# kbuffer_read_event

## 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.
    


## 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
    


* * *