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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | EXAMPLE | FILES | SEE ALSO | AUTHOR | REPORTING BUGS | LICENSE | RESOURCES | NOTES | COLOPHON |
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LIBTRACEEVENT(3) libtraceevent Manual LIBTRACEEVENT(3)
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
#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 *kbuffer_alloc(enum kbuffer_long_size size, enum kbuffer_endian endian);
struct kbuffer *kbuffer_dup(struct kbuffer *kbuf);
void kbuffer_free(struct kbuffer *kbuf);
int kbuffer_load_subbuffer(struct kbuffer *kbuf, void *subbuffer);
int kbuffer_subbuffer_size(struct kbuffer *kbuf);
int kbuffer_refresh(struct kbuffer *kbuf);
int kbuffer_start_of_data(struct kbuffer *kbuf);
void *kbuffer_subbuffer(struct kbuffer *_kbuf);
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.
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().
#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;
}
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.
libtraceevent(3), trace-cmd(1)
Steven Rostedt <rostedt@goodmis.org[1]>, author of libtraceevent.
Report bugs to <linux-trace-devel@vger.kernel.org[2]>
libtraceevent is Free Software licensed under the GNU LGPL 2.1
https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/
1. rostedt@goodmis.org
mailto:rostedt@goodmis.org
2. linux-trace-devel@vger.kernel.org
mailto:linux-trace-devel@vger.kernel.org
This page is part of the libtraceevent (Linux kernel trace event
library) project. Information about the project can be found at
⟨https://www.trace-cmd.org/⟩. If you have a bug report for this
manual page, see ⟨https://www.trace-cmd.org/⟩. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git⟩
on 2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-05-30.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
libtraceevent 1.8.2 06/07/2024 LIBTRACEEVENT(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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