1LIBTRACEEVENT(3) libtraceevent Manual LIBTRACEEVENT(3)
2
6 kbuffer_read_event, kbuffer_next_event, kbuffer_missed_events,
7 kbuffer_event_size, kbuffer_curr_size, kbuffer_curr_offset,
8 kbuffer_curr_index - Functions to read through the kbuffer sub buffer.
9
11 #include <kbuffer.h>
12 void *kbuffer_read_event(struct kbuffer *kbuf, unsigned long long *ts);
14 void *kbuffer_next_event(struct kbuffer *kbuf, unsigned long long *ts);
15 void *kbuffer_read_at_offset(struct kbuffer *kbuf, int offset, unsigned long long *ts);
16 int kbuffer_missed_events(struct kbuffer *kbuf);
17 int kbuffer_event_size(struct kbuffer *kbuf);
18 int kbuffer_curr_size(struct kbuffer *kbuf);
19 int kbuffer_curr_offset(struct kbuffer *kbuf);
20 int kbuffer_curr_index(struct kbuffer *kbuf);
21
23 The function kbuffer_read_event() reads the next event in the kbuf
24 descriptor and if ts is non NULL, will place its timestamp into it.
25 This does not modify the kbuf descriptor, and calling this function
26 mulitple times will return the same result.
27 The function kbuffer_next_event() will return the next event in the
29 kbuf descriptor. It will also set the ts to the timestamp of the
30 returned event. NULL is returned if there are no more events and ts
31 will be undefined. Note, if this is called directly after a
32 kbuffer_load_subbuffer() then it will likely give an unexpected result,
33 as it will return the second event and not the first event. Usually
34 this function is only used to move to the next event and to know if
35 there’s any more events to read, and kbuffer_read_event() is always
36 called first.
37 The function kbuffer_read_at_offset() returns the event located at a
39 given offset from the beginning of the sub-buffer. This offset can be
40 retrieved by kbuffer_curr_offset(). If ts points to an unsigned long
41 long, then it will be set to the event at the given offset’s timestamp.
42 If the sub-buffer had missed events before it, then
44 kbuffer_missed_events() will return the non zero. If it returns -1,
45 that means there were missed events, but the exact number of missed
46 events is unknown. If it returns a positive number, then the number of
47 missed events is the return value.
48 The kbuffer_event_size() function returns the size of the data portion
50 of the current event (the one that would be returned by
51 kbuffer_read_event().
52 The kbuffer_curr_size() function returns the entire record size of the
54 current event (the one that would be returned by kbuffer_read_event().
55 The difference here is that the return value includes the size of the
56 event record meta data that is not part of what is returned by
57 kbuffer_read_event().
58 The kbuffer_curr_offset() function returns the offset from the
60 beginning of the sub-buffer of where the current event’s meta data for
61 the record begins. The first event will not be at offset zero. This
62 offset can be used to retrieve the event with kbuffer_read_at_offset().
63 The kbuffer_curr_index() function returns the index from the beginning
65 of the data portion of the sub-buffer where the current evnet’s meta
66 data is located. The first event will likely be zero, but may not be if
67 there’s a timestamp attached to it.
68
70 kbuffer_read_event() returns the event that the kbuf descriptor is
71 currently at, or NULL if the last event was passed (by
72 kbuffer_next_event()).
73 kbuffer_next_event() returns the next event after the current event or
75 NULL if there are no more events.
76 kbuffer_read_at_offset() returns the event at a given offset from the
78 start of the sub-buffer stored in kbuf, or NULL if there exists no
79 event. Note, offset only needs to be an offset that lands on the
80 record, or is at the start of it. It does not need to be exactly at the
81 beginning of the record.
82 kbuffer_missed_events() returns 0 if there were no missed events before
84 loaded sub-buffer. Returns -1 if there were an unknown number of missed
85 events, or if the number of missed events is known, that number will be
86 returned.
87 kbuffer_event_size() returns the size of the data payload of the
89 current event of kbuf.
90 kbuffer_curr_size() returns the size of the entire record of the
92 current event of kbuf. This includes the size of the meta data for that
93 record.
94 kbuf_curr_offset() returns the offset of the current record from the
96 beginning of the kbuf sub-buffer.
97 kbuf_curr_index() returns the index of the current record from the
99 beginning of the kbuf data section.
100
102 #include <stdio.h>
103 #include <stdlib.h>
104 #include <fcntl.h>
105 #include <unistd.h>
106 #include <sys/stat.h>
107 #include <kbuffer.h>
109 int main (int argc, char **argv)
111 {
112 unsigned long long ts;
113 struct kbuffer *kbuf;
114 struct stat st;
115 char *buf;
116 void *event;
117 int save_offset = -1;
118 int record_size;
119 int offset;
120 int index;
121 int size;
122 int ret;
123 int fd;
124 int i = 0;
125 if (argc < 2) {
127 printf("usage: %s raw-subbuffer-page\n", argv[0]);
128 printf(" Try: dd count=1 bs=4096 if=/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw of=/tmp/file\n");
129 exit(0);
130 }
131 if (stat(argv[1], &st) < 0) {
133 perror("stat");
134 exit(-1);
135 }
136 buf = malloc(st.st_size);
138 if (!buf) {
139 perror("Allocating buffer");
140 exit(-1);
141 }
142 fd = open(argv[1], O_RDONLY);
144 if (fd < 0) {
145 perror(argv[1]);
146 exit(-1);
147 }
148 ret = read(fd, buf, st.st_size);
150 if (ret < 0) {
151 perror("Reading buffer");
152 exit(-1);
153 }
154 close(fd);
155 kbuf = kbuffer_alloc(KBUFFER_ENDIAN_SAME_AS_HOST,
157 KBUFFER_LSIZE_SAME_AS_HOST);
158 if (!kbuf) {
159 perror("Creating kbuffer");
160 exit(-1);
161 }
162 ret = kbuffer_load_subbuffer(kbuf, buf);
163 if (ret < 0) {
164 perror("Loading sub bufer");
165 exit(-1);
166 }
167 if (kbuffer_subbuffer_size(kbuf) > st.st_size) {
169 fprintf(stderr, "kbuffer is bigger than raw size %d > %ld\n",
170 kbuffer_subbuffer_size(kbuf), st.st_size);
171 exit(-1);
172 }
173 ret = kbuffer_missed_events(kbuf);
175 if (ret) {
176 if (ret > 0)
177 printf("Missed %d events before this buffer\n", ret);
178 else
179 printf("Missed unknown number of events before this buffer\n");
180 }
181 do {
182 event = kbuffer_read_event(kbuf, &ts);
183 if (event) {
184 record_size = kbuffer_curr_size(kbuf);
185 offset = kbuffer_curr_offset(kbuf);
186 index = kbuffer_curr_index(kbuf);
187 size = kbuffer_event_size(kbuf);
188 if (i == 20)
190 save_offset = offset;
191 printf(" event %3d ts:%lld\trecord_size:%d size:%d\tindex:%d offset:%d\n",
192 i++, ts, record_size, size, index, offset);
193 event = kbuffer_next_event(kbuf, NULL);
194 }
195 } while (event);
196 if (!event)
198 printf("Finished sub buffer\n");
199 if (save_offset > 0) {
201 event = kbuffer_read_at_offset(kbuf, save_offset, &ts);
202 if (!event) {
203 fprintf(stderr, "Funny, can't find event 20 at offset %d\n", save_offset);
204 exit(-1);
205 }
206 record_size = kbuffer_curr_size(kbuf);
207 offset = kbuffer_curr_offset(kbuf);
208 index = kbuffer_curr_index(kbuf);
209 size = kbuffer_event_size(kbuf);
210 printf("\n saved event 20 ts:%lld\trecord_size:%d size:%d\tindex:%d offset:%d\n\n",
212 ts, record_size, size, index, offset);
213 }
214 kbuffer_free(kbuf);
215 return 0;
217 }
218
220 event-parse.h
221 Header file to include in order to have access to the library APIs.
222 -ltraceevent
223 Linker switch to add when building a program that uses the library.
224
226 libtraceevent(3), trace-cmd(1)
227
229 Steven Rostedt <rostedt@goodmis.org[1]>, author of libtraceevent.
230
232 Report bugs to <linux-trace-devel@vger.kernel.org[2]>
233
235 libtraceevent is Free Software licensed under the GNU LGPL 2.1
236
238 https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/
239
241 1. rostedt@goodmis.org
242 mailto:rostedt@goodmis.org
243 2. linux-trace-devel@vger.kernel.org
245 mailto:linux-trace-devel@vger.kernel.org
246libtraceevent 1.7.2 07/20/2023 LIBTRACEEVENT(3)