Update proposal: remove duplicate styles for array/seq
[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:
206
207 unit_type name:size:
208
209 As an example, the following structure declared in C compiled by GCC:
210
211 struct example {
212 short a:12;
213 short b:5;
214 };
215
216 The example structure is aligned on the largest element (short). The second
217 bitfield would be aligned on the next unit boundary, because it would not fit in
218 the current unit.
219
220 4.1.7 Floating point
221
222 The floating point values byte ordering is defined in the metadata.
223
224 Floating point values follow the IEEE 754-2008 standard interchange formats.
225 Description of the floating point values include the exponent and mantissa size
226 in bits. Some requirements are imposed on the floating point values:
227
228 - FLT_RADIX must be 2.
229 - mant_dig is the number of digits represented in the mantissa. It is specified
230 by the ISO C99 standard, section 5.2.4, as FLT_MANT_DIG, DBL_MANT_DIG and
231 LDBL_MANT_DIG as defined by <float.h>.
232 - exp_dig is the number of digits represented in the exponent. Given that
233 mant_dig is one bit more than its actual size in bits (leading 1 is not
234 needed) and also given that the sign bit always takes one bit, exp_dig can be
235 specified as:
236
237 - sizeof(float) * CHAR_BIT - FLT_MANT_DIG
238 - sizeof(double) * CHAR_BIT - DBL_MANT_DIG
239 - sizeof(long double) * CHAR_BIT - LDBL_MANT_DIG
240
241 Metadata representation:
242
243 floating_point {
244 exp_dig = value;
245 mant_dig = value;
246 byte_order = native OR network OR be OR le;
247 }
248
249 Example of type inheritance:
250
251 typedef floating_point {
252 exp_dig = 8; /* sizeof(float) * CHAR_BIT - FLT_MANT_DIG */
253 mant_dig = 24; /* FLT_MANT_DIG */
254 byte_order = native;
255 } float;
256
257 TODO: define NaN, +inf, -inf behavior.
258
259 4.1.8 Enumerations
260
261 Enumerations are a mapping between an integer type and a table of strings. The
262 numerical representation of the enumeration follows the integer type specified
263 by the metadata. The enumeration mapping table is detailed in the enumeration
264 description within the metadata. The mapping table maps inclusive value ranges
265 (or single values) to strings. Instead of being limited to simple
266 "value -> string" mappings, these enumerations map
267 "[ start_value ... end_value ] -> string", which map inclusive ranges of
268 values to strings. An enumeration from the C language can be represented in
269 this format by having the same start_value and end_value for each element, which
270 is in fact a range of size 1. This single-value range is supported without
271 repeating the start and end values with the value = string declaration. If the
272 <integer_type> is omitted, the type chosen by the C compiler to hold the
273 enumeration is used. The <integer_type> specifier can only be omitted for
274 enumerations containing only simple "value -> string" mappings (compatible with
275 C).
276
277 enum <integer_type> name {
278 string = start_value1 ... end_value1,
279 "other string" = start_value2 ... end_value2,
280 yet_another_string, /* will be assigned to end_value2 + 1 */
281 "some other string" = value,
282 ...
283 };
284
285 If the values are omitted, the enumeration starts at 0 and increment of 1 for
286 each entry:
287
288 enum name {
289 ZERO,
290 ONE,
291 TWO,
292 TEN = 10,
293 ELEVEN,
294 };
295
296 Overlapping ranges within a single enumeration are implementation defined.
297
298 A nameless enumeration can be declared as a field type or as part of a typedef:
299
300 enum <integer_type> {
301 ...
302 }
303
304 4.2 Compound types
305
306 4.2.1 Structures
307
308 Structures are aligned on the largest alignment required by basic types
309 contained within the structure. (This follows the ISO/C standard for structures)
310
311 Metadata representation of a named structure:
312
313 struct name {
314 field_type field_name;
315 field_type field_name;
316 ...
317 };
318
319 Example:
320
321 struct example {
322 integer { /* Nameless type */
323 size = 16;
324 signed = true;
325 align = 16;
326 } first_field_name;
327 uint64_t second_field_name; /* Named type declared in the metadata */
328 };
329
330 The fields are placed in a sequence next to each other. They each possess a
331 field name, which is a unique identifier within the structure.
332
333 A nameless structure can be declared as a field type or as part of a typedef:
334
335 struct {
336 ...
337 }
338
339 4.2.2 Arrays
340
341 Arrays are fixed-length. Their length is declared in the type declaration within
342 the metadata. They contain an array of "inner type" elements, which can refer to
343 any type not containing the type of the array being declared (no circular
344 dependency). The length is the number of elements in an array.
345
346 Metadata representation of a named array:
347
348 typedef elem_type name[length];
349
350 A nameless array can be declared as a field type within a structure, e.g.:
351
352 uint8_t field_name[10];
353
354
355 4.2.3 Sequences
356
357 Sequences are dynamically-sized arrays. They start with an integer that specify
358 the length of the sequence, followed by an array of "inner type" elements.
359 The length is the number of elements in the sequence.
360
361 Metadata representation for a named sequence:
362
363 typedef elem_type name[length_type];
364
365 A nameless sequence can be declared as a field type, e.g.:
366
367 long field_name[int];
368
369 The length type follows the integer types specifications, and the sequence
370 elements follow the "array" specifications.
371
372 4.2.4 Strings
373
374 Strings are an array of bytes of variable size and are terminated by a '\0'
375 "NULL" character. Their encoding is described in the metadata. In absence of
376 encoding attribute information, the default encoding is UTF-8.
377
378 Metadata representation of a named string type:
379
380 typedef string {
381 encoding = UTF8 OR ASCII;
382 } name;
383
384 A nameless string type can be declared as a field type:
385
386 string field_name; /* Use default UTF8 encoding */
387
388 5. Event Packet Header
389
390 The event packet header consists of two part: one is mandatory and have a fixed
391 layout. The second part, the "event packet context", has its layout described in
392 the metadata.
393
394 - Aligned on page size. Fixed size. Fields either aligned or packed (depending
395 on the architecture preference).
396 No padding at the end of the event packet header. Native architecture byte
397 ordering.
398
399 Fixed layout (event packet header):
400
401 - Magic number (CTF magic numbers: 0xC1FC1FC1 and its reverse endianness
402 representation: 0xC11FFCC1) It needs to have a non-symmetric bytewise
403 representation. Used to distinguish between big and little endian traces (this
404 information is determined by knowing the endianness of the architecture
405 reading the trace and comparing the magic number against its value and the
406 reverse, 0xC11FFCC1). This magic number specifies that we use the CTF metadata
407 description language described in this document. Different magic numbers
408 should be used for other metadata description languages.
409 - Trace UUID, used to ensure the event packet match the metadata used.
410 (note: we cannot use a metadata checksum because metadata can be appended to
411 while tracing is active)
412 - Stream ID, used as reference to stream description in metadata.
413
414 Metadata-defined layout (event packet context):
415
416 - Event packet content size (in bytes).
417 - Event packet size (in bytes, includes padding).
418 - Event packet content checksum (optional). Checksum excludes the event packet
419 header.
420 - Per-stream event packet sequence count (to deal with UDP packet loss). The
421 number of significant sequence counter bits should also be present, so
422 wrap-arounds are deal with correctly.
423 - Timestamp at the beginning and timestamp at the end of the event packet.
424 Both timestamps are written in the packet header, but sampled respectively
425 while (or before) writing the first event and while (or after) writing the
426 last event in the packet. The inclusive range between these timestamps should
427 include all event timestamps assigned to events contained within the packet.
428 - Events discarded count
429 - Snapshot of a per-stream free-running counter, counting the number of
430 events discarded that were supposed to be written in the stream prior to
431 the first event in the event packet.
432 * Note: producer-consumer buffer full condition should fill the current
433 event packet with padding so we know exactly where events have been
434 discarded.
435 - Lossless compression scheme used for the event packet content. Applied
436 directly to raw data. New types of compression can be added in following
437 versions of the format.
438 0: no compression scheme
439 1: bzip2
440 2: gzip
441 3: xz
442 - Cypher used for the event packet content. Applied after compression.
443 0: no encryption
444 1: AES
445 - Checksum scheme used for the event packet content. Applied after encryption.
446 0: no checksum
447 1: md5
448 2: sha1
449 3: crc32
450
451 5.1 Event Packet Header Fixed Layout Description
452
453 struct event_packet_header {
454 uint32_t magic;
455 uint8_t trace_uuid[16];
456 uint32_t stream_id;
457 };
458
459 5.2 Event Packet Context Description
460
461 Event packet context example. These are declared within the stream declaration
462 in the metadata. All these fields are optional except for "content_size" and
463 "packet_size", which must be present in the context.
464
465 An example event packet context type:
466
467 struct event_packet_context {
468 uint64_t timestamp_begin;
469 uint64_t timestamp_end;
470 uint32_t checksum;
471 uint32_t stream_packet_count;
472 uint32_t events_discarded;
473 uint32_t cpu_id;
474 uint32_t/uint16_t content_size;
475 uint32_t/uint16_t packet_size;
476 uint8_t stream_packet_count_bits; /* Significant counter bits */
477 uint8_t compression_scheme;
478 uint8_t encryption_scheme;
479 uint8_t checksum;
480 };
481
482 6. Event Structure
483
484 The overall structure of an event is:
485
486 - Event Header (as specifed by the stream metadata)
487 - Extended Event Header (as specified by the event header)
488 - Event Context (as specified by the stream metadata)
489 - Event Payload (as specified by the event metadata)
490
491
492 6.1 Event Header
493
494 One major factor can vary between streams: the number of event IDs assigned to
495 a stream. Luckily, this information tends to stay relatively constant (modulo
496 event registration while trace is being recorded), so we can specify different
497 representations for streams containing few event IDs and streams containing
498 many event IDs, so we end up representing the event ID and timestamp as densely
499 as possible in each case.
500
501 We therefore provide two types of events headers. Type 1 accommodates streams
502 with less than 31 event IDs. Type 2 accommodates streams with 31 or more event
503 IDs.
504
505 The "extended headers" are used in the rare occasions where the information
506 cannot be represented in the ranges available in the event header. They are also
507 used in the rare occasions where the data required for a field could not be
508 collected: the flag corresponding to the missing field within the missing_fields
509 array is then set to 1.
510
511 Types uintX_t represent an X-bit unsigned integer.
512
513
514 6.1.1 Type 1 - Few event IDs
515
516 - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
517 preference).
518 - Fixed size: 32 bits.
519 - Native architecture byte ordering.
520
521 struct event_header_1 {
522 uint5_t id; /*
523 * id: range: 0 - 30.
524 * id 31 is reserved to indicate a following
525 * extended header.
526 */
527 uint27_t timestamp;
528 };
529
530 The end of a type 1 header is aligned on a 32-bit boundary (or packed).
531
532
533 6.1.2 Extended Type 1 Event Header
534
535 - Follows struct event_header_1, which is aligned on 32-bit, so no need to
536 realign.
537 - Variable size (depends on the number of fields per event).
538 - Native architecture byte ordering.
539 - NR_FIELDS is the number of fields within the event.
540
541 struct event_header_1_ext {
542 uint32_t id; /* 32-bit event IDs */
543 uint64_t timestamp; /* 64-bit timestamps */
544 uint1_t missing_fields[NR_FIELDS]; /* missing event fields bitmap */
545 };
546
547
548 6.1.3 Type 2 - Many event IDs
549
550 - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
551 preference).
552 - Fixed size: 48 bits.
553 - Native architecture byte ordering.
554
555 struct event_header_2 {
556 uint32_t timestamp;
557 uint16_t id; /*
558 * id: range: 0 - 65534.
559 * id 65535 is reserved to indicate a following
560 * extended header.
561 */
562 };
563
564 The end of a type 2 header is aligned on a 16-bit boundary (or 8-bit if
565 byte-packed).
566
567
568 6.1.4 Extended Type 2 Event Header
569
570 - Follows struct event_header_2, which alignment end on a 16-bit boundary, so
571 we need to align on 64-bit integer architecture alignment (or 8-bit if
572 byte-packed).
573 - Variable size (depends on the number of fields per event).
574 - Native architecture byte ordering.
575 - NR_FIELDS is the number of fields within the event.
576
577 struct event_header_2_ext {
578 uint64_t timestamp; /* 64-bit timestamps */
579 uint32_t id; /* 32-bit event IDs */
580 uint1_t missing_fields[NR_FIELDS]; /* missing event fields bitmap */
581 };
582
583
584 6.2 Event Context
585
586 The event context contains information relative to the current event. The choice
587 and meaning of this information is specified by the metadata "stream"
588 information. For this trace format, event context is usually empty, except when
589 the metadata "stream" information specifies otherwise by declaring a non-empty
590 structure for the event context. An example of event context is to save the
591 event payload size with each event, or to save the current PID with each event.
592 These are declared within the stream declaration within the metadata.
593
594 An example event context type:
595
596 struct event_context {
597 uint pid;
598 uint16_t payload_size;
599 };
600
601
602 6.3 Event Payload
603
604 An event payload contains fields specific to a given event type. The fields
605 belonging to an event type are described in the event-specific metadata
606 within a structure type.
607
608 6.3.1 Padding
609
610 No padding at the end of the event payload. This differs from the ISO/C standard
611 for structures, but follows the CTF standard for structures. In a trace, even
612 though it makes sense to align the beginning of a structure, it really makes no
613 sense to add padding at the end of the structure, because structures are usually
614 not followed by a structure of the same type.
615
616 This trick can be done by adding a zero-length "end" field at the end of the C
617 structures, and by using the offset of this field rather than using sizeof()
618 when calculating the size of a structure (see Appendix "A. Helper macros").
619
620 6.3.2 Alignment
621
622 The event payload is aligned on the largest alignment required by types
623 contained within the payload. (This follows the ISO/C standard for structures)
624
625
626
627 7. Metadata
628
629 The meta-data is located in a stream named "metadata". It is made of "event
630 packets", which each start with an event packet header. The event type within
631 the metadata stream have no event header nor event context. Each event only
632 contains a null-terminated "string" payload, which is a metadata description
633 entry. The events are packed one next to another. Each event packet start with
634 an event packet header, which contains, amongst other fields, the magic number
635 and trace UUID.
636
637 The metadata can be parsed by reading through the metadata strings, skipping
638 newlines and null-characters. Type names may contain spaces.
639
640 trace {
641 major = value; /* Trace format version */
642 minor = value;
643 uuid = value; /* Trace UUID */
644 word_size = value;
645 };
646
647 stream {
648 id = stream_id;
649 event {
650 /* Type 1 - Few event IDs; Type 2 - Many event IDs. See section 6.1. */
651 header_type = event_header_1 OR event_header_2;
652 /*
653 * Extended event header type. Only present if specified in event header
654 * on a per-event basis.
655 */
656 header_type_ext = event_header_1_ext OR event_header_2_ext;
657 context_type = struct {
658 ...
659 };
660 };
661 packet {
662 context_type = struct {
663 ...
664 };
665 };
666 };
667
668 event {
669 name = event_name;
670 id = value; /* Numeric identifier within the stream */
671 stream = stream_id;
672 fields = struct {
673 ...
674 };
675 };
676
677 /* More detail on types in section 4. Types */
678
679 /*
680 * Named types:
681 *
682 * A named type can only have a prefix and postfix if it aliases a CTF basic
683 * type. A type name aliasing another type name cannot have prefix nor postfix,
684 * but the type aliased can have a prefix and/or postfix.
685 */
686
687 typedef aliased_type_prefix aliased_type new_type aliased_type_postfix;
688
689 /* e.g.: typedef struct example new_type_name[10]; */
690
691 typedef type_class {
692 ...
693 } new_type_prefix new_type new_type_postfix;
694
695 /*
696 * e.g.:
697 * typedef integer {
698 * size = 32;
699 * align = 32;
700 * signed = false;
701 * } struct page *;
702 */
703
704 struct name {
705 ...
706 };
707
708 enum <integer_type> name {
709 ...
710 };
711
712
713 /* Unnamed types, contained within compound type fields or typedef. */
714
715 struct {
716 ...
717 }
718
719 enum <integer_type> {
720 ...
721 }
722
723 typedef type new_type[length];
724
725 struct {
726 type field_name[length];
727 }
728
729 typedef type new_type[length_type];
730
731 struct {
732 type field_name[length_type];
733 }
734
735 integer {
736 ...
737 }
738
739 floating_point {
740 ...
741 }
742
743 struct {
744 integer_type field_name:size; /* GNU/C bitfield */
745 }
746
747 struct {
748 string field_name;
749 }
750
751 A. Helper macros
752
753 The two following macros keep track of the size of a GNU/C structure without
754 padding at the end by placing HEADER_END as the last field. A one byte end field
755 is used for C90 compatibility (C99 flexible arrays could be used here). Note
756 that this does not affect the effective structure size, which should always be
757 calculated with the header_sizeof() helper.
758
759 #define HEADER_END char end_field
760 #define header_sizeof(type) offsetof(typeof(type), end_field)
761
762
763 B. Stream Header Rationale
764
765 An event stream is divided in contiguous event packets of variable size. These
766 subdivisions allow the trace analyzer to perform a fast binary search by time
767 within the stream (typically requiring to index only the event packet headers)
768 without reading the whole stream. These subdivisions have a variable size to
769 eliminate the need to transfer the event packet padding when partially filled
770 event packets must be sent when streaming a trace for live viewing/analysis.
771 An event packet can contain a certain amount of padding at the end. Dividing
772 streams into event packets is also useful for network streaming over UDP and
773 flight recorder mode tracing (a whole event packet can be swapped out of the
774 buffer atomically for reading).
775
776 The stream header is repeated at the beginning of each event packet to allow
777 flexibility in terms of:
778
779 - streaming support,
780 - allowing arbitrary buffers to be discarded without making the trace
781 unreadable,
782 - allow UDP packet loss handling by either dealing with missing event packet
783 or asking for re-transmission.
784 - transparently support flight recorder mode,
785 - transparently support crash dump.
786
787 The event stream header will therefore be referred to as the "event packet
788 header" throughout the rest of this document.
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