Update named types
[ctf.git] / common-trace-format-proposal.txt
1
2 RFC: Common Trace Format (CTF) Proposal (v1.6)
3
4 Mathieu Desnoyers, EfficiOS Inc.
5
6 The goal of the present document is to propose a trace format that suits the
7 needs of the embedded, telecom, high-performance and kernel communities. It is
8 based on the Common Trace Format Requirements (v1.4) document. It is designed to
9 allow traces to be natively generated by the Linux kernel, Linux user-space
10 applications written in C/C++, and hardware components.
11
12 The latest version of this document can be found at:
13
14 git tree: git://git.efficios.com/ctf.git
15 gitweb: http://git.efficios.com/?p=ctf.git
16
17 A reference implementation of a library to read and write this trace format is
18 being implemented within the BabelTrace project, a converter between trace
19 formats. The development tree is available at:
20
21 git tree: git://git.efficios.com/babeltrace.git
22 gitweb: http://git.efficios.com/?p=babeltrace.git
23
24
25 1. Preliminary definitions
26
27 - Event Trace: An ordered sequence of events.
28 - Event Stream: An ordered sequence of events, containing a subset of the
29 trace event types.
30 - Event Packet: A sequence of physically contiguous events within an event
31 stream.
32 - Event: This is the basic entry in a trace. (aka: a trace record).
33 - An event identifier (ID) relates to the class (a type) of event within
34 an event stream.
35 e.g. event: irq_entry.
36 - An event (or event record) relates to a specific instance of an event
37 class.
38 e.g. event: irq_entry, at time X, on CPU Y
39 - Source Architecture: Architecture writing the trace.
40 - Reader Architecture: Architecture reading the trace.
41
42
43 2. High-level representation of a trace
44
45 A trace is divided into multiple event streams. Each event stream contains a
46 subset of the trace event types.
47
48 The final output of the trace, after its generation and optional transport over
49 the network, is expected to be either on permanent or temporary storage in a
50 virtual file system. Because each event stream is appended to while a trace is
51 being recorded, each is associated with a separate file for output. Therefore,
52 a stored trace can be represented as a directory containing one file per stream.
53
54 A metadata event stream contains information on trace event types. It describes:
55
56 - Trace version.
57 - Types available.
58 - Per-stream event header description.
59 - Per-stream event header selection.
60 - Per-stream event context fields.
61 - Per-event
62 - Event type to stream mapping.
63 - Event type to name mapping.
64 - Event type to ID mapping.
65 - Event fields description.
66
67
68 3. Event stream
69
70 An event stream is divided in contiguous event packets of variable size. These
71 subdivisions have a variable size. An event packet can contain a certain amount
72 of padding at the end. The rationale for the event stream design choices is
73 explained in Appendix B. Stream Header Rationale.
74
75 An event stream is divided in contiguous event packets of variable size. These
76 subdivisions have a variable size. An event packet can contain a certain amount
77 of padding at the end. The stream header is repeated at the beginning of each
78 event packet.
79
80 The event stream header will therefore be referred to as the "event packet
81 header" throughout the rest of this document.
82
83
84 4. Types
85
86 4.1 Basic types
87
88 A basic type is a scalar type, as described in this section.
89
90 4.1.1 Type inheritance
91
92 Type specifications can be inherited to allow deriving types from a
93 type class. For example, see the uint32_t named type derived from the "integer"
94 type class below ("Integers" section). Types have a precise binary
95 representation in the trace. A type class has methods to read and write these
96 types, but must be derived into a type to be usable in an event field.
97
98 4.1.2 Alignment
99
100 We define "byte-packed" types as aligned on the byte size, namely 8-bit.
101 We define "bit-packed" types as following on the next bit, as defined by the
102 "bitfields" section.
103
104 All basic types, except bitfields, are either aligned on an architecture-defined
105 specific alignment or byte-packed, depending on the architecture preference.
106 Architectures providing fast unaligned write byte-packed basic types to save
107 space, aligning each type on byte boundaries (8-bit). Architectures with slow
108 unaligned writes align types on specific alignment values. If no specific
109 alignment is declared for a type nor its parents, it is assumed to be bit-packed
110 for bitfields and byte-packed for other types.
111
112 Metadata attribute representation of a specific alignment:
113
114 align = value; /* value in bits */
115
116 4.1.3 Byte order
117
118 By default, the native endianness of the source architecture the trace is used.
119 Byte order can be overridden for a basic type by specifying a "byte_order"
120 attribute. Typical use-case is to specify the network byte order (big endian:
121 "be") to save data captured from the network into the trace without conversion.
122 If not specified, the byte order is native.
123
124 Metadata representation:
125
126 byte_order = native OR network OR be OR le; /* network and be are aliases */
127
128 4.1.4 Size
129
130 Type size, in bits, for integers and floats is that returned by "sizeof()" in C
131 multiplied by CHAR_BIT.
132 We require the size of "char" and "unsigned char" types (CHAR_BIT) to be fixed
133 to 8 bits for cross-endianness compatibility.
134
135 Metadata representation:
136
137 size = value; (value is in bits)
138
139 4.1.5 Integers
140
141 Signed integers are represented in two-complement. Integer alignment, size,
142 signedness and byte ordering are defined in the metadata. Integers aligned on
143 byte size (8-bit) and with length multiple of byte size (8-bit) correspond to
144 the C99 standard integers. In addition, integers with alignment and/or size that
145 are _not_ a multiple of the byte size are permitted; these correspond to the C99
146 standard bitfields, with the added specification that the CTF integer bitfields
147 have a fixed binary representation. A MIT-licensed reference implementation of
148 the CTF portable bitfields is available at:
149
150 http://git.efficios.com/?p=babeltrace.git;a=blob;f=include/babeltrace/bitfield.h
151
152 Binary representation of integers:
153
154 - On little and big endian:
155 - Within a byte, high bits correspond to an integer high bits, and low bits
156 correspond to low bits.
157 - On little endian:
158 - Integer across multiple bytes are placed from the less significant to the
159 most significant.
160 - Consecutive integers are placed from lower bits to higher bits (even within
161 a byte).
162 - On big endian:
163 - Integer across multiple bytes are placed from the most significant to the
164 less significant.
165 - Consecutive integers are placed from higher bits to lower bits (even within
166 a byte).
167
168 This binary representation is derived from the bitfield implementation in GCC
169 for little and big endian. However, contrary to what GCC does, integers can
170 cross units boundaries (no padding is required). Padding can be explicitely
171 added (see 4.1.6 GNU/C bitfields) to follow the GCC layout if needed.
172
173 Metadata representation:
174
175 integer {
176 signed = true OR false; /* default false */
177 byte_order = native OR network OR be OR le; /* default native */
178 size = value; /* value in bits, no default */
179 align = value; /* value in bits */
180 };
181
182 Example of type inheritance (creation of a uint32_t named type):
183
184 typedef integer {
185 size = 32;
186 signed = false;
187 align = 32;
188 } uint32_t;
189
190 Definition of a named 5-bit signed bitfield:
191
192 typedef integer {
193 size = 5;
194 signed = true;
195 align = 1;
196 } int5_t;
197
198 4.1.6 GNU/C bitfields
199
200 The GNU/C bitfields follow closely the integer representation, with a
201 particularity on alignment: if a bitfield cannot fit in the current unit, the
202 unit is padded and the bitfield starts at the following unit. The unit size is
203 defined by the size of the type "unit_type".
204
205 Metadata representation. Either:
206
207 gcc_bitfield {
208 unit_type = integer {
209 ...
210 };
211 size = value;
212 };
213
214 Or bitfield within structures as specified by the C standard
215
216 unit_type name:size:
217
218 As an example, the following structure declared in C compiled by GCC:
219
220 struct example {
221 short a:12;
222 short b:5;
223 };
224
225 is equivalent to the following structure declaration, aligned on the largest
226 element (short). The second bitfield would be aligned on the next unit boundary,
227 because it would not fit in the current unit. The two declarations (C
228 declaration above or CTF declaration with "type gcc_bitfield") are strictly
229 equivalent.
230
231 struct example {
232 gcc_bitfield {
233 unit_type = short;
234 size = 12;
235 } a;
236 gcc_bitfield {
237 unit_type = short;
238 size = 5;
239 } b;
240 };
241
242 4.1.7 Floating point
243
244 The floating point values byte ordering is defined in the metadata.
245
246 Floating point values follow the IEEE 754-2008 standard interchange formats.
247 Description of the floating point values include the exponent and mantissa size
248 in bits. Some requirements are imposed on the floating point values:
249
250 - FLT_RADIX must be 2.
251 - mant_dig is the number of digits represented in the mantissa. It is specified
252 by the ISO C99 standard, section 5.2.4, as FLT_MANT_DIG, DBL_MANT_DIG and
253 LDBL_MANT_DIG as defined by <float.h>.
254 - exp_dig is the number of digits represented in the exponent. Given that
255 mant_dig is one bit more than its actual size in bits (leading 1 is not
256 needed) and also given that the sign bit always takes one bit, exp_dig can be
257 specified as:
258
259 - sizeof(float) * CHAR_BIT - FLT_MANT_DIG
260 - sizeof(double) * CHAR_BIT - DBL_MANT_DIG
261 - sizeof(long double) * CHAR_BIT - LDBL_MANT_DIG
262
263 Metadata representation:
264
265 floating_point {
266 exp_dig = value;
267 mant_dig = value;
268 byte_order = native OR network OR be OR le;
269 };
270
271 Example of type inheritance:
272
273 typedef floating_point {
274 exp_dig = 8; /* sizeof(float) * CHAR_BIT - FLT_MANT_DIG */
275 mant_dig = 24; /* FLT_MANT_DIG */
276 byte_order = native;
277 } float;
278
279 TODO: define NaN, +inf, -inf behavior.
280
281 4.1.8 Enumerations
282
283 Enumerations are a mapping between an integer type and a table of strings. The
284 numerical representation of the enumeration follows the integer type specified
285 by the metadata. The enumeration mapping table is detailed in the enumeration
286 description within the metadata. The mapping table maps inclusive value ranges
287 (or single values) to strings. Instead of being limited to simple
288 "value -> string" mappings, these enumerations map
289 "[ start_value ... end_value ] -> string", which map inclusive ranges of
290 values to strings. An enumeration from the C language can be represented in
291 this format by having the same start_value and end_value for each element, which
292 is in fact a range of size 1. This single-value range is supported without
293 repeating the start and end values with the value = string declaration. If the
294 <integer_type> is omitted, the type chosen by the C compiler to hold the
295 enumeration is used. The <integer_type> specifier can only be omitted for
296 enumerations containing only simple "value -> string" mappings (compatible with
297 C).
298
299 enum <integer_type> name {
300 string = start_value1 ... end_value1,
301 "other string" = start_value2 ... end_value2,
302 yet_another_string, /* will be assigned to end_value2 + 1 */
303 "some other string" = value,
304 ...
305 };
306
307 If the values are omitted, the enumeration starts at 0 and increment of 1 for
308 each entry:
309
310 enum {
311 ZERO,
312 ONE,
313 TWO,
314 TEN = 10,
315 ELEVEN,
316 };
317
318 Overlapping ranges within a single enumeration are implementation defined.
319
320 4.2 Compound types
321
322 4.2.1 Structures
323
324 Structures are aligned on the largest alignment required by basic types
325 contained within the structure. (This follows the ISO/C standard for structures)
326
327 Metadata representation of a named structure:
328
329 struct name {
330 field_type field_name;
331 field_type field_name;
332 ...
333 };
334
335 Example:
336
337 struct example {
338 integer { /* Nameless type */
339 size = 16;
340 signed = true;
341 align = 16;
342 } first_field_name;
343 uint64_t second_field_name; /* Named type declared in the metadata */
344 };
345
346 The fields are placed in a sequence next to each other. They each possess a
347 field name, which is a unique identifier within the structure.
348
349 A nameless structure can be declared as a field type:
350
351 struct {
352 ...
353 } field_name;
354
355 4.2.2 Arrays
356
357 Arrays are fixed-length. Their length is declared in the type declaration within
358 the metadata. They contain an array of "inner type" elements, which can refer to
359 any type not containing the type of the array being declared (no circular
360 dependency). The length is the number of elements in an array.
361
362 Metadata representation of a named array, either:
363
364 typedef array {
365 length = value;
366 elem_type = type;
367 } name;
368
369 or:
370
371 typedef elem_type name[length];
372
373 E.g.:
374
375 typedef array {
376 length = 10;
377 elem_type = uint32_t;
378 } example;
379
380 A nameless array can be declared as a field type, e.g.:
381
382 array {
383 length = 5;
384 elem_type = uint8_t;
385 } field_name;
386
387 or
388
389 uint8_t field_name[10];
390
391
392 4.2.3 Sequences
393
394 Sequences are dynamically-sized arrays. They start with an integer that specify
395 the length of the sequence, followed by an array of "inner type" elements.
396 The length is the number of elements in the sequence.
397
398 Metadata representation for a named sequence, either:
399
400 typedef sequence {
401 length_type = type; /* integer class */
402 elem_type = type;
403 } name;
404
405 or:
406
407 typedef elem_type name[length_type];
408
409 A nameless sequence can be declared as a field type, e.g.:
410
411 sequence {
412 length_type = int;
413 elem_type = long;
414 } field_name;
415
416 or
417
418 long field_name[int];
419
420 The length type follows the integer types specifications, and the sequence
421 elements follow the "array" specifications.
422
423 4.2.4 Strings
424
425 Strings are an array of bytes of variable size and are terminated by a '\0'
426 "NULL" character. Their encoding is described in the metadata. In absence of
427 encoding attribute information, the default encoding is UTF-8.
428
429 Metadata representation of a named string type:
430
431 typedef string {
432 encoding = UTF8 OR ASCII;
433 } name;
434
435 A nameless string type can be declared as a field type:
436
437 string field_name; /* Use default UTF8 encoding */
438
439 5. Event Packet Header
440
441 The event packet header consists of two part: one is mandatory and have a fixed
442 layout. The second part, the "event packet context", has its layout described in
443 the metadata.
444
445 - Aligned on page size. Fixed size. Fields either aligned or packed (depending
446 on the architecture preference).
447 No padding at the end of the event packet header. Native architecture byte
448 ordering.
449
450 Fixed layout (event packet header):
451
452 - Magic number (CTF magic numbers: 0xC1FC1FC1 and its reverse endianness
453 representation: 0xC11FFCC1) It needs to have a non-symmetric bytewise
454 representation. Used to distinguish between big and little endian traces (this
455 information is determined by knowing the endianness of the architecture
456 reading the trace and comparing the magic number against its value and the
457 reverse, 0xC11FFCC1). This magic number specifies that we use the CTF metadata
458 description language described in this document. Different magic numbers
459 should be used for other metadata description languages.
460 - Trace UUID, used to ensure the event packet match the metadata used.
461 (note: we cannot use a metadata checksum because metadata can be appended to
462 while tracing is active)
463 - Stream ID, used as reference to stream description in metadata.
464
465 Metadata-defined layout (event packet context):
466
467 - Event packet content size (in bytes).
468 - Event packet size (in bytes, includes padding).
469 - Event packet content checksum (optional). Checksum excludes the event packet
470 header.
471 - Per-stream event packet sequence count (to deal with UDP packet loss). The
472 number of significant sequence counter bits should also be present, so
473 wrap-arounds are deal with correctly.
474 - Timestamp at the beginning and timestamp at the end of the event packet.
475 Both timestamps are written in the packet header, but sampled respectively
476 while (or before) writing the first event and while (or after) writing the
477 last event in the packet. The inclusive range between these timestamps should
478 include all event timestamps assigned to events contained within the packet.
479 - Events discarded count
480 - Snapshot of a per-stream free-running counter, counting the number of
481 events discarded that were supposed to be written in the stream prior to
482 the first event in the event packet.
483 * Note: producer-consumer buffer full condition should fill the current
484 event packet with padding so we know exactly where events have been
485 discarded.
486 - Lossless compression scheme used for the event packet content. Applied
487 directly to raw data. New types of compression can be added in following
488 versions of the format.
489 0: no compression scheme
490 1: bzip2
491 2: gzip
492 3: xz
493 - Cypher used for the event packet content. Applied after compression.
494 0: no encryption
495 1: AES
496 - Checksum scheme used for the event packet content. Applied after encryption.
497 0: no checksum
498 1: md5
499 2: sha1
500 3: crc32
501
502 5.1 Event Packet Header Fixed Layout Description
503
504 struct event_packet_header {
505 uint32_t magic;
506 uint8_t trace_uuid[16];
507 uint32_t stream_id;
508 };
509
510 5.2 Event Packet Context Description
511
512 Event packet context example. These are declared within the stream declaration
513 in the metadata. All these fields are optional except for "content_size" and
514 "packet_size", which must be present in the context.
515
516 An example event packet context type:
517
518 struct event_packet_context {
519 uint64_t timestamp_begin;
520 uint64_t timestamp_end;
521 uint32_t checksum;
522 uint32_t stream_packet_count;
523 uint32_t events_discarded;
524 uint32_t cpu_id;
525 uint32_t/uint16_t content_size;
526 uint32_t/uint16_t packet_size;
527 uint8_t stream_packet_count_bits; /* Significant counter bits */
528 uint8_t compression_scheme;
529 uint8_t encryption_scheme;
530 uint8_t checksum;
531 };
532
533 6. Event Structure
534
535 The overall structure of an event is:
536
537 - Event Header (as specifed by the stream metadata)
538 - Extended Event Header (as specified by the event header)
539 - Event Context (as specified by the stream metadata)
540 - Event Payload (as specified by the event metadata)
541
542
543 6.1 Event Header
544
545 One major factor can vary between streams: the number of event IDs assigned to
546 a stream. Luckily, this information tends to stay relatively constant (modulo
547 event registration while trace is being recorded), so we can specify different
548 representations for streams containing few event IDs and streams containing
549 many event IDs, so we end up representing the event ID and timestamp as densely
550 as possible in each case.
551
552 We therefore provide two types of events headers. Type 1 accommodates streams
553 with less than 31 event IDs. Type 2 accommodates streams with 31 or more event
554 IDs.
555
556 The "extended headers" are used in the rare occasions where the information
557 cannot be represented in the ranges available in the event header. They are also
558 used in the rare occasions where the data required for a field could not be
559 collected: the flag corresponding to the missing field within the missing_fields
560 array is then set to 1.
561
562 Types uintX_t represent an X-bit unsigned integer.
563
564
565 6.1.1 Type 1 - Few event IDs
566
567 - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
568 preference).
569 - Fixed size: 32 bits.
570 - Native architecture byte ordering.
571
572 struct event_header_1 {
573 uint5_t id; /*
574 * id: range: 0 - 30.
575 * id 31 is reserved to indicate a following
576 * extended header.
577 */
578 uint27_t timestamp;
579 };
580
581 The end of a type 1 header is aligned on a 32-bit boundary (or packed).
582
583
584 6.1.2 Extended Type 1 Event Header
585
586 - Follows struct event_header_1, which is aligned on 32-bit, so no need to
587 realign.
588 - Variable size (depends on the number of fields per event).
589 - Native architecture byte ordering.
590 - NR_FIELDS is the number of fields within the event.
591
592 struct event_header_1_ext {
593 uint32_t id; /* 32-bit event IDs */
594 uint64_t timestamp; /* 64-bit timestamps */
595 uint1_t missing_fields[NR_FIELDS]; /* missing event fields bitmap */
596 };
597
598
599 6.1.3 Type 2 - Many event IDs
600
601 - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
602 preference).
603 - Fixed size: 48 bits.
604 - Native architecture byte ordering.
605
606 struct event_header_2 {
607 uint32_t timestamp;
608 uint16_t id; /*
609 * id: range: 0 - 65534.
610 * id 65535 is reserved to indicate a following
611 * extended header.
612 */
613 };
614
615 The end of a type 2 header is aligned on a 16-bit boundary (or 8-bit if
616 byte-packed).
617
618
619 6.1.4 Extended Type 2 Event Header
620
621 - Follows struct event_header_2, which alignment end on a 16-bit boundary, so
622 we need to align on 64-bit integer architecture alignment (or 8-bit if
623 byte-packed).
624 - Variable size (depends on the number of fields per event).
625 - Native architecture byte ordering.
626 - NR_FIELDS is the number of fields within the event.
627
628 struct event_header_2_ext {
629 uint64_t timestamp; /* 64-bit timestamps */
630 uint32_t id; /* 32-bit event IDs */
631 uint1_t missing_fields[NR_FIELDS]; /* missing event fields bitmap */
632 };
633
634
635 6.2 Event Context
636
637 The event context contains information relative to the current event. The choice
638 and meaning of this information is specified by the metadata "stream"
639 information. For this trace format, event context is usually empty, except when
640 the metadata "stream" information specifies otherwise by declaring a non-empty
641 structure for the event context. An example of event context is to save the
642 event payload size with each event, or to save the current PID with each event.
643 These are declared within the stream declaration within the metadata.
644
645 An example event context type:
646
647 struct event_context {
648 uint pid;
649 uint16_t payload_size;
650 };
651
652
653 6.3 Event Payload
654
655 An event payload contains fields specific to a given event type. The fields
656 belonging to an event type are described in the event-specific metadata
657 within a structure type.
658
659 6.3.1 Padding
660
661 No padding at the end of the event payload. This differs from the ISO/C standard
662 for structures, but follows the CTF standard for structures. In a trace, even
663 though it makes sense to align the beginning of a structure, it really makes no
664 sense to add padding at the end of the structure, because structures are usually
665 not followed by a structure of the same type.
666
667 This trick can be done by adding a zero-length "end" field at the end of the C
668 structures, and by using the offset of this field rather than using sizeof()
669 when calculating the size of a structure (see Appendix "A. Helper macros").
670
671 6.3.2 Alignment
672
673 The event payload is aligned on the largest alignment required by types
674 contained within the payload. (This follows the ISO/C standard for structures)
675
676
677
678 7. Metadata
679
680 The meta-data is located in a stream named "metadata". It is made of "event
681 packets", which each start with an event packet header. The event type within
682 the metadata stream have no event header nor event context. Each event only
683 contains a null-terminated "string" payload, which is a metadata description
684 entry. The events are packed one next to another. Each event packet start with
685 an event packet header, which contains, amongst other fields, the magic number
686 and trace UUID.
687
688 The metadata can be parsed by reading through the metadata strings, skipping
689 newlines and null-characters. Type names may contain spaces.
690
691 trace {
692 major = value; /* Trace format version */
693 minor = value;
694 uuid = value; /* Trace UUID */
695 word_size = value;
696 };
697
698 stream {
699 id = stream_id;
700 event {
701 /* Type 1 - Few event IDs; Type 2 - Many event IDs. See section 6.1. */
702 header_type = event_header_1 OR event_header_2;
703 /*
704 * Extended event header type. Only present if specified in event header
705 * on a per-event basis.
706 */
707 header_type_ext = event_header_1_ext OR event_header_2_ext;
708 context_type = struct {
709 ...
710 };
711 };
712 packet {
713 context_type = struct {
714 ...
715 };
716 };
717 };
718
719 event {
720 name = event_name;
721 id = value; /* Numeric identifier within the stream */
722 stream = stream_id;
723 fields = struct {
724 ...
725 };
726 };
727
728 /* More detail on types in section 4. Types */
729
730 /*
731 * Named types:
732 *
733 * A named type can only have a prefix and postfix if it aliases a CTF basic
734 * type. A type name aliasing another type name cannot have prefix nor postfix,
735 * but the type aliased can have a prefix and/or postfix.
736 */
737
738 typedef aliased_type_prefix aliased_type new_type aliased_type_postfix;
739 /* e.g.: typedef struct example new_type_name[10]; */
740
741 typedef type_class {
742 ...
743 } new_type_prefix new_type new_type_postfix;
744 /*
745 * e.g.:
746 * typedef integer {
747 * size = 32;
748 * align = 32;
749 * signed = false;
750 * } struct page *;
751 */
752
753 struct name {
754 ...
755 };
756
757 enum <integer_type> name {
758 ...
759 };
760
761 /* Unnamed types, contained within compound type fields or type assignments. */
762 struct {
763 ...
764 };
765
766 enum <integer_type> {
767 ...
768 };
769
770 array {
771 ...
772 };
773
774 sequence {
775 ...
776 };
777
778 A. Helper macros
779
780 The two following macros keep track of the size of a GNU/C structure without
781 padding at the end by placing HEADER_END as the last field. A one byte end field
782 is used for C90 compatibility (C99 flexible arrays could be used here). Note
783 that this does not affect the effective structure size, which should always be
784 calculated with the header_sizeof() helper.
785
786 #define HEADER_END char end_field
787 #define header_sizeof(type) offsetof(typeof(type), end_field)
788
789
790 B. Stream Header Rationale
791
792 An event stream is divided in contiguous event packets of variable size. These
793 subdivisions allow the trace analyzer to perform a fast binary search by time
794 within the stream (typically requiring to index only the event packet headers)
795 without reading the whole stream. These subdivisions have a variable size to
796 eliminate the need to transfer the event packet padding when partially filled
797 event packets must be sent when streaming a trace for live viewing/analysis.
798 An event packet can contain a certain amount of padding at the end. Dividing
799 streams into event packets is also useful for network streaming over UDP and
800 flight recorder mode tracing (a whole event packet can be swapped out of the
801 buffer atomically for reading).
802
803 The stream header is repeated at the beginning of each event packet to allow
804 flexibility in terms of:
805
806 - streaming support,
807 - allowing arbitrary buffers to be discarded without making the trace
808 unreadable,
809 - allow UDP packet loss handling by either dealing with missing event packet
810 or asking for re-transmission.
811 - transparently support flight recorder mode,
812 - transparently support crash dump.
813
814 The event stream header will therefore be referred to as the "event packet
815 header" throughout the rest of this document.
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