Provided by: libtraceevent-doc_1.8.2-1ubuntu2_all 
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
#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);
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
#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
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
Steven Rostedt <rostedt@goodmis.org[1]>, author of libtraceevent.
REPORTING BUGS
Report bugs to <linux-trace-devel@vger.kernel.org[2]>
LICENSE
libtraceevent is Free Software licensed under the GNU LGPL 2.1
RESOURCES
https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/
NOTES
1. rostedt@goodmis.org
mailto:rostedt@goodmis.org
2. linux-trace-devel@vger.kernel.org
mailto:linux-trace-devel@vger.kernel.org