--- name: "kbuffer_event_size" section: "3" platform: "linux" package: "libtraceevent-doc" version: "1:1.8.4-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/kbuffer_read_event" --- # kbuffer\_event\_size(3) Functions to read through the kbuffer sub buffer. Part of [kbuffer\_read\_event(3)](/kbuffer_read_event/3.md). ## 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 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 #include #include #include #include #include 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)](/libtraceevent/3.md), [**trace-cmd**(1)](/trace-cmd/1.md) ## AUTHOR ``` Steven Rostedt , 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