f6a8912852c892d18457e4bafb554)

libtracecmd(3)
=============

NAME
----
tracecmd_open, tracecmd_open_fd, tracecmd_open_head, tracecmd_init_data,
tracecmd_close - Open and close a trace file.

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

struct tracecmd_input pass:[*]*tracecmd_open*(const char pass:[*]_file_, int _flags_);
struct tracecmd_input pass:[*]*tracecmd_open_fd*(int _fd_, int _flags_);
struct tracecmd_input pass:[*]*tracecmd_open_head*(const char pass:[*]_file_, int _flags_);
int *tracecmd_init_data*(struct tracecmd_input pass:[*]_handle_);
void *tracecmd_close*(struct tracecmd_input pass:[*]_handle_);
--

DESCRIPTION
-----------
This set of APIs can be used to open and close a trace file recorded by
_trace-cmd(1)_ and containing tracing information from ftrace, the official
Linux kernel tracer. The opened file is represented by a _tracecmd_input_
structure, all other library APIs that work with the file require a pointer
to the structure. The APIs for opening a trace file have a _flag_ input
parameter, which controls how the file will be opened and parsed. The _flag_
is a combination of these options:

 TRACECMD_FL_LOAD_NO_PLUGINS - Do not load any plugins
 TRACECMD_FL_LOAD_NO_SYSTEM_PLUGINS - Do not load system wide plugins, load only "local only"
					plugins from user's home directory.

The _tracecmd_open()_ function opens a given trace _file_, parses the
metadata headers from the file, allocates and initializes а _tracecmd_input_
handler structure representing the file. It also initializes the handler
for reading trace data from the file. The returned handler is ready to be
used with _tracecmd_read__ APIs.

The _tracecmd_open_fd()_ function does the same as _tracecmd_open()_, but
works with a file descriptor to a trace file, opened for reading.

The _tracecmd_open_head()_ function is the same as _tracecmd_open()_, but
does not initialize the handler for reading trace data. It reads and parses
the metadata headers only. The _tracecmd_init_data()_ should be used before
using the _tracecmd_read__ APIs.

The _tracecmd_init_data()_ function initializes a _handle_, allocated with
_tracecmd_open_head()_, for reading trace data from the file associated with
it. This API must be called before any of the _tracecmd_read__ APIs.

The _tracecmd_close()_ function frees a _handle_, pointer to tracecmd_input
structure, previously allocated with _tracecmd_open()_, _tracecmd_open_fd()_
or _tracecmd_open_head()_ APIs.

RETURN VALUE
------------
The _tracecmd_open()_, _tracecmd_open_fd()_ and _tracecmd_open_head()_
functions return a pointer to tracecmd_input structure or NULL in case of
an error. The returned structure must be free with _tracecmd_close()_.
Note that if _tracecmd_open_fd()_ is used to allocate a tracecmd_input handler,
when _tracecmd_close()_ is called to close it, that fd will be closed also.

The _tracecmd_init_data()_ function returns -1 in case of an error or
0 otherwise.

EXAMPLE
-------
[source,c]
--
The are two different use patterns for opening and reading trace data from
a trace file, which can be used depending on the use case.

1. Open and initialise the trace file in а single step:

#include <trace-cmd.h>
...
struct tracecmd_input *handle = tracecmd_open("trace.dat");
	if (!handle) {
		/* Failed to open trace.dat file */
	}
...
	/* Read tracing data from the file, using the handle */
...
	tracecmd_close(handle);
...
int fd;
	fd = = open("trace.dat", O_RDONLY);
	if (fd < 0) {
		/* Failed to open trace file for reading */
	}
	handle = tracecmd_open_fd(fd);
	if (!handle) {
		close(fd);
		/* Failed to initialise handler for reading the trace file */
	}
...
	/* Read tracing data from the file, using the handle */
...
	tracecmd_close(handle);
...

2. Open and initialise the trace file in two steps. This allows to perform
some processing based on metadata, read from the file, before initialising
the trace data for reading. Example for such use case is when opening multiple
trace files recorded in a same trace session. In that case timestamps of all
trace events must be adjusted based on the information from  the file's metadata
and before reading the trace data.

#include <trace-cmd.h>
...
struct tracecmd_input *handle = tracecmd_open_head("trace.dat");
	if (!handle) {
		/* Failed to open trace.dat file */
	}
...
	/* do some processing, before initialising the trace data for reading */
...
	if (tracecmd_init_data(handle) < 0) {
		/* Failed to initialize hadle for reading the trace data */
	}
...
	/* Read tracing data from the file, using the handle */
...
	tracecmd_close(handle);
...
--
FILES
-----
[verse]
--
*trace-cmd.h*
	Header file to include in order to have access to the library APIs.
*-ltracecmd*
	Linker switch to add when building a program that uses the library.
--

SEE ALSO
--------
_libtracefs(3)_,
_libtraceevent(3)_,
_trace-cmd(1)_
_trace-cmd.dat(5)_

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

LICENSE
-------
libtracecmd is Free Software licensed under the GNU LGPL 2.1

RESOURCES
---------
https://git.kernel.org/pub/scm/utils/trace-cmd/trace-cmd.git/

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