xref: /aosp_15_r20/external/libtraceevent/Documentation/libtraceevent-kbuffer-read.txt (revision 436bf2bcd5202612ffffe471bbcc1f277cc8d28e)

libtraceevent(3)
================

NAME
----
kbuffer_read_event, kbuffer_next_event, kbuffer_missed_events, kbuffer_event_size, kbuffer_curr_size,
kbuffer_curr_offset, kbuffer_curr_index, kbuffer_read_buffer -
Functions to read through the kbuffer sub buffer.

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

void pass:[*]*kbuffer_read_event*(struct kbuffer pass:[*]_kbuf_, unsigned long long pass:[*]_ts_);
void pass:[*]*kbuffer_next_event*(struct kbuffer pass:[*]_kbuf_, unsigned long long pass:[*]_ts_);
void pass:[*]*kbuffer_read_at_offset*(struct kbuffer pass:[*]_kbuf_, int _offset_, unsigned long long pass:[*]_ts_);
int *kbuffer_missed_events*(struct kbuffer pass:[*]_kbuf_);
int *kbuffer_event_size*(struct kbuffer pass:[*]_kbuf_);
int *kbuffer_curr_size*(struct kbuffer pass:[*]_kbuf_);
int *kbuffer_curr_offset*(struct kbuffer pass:[*]_kbuf_);
int *kbuffer_curr_index*(struct kbuffer pass:[*]_kbuf_);
int *kbuffer_read_buffer*(struct kbuffer pass:[*]_kbuf_, void pass:[*]_buffer_, int _len_);
--

DESCRIPTION
-----------
The function *kbuffer_read_event()* reads the next event in the _kbuf_ descriptor
and if _ts_ is non NULL, will place its timestamp into it. This does not modify the _kbuf_
descriptor, and calling this function mulitple times will return the same result.

The function *kbuffer_next_event()* will return the next event in the _kbuf_ descriptor.
It will also set the _ts_ to the timestamp of the returned event. NULL is returned
if there are no more events and _ts_ will be undefined. Note, if this is called directly
after a *kbuffer_load_subbuffer()* then it will likely give an unexpected result, as it
will return the second event and not the first event. Usually this function is only used
to move to the next event and to know if there's any more events to read, and
*kbuffer_read_event()* is always called first.

The function *kbuffer_read_at_offset()* returns the event located at a given _offset_ from
the beginning of the sub-buffer. This offset can be retrieved by *kbuffer_curr_offset()*.
If _ts_ points to an unsigned long long, then it will be set to the event at the given
offset's timestamp.

If the sub-buffer had missed events before it, then *kbuffer_missed_events()* will return
the non zero. If it returns -1, that means there were missed events, but the exact number
of missed events is unknown. If it returns a positive number, then the number of missed events
is the return value.

The *kbuffer_event_size()* function returns the size of the data portion of the current event
(the one that would be returned by *kbuffer_read_event()*.

The *kbuffer_curr_size()* function returns the entire record size of the current event
(the one that would be returned by *kbuffer_read_event()*. The difference here is that the
return value includes the size of the event record meta data that is not part of what
is returned by *kbuffer_read_event()*.

The *kbuffer_curr_offset()* function returns the offset from the beginning of the sub-buffer
of where the current event's meta data for the record begins. The first event will
not be at offset zero. This offset can be used to retrieve the event with
*kbuffer_read_at_offset()*.

The *kbuffer_curr_index()* function returns the index from the beginning of the data
portion of the sub-buffer where the current evnet's meta data is located.
The first event will likely be zero, but may not be if there's a timestamp attached to it.

The *kbuffer_read_buffer()* function will fill the given _buffer_ from the _kbuf_ the same
way the kernel would do a read system call. That is, if the length _len_ is less than the
sub buffer size, or the kbuffer current index is non-zero, it will start copying from the
_kbuf_ current event and create _buffer_ as a new sub buffer (with a timestamp
and commit header) with that event that was found and including all events after that can
fit within _len_. The _len_ must include the size of the sub buffer header as well as the
events to include. That is, _len_ is the allocate size of _buffer_ that can be filled.
The return from this function is the index of the end of the last event that was added.
If there are no more events then zero is returned, and if the buffer can not
copy any events because _len_ was too small, then -1 is returned.


RETURN VALUE
------------
*kbuffer_read_event()* returns the event that the _kbuf_ descriptor is currently at,
or NULL if the last event was passed (by *kbuffer_next_event()*).

*kbuffer_next_event()* returns the next event after the current event or NULL if there
are no more events.

*kbuffer_read_at_offset()* returns the event at a given _offset_ from the start of
the sub-buffer stored in _kbuf_, or NULL if there exists no event. Note, _offset_
only needs to be an offset that lands on the record, or is at the start of it. It does
not need to be exactly at the beginning of the record.

*kbuffer_missed_events()* returns 0 if there were no missed events before loaded sub-buffer.
Returns -1 if there were an unknown number of missed events, or if the number of missed events
is known, that number will be returned.

*kbuffer_event_size()* returns the size of the data payload of the current event of _kbuf_.

*kbuffer_curr_size()* returns the size of the entire record of the current event of _kbuf_.
This includes the size of the meta data for that record.

*kbuf_curr_offset()* returns the offset of the current record from the beginning of the _kbuf_
sub-buffer.

*kbuf_curr_index()* returns the index of the current record from the beginning of the _kbuf_
data section.

*kbuf_read_buffer()* returns the index of the end of the last event that was filled in
_buffer_. If there are no more events to copy from _start_ then 0 is returned. If _len_
is not big enough to hold any events, then -1 is returned.

EXAMPLE
-------
[source,c]
--
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>

#include <kbuffer.h>

int main (int argc, char **argv)
{
	unsigned long long ts;
	struct kbuffer *kbuf;
	struct stat st;
	char *buf;
	void *event;
	int save_offset = -1;
	int record_size;
	int offset;
	int index;
	int size;
	int ret;
	int fd;
	int i = 0;

	if (argc < 2) {
		printf("usage: %s raw-subbuffer-page\n", argv[0]);
		printf(" Try: dd count=1 bs=4096 if=/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw of=/tmp/file\n");
		exit(0);
	}

	if (stat(argv[1], &st) < 0) {
		perror("stat");
		exit(-1);
	}

	buf = malloc(st.st_size);
	if (!buf) {
		perror("Allocating buffer");
		exit(-1);
	}

	fd = open(argv[1], O_RDONLY);
	if (fd < 0) {
		perror(argv[1]);
		exit(-1);
	}

	ret = read(fd, buf, st.st_size);
	if (ret < 0) {
		perror("Reading buffer");
		exit(-1);
	}
	close(fd);

	kbuf = kbuffer_alloc(KBUFFER_ENDIAN_SAME_AS_HOST,
			     KBUFFER_LSIZE_SAME_AS_HOST);
	if (!kbuf) {
		perror("Creating kbuffer");
		exit(-1);
	}
	ret = kbuffer_load_subbuffer(kbuf, buf);
	if (ret < 0) {
		perror("Loading sub bufer");
		exit(-1);
	}

	if (kbuffer_subbuffer_size(kbuf) > st.st_size) {
		fprintf(stderr, "kbuffer is bigger than raw size %d > %ld\n",
			kbuffer_subbuffer_size(kbuf), st.st_size);
		exit(-1);
	}

	ret = kbuffer_missed_events(kbuf);
	if (ret) {
		if (ret > 0)
			printf("Missed %d events before this buffer\n", ret);
		else
			printf("Missed unknown number of events before this buffer\n");
	}
	do {
		event = kbuffer_read_event(kbuf, &ts);
		if (event) {
			record_size = kbuffer_curr_size(kbuf);
			offset = kbuffer_curr_offset(kbuf);
			index = kbuffer_curr_index(kbuf);
			size = kbuffer_event_size(kbuf);

			if (i == 20)
				save_offset = offset;
			printf(" event %3d ts:%lld\trecord_size:%d size:%d\tindex:%d offset:%d\n",
			       i++, ts, record_size, size, index, offset);
			event = kbuffer_next_event(kbuf, NULL);
		}
	} while (event);

	if (!event)
		printf("Finished sub buffer\n");

	if (save_offset > 0) {
		event = kbuffer_read_at_offset(kbuf, save_offset, &ts);
		if (!event) {
			fprintf(stderr, "Funny, can't find event 20 at offset %d\n", save_offset);
			exit(-1);
		}
		record_size = kbuffer_curr_size(kbuf);
		offset = kbuffer_curr_offset(kbuf);
		index = kbuffer_curr_index(kbuf);
		size = kbuffer_event_size(kbuf);

		printf("\n saved event 20 ts:%lld\trecord_size:%d size:%d\tindex:%d offset:%d\n\n",
		       ts, record_size, size, index, offset);
	}
	kbuffer_free(kbuf);

	return 0;
}
--
FILES
-----
[verse]
--
*event-parse.h*
	Header file to include in order to have access to the library APIs.
*-ltraceevent*
	Linker switch to add when building a program that uses the library.
--

SEE ALSO
--------
*libtraceevent*(3), *trace-cmd*(1)

AUTHOR
------
[verse]
--
*Steven Rostedt* <[email protected]>, author of *libtraceevent*.
--
REPORTING BUGS
--------------
Report bugs to  <[email protected]>

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

RESOURCES
---------
https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/