Explain dynamic scope vs current context
[ctf.git] / common-trace-format-proposal.txt
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5ba9f198 1
4767a9e7 2RFC: Common Trace Format (CTF) Proposal (pre-v1.7)
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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
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86Types are organized as type classes. Each type class belong to either of two
87kind of types: basic types or compound types.
88
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894.1 Basic types
90
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91A basic type is a scalar type, as described in this section. It includes
92integers, GNU/C bitfields, enumerations, and floating point values.
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93
944.1.1 Type inheritance
95
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96Type specifications can be inherited to allow deriving types from a
97type class. For example, see the uint32_t named type derived from the "integer"
98type class below ("Integers" section). Types have a precise binary
99representation in the trace. A type class has methods to read and write these
100types, but must be derived into a type to be usable in an event field.
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101
1024.1.2 Alignment
103
104We define "byte-packed" types as aligned on the byte size, namely 8-bit.
105We define "bit-packed" types as following on the next bit, as defined by the
106"bitfields" section.
5ba9f198 107
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108All basic types, except bitfields, are either aligned on an architecture-defined
109specific alignment or byte-packed, depending on the architecture preference.
110Architectures providing fast unaligned write byte-packed basic types to save
5ba9f198 111space, aligning each type on byte boundaries (8-bit). Architectures with slow
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112unaligned writes align types on specific alignment values. If no specific
113alignment is declared for a type nor its parents, it is assumed to be bit-packed
114for bitfields and byte-packed for other types.
5ba9f198 115
3bf79539 116Metadata attribute representation of a specific alignment:
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117
118 align = value; /* value in bits */
119
1204.1.3 Byte order
121
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122By default, the native endianness of the source architecture the trace is used.
123Byte order can be overridden for a basic type by specifying a "byte_order"
124attribute. Typical use-case is to specify the network byte order (big endian:
125"be") to save data captured from the network into the trace without conversion.
126If not specified, the byte order is native.
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127
128Metadata representation:
129
130 byte_order = native OR network OR be OR le; /* network and be are aliases */
131
1324.1.4 Size
133
134Type size, in bits, for integers and floats is that returned by "sizeof()" in C
135multiplied by CHAR_BIT.
136We require the size of "char" and "unsigned char" types (CHAR_BIT) to be fixed
137to 8 bits for cross-endianness compatibility.
138
139Metadata representation:
140
141 size = value; (value is in bits)
142
1434.1.5 Integers
144
145Signed integers are represented in two-complement. Integer alignment, size,
146signedness and byte ordering are defined in the metadata. Integers aligned on
147byte size (8-bit) and with length multiple of byte size (8-bit) correspond to
148the C99 standard integers. In addition, integers with alignment and/or size that
149are _not_ a multiple of the byte size are permitted; these correspond to the C99
150standard bitfields, with the added specification that the CTF integer bitfields
151have a fixed binary representation. A MIT-licensed reference implementation of
152the CTF portable bitfields is available at:
153
154 http://git.efficios.com/?p=babeltrace.git;a=blob;f=include/babeltrace/bitfield.h
155
156Binary representation of integers:
157
158- On little and big endian:
159 - Within a byte, high bits correspond to an integer high bits, and low bits
160 correspond to low bits.
161- On little endian:
162 - Integer across multiple bytes are placed from the less significant to the
163 most significant.
164 - Consecutive integers are placed from lower bits to higher bits (even within
165 a byte).
166- On big endian:
167 - Integer across multiple bytes are placed from the most significant to the
168 less significant.
169 - Consecutive integers are placed from higher bits to lower bits (even within
170 a byte).
171
172This binary representation is derived from the bitfield implementation in GCC
173for little and big endian. However, contrary to what GCC does, integers can
174cross units boundaries (no padding is required). Padding can be explicitely
175added (see 4.1.6 GNU/C bitfields) to follow the GCC layout if needed.
176
177Metadata representation:
178
80fd2569 179 integer {
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180 signed = true OR false; /* default false */
181 byte_order = native OR network OR be OR le; /* default native */
182 size = value; /* value in bits, no default */
183 align = value; /* value in bits */
2152348f 184 }
5ba9f198 185
80fd2569 186Example of type inheritance (creation of a uint32_t named type):
5ba9f198 187
359894ac 188typealias integer {
9e4e34e9 189 size = 32;
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190 signed = false;
191 align = 32;
359894ac 192} : uint32_t;
5ba9f198 193
80fd2569 194Definition of a named 5-bit signed bitfield:
5ba9f198 195
359894ac 196typealias integer {
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197 size = 5;
198 signed = true;
199 align = 1;
359894ac 200} : int5_t;
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201
2024.1.6 GNU/C bitfields
203
204The GNU/C bitfields follow closely the integer representation, with a
205particularity on alignment: if a bitfield cannot fit in the current unit, the
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206unit is padded and the bitfield starts at the following unit. The unit size is
207defined by the size of the type "unit_type".
5ba9f198 208
2152348f 209Metadata representation:
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210
211 unit_type name:size:
212
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213As an example, the following structure declared in C compiled by GCC:
214
215struct example {
216 short a:12;
217 short b:5;
218};
219
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220The example structure is aligned on the largest element (short). The second
221bitfield would be aligned on the next unit boundary, because it would not fit in
222the current unit.
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223
2244.1.7 Floating point
225
226The floating point values byte ordering is defined in the metadata.
227
228Floating point values follow the IEEE 754-2008 standard interchange formats.
229Description of the floating point values include the exponent and mantissa size
230in bits. Some requirements are imposed on the floating point values:
231
232- FLT_RADIX must be 2.
233- mant_dig is the number of digits represented in the mantissa. It is specified
234 by the ISO C99 standard, section 5.2.4, as FLT_MANT_DIG, DBL_MANT_DIG and
235 LDBL_MANT_DIG as defined by <float.h>.
236- exp_dig is the number of digits represented in the exponent. Given that
237 mant_dig is one bit more than its actual size in bits (leading 1 is not
238 needed) and also given that the sign bit always takes one bit, exp_dig can be
239 specified as:
240
241 - sizeof(float) * CHAR_BIT - FLT_MANT_DIG
242 - sizeof(double) * CHAR_BIT - DBL_MANT_DIG
243 - sizeof(long double) * CHAR_BIT - LDBL_MANT_DIG
244
245Metadata representation:
246
80fd2569 247floating_point {
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248 exp_dig = value;
249 mant_dig = value;
250 byte_order = native OR network OR be OR le;
2152348f 251}
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252
253Example of type inheritance:
254
359894ac 255typealias floating_point {
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256 exp_dig = 8; /* sizeof(float) * CHAR_BIT - FLT_MANT_DIG */
257 mant_dig = 24; /* FLT_MANT_DIG */
258 byte_order = native;
359894ac 259} : float;
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260
261TODO: define NaN, +inf, -inf behavior.
262
2634.1.8 Enumerations
264
265Enumerations are a mapping between an integer type and a table of strings. The
266numerical representation of the enumeration follows the integer type specified
267by the metadata. The enumeration mapping table is detailed in the enumeration
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268description within the metadata. The mapping table maps inclusive value ranges
269(or single values) to strings. Instead of being limited to simple
270"value -> string" mappings, these enumerations map
80fd2569 271"[ start_value ... end_value ] -> string", which map inclusive ranges of
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272values to strings. An enumeration from the C language can be represented in
273this format by having the same start_value and end_value for each element, which
274is in fact a range of size 1. This single-value range is supported without
4767a9e7 275repeating the start and end values with the value = string declaration.
80fd2569 276
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277If a numeric value is encountered between < >, it represents the integer type
278size used to hold the enumeration, in bits.
279
cfc73fdc 280enum name <integer_type OR size> {
359894ac 281 somestring = start_value1 ... end_value1,
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282 "other string" = start_value2 ... end_value2,
283 yet_another_string, /* will be assigned to end_value2 + 1 */
284 "some other string" = value,
285 ...
286};
287
288If the values are omitted, the enumeration starts at 0 and increment of 1 for
289each entry:
290
cfc73fdc 291enum name <32> {
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292 ZERO,
293 ONE,
294 TWO,
295 TEN = 10,
296 ELEVEN,
3bf79539 297};
5ba9f198 298
80fd2569 299Overlapping ranges within a single enumeration are implementation defined.
5ba9f198 300
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301A nameless enumeration can be declared as a field type or as part of a typedef:
302
303enum <integer_type> {
304 ...
305}
306
1fad7a85 307
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3084.2 Compound types
309
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310Compound are aggregation of type declarations. Compound types include
311structures, variant, arrays, sequences, and strings.
312
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3134.2.1 Structures
314
315Structures are aligned on the largest alignment required by basic types
316contained within the structure. (This follows the ISO/C standard for structures)
317
80fd2569 318Metadata representation of a named structure:
5ba9f198 319
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320struct name {
321 field_type field_name;
322 field_type field_name;
323 ...
324};
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325
326Example:
327
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328struct example {
329 integer { /* Nameless type */
330 size = 16;
331 signed = true;
332 align = 16;
333 } first_field_name;
334 uint64_t second_field_name; /* Named type declared in the metadata */
3bf79539 335};
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336
337The fields are placed in a sequence next to each other. They each possess a
338field name, which is a unique identifier within the structure.
339
2152348f 340A nameless structure can be declared as a field type or as part of a typedef:
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341
342struct {
343 ...
2152348f 344}
80fd2569 345
77a98c82 3464.2.2 Variants (Discriminated/Tagged Unions)
fcba70d4 347
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348A CTF variant is a selection between different types. A CTF variant must
349always be defined within the scope of a structure or within fields
350contained within a structure (defined recursively). A "tag" enumeration
351field must appear in either the same lexical scope, prior to the variant
352field (in field declaration order), in an uppermost lexical scope (see
353Section 7.2.1), or in an uppermost dynamic scope (see Section 7.2.2).
354The type selection is indicated by the mapping from the enumeration
355value to the string used as variant type selector. The field to use as
356tag is specified by the "tag_field", specified between "< >" after the
357"variant" keyword for unnamed variants, and after "variant name" for
358named variants.
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359
360The alignment of the variant is the alignment of the type as selected by the tag
361value for the specific instance of the variant. The alignment of the type
362containing the variant is independent of the variant alignment. The size of the
363variant is the size as selected by the tag value for the specific instance of
364the variant.
365
366A named variant declaration followed by its definition within a structure
367declaration:
368
369variant name {
370 field_type sel1;
371 field_type sel2;
372 field_type sel3;
373 ...
374};
375
376struct {
377 enum <integer_type or size> { sel1, sel2, sel3, ... } tag_field;
378 ...
379 variant name <tag_field> v;
380}
381
382An unnamed variant definition within a structure is expressed by the following
383metadata:
384
385struct {
386 enum <integer_type or size> { sel1, sel2, sel3, ... } tag_field;
387 ...
388 variant <tag_field> {
389 field_type sel1;
390 field_type sel2;
391 field_type sel3;
392 ...
393 } v;
394}
395
396Example of a named variant within a sequence that refers to a single tag field:
397
398variant example {
399 uint32_t a;
400 uint64_t b;
401 short c;
402};
403
404struct {
405 enum <uint2_t> { a, b, c } choice;
15850440 406 variant example <choice> v[unsigned int];
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407}
408
409Example of an unnamed variant:
410
411struct {
412 enum <uint2_t> { a, b, c, d } choice;
413 /* Unrelated fields can be added between the variant and its tag */
414 int32_t somevalue;
415 variant <choice> {
416 uint32_t a;
417 uint64_t b;
418 short c;
419 struct {
420 unsigned int field1;
421 uint64_t field2;
422 } d;
423 } s;
424}
425
426Example of an unnamed variant within an array:
427
428struct {
429 enum <uint2_t> { a, b, c } choice;
430 variant <choice> {
431 uint32_t a;
432 uint64_t b;
433 short c;
15850440 434 } v[10];
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435}
436
437Example of a variant type definition within a structure, where the defined type
438is then declared within an array of structures. This variant refers to a tag
439located in an upper lexical scope. This example clearly shows that a variant
440type definition referring to the tag "x" uses the closest preceding field from
441the lexical scope of the type definition.
442
443struct {
444 enum <uint2_t> { a, b, c, d } x;
445
446 typedef variant <x> { /*
447 * "x" refers to the preceding "x" enumeration in the
448 * lexical scope of the type definition.
449 */
450 uint32_t a;
451 uint64_t b;
452 short c;
453 } example_variant;
454
455 struct {
456 enum <int> { x, y, z } x; /* This enumeration is not used by "v". */
457 example_variant v; /*
458 * "v" uses the "enum <uint2_t> { a, b, c, d }"
459 * tag.
460 */
461 } a[10];
462}
463
4644.2.3 Arrays
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465
466Arrays are fixed-length. Their length is declared in the type declaration within
467the metadata. They contain an array of "inner type" elements, which can refer to
468any type not containing the type of the array being declared (no circular
3bf79539 469dependency). The length is the number of elements in an array.
5ba9f198 470
2152348f 471Metadata representation of a named array:
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472
473typedef elem_type name[length];
5ba9f198 474
2152348f 475A nameless array can be declared as a field type within a structure, e.g.:
5ba9f198 476
2152348f 477 uint8_t field_name[10];
80fd2569 478
5ba9f198 479
fcba70d4 4804.2.4 Sequences
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481
482Sequences are dynamically-sized arrays. They start with an integer that specify
483the length of the sequence, followed by an array of "inner type" elements.
3bf79539 484The length is the number of elements in the sequence.
5ba9f198 485
2152348f 486Metadata representation for a named sequence:
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487
488typedef elem_type name[length_type];
489
490A nameless sequence can be declared as a field type, e.g.:
491
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492long field_name[int];
493
494The length type follows the integer types specifications, and the sequence
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495elements follow the "array" specifications.
496
fcba70d4 4974.2.5 Strings
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498
499Strings are an array of bytes of variable size and are terminated by a '\0'
500"NULL" character. Their encoding is described in the metadata. In absence of
501encoding attribute information, the default encoding is UTF-8.
502
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503Metadata representation of a named string type:
504
359894ac 505typealias string {
5ba9f198 506 encoding = UTF8 OR ASCII;
359894ac 507} : name;
5ba9f198 508
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509A nameless string type can be declared as a field type:
510
511string field_name; /* Use default UTF8 encoding */
5ba9f198 512
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5135. Event Packet Header
514
515The event packet header consists of two part: one is mandatory and have a fixed
516layout. The second part, the "event packet context", has its layout described in
517the metadata.
5ba9f198 518
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519- Aligned on page size. Fixed size. Fields either aligned or packed (depending
520 on the architecture preference).
521 No padding at the end of the event packet header. Native architecture byte
5ba9f198 522 ordering.
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523
524Fixed layout (event packet header):
525
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526- Magic number (CTF magic numbers: 0xC1FC1FC1 and its reverse endianness
527 representation: 0xC11FFCC1) It needs to have a non-symmetric bytewise
528 representation. Used to distinguish between big and little endian traces (this
529 information is determined by knowing the endianness of the architecture
530 reading the trace and comparing the magic number against its value and the
531 reverse, 0xC11FFCC1). This magic number specifies that we use the CTF metadata
532 description language described in this document. Different magic numbers
533 should be used for other metadata description languages.
3bf79539 534- Trace UUID, used to ensure the event packet match the metadata used.
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535 (note: we cannot use a metadata checksum because metadata can be appended to
536 while tracing is active)
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537- Stream ID, used as reference to stream description in metadata.
538
539Metadata-defined layout (event packet context):
540
541- Event packet content size (in bytes).
542- Event packet size (in bytes, includes padding).
543- Event packet content checksum (optional). Checksum excludes the event packet
544 header.
545- Per-stream event packet sequence count (to deal with UDP packet loss). The
546 number of significant sequence counter bits should also be present, so
547 wrap-arounds are deal with correctly.
548- Timestamp at the beginning and timestamp at the end of the event packet.
549 Both timestamps are written in the packet header, but sampled respectively
550 while (or before) writing the first event and while (or after) writing the
551 last event in the packet. The inclusive range between these timestamps should
552 include all event timestamps assigned to events contained within the packet.
5ba9f198 553- Events discarded count
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554 - Snapshot of a per-stream free-running counter, counting the number of
555 events discarded that were supposed to be written in the stream prior to
556 the first event in the event packet.
5ba9f198 557 * Note: producer-consumer buffer full condition should fill the current
3bf79539 558 event packet with padding so we know exactly where events have been
5ba9f198 559 discarded.
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560- Lossless compression scheme used for the event packet content. Applied
561 directly to raw data. New types of compression can be added in following
562 versions of the format.
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563 0: no compression scheme
564 1: bzip2
565 2: gzip
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566 3: xz
567- Cypher used for the event packet content. Applied after compression.
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568 0: no encryption
569 1: AES
3bf79539 570- Checksum scheme used for the event packet content. Applied after encryption.
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571 0: no checksum
572 1: md5
573 2: sha1
574 3: crc32
575
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5765.1 Event Packet Header Fixed Layout Description
577
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578struct event_packet_header {
579 uint32_t magic;
580 uint8_t trace_uuid[16];
3bf79539 581 uint32_t stream_id;
80fd2569 582};
5ba9f198 583
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5845.2 Event Packet Context Description
585
586Event packet context example. These are declared within the stream declaration
587in the metadata. All these fields are optional except for "content_size" and
588"packet_size", which must be present in the context.
589
590An example event packet context type:
591
80fd2569 592struct event_packet_context {
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593 uint64_t timestamp_begin;
594 uint64_t timestamp_end;
595 uint32_t checksum;
596 uint32_t stream_packet_count;
597 uint32_t events_discarded;
598 uint32_t cpu_id;
599 uint32_t/uint16_t content_size;
600 uint32_t/uint16_t packet_size;
601 uint8_t stream_packet_count_bits; /* Significant counter bits */
602 uint8_t compression_scheme;
603 uint8_t encryption_scheme;
3b0f8e4d 604 uint8_t checksum_scheme;
3bf79539 605};
5ba9f198 606
fcba70d4 607
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6086. Event Structure
609
610The overall structure of an event is:
611
fcba70d4 6121 - Stream Packet Context (as specified by the stream metadata)
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613 2 - Event Header (as specified by the stream metadata)
614 3 - Stream Event Context (as specified by the stream metadata)
615 4 - Event Context (as specified by the event metadata)
616 5 - Event Payload (as specified by the event metadata)
5ba9f198 617
fdf2bb05 618This structure defines an implicit dynamic scoping, where variants
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619located in inner structures (those with a higher number in the listing
620above) can refer to the fields of outer structures (with lower number in
621the listing above). See Section 7.2 Metadata Scopes for more detail.
5ba9f198 622
fdf2bb05 6236.1 Event Header
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624
625Event headers can be described within the metadata. We hereby propose, as an
626example, two types of events headers. Type 1 accommodates streams with less than
62731 event IDs. Type 2 accommodates streams with 31 or more event IDs.
5ba9f198 628
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629One major factor can vary between streams: the number of event IDs assigned to
630a stream. Luckily, this information tends to stay relatively constant (modulo
5ba9f198 631event registration while trace is being recorded), so we can specify different
3bf79539 632representations for streams containing few event IDs and streams containing
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633many event IDs, so we end up representing the event ID and timestamp as densely
634as possible in each case.
635
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636The header is extended in the rare occasions where the information cannot be
637represented in the ranges available in the standard event header. They are also
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638used in the rare occasions where the data required for a field could not be
639collected: the flag corresponding to the missing field within the missing_fields
640array is then set to 1.
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641
642Types uintX_t represent an X-bit unsigned integer.
643
644
fdf2bb05 6456.1.1 Type 1 - Few event IDs
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646
647 - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
648 preference).
5ba9f198 649 - Native architecture byte ordering.
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650 - For "compact" selection
651 - Fixed size: 32 bits.
652 - For "extended" selection
653 - Size depends on the architecture and variant alignment.
5ba9f198 654
80fd2569 655struct event_header_1 {
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656 /*
657 * id: range: 0 - 30.
658 * id 31 is reserved to indicate an extended header.
659 */
660 enum <uint5_t> { compact = 0 ... 30, extended = 31 } id;
661 variant <id> {
662 struct {
663 uint27_t timestamp;
664 } compact;
665 struct {
666 uint32_t id; /* 32-bit event IDs */
667 uint64_t timestamp; /* 64-bit timestamps */
668 } extended;
669 } v;
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670};
671
5ba9f198 672
fdf2bb05 6736.1.2 Type 2 - Many event IDs
5ba9f198 674
fcba70d4 675 - Aligned on 16-bit (or 8-bit if byte-packed, depending on the architecture
5ba9f198 676 preference).
5ba9f198 677 - Native architecture byte ordering.
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678 - For "compact" selection
679 - Size depends on the architecture and variant alignment.
680 - For "extended" selection
681 - Size depends on the architecture and variant alignment.
5ba9f198 682
80fd2569 683struct event_header_2 {
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684 /*
685 * id: range: 0 - 65534.
686 * id 65535 is reserved to indicate an extended header.
687 */
688 enum <uint16_t> { compact = 0 ... 65534, extended = 65535 } id;
689 variant <id> {
690 struct {
691 uint32_t timestamp;
692 } compact;
693 struct {
694 uint32_t id; /* 32-bit event IDs */
695 uint64_t timestamp; /* 64-bit timestamps */
696 } extended;
697 } v;
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698};
699
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700
7016.2 Event Context
702
703The event context contains information relative to the current event. The choice
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704and meaning of this information is specified by the metadata "stream" and
705"event" information. The "stream" context is applied to all events within the
706stream. The "stream" context structure follows the event header. The "event"
707context is applied to specific events. Its structure follows the "stream"
708context stucture.
5ba9f198 709
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710An example of stream-level event context is to save the event payload size with
711each event, or to save the current PID with each event. These are declared
712within the stream declaration within the metadata:
5ba9f198 713
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714 stream {
715 ...
716 event {
717 ...
4fa992a5 718 context := struct {
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719 uint pid;
720 uint16_t payload_size;
3bf79539 721 };
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722 }
723 };
724
725An example of event-specific event context is to declare a bitmap of missing
726fields, only appended after the stream event context if the extended event
727header is selected. NR_FIELDS is the number of fields within the event (a
728numeric value).
5ba9f198 729
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730 event {
731 context = struct {
732 variant <id> {
733 struct { } compact;
734 struct {
735 uint1_t missing_fields[NR_FIELDS]; /* missing event fields bitmap */
736 } extended;
737 } v;
738 };
739 ...
740 }
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741
7426.3 Event Payload
743
744An event payload contains fields specific to a given event type. The fields
745belonging to an event type are described in the event-specific metadata
746within a structure type.
747
7486.3.1 Padding
749
750No padding at the end of the event payload. This differs from the ISO/C standard
751for structures, but follows the CTF standard for structures. In a trace, even
752though it makes sense to align the beginning of a structure, it really makes no
753sense to add padding at the end of the structure, because structures are usually
754not followed by a structure of the same type.
755
756This trick can be done by adding a zero-length "end" field at the end of the C
757structures, and by using the offset of this field rather than using sizeof()
3bf79539 758when calculating the size of a structure (see Appendix "A. Helper macros").
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759
7606.3.2 Alignment
761
762The event payload is aligned on the largest alignment required by types
763contained within the payload. (This follows the ISO/C standard for structures)
764
765
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7667. Metadata
767
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768The meta-data is located in a stream named "metadata". It is made of "event
769packets", which each start with an event packet header. The event type within
770the metadata stream have no event header nor event context. Each event only
5ba9f198 771contains a null-terminated "string" payload, which is a metadata description
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772entry. The events are packed one next to another. Each event packet start with
773an event packet header, which contains, amongst other fields, the magic number
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774and trace UUID. The trace UUID is represented as a string of hexadecimal digits
775and dashes "-".
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776
777The metadata can be parsed by reading through the metadata strings, skipping
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778newlines and null-characters. Type names are made of a single identifier, and
779can be surrounded by prefix/postfix. Text contained within "/*" and "*/", as
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780well as within "//" and end of line, are treated as comments. Boolean values can
781be represented as true, TRUE, or 1 for true, and false, FALSE, or 0 for false.
fcba70d4 782
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783
7847.1 Declaration vs Definition
785
786A declaration associates a layout to a type, without specifying where
787this type is located in the event structure hierarchy (see Section 6).
788This therefore includes typedef, typealias, as well as all type
789specifiers. In certain circumstances (typedef, structure field and
790variant field), a declaration is followed by a declarator, which specify
791the newly defined type name (for typedef), or the field name (for
792declarations located within structure and variants). Array and sequence,
793declared with square brackets ("[" "]"), are part of the declarator,
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794similarly to C99. The enumeration type specifier and variant tag name
795(both specified with "<" ">") are part of the type specifier.
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796
797A definition associates a type to a location in the event structure
798hierarchy (see Section 6).
799
800
8017.2 Metadata Scopes
802
803CTF metadata uses two different types of scoping: a lexical scope is
804used for declarations and type definitions, and a dynamic scope is used
805for variants references to tag fields.
806
8077.2.1 Lexical Scope
808
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809Each of "trace", "stream", "event", "struct" and "variant" have their own
810nestable declaration scope, within which types can be declared using "typedef"
fdf2bb05 811and "typealias". A root declaration scope also contains all declarations
7d9d7e92 812located outside of any of the aforementioned declarations. An inner
fdf2bb05 813declaration scope can refer to type declared within its container
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814lexical scope prior to the inner declaration scope. Redefinition of a
815typedef or typealias is not valid, although hiding an upper scope
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816typedef or typealias is allowed within a sub-scope.
817
8187.2.2 Dynamic Scope
819
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820A dynamic scope consists in the lexical scope augmented with the
821implicit event structure definition hierarchy presented at Section 6.
822The dynamic scope is only used for variant tag definitions. It is used
823at definition time to look up the location of the tag field associated
824with a variant.
825
826Therefore, variants in lower levels in the dynamic scope (e.g. event
827context) can refer to a tag field located in upper levels (e.g. in the
828event header) by specifying, in this case, the associated tag with
829<header.field_name>. This allows, for instance, the event context to
830define a variant referring to the "id" field of the event header as
831selector.
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832
833The target dynamic scope must be specified explicitly when referring to
834a field outside of the local static scope. The dynamic scope prefixes
835are thus:
836
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837 - Stream Packet Context: <stream.packet.context. >,
838 - Event Header: <stream.event.header. >,
839 - Stream Event Context: <stream.event.context. >,
840 - Event Context: <event.context. >,
841 - Event Payload: <event.fields. >.
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842
843Multiple declarations of the same field name within a single scope is
844not valid. It is however valid to re-use the same field name in
845different scopes. There is no possible conflict, because the dynamic
846scope must be specified when a variant refers to a tag field located in
847a different dynamic scope.
848
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849The information available in the dynamic scopes can be thought of as the
850current tracing context. At trace production, information about the
851current context is saved into the specified scope field levels. At trace
852consumption, for each event, the current trace context is therefore
853readable by accessing the upper dynamic scopes.
854
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855
8567.2 Metadata Examples
d285084f 857
fcba70d4 858The grammar representing the CTF metadata is presented in
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859Appendix C. CTF Metadata Grammar. This section presents a rather ligher
860reading that consists in examples of CTF metadata, with template values:
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861
862trace {
fdf2bb05 863 major = value; /* Trace format version */
5ba9f198 864 minor = value;
fdf2bb05 865 uuid = "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"; /* Trace UUID */
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866 word_size = value;
867};
5ba9f198 868
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869stream {
870 id = stream_id;
fdf2bb05 871 /* Type 1 - Few event IDs; Type 2 - Many event IDs. See section 6.1. */
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872 event.header := event_header_1 OR event_header_2;
873 event.context := struct {
77a98c82 874 ...
3bf79539 875 };
4fa992a5 876 packet.context := struct {
77a98c82 877 ...
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878 };
879};
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880
881event {
3d13ef1a 882 name = event_name;
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883 id = value; /* Numeric identifier within the stream */
884 stream = stream_id;
4fa992a5 885 context := struct {
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886 ...
887 };
4fa992a5 888 fields := struct {
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889 ...
890 };
3bf79539 891};
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892
893/* More detail on types in section 4. Types */
894
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895/*
896 * Named types:
897 *
4fa992a5 898 * Type declarations behave similarly to the C standard.
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899 */
900
901typedef aliased_type_prefix aliased_type new_type aliased_type_postfix;
2152348f 902
3d13ef1a 903/* e.g.: typedef struct example new_type_name[10]; */
80fd2569 904
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905/*
906 * typealias
907 *
908 * The "typealias" declaration can be used to give a name (including
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909 * prefix/postfix) to a type. It should also be used to map basic C types
910 * (float, int, unsigned long, ...) to a CTF type. Typealias is a superset of
911 * "typedef": it also allows assignment of a simple variable identifier to a
912 * type.
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913 */
914
915typealias type_class {
80fd2569 916 ...
fcba70d4 917} : new_type_prefix new_type new_type_postfix;
2152348f 918
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919/*
920 * e.g.:
4fa992a5 921 * typealias integer {
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922 * size = 32;
923 * align = 32;
924 * signed = false;
fcba70d4 925 * } : struct page *;
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926 *
927 * typealias integer {
928 * size = 32;
929 * align = 32;
930 * signed = true;
931 * } : int;
3d13ef1a 932 */
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933
934struct name {
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935 ...
936};
5ba9f198 937
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938variant name {
939 ...
940};
941
cfc73fdc 942enum name <integer_type or size> {
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943 ...
944};
945
2152348f 946
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947/*
948 * Unnamed types, contained within compound type fields, typedef or typealias.
949 */
2152348f 950
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951struct {
952 ...
2152348f 953}
5ba9f198 954
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955variant {
956 ...
957}
958
4767a9e7 959enum <integer_type or size> {
80fd2569 960 ...
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961}
962
963typedef type new_type[length];
3bf79539 964
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965struct {
966 type field_name[length];
967}
968
969typedef type new_type[length_type];
970
971struct {
972 type field_name[length_type];
973}
974
975integer {
80fd2569 976 ...
2152348f 977}
3bf79539 978
2152348f 979floating_point {
80fd2569 980 ...
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981}
982
983struct {
984 integer_type field_name:size; /* GNU/C bitfield */
985}
986
987struct {
988 string field_name;
989}
3bf79539 990
fcba70d4 991
3bf79539 992A. Helper macros
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993
994The two following macros keep track of the size of a GNU/C structure without
995padding at the end by placing HEADER_END as the last field. A one byte end field
996is used for C90 compatibility (C99 flexible arrays could be used here). Note
997that this does not affect the effective structure size, which should always be
998calculated with the header_sizeof() helper.
999
1000#define HEADER_END char end_field
1001#define header_sizeof(type) offsetof(typeof(type), end_field)
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1002
1003
1004B. Stream Header Rationale
1005
1006An event stream is divided in contiguous event packets of variable size. These
1007subdivisions allow the trace analyzer to perform a fast binary search by time
1008within the stream (typically requiring to index only the event packet headers)
1009without reading the whole stream. These subdivisions have a variable size to
1010eliminate the need to transfer the event packet padding when partially filled
1011event packets must be sent when streaming a trace for live viewing/analysis.
1012An event packet can contain a certain amount of padding at the end. Dividing
1013streams into event packets is also useful for network streaming over UDP and
1014flight recorder mode tracing (a whole event packet can be swapped out of the
1015buffer atomically for reading).
1016
1017The stream header is repeated at the beginning of each event packet to allow
1018flexibility in terms of:
1019
1020 - streaming support,
1021 - allowing arbitrary buffers to be discarded without making the trace
1022 unreadable,
1023 - allow UDP packet loss handling by either dealing with missing event packet
1024 or asking for re-transmission.
1025 - transparently support flight recorder mode,
1026 - transparently support crash dump.
1027
1028The event stream header will therefore be referred to as the "event packet
1029header" throughout the rest of this document.
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1030
1031C. CTF Metadata Grammar
1032
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1033/*
1034 * Common Trace Format (CTF) Metadata Grammar.
1035 *
1036 * Inspired from the C99 grammar:
1037 * http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf (Annex A)
1038 *
1039 * Specialized for CTF needs by including only constant and declarations from
1040 * C99 (excluding function declarations), and by adding support for variants,
1041 * sequences and CTF-specific specifiers.
1042 */
1043
10441) Lexical grammar
1045
10461.1) Lexical elements
1047
1048token:
1049 keyword
1050 identifier
1051 constant
1052 string-literal
1053 punctuator
1054
10551.2) Keywords
1056
1057keyword: is one of
1058
1059const
1060char
1061double
1062enum
1063event
1064floating_point
1065float
1066integer
1067int
1068long
1069short
1070signed
1071stream
1072string
1073struct
1074trace
3e1e1a78 1075typealias
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1076typedef
1077unsigned
1078variant
1079void
1080_Bool
1081_Complex
1082_Imaginary
1083
1084
10851.3) Identifiers
1086
1087identifier:
1088 identifier-nondigit
1089 identifier identifier-nondigit
1090 identifier digit
1091
1092identifier-nondigit:
1093 nondigit
1094 universal-character-name
1095 any other implementation-defined characters
1096
1097nondigit:
1098 _
1099 [a-zA-Z] /* regular expression */
1100
1101digit:
1102 [0-9] /* regular expression */
1103
11041.4) Universal character names
1105
1106universal-character-name:
1107 \u hex-quad
1108 \U hex-quad hex-quad
1109
1110hex-quad:
1111 hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit
1112
11131.5) Constants
1114
1115constant:
1116 integer-constant
1117 enumeration-constant
1118 character-constant
1119
1120integer-constant:
1121 decimal-constant integer-suffix-opt
1122 octal-constant integer-suffix-opt
1123 hexadecimal-constant integer-suffix-opt
1124
1125decimal-constant:
1126 nonzero-digit
1127 decimal-constant digit
1128
1129octal-constant:
1130 0
1131 octal-constant octal-digit
1132
1133hexadecimal-constant:
1134 hexadecimal-prefix hexadecimal-digit
1135 hexadecimal-constant hexadecimal-digit
1136
1137hexadecimal-prefix:
1138 0x
1139 0X
1140
1141nonzero-digit:
1142 [1-9]
1143
1144integer-suffix:
1145 unsigned-suffix long-suffix-opt
1146 unsigned-suffix long-long-suffix
1147 long-suffix unsigned-suffix-opt
1148 long-long-suffix unsigned-suffix-opt
1149
1150unsigned-suffix:
1151 u
1152 U
1153
1154long-suffix:
1155 l
1156 L
1157
1158long-long-suffix:
1159 ll
1160 LL
1161
1162digit-sequence:
1163 digit
1164 digit-sequence digit
1165
1166hexadecimal-digit-sequence:
1167 hexadecimal-digit
1168 hexadecimal-digit-sequence hexadecimal-digit
1169
1170enumeration-constant:
1171 identifier
1172 string-literal
1173
1174character-constant:
1175 ' c-char-sequence '
1176 L' c-char-sequence '
1177
1178c-char-sequence:
1179 c-char
1180 c-char-sequence c-char
1181
1182c-char:
1183 any member of source charset except single-quote ('), backslash
1184 (\), or new-line character.
1185 escape-sequence
1186
1187escape-sequence:
1188 simple-escape-sequence
1189 octal-escape-sequence
1190 hexadecimal-escape-sequence
1191 universal-character-name
1192
1193simple-escape-sequence: one of
1194 \' \" \? \\ \a \b \f \n \r \t \v
1195
1196octal-escape-sequence:
1197 \ octal-digit
1198 \ octal-digit octal-digit
1199 \ octal-digit octal-digit octal-digit
1200
1201hexadecimal-escape-sequence:
1202 \x hexadecimal-digit
1203 hexadecimal-escape-sequence hexadecimal-digit
1204
12051.6) String literals
1206
1207string-literal:
1208 " s-char-sequence-opt "
1209 L" s-char-sequence-opt "
1210
1211s-char-sequence:
1212 s-char
1213 s-char-sequence s-char
1214
1215s-char:
1216 any member of source charset except double-quote ("), backslash
1217 (\), or new-line character.
1218 escape-sequence
1219
12201.7) Punctuators
1221
1222punctuator: one of
1223 [ ] ( ) { } . -> * + - < > : ; ... = ,
1224
1225
12262) Phrase structure grammar
1227
1228primary-expression:
1229 identifier
1230 constant
1231 string-literal
1232 ( unary-expression )
1233
1234postfix-expression:
1235 primary-expression
1236 postfix-expression [ unary-expression ]
1237 postfix-expression . identifier
1238 postfix-expressoin -> identifier
1239
1240unary-expression:
1241 postfix-expression
1242 unary-operator postfix-expression
1243
1244unary-operator: one of
1245 + -
1246
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1247assignment-operator:
1248 =
1249
1250constant-expression:
1251 unary-expression
1252
1253constant-expression-range:
1254 constant-expression ... constant-expression
1255
12562.2) Declarations:
1257
1258declaration:
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1259 declaration-specifiers ;
1260 declaration-specifiers storage-class-specifier declaration-specifiers declarator-list ;
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1261 ctf-specifier ;
1262
1263declaration-specifiers:
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1264 type-specifier declaration-specifiers-opt
1265 type-qualifier declaration-specifiers-opt
1266
1267declarator-list:
1268 declarator
1269 declarator-list , declarator
1270
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1271abstract-declarator-list:
1272 abstract-declarator
1273 abstract-declarator-list , abstract-declarator
1274
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1275storage-class-specifier:
1276 typedef
1277
1278type-specifier:
1279 void
1280 char
1281 short
1282 int
1283 long
1284 float
1285 double
1286 signed
1287 unsigned
1288 _Bool
1289 _Complex
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1290 struct-specifier
1291 variant-specifier
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1292 enum-specifier
1293 typedef-name
1294 ctf-type-specifier
1295
1296struct-specifier:
3b0f8e4d 1297 struct identifier-opt { struct-or-variant-declaration-list-opt }
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1298 struct identifier
1299
1300struct-or-variant-declaration-list:
1301 struct-or-variant-declaration
1302 struct-or-variant-declaration-list struct-or-variant-declaration
1303
1304struct-or-variant-declaration:
1305 specifier-qualifier-list struct-or-variant-declarator-list ;
550aca33 1306 declaration-specifiers storage-class-specifier declaration-specifiers declarator-list ;
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1307 typealias declaration-specifiers abstract-declarator-list : declaration-specifiers abstract-declarator-list ;
1308 typealias declaration-specifiers abstract-declarator-list : declarator-list ;
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1309
1310specifier-qualifier-list:
1311 type-specifier specifier-qualifier-list-opt
1312 type-qualifier specifier-qualifier-list-opt
1313
1314struct-or-variant-declarator-list:
1315 struct-or-variant-declarator
1316 struct-or-variant-declarator-list , struct-or-variant-declarator
1317
1318struct-or-variant-declarator:
1319 declarator
1320 declarator-opt : constant-expression
1321
1322variant-specifier:
1323 variant identifier-opt variant-tag-opt { struct-or-variant-declaration-list }
1324 variant identifier variant-tag
1325
1326variant-tag:
1327 < identifier >
1328
1329enum-specifier:
1330 enum identifier-opt { enumerator-list }
1331 enum identifier-opt { enumerator-list , }
1332 enum identifier
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1333 enum identifier-opt < declaration-specifiers > { enumerator-list }
1334 enum identifier-opt < declaration-specifiers > { enumerator-list , }
1335 enum identifier < declaration-specifiers >
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1336 enum identifier-opt < integer-constant > { enumerator-list }
1337 enum identifier-opt < integer-constant > { enumerator-list , }
1338 enum identifier < integer-constant >
1339
1340enumerator-list:
1341 enumerator
1342 enumerator-list , enumerator
1343
1344enumerator:
1345 enumeration-constant
1346 enumeration-constant = constant-expression
1347 enumeration-constant = constant-expression-range
1348
1349type-qualifier:
1350 const
1351
1352declarator:
1353 pointer-opt direct-declarator
1354
1355direct-declarator:
1356 identifier
1357 ( declarator )
1358 direct-declarator [ type-specifier ]
1359 direct-declarator [ constant-expression ]
1360
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1361abstract-declarator:
1362 pointer-opt direct-abstract-declarator
1363
1364direct-abstract-declarator:
1365 identifier-opt
1366 ( abstract-declarator )
1367 direct-abstract-declarator [ type-specifier ]
1368 direct-abstract-declarator [ constant-expression ]
1369 direct-abstract-declarator [ ]
1370
4fa992a5 1371pointer:
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1372 * type-qualifier-list-opt
1373 * type-qualifier-list-opt pointer
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1374
1375type-qualifier-list:
1376 type-qualifier
1377 type-qualifier-list type-qualifier
1378
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1379typedef-name:
1380 identifier
1381
13822.3) CTF-specific declarations
1383
1384ctf-specifier:
1385 event { ctf-assignment-expression-list-opt }
1386 stream { ctf-assignment-expression-list-opt }
1387 trace { ctf-assignment-expression-list-opt }
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1388 typealias declaration-specifiers abstract-declarator-list : declaration-specifiers abstract-declarator-list ;
1389 typealias declaration-specifiers abstract-declarator-list : declarator-list ;
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1390
1391ctf-type-specifier:
1392 floating_point { ctf-assignment-expression-list-opt }
1393 integer { ctf-assignment-expression-list-opt }
1394 string { ctf-assignment-expression-list-opt }
1395
1396ctf-assignment-expression-list:
1397 ctf-assignment-expression
1398 ctf-assignment-expression-list ; ctf-assignment-expression
1399
1400ctf-assignment-expression:
1401 unary-expression assignment-operator unary-expression
1402 unary-expression type-assignment-operator type-specifier
550aca33 1403 declaration-specifiers storage-class-specifier declaration-specifiers declarator-list
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1404 typealias declaration-specifiers abstract-declarator-list : declaration-specifiers abstract-declarator-list
1405 typealias declaration-specifiers abstract-declarator-list : declarator-list
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