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[ctf.git] / common-trace-format-proposal.txt
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5ba9f198 1
dce2dd9a 2RFC: Common Trace Format (CTF) Proposal (v1.6)
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3
4Mathieu Desnoyers, EfficiOS Inc.
5
6The goal of the present document is to propose a trace format that suits the
cc089c3a 7needs of the embedded, telecom, high-performance and kernel communities. It is
5ba9f198 8based on the Common Trace Format Requirements (v1.4) document. It is designed to
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9allow traces to be natively generated by the Linux kernel, Linux user-space
10applications written in C/C++, and hardware components.
11
12The 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
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16
17A reference implementation of a library to read and write this trace format is
18being implemented within the BabelTrace project, a converter between trace
19formats. 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
251. Preliminary definitions
26
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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.
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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
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34 an event stream.
35 e.g. event: irq_entry.
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36 - An event (or event record) relates to a specific instance of an event
37 class.
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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.
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41
42
432. High-level representation of a trace
44
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45A trace is divided into multiple event streams. Each event stream contains a
46subset of the trace event types.
5ba9f198 47
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48The final output of the trace, after its generation and optional transport over
49the network, is expected to be either on permanent or temporary storage in a
50virtual file system. Because each event stream is appended to while a trace is
51being recorded, each is associated with a separate file for output. Therefore,
52a stored trace can be represented as a directory containing one file per stream.
5ba9f198 53
3bf79539 54A metadata event stream contains information on trace event types. It describes:
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55
56- Trace version.
57- Types available.
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58- Per-stream event header description.
59- Per-stream event header selection.
60- Per-stream event context fields.
5ba9f198 61- Per-event
3bf79539 62 - Event type to stream mapping.
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63 - Event type to name mapping.
64 - Event type to ID mapping.
65 - Event fields description.
66
67
3bf79539 683. Event stream
5ba9f198 69
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70An event stream is divided in contiguous event packets of variable size. These
71subdivisions have a variable size. An event packet can contain a certain amount
72of padding at the end. The rationale for the event stream design choices is
73explained in Appendix B. Stream Header Rationale.
5ba9f198 74
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75An event stream is divided in contiguous event packets of variable size. These
76subdivisions have a variable size. An event packet can contain a certain amount
77of padding at the end. The stream header is repeated at the beginning of each
78event packet.
5ba9f198 79
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80The event stream header will therefore be referred to as the "event packet
81header" throughout the rest of this document.
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82
83
844. Types
85
864.1 Basic types
87
88A basic type is a scalar type, as described in this section.
89
904.1.1 Type inheritance
91
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92Type specifications can be inherited to allow deriving types from a
93type class. For example, see the uint32_t named type derived from the "integer"
94type class below ("Integers" section). Types have a precise binary
95representation in the trace. A type class has methods to read and write these
96types, but must be derived into a type to be usable in an event field.
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97
984.1.2 Alignment
99
100We define "byte-packed" types as aligned on the byte size, namely 8-bit.
101We define "bit-packed" types as following on the next bit, as defined by the
102"bitfields" section.
5ba9f198 103
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104All basic types, except bitfields, are either aligned on an architecture-defined
105specific alignment or byte-packed, depending on the architecture preference.
106Architectures providing fast unaligned write byte-packed basic types to save
5ba9f198 107space, aligning each type on byte boundaries (8-bit). Architectures with slow
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108unaligned writes align types on specific alignment values. If no specific
109alignment is declared for a type nor its parents, it is assumed to be bit-packed
110for bitfields and byte-packed for other types.
5ba9f198 111
3bf79539 112Metadata attribute representation of a specific alignment:
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113
114 align = value; /* value in bits */
115
1164.1.3 Byte order
117
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118By default, the native endianness of the source architecture the trace is used.
119Byte order can be overridden for a basic type by specifying a "byte_order"
120attribute. 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.
122If not specified, the byte order is native.
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123
124Metadata representation:
125
126 byte_order = native OR network OR be OR le; /* network and be are aliases */
127
1284.1.4 Size
129
130Type size, in bits, for integers and floats is that returned by "sizeof()" in C
131multiplied by CHAR_BIT.
132We require the size of "char" and "unsigned char" types (CHAR_BIT) to be fixed
133to 8 bits for cross-endianness compatibility.
134
135Metadata representation:
136
137 size = value; (value is in bits)
138
1394.1.5 Integers
140
141Signed integers are represented in two-complement. Integer alignment, size,
142signedness and byte ordering are defined in the metadata. Integers aligned on
143byte size (8-bit) and with length multiple of byte size (8-bit) correspond to
144the C99 standard integers. In addition, integers with alignment and/or size that
145are _not_ a multiple of the byte size are permitted; these correspond to the C99
146standard bitfields, with the added specification that the CTF integer bitfields
147have a fixed binary representation. A MIT-licensed reference implementation of
148the CTF portable bitfields is available at:
149
150 http://git.efficios.com/?p=babeltrace.git;a=blob;f=include/babeltrace/bitfield.h
151
152Binary 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
168This binary representation is derived from the bitfield implementation in GCC
169for little and big endian. However, contrary to what GCC does, integers can
170cross units boundaries (no padding is required). Padding can be explicitely
171added (see 4.1.6 GNU/C bitfields) to follow the GCC layout if needed.
172
173Metadata representation:
174
80fd2569 175 integer {
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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 */
3bf79539 180 };
5ba9f198 181
80fd2569 182Example of type inheritance (creation of a uint32_t named type):
5ba9f198 183
80fd2569 184typedef integer {
9e4e34e9 185 size = 32;
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186 signed = false;
187 align = 32;
80fd2569 188} uint32_t;
5ba9f198 189
80fd2569 190Definition of a named 5-bit signed bitfield:
5ba9f198 191
80fd2569 192typedef integer {
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193 size = 5;
194 signed = true;
195 align = 1;
80fd2569 196} int5_t;
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197
1984.1.6 GNU/C bitfields
199
200The GNU/C bitfields follow closely the integer representation, with a
201particularity on alignment: if a bitfield cannot fit in the current unit, the
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202unit is padded and the bitfield starts at the following unit. The unit size is
203defined by the size of the type "unit_type".
5ba9f198 204
80fd2569 205Metadata representation. Either:
5ba9f198 206
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207gcc_bitfield {
208 unit_type = integer {
209 ...
210 };
211 size = value;
3bf79539 212};
5ba9f198 213
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214Or bitfield within structures as specified by the C standard
215
216 unit_type name:size:
217
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218As an example, the following structure declared in C compiled by GCC:
219
220struct example {
221 short a:12;
222 short b:5;
223};
224
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225is equivalent to the following structure declaration, aligned on the largest
226element (short). The second bitfield would be aligned on the next unit boundary,
227because it would not fit in the current unit. The two declarations (C
228declaration above or CTF declaration with "type gcc_bitfield") are strictly
229equivalent.
230
231struct 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;
3bf79539 240};
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241
2424.1.7 Floating point
243
244The floating point values byte ordering is defined in the metadata.
245
246Floating point values follow the IEEE 754-2008 standard interchange formats.
247Description of the floating point values include the exponent and mantissa size
248in 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
263Metadata representation:
264
80fd2569 265floating_point {
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266 exp_dig = value;
267 mant_dig = value;
268 byte_order = native OR network OR be OR le;
3bf79539 269};
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270
271Example of type inheritance:
272
80fd2569 273typedef floating_point {
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274 exp_dig = 8; /* sizeof(float) * CHAR_BIT - FLT_MANT_DIG */
275 mant_dig = 24; /* FLT_MANT_DIG */
276 byte_order = native;
80fd2569 277} float;
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278
279TODO: define NaN, +inf, -inf behavior.
280
2814.1.8 Enumerations
282
283Enumerations are a mapping between an integer type and a table of strings. The
284numerical representation of the enumeration follows the integer type specified
285by the metadata. The enumeration mapping table is detailed in the enumeration
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286description 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
80fd2569 289"[ start_value ... end_value ] -> string", which map inclusive ranges of
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290values to strings. An enumeration from the C language can be represented in
291this format by having the same start_value and end_value for each element, which
292is in fact a range of size 1. This single-value range is supported without
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293repeating 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
295enumeration is used. The <integer_type> specifier can only be omitted for
296enumerations containing only simple "value -> string" mappings (compatible with
297C).
298
299enum <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
307If the values are omitted, the enumeration starts at 0 and increment of 1 for
308each entry:
309
310enum {
311 ZERO,
312 ONE,
313 TWO,
314 TEN = 10,
315 ELEVEN,
3bf79539 316};
5ba9f198 317
80fd2569 318Overlapping ranges within a single enumeration are implementation defined.
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319
3204.2 Compound types
321
3224.2.1 Structures
323
324Structures are aligned on the largest alignment required by basic types
325contained within the structure. (This follows the ISO/C standard for structures)
326
80fd2569 327Metadata representation of a named structure:
5ba9f198 328
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329struct name {
330 field_type field_name;
331 field_type field_name;
332 ...
333};
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334
335Example:
336
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337struct 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 */
3bf79539 344};
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345
346The fields are placed in a sequence next to each other. They each possess a
347field name, which is a unique identifier within the structure.
348
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349A nameless structure can be declared as a field type:
350
351struct {
352 ...
353} field_name;
354
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3554.2.2 Arrays
356
357Arrays are fixed-length. Their length is declared in the type declaration within
358the metadata. They contain an array of "inner type" elements, which can refer to
359any type not containing the type of the array being declared (no circular
3bf79539 360dependency). The length is the number of elements in an array.
5ba9f198 361
80fd2569 362Metadata representation of a named array, either:
5ba9f198 363
80fd2569 364typedef array {
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365 length = value;
366 elem_type = type;
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367} name;
368
369or:
370
371typedef elem_type name[length];
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372
373E.g.:
374
80fd2569 375typedef array {
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376 length = 10;
377 elem_type = uint32_t;
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378} example;
379
380A nameless array can be declared as a field type, e.g.:
381
382array {
383 length = 5;
384 elem_type = uint8_t;
385} field_name;
386
387or
388
389uint8_t field_name[10];
390
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391
3924.2.3 Sequences
393
394Sequences are dynamically-sized arrays. They start with an integer that specify
395the length of the sequence, followed by an array of "inner type" elements.
3bf79539 396The length is the number of elements in the sequence.
5ba9f198 397
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398Metadata representation for a named sequence, either:
399
400typedef sequence {
401 length_type = type; /* integer class */
5ba9f198 402 elem_type = type;
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403} name;
404
405or:
406
407typedef elem_type name[length_type];
408
409A nameless sequence can be declared as a field type, e.g.:
410
411sequence {
412 length_type = int;
413 elem_type = long;
414} field_name;
415
416or
5ba9f198 417
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418long field_name[int];
419
420The length type follows the integer types specifications, and the sequence
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421elements follow the "array" specifications.
422
4234.2.4 Strings
424
425Strings 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
427encoding attribute information, the default encoding is UTF-8.
428
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429Metadata representation of a named string type:
430
431typedef string {
5ba9f198 432 encoding = UTF8 OR ASCII;
80fd2569 433} name;
5ba9f198 434
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435A nameless string type can be declared as a field type:
436
437string field_name; /* Use default UTF8 encoding */
5ba9f198 438
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4395. Event Packet Header
440
441The event packet header consists of two part: one is mandatory and have a fixed
442layout. The second part, the "event packet context", has its layout described in
443the metadata.
5ba9f198 444
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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
5ba9f198 448 ordering.
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449
450Fixed layout (event packet header):
451
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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.
3bf79539 460- Trace UUID, used to ensure the event packet match the metadata used.
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461 (note: we cannot use a metadata checksum because metadata can be appended to
462 while tracing is active)
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463- Stream ID, used as reference to stream description in metadata.
464
465Metadata-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.
5ba9f198 479- Events discarded count
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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.
5ba9f198 483 * Note: producer-consumer buffer full condition should fill the current
3bf79539 484 event packet with padding so we know exactly where events have been
5ba9f198 485 discarded.
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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.
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489 0: no compression scheme
490 1: bzip2
491 2: gzip
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492 3: xz
493- Cypher used for the event packet content. Applied after compression.
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494 0: no encryption
495 1: AES
3bf79539 496- Checksum scheme used for the event packet content. Applied after encryption.
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497 0: no checksum
498 1: md5
499 2: sha1
500 3: crc32
501
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5025.1 Event Packet Header Fixed Layout Description
503
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504struct event_packet_header {
505 uint32_t magic;
506 uint8_t trace_uuid[16];
3bf79539 507 uint32_t stream_id;
80fd2569 508};
5ba9f198 509
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5105.2 Event Packet Context Description
511
512Event packet context example. These are declared within the stream declaration
513in the metadata. All these fields are optional except for "content_size" and
514"packet_size", which must be present in the context.
515
516An example event packet context type:
517
80fd2569 518struct event_packet_context {
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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};
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532
5336. Event Structure
534
535The overall structure of an event is:
536
3bf79539 537 - Event Header (as specifed by the stream metadata)
5ba9f198 538 - Extended Event Header (as specified by the event header)
3bf79539 539 - Event Context (as specified by the stream metadata)
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540 - Event Payload (as specified by the event metadata)
541
542
5436.1 Event Header
544
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545One major factor can vary between streams: the number of event IDs assigned to
546a stream. Luckily, this information tends to stay relatively constant (modulo
5ba9f198 547event registration while trace is being recorded), so we can specify different
3bf79539 548representations for streams containing few event IDs and streams containing
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549many event IDs, so we end up representing the event ID and timestamp as densely
550as possible in each case.
551
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552We therefore provide two types of events headers. Type 1 accommodates streams
553with less than 31 event IDs. Type 2 accommodates streams with 31 or more event
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554IDs.
555
556The "extended headers" are used in the rare occasions where the information
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557cannot be represented in the ranges available in the event header. They are also
558used in the rare occasions where the data required for a field could not be
559collected: the flag corresponding to the missing field within the missing_fields
560array is then set to 1.
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561
562Types uintX_t represent an X-bit unsigned integer.
563
564
5656.1.1 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
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572struct event_header_1 {
573 uint5_t id; /*
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574 * id: range: 0 - 30.
575 * id 31 is reserved to indicate a following
576 * extended header.
577 */
80fd2569 578 uint27_t timestamp;
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579};
580
581The end of a type 1 header is aligned on a 32-bit boundary (or packed).
582
583
5846.1.2 Extended Type 1 Event Header
585
586 - Follows struct event_header_1, which is aligned on 32-bit, so no need to
587 realign.
3bf79539 588 - Variable size (depends on the number of fields per event).
5ba9f198 589 - Native architecture byte ordering.
80fd2569 590 - NR_FIELDS is the number of fields within the event.
5ba9f198 591
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592struct 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 */
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596};
597
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598
5996.1.3 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
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606struct event_header_2 {
607 uint32_t timestamp;
608 uint16_t id; /*
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609 * id: range: 0 - 65534.
610 * id 65535 is reserved to indicate a following
611 * extended header.
612 */
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613};
614
615The end of a type 2 header is aligned on a 16-bit boundary (or 8-bit if
616byte-packed).
617
618
6196.1.4 Extended Type 2 Event Header
620
621 - Follows struct event_header_2, which alignment end on a 16-bit boundary, so
3bf79539 622 we need to align on 64-bit integer architecture alignment (or 8-bit if
5ba9f198 623 byte-packed).
3bf79539 624 - Variable size (depends on the number of fields per event).
5ba9f198 625 - Native architecture byte ordering.
80fd2569 626 - NR_FIELDS is the number of fields within the event.
5ba9f198 627
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628struct 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 */
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632};
633
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634
6356.2 Event Context
636
637The event context contains information relative to the current event. The choice
3bf79539 638and meaning of this information is specified by the metadata "stream"
5ba9f198 639information. For this trace format, event context is usually empty, except when
3bf79539 640the metadata "stream" information specifies otherwise by declaring a non-empty
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641structure for the event context. An example of event context is to save the
642event payload size with each event, or to save the current PID with each event.
3bf79539 643These are declared within the stream declaration within the metadata.
5ba9f198 644
3bf79539 645An example event context type:
5ba9f198 646
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647 struct event_context {
648 uint pid;
649 uint16_t payload_size;
3bf79539 650 };
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651
652
6536.3 Event Payload
654
655An event payload contains fields specific to a given event type. The fields
656belonging to an event type are described in the event-specific metadata
657within a structure type.
658
6596.3.1 Padding
660
661No padding at the end of the event payload. This differs from the ISO/C standard
662for structures, but follows the CTF standard for structures. In a trace, even
663though it makes sense to align the beginning of a structure, it really makes no
664sense to add padding at the end of the structure, because structures are usually
665not followed by a structure of the same type.
666
667This trick can be done by adding a zero-length "end" field at the end of the C
668structures, and by using the offset of this field rather than using sizeof()
3bf79539 669when calculating the size of a structure (see Appendix "A. Helper macros").
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670
6716.3.2 Alignment
672
673The event payload is aligned on the largest alignment required by types
674contained within the payload. (This follows the ISO/C standard for structures)
675
676
677
6787. Metadata
679
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680The meta-data is located in a stream named "metadata". It is made of "event
681packets", which each start with an event packet header. The event type within
682the metadata stream have no event header nor event context. Each event only
5ba9f198 683contains a null-terminated "string" payload, which is a metadata description
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684entry. The events are packed one next to another. Each event packet start with
685an event packet header, which contains, amongst other fields, the magic number
686and trace UUID.
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687
688The metadata can be parsed by reading through the metadata strings, skipping
3bf79539 689newlines and null-characters. Type names may contain spaces.
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690
691trace {
692 major = value; /* Trace format version */
693 minor = value;
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694 uuid = value; /* Trace UUID */
695 word_size = value;
696};
5ba9f198 697
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698stream {
699 id = stream_id;
5ba9f198 700 event {
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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;
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708 context_type = struct {
709 ...
710 };
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711 };
712 packet {
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713 context_type = struct {
714 ...
715 };
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716 };
717};
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718
719event {
3d13ef1a 720 name = event_name;
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721 id = value; /* Numeric identifier within the stream */
722 stream = stream_id;
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723 fields = struct {
724 ...
725 };
3bf79539 726};
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727
728/* More detail on types in section 4. Types */
729
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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
738typedef aliased_type_prefix aliased_type new_type aliased_type_postfix;
739/* e.g.: typedef struct example new_type_name[10]; */
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740
741typedef type_class {
742 ...
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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 */
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752
753struct name {
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754 ...
755};
5ba9f198 756
3d13ef1a 757enum <integer_type> name {
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758 ...
759};
760
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761/* Unnamed types, contained within compound type fields or type assignments. */
762struct {
763 ...
764};
5ba9f198 765
3d13ef1a 766enum <integer_type> {
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767 ...
768};
3bf79539 769
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770array {
771 ...
772};
3bf79539 773
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774sequence {
775 ...
776};
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777
778A. Helper macros
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779
780The two following macros keep track of the size of a GNU/C structure without
781padding at the end by placing HEADER_END as the last field. A one byte end field
782is used for C90 compatibility (C99 flexible arrays could be used here). Note
783that this does not affect the effective structure size, which should always be
784calculated with the header_sizeof() helper.
785
786#define HEADER_END char end_field
787#define header_sizeof(type) offsetof(typeof(type), end_field)
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788
789
790B. Stream Header Rationale
791
792An event stream is divided in contiguous event packets of variable size. These
793subdivisions allow the trace analyzer to perform a fast binary search by time
794within the stream (typically requiring to index only the event packet headers)
795without reading the whole stream. These subdivisions have a variable size to
796eliminate the need to transfer the event packet padding when partially filled
797event packets must be sent when streaming a trace for live viewing/analysis.
798An event packet can contain a certain amount of padding at the end. Dividing
799streams into event packets is also useful for network streaming over UDP and
800flight recorder mode tracing (a whole event packet can be swapped out of the
801buffer atomically for reading).
802
803The stream header is repeated at the beginning of each event packet to allow
804flexibility 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
814The event stream header will therefore be referred to as the "event packet
815header" throughout the rest of this document.
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