Documentation/libtracefs-cpu-buf.txt

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

NAME
----
tracefs_cpu_read_buf, tracefs_cpu_buffered_read_buf, tracefs_cpu_flush_buf
- Reading trace_pipe_raw data returning a kbuffer

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

struct kbuffer pass:[*]*tracefs_cpu_read_buf*(struct tracefs_cpu pass:[*]_tcpu_, bool _nonblock_);
struct kbuffer pass:[*]*tracefs_cpu_buffered_read_buf*(struct tracefs_cpu pass:[*]_tcpu_, bool _nonblock_);
struct kbuffer pass:[*]*tracefs_cpu_flush_buf*(struct tracefs_cpu pass:[*]_tcpu_);
--

DESCRIPTION
-----------
This set of APIs can be used to read the raw data from the trace_pipe_raw
files in the tracefs file system and return a kbuffer structure to read it with.

The *tracefs_cpu_read_buf()* reads the trace_pipe_raw files associated to _tcpu_
and returns a kbuffer structure that can be used to iterate the events.
If _nonblock_ is set, and there's no data available, it will return immediately.
Otherwise depending on how _tcpu_ was opened, it will block. If _tcpu_ was
opened with nonblock set, then this _nonblock_ will make no difference.

The *tracefs_cpu_buffered_read_buf()* is basically the same as *tracefs_cpu_read_buf()*
except that it uses a pipe through splice to buffer reads. This will batch
reads keeping the reading from the ring buffer less intrusive to the system,
as just reading all the time can cause quite a disturbance. Note, one
difference between this and *tracefs_cpu_read()* is that it will read only in
sub buffer pages. If the ring buffer has not filled a page, then it will not
return anything, even with _nonblock_ set.  Calls to *tracefs_cpu_flush_buf()*
or *tracefs_cpu_flush()* should be done to read the rest of the file at the
end of the trace.

The *tracefs_cpu_flush_buf()* reads the trace_pipe_raw file associated by the
_tcpu_ and puts it into _buffer_, which must be the size of the sub buffer
which is retrieved.  This should be called at the end of tracing
to get the rest of the data. This call will convert the file descriptor of
trace_pipe_raw into non-blocking mode.

RETURN VALUE
------------
The functions *tracefs_cpu_read_buf()*, tracefs_cpu_buffered_read_buf()* and
*tracefs_cpu_flush()* returns a kbuffer descriptor that can be iterated
over to find the events. Note, this descriptor is part of the tracefs_cpu structure
and should not be freed. It will be freed. It returns NULL on error or if nonblock
is set and there are no events available. In the case of no events, errno will be
set with EAGAIN.

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

static void read_page(struct tep_handle *tep, struct kbuffer *kbuf)
{
	static struct trace_seq seq;
	struct tep_record record;

	if (seq.buffer)
		trace_seq_reset(&seq);
	else
		trace_seq_init(&seq);

	while ((record.data = kbuffer_read_event(kbuf, &record.ts))) {
		record.size = kbuffer_event_size(kbuf);
		kbuffer_next_event(kbuf, NULL);
		tep_print_event(tep, &seq, &record,
				"%s-%d %9d\t%s: %s\n",
				TEP_PRINT_COMM,
				TEP_PRINT_PID,
				TEP_PRINT_TIME,
				TEP_PRINT_NAME,
				TEP_PRINT_INFO);
		trace_seq_do_printf(&seq);
		trace_seq_reset(&seq);
	}
}

int main (int argc, char **argv)
{
	struct tracefs_cpu *tcpu;
	struct tep_handle *tep;
	struct kbuffer *kbuf;
	int cpu;

	if (argc < 2 || !isdigit(argv[1][0])) {
		printf("usage: %s cpu\n\n", argv[0]);
		exit(-1);
	}

	cpu = atoi(argv[1]);

	tep = tracefs_local_events(NULL);
	if (!tep) {
		perror("Reading trace event formats");
		exit(-1);
	}

	tcpu = tracefs_cpu_open(NULL, cpu, 0);
	if (!tcpu) {
		perror("Open CPU 0 file");
		exit(-1);
	}

	while ((kbuf = tracefs_cpu_buffered_read_buf(tcpu, true))) {
		read_page(tep, kbuf);
	}

	kbuf = tracefs_cpu_flush_buf(tcpu);
	if (kbuf)
		read_page(tep, kbuf);

	tracefs_cpu_close(tcpu);
	tep_free(tep);

	return 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_cpu_open*(3)
*tracefs_cpu_close*(3)
*tracefs_cpu_read*(3)
*tracefs_cpu_buffered_read*(3)
*tracefs_cpu_flush*(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) 2022 Google, Inc. Free use of this software is granted under
the terms of the GNU Public License (GPL).