Update proposal: remove duplicate styles for array/seq
[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 */
2152348f 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
2152348f 205Metadata representation:
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206
207 unit_type name:size:
208
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209As an example, the following structure declared in C compiled by GCC:
210
211struct example {
212 short a:12;
213 short b:5;
214};
215
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216The example structure is aligned on the largest element (short). The second
217bitfield would be aligned on the next unit boundary, because it would not fit in
218the current unit.
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219
2204.1.7 Floating point
221
222The floating point values byte ordering is defined in the metadata.
223
224Floating point values follow the IEEE 754-2008 standard interchange formats.
225Description of the floating point values include the exponent and mantissa size
226in 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
241Metadata representation:
242
80fd2569 243floating_point {
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244 exp_dig = value;
245 mant_dig = value;
246 byte_order = native OR network OR be OR le;
2152348f 247}
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248
249Example of type inheritance:
250
80fd2569 251typedef floating_point {
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252 exp_dig = 8; /* sizeof(float) * CHAR_BIT - FLT_MANT_DIG */
253 mant_dig = 24; /* FLT_MANT_DIG */
254 byte_order = native;
80fd2569 255} float;
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256
257TODO: define NaN, +inf, -inf behavior.
258
2594.1.8 Enumerations
260
261Enumerations are a mapping between an integer type and a table of strings. The
262numerical representation of the enumeration follows the integer type specified
263by the metadata. The enumeration mapping table is detailed in the enumeration
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264description 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
80fd2569 267"[ start_value ... end_value ] -> string", which map inclusive ranges of
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268values to strings. An enumeration from the C language can be represented in
269this format by having the same start_value and end_value for each element, which
270is in fact a range of size 1. This single-value range is supported without
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271repeating 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
273enumeration is used. The <integer_type> specifier can only be omitted for
274enumerations containing only simple "value -> string" mappings (compatible with
275C).
276
277enum <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
285If the values are omitted, the enumeration starts at 0 and increment of 1 for
286each entry:
287
2152348f 288enum name {
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289 ZERO,
290 ONE,
291 TWO,
292 TEN = 10,
293 ELEVEN,
3bf79539 294};
5ba9f198 295
80fd2569 296Overlapping ranges within a single enumeration are implementation defined.
5ba9f198 297
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298A nameless enumeration can be declared as a field type or as part of a typedef:
299
300enum <integer_type> {
301 ...
302}
303
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3044.2 Compound types
305
3064.2.1 Structures
307
308Structures are aligned on the largest alignment required by basic types
309contained within the structure. (This follows the ISO/C standard for structures)
310
80fd2569 311Metadata representation of a named structure:
5ba9f198 312
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313struct name {
314 field_type field_name;
315 field_type field_name;
316 ...
317};
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318
319Example:
320
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321struct 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 */
3bf79539 328};
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329
330The fields are placed in a sequence next to each other. They each possess a
331field name, which is a unique identifier within the structure.
332
2152348f 333A nameless structure can be declared as a field type or as part of a typedef:
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334
335struct {
336 ...
2152348f 337}
80fd2569 338
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3394.2.2 Arrays
340
341Arrays are fixed-length. Their length is declared in the type declaration within
342the metadata. They contain an array of "inner type" elements, which can refer to
343any type not containing the type of the array being declared (no circular
3bf79539 344dependency). The length is the number of elements in an array.
5ba9f198 345
2152348f 346Metadata representation of a named array:
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347
348typedef elem_type name[length];
5ba9f198 349
2152348f 350A nameless array can be declared as a field type within a structure, e.g.:
5ba9f198 351
2152348f 352 uint8_t field_name[10];
80fd2569 353
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354
3554.2.3 Sequences
356
357Sequences are dynamically-sized arrays. They start with an integer that specify
358the length of the sequence, followed by an array of "inner type" elements.
3bf79539 359The length is the number of elements in the sequence.
5ba9f198 360
2152348f 361Metadata representation for a named sequence:
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362
363typedef elem_type name[length_type];
364
365A nameless sequence can be declared as a field type, e.g.:
366
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367long field_name[int];
368
369The length type follows the integer types specifications, and the sequence
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370elements follow the "array" specifications.
371
3724.2.4 Strings
373
374Strings 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
376encoding attribute information, the default encoding is UTF-8.
377
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378Metadata representation of a named string type:
379
380typedef string {
5ba9f198 381 encoding = UTF8 OR ASCII;
80fd2569 382} name;
5ba9f198 383
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384A nameless string type can be declared as a field type:
385
386string field_name; /* Use default UTF8 encoding */
5ba9f198 387
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3885. Event Packet Header
389
390The event packet header consists of two part: one is mandatory and have a fixed
391layout. The second part, the "event packet context", has its layout described in
392the metadata.
5ba9f198 393
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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
5ba9f198 397 ordering.
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398
399Fixed layout (event packet header):
400
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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.
3bf79539 409- Trace UUID, used to ensure the event packet match the metadata used.
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410 (note: we cannot use a metadata checksum because metadata can be appended to
411 while tracing is active)
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412- Stream ID, used as reference to stream description in metadata.
413
414Metadata-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.
5ba9f198 428- Events discarded count
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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.
5ba9f198 432 * Note: producer-consumer buffer full condition should fill the current
3bf79539 433 event packet with padding so we know exactly where events have been
5ba9f198 434 discarded.
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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.
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438 0: no compression scheme
439 1: bzip2
440 2: gzip
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441 3: xz
442- Cypher used for the event packet content. Applied after compression.
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443 0: no encryption
444 1: AES
3bf79539 445- Checksum scheme used for the event packet content. Applied after encryption.
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446 0: no checksum
447 1: md5
448 2: sha1
449 3: crc32
450
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4515.1 Event Packet Header Fixed Layout Description
452
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453struct event_packet_header {
454 uint32_t magic;
455 uint8_t trace_uuid[16];
3bf79539 456 uint32_t stream_id;
80fd2569 457};
5ba9f198 458
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4595.2 Event Packet Context Description
460
461Event packet context example. These are declared within the stream declaration
462in the metadata. All these fields are optional except for "content_size" and
463"packet_size", which must be present in the context.
464
465An example event packet context type:
466
80fd2569 467struct event_packet_context {
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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};
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481
4826. Event Structure
483
484The overall structure of an event is:
485
3bf79539 486 - Event Header (as specifed by the stream metadata)
5ba9f198 487 - Extended Event Header (as specified by the event header)
3bf79539 488 - Event Context (as specified by the stream metadata)
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489 - Event Payload (as specified by the event metadata)
490
491
4926.1 Event Header
493
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494One major factor can vary between streams: the number of event IDs assigned to
495a stream. Luckily, this information tends to stay relatively constant (modulo
5ba9f198 496event registration while trace is being recorded), so we can specify different
3bf79539 497representations for streams containing few event IDs and streams containing
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498many event IDs, so we end up representing the event ID and timestamp as densely
499as possible in each case.
500
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501We therefore provide two types of events headers. Type 1 accommodates streams
502with less than 31 event IDs. Type 2 accommodates streams with 31 or more event
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503IDs.
504
505The "extended headers" are used in the rare occasions where the information
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506cannot be represented in the ranges available in the event header. They are also
507used in the rare occasions where the data required for a field could not be
508collected: the flag corresponding to the missing field within the missing_fields
509array is then set to 1.
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510
511Types uintX_t represent an X-bit unsigned integer.
512
513
5146.1.1 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
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521struct event_header_1 {
522 uint5_t id; /*
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523 * id: range: 0 - 30.
524 * id 31 is reserved to indicate a following
525 * extended header.
526 */
80fd2569 527 uint27_t timestamp;
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528};
529
530The end of a type 1 header is aligned on a 32-bit boundary (or packed).
531
532
5336.1.2 Extended Type 1 Event Header
534
535 - Follows struct event_header_1, which is aligned on 32-bit, so no need to
536 realign.
3bf79539 537 - Variable size (depends on the number of fields per event).
5ba9f198 538 - Native architecture byte ordering.
80fd2569 539 - NR_FIELDS is the number of fields within the event.
5ba9f198 540
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541struct 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 */
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545};
546
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547
5486.1.3 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
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555struct event_header_2 {
556 uint32_t timestamp;
557 uint16_t id; /*
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558 * id: range: 0 - 65534.
559 * id 65535 is reserved to indicate a following
560 * extended header.
561 */
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562};
563
564The end of a type 2 header is aligned on a 16-bit boundary (or 8-bit if
565byte-packed).
566
567
5686.1.4 Extended Type 2 Event Header
569
570 - Follows struct event_header_2, which alignment end on a 16-bit boundary, so
3bf79539 571 we need to align on 64-bit integer architecture alignment (or 8-bit if
5ba9f198 572 byte-packed).
3bf79539 573 - Variable size (depends on the number of fields per event).
5ba9f198 574 - Native architecture byte ordering.
80fd2569 575 - NR_FIELDS is the number of fields within the event.
5ba9f198 576
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577struct 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 */
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581};
582
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583
5846.2 Event Context
585
586The event context contains information relative to the current event. The choice
3bf79539 587and meaning of this information is specified by the metadata "stream"
5ba9f198 588information. For this trace format, event context is usually empty, except when
3bf79539 589the metadata "stream" information specifies otherwise by declaring a non-empty
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590structure for the event context. An example of event context is to save the
591event payload size with each event, or to save the current PID with each event.
3bf79539 592These are declared within the stream declaration within the metadata.
5ba9f198 593
3bf79539 594An example event context type:
5ba9f198 595
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596 struct event_context {
597 uint pid;
598 uint16_t payload_size;
3bf79539 599 };
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600
601
6026.3 Event Payload
603
604An event payload contains fields specific to a given event type. The fields
605belonging to an event type are described in the event-specific metadata
606within a structure type.
607
6086.3.1 Padding
609
610No padding at the end of the event payload. This differs from the ISO/C standard
611for structures, but follows the CTF standard for structures. In a trace, even
612though it makes sense to align the beginning of a structure, it really makes no
613sense to add padding at the end of the structure, because structures are usually
614not followed by a structure of the same type.
615
616This trick can be done by adding a zero-length "end" field at the end of the C
617structures, and by using the offset of this field rather than using sizeof()
3bf79539 618when calculating the size of a structure (see Appendix "A. Helper macros").
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619
6206.3.2 Alignment
621
622The event payload is aligned on the largest alignment required by types
623contained within the payload. (This follows the ISO/C standard for structures)
624
625
626
6277. Metadata
628
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629The meta-data is located in a stream named "metadata". It is made of "event
630packets", which each start with an event packet header. The event type within
631the metadata stream have no event header nor event context. Each event only
5ba9f198 632contains a null-terminated "string" payload, which is a metadata description
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633entry. The events are packed one next to another. Each event packet start with
634an event packet header, which contains, amongst other fields, the magic number
635and trace UUID.
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636
637The metadata can be parsed by reading through the metadata strings, skipping
3bf79539 638newlines and null-characters. Type names may contain spaces.
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639
640trace {
641 major = value; /* Trace format version */
642 minor = value;
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643 uuid = value; /* Trace UUID */
644 word_size = value;
645};
5ba9f198 646
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647stream {
648 id = stream_id;
5ba9f198 649 event {
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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;
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657 context_type = struct {
658 ...
659 };
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660 };
661 packet {
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662 context_type = struct {
663 ...
664 };
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665 };
666};
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667
668event {
3d13ef1a 669 name = event_name;
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670 id = value; /* Numeric identifier within the stream */
671 stream = stream_id;
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672 fields = struct {
673 ...
674 };
3bf79539 675};
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676
677/* More detail on types in section 4. Types */
678
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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
687typedef aliased_type_prefix aliased_type new_type aliased_type_postfix;
2152348f 688
3d13ef1a 689/* e.g.: typedef struct example new_type_name[10]; */
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690
691typedef type_class {
692 ...
3d13ef1a 693} new_type_prefix new_type new_type_postfix;
2152348f 694
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695/*
696 * e.g.:
697 * typedef integer {
698 * size = 32;
699 * align = 32;
700 * signed = false;
701 * } struct page *;
702 */
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703
704struct name {
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705 ...
706};
5ba9f198 707
3d13ef1a 708enum <integer_type> name {
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709 ...
710};
711
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712
713/* Unnamed types, contained within compound type fields or typedef. */
714
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715struct {
716 ...
2152348f 717}
5ba9f198 718
3d13ef1a 719enum <integer_type> {
80fd2569 720 ...
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721}
722
723typedef type new_type[length];
3bf79539 724
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725struct {
726 type field_name[length];
727}
728
729typedef type new_type[length_type];
730
731struct {
732 type field_name[length_type];
733}
734
735integer {
80fd2569 736 ...
2152348f 737}
3bf79539 738
2152348f 739floating_point {
80fd2569 740 ...
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741}
742
743struct {
744 integer_type field_name:size; /* GNU/C bitfield */
745}
746
747struct {
748 string field_name;
749}
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750
751A. Helper macros
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752
753The two following macros keep track of the size of a GNU/C structure without
754padding at the end by placing HEADER_END as the last field. A one byte end field
755is used for C90 compatibility (C99 flexible arrays could be used here). Note
756that this does not affect the effective structure size, which should always be
757calculated with the header_sizeof() helper.
758
759#define HEADER_END char end_field
760#define header_sizeof(type) offsetof(typeof(type), end_field)
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761
762
763B. Stream Header Rationale
764
765An event stream is divided in contiguous event packets of variable size. These
766subdivisions allow the trace analyzer to perform a fast binary search by time
767within the stream (typically requiring to index only the event packet headers)
768without reading the whole stream. These subdivisions have a variable size to
769eliminate the need to transfer the event packet padding when partially filled
770event packets must be sent when streaming a trace for live viewing/analysis.
771An event packet can contain a certain amount of padding at the end. Dividing
772streams into event packets is also useful for network streaming over UDP and
773flight recorder mode tracing (a whole event packet can be swapped out of the
774buffer atomically for reading).
775
776The stream header is repeated at the beginning of each event packet to allow
777flexibility 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
787The event stream header will therefore be referred to as the "event packet
788header" throughout the rest of this document.
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