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

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

NAME
----
kbuffer_alloc, kbuffer_dup, kbuffer_free, kbuffer_load_subbuffer, kbuffer_subbuffer,
kbuffer_refresh, kbuffer_subbuffer_size, kbuffer_start_of_data - Creating of kbuffer element to parse
the Linux kernel tracing ring buffer

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

enum kbuffer_endian {
	KBUFFER_ENDIAN_BIG,
	KBUFFER_ENDIAN_LITTLE,
	KBUFFER_ENDIAN_SAME_AS_HOST,
};

enum kbuffer_long_size {
	KBUFFER_LSIZE_4,
	KBUFFER_LSIZE_8,
	KBUFFER_LSIZE_SAME_AS_HOST,
};

struct kbuffer;
struct tep_handle;

struct kbuffer pass:[*]*kbuffer_alloc*(enum kbuffer_long_size _size_, enum kbuffer_endian _endian_);
struct kbuffer pass:[*]*kbuffer_dup*(struct kbuffer pass:[*]_kbuf_);
void *kbuffer_free*(struct kbuffer pass:[*]_kbuf_);
int *kbuffer_load_subbuffer*(struct kbuffer pass:[*]_kbuf_, void pass:[*]_subbuffer_);
int *kbuffer_subbuffer_size*(struct kbuffer pass:[*]_kbuf_);
int *kbuffer_refresh*(struct kbuffer pass:[*]_kbuf_);
int *kbuffer_start_of_data*(struct kbuffer pass:[*]_kbuf_);
void pass:[*]*kbuffer_subbuffer*(struct kbuffer pass:[*]_kbuf);
--

DESCRIPTION
-----------
These functions create a _kbuffer_ handle that can be used to parse the raw sub buffers
of the Linux kernel tracing ring buffer. The ring buffer is found in the tracefs
directory, and can be retrieved by *tracefs_instance_get_file(3)* at
*per_cpu/cpuX/trace_pipe_raw* where *X* is replaced by the per CPU number of
the specified ring buffer. The ring buffer inside the kernel is split up per
CPU, such that the raw ring buffer must be retrieved per CPU as well.

The *kbuffer_alloc()* will create a descriptor that can be used to manage a sub buffer
read by the ring buffer. The _size_ parameter denotes what the word size is
for the given buffer (note, this works from reading raw data from machines other
than the machine that is calling this function). The _endian_ denotes the endian
for the machine.

If _endian_ is set to _KBUFFER_ENDIAN_SAME_AS_HOST_ the endian will be set to the same
as the host endianess, which is useful when the application is reading the
ring buffer data directly from the same machine it is running on.

If _size_ is set to _KBUFFER_LSIZE_SAME_AS_HOST_, if the word size is 8, it will
set the kbuffer descriptor to long size of 8. But if the size is 4, then it
will then perform a *uname(2)* call, and if the _machine_ field has the string "64"
in it, it will be set to 8 byte long size and not 4 byte. This is because the
ring buffer long size is dependent on the kernel and not user space.

The *kbuffer_dup()* function will duplicate an existing kbuffer structure with
an allocated new one. It will have all the properties of the passed in _kbuf_,
including pointing to the same subbuffer that was loaded in the _kbuf_.
It must be freed with *kbuffer_free()*.

The *kbuffer_free()* function will free the resources created by *kbuffer_alloc()*.

The *kbuffer_load_subbuffer()* will take a _subbuffer_ which is a raw data blob
from the tracefs *trace_pipe_raw* file. The Linux tracing ring buffer is broken up
into sub buffers. Each sub buffer is as stand alone data segment that has all the
information to split out the individual events and time stamps. This sub buffer
is what kbuffer uses to walk the events.

The *kbuffer_subbuffer_size()* returns the location of the end of the last event
on the sub-buffer. It does not return the size of the sub-buffer itself.

The *kbuffer_refresh()* is to be used if more writes were done on the loaded kbuffer
where the size of the kbuffer needs to be refreshed to be able to read the new
events that were written since the last *kbuffer_load_subbuffer()* was called on it.

Note, no memory barriers are implemented with this function and any synchronization
with the writer is the responsibility of the application.

The *kbuffer_start_of_data()* function returns the offset of where the actual
data load of the sub-buffer begins.

The *kbuffer_subbuffer()* function returns the pointer to the currently loaded
subbuffer. That is, the last subbuffer that was loaded by *kbuffer_load_subbuffer()*.
If no subbuffer was loaded NULL is returned.

RETURN VALUE
------------
*kbuffer_alloc()* returns an allocated kbuffer descriptor or NULL on error.
The returned descriptor must be freed with *kbuffer_free()*

*kbuffer_load_subbuffer()* returns 0 on success and -1 on error.

*kbuffer_subbuffer_size()* returns the index on the subbuffer where the end
of the last event is located.

*kbuffer_start_of_data()* returns the offset of where the data begins on the
sub-buffer loaded in _kbuf_.

*kbuffer_subbuffer()* returns the last loaded subbuffer to _kbuf_ that was loaded
by *kbuffer_load_subbuffer()* or NULL if none was loaded.

*kbuffer_refresh()* returns 0 on success and -1 if _kbuf_ is NULL or it does not
have a subbuffer loaded via *kbuffer_load_subbuffer()*.

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 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);
	}

	printf("Kbuffer data starts at %d\n", kbuffer_start_of_data(kbuf));
	do {
		event = kbuffer_read_event(kbuf, &ts);
		if (event) {
			printf(" event %3d ts:%lld\n", i++, ts);
			event = kbuffer_next_event(kbuf, NULL);
		}
	} while (event);

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

	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/