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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_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.
#include <kbuffer.h>
void *kbuffer_read_event(struct kbuffer *kbuf, unsigned long long *ts);
void *kbuffer_next_event(struct kbuffer *kbuf, unsigned long long *ts);
void *kbuffer_read_at_offset(struct kbuffer *kbuf, int offset, unsigned long long *ts);
int kbuffer_missed_events(struct kbuffer *kbuf);
int kbuffer_event_size(struct kbuffer *kbuf);
int kbuffer_curr_size(struct kbuffer *kbuf);
int kbuffer_curr_offset(struct kbuffer *kbuf);
int kbuffer_curr_index(struct kbuffer *kbuf);
int kbuffer_read_buffer(struct kbuffer *kbuf, void *buffer, int len);
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.
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.
#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;
}
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 2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2025-01-22.) 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 2025-02-02 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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