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5ba9f198 MD |
1 | |
2 | RFC: Common Trace Format Proposal for Linux (v1) | |
3 | ||
4 | Mathieu Desnoyers, EfficiOS Inc. | |
5 | ||
6 | The goal of the present document is to propose a trace format that suits the | |
7 | needs of the embedded, telecom, high-performance and kernel communities. It is | |
8 | based on the Common Trace Format Requirements (v1.4) document. It is designed to | |
9 | be natively generated by tracing of a Linux kernel and Linux user-space | |
10 | applications written in C/C++. | |
11 | ||
12 | A reference implementation of a library to read and write this trace format is | |
13 | being implemented within the BabelTrace project, a converter between trace | |
14 | formats. The development tree is available at: | |
15 | ||
16 | git tree: git://git.efficios.com/babeltrace.git | |
17 | gitweb: http://git.efficios.com/?p=babeltrace.git | |
18 | ||
19 | ||
20 | 1. Preliminary definitions | |
21 | ||
22 | - Trace: An ordered sequence of events. | |
23 | - Section: Group of events, containing a subset of the trace event types. | |
24 | - Packet: A sequence of physically contiguous events within a section. | |
25 | - Event: This is the basic entry in a trace. (aka: a trace record). | |
26 | - An event identifier (ID) relates to the class (a type) of event within | |
27 | a section. | |
28 | e.g. section: high_throughput, event: irq_entry. | |
29 | - An event (or event record) relates to a specific instance of an event | |
30 | class. | |
31 | e.g. section: high_throughput, event: irq_entry, at time X, on CPU Y | |
32 | ||
33 | ||
34 | 2. High-level representation of a trace | |
35 | ||
36 | A trace is divided into multiple trace streams, each representing an information | |
37 | stream specific to: | |
38 | ||
39 | - a section, | |
40 | - a processor. | |
41 | ||
42 | A trace "section" consists of a collection of trace streams (typically one trace | |
43 | stream per cpu) containing a subset of the trace event types. | |
44 | ||
45 | Because each trace stream is appended to while a trace is being recorded, each | |
46 | is associated with a separate file for disk output. Therefore, a trace stored to | |
47 | disk can be represented as a directory containing one file per section. | |
48 | ||
49 | A metadata section contains information on trace event types. It describes: | |
50 | ||
51 | - Trace version. | |
52 | - Types available. | |
53 | - Per-section event header description. | |
54 | - Per-section event header selection. | |
55 | - Per-section event context fields. | |
56 | - Per-event | |
57 | - Event type to section mapping. | |
58 | - Event type to name mapping. | |
59 | - Event type to ID mapping. | |
60 | - Event fields description. | |
61 | ||
62 | ||
63 | 3. Trace Section | |
64 | ||
65 | A trace section is divided in contiguous packets of variable size. These | |
66 | subdivisions allow the trace analyzer to perform a fast binary search by time | |
67 | within the section (typically requiring to index only the packet headers) | |
68 | without reading the whole section. These subdivisions have a variable size to | |
69 | eliminate the need to transfer the packet padding when partially filled packets | |
70 | must be sent when streaming a trace for live viewing/analysis. Dividing sections | |
71 | into packets is also useful for network streaming over UDP and flight recorder | |
72 | mode tracing (a whole packet can be swapped out of the buffer atomically for | |
73 | reading). | |
74 | ||
75 | The section header is repeated at the beginning of each packet to allow | |
76 | flexibility in terms of: | |
77 | ||
78 | - streaming support, | |
79 | - allowing arbitrary buffers to be discarded without making the trace | |
80 | unreadable, | |
81 | - allow UDP packet loss handling by either dealing with missing packet or | |
82 | asking for re-transmission. | |
83 | - transparently support flight recorder mode, | |
84 | - transparently support crash dump. | |
85 | ||
86 | The section header will therefore be referred to as the "packet header" | |
87 | thorough the rest of this document. | |
88 | ||
89 | ||
90 | 4. Types | |
91 | ||
92 | 4.1 Basic types | |
93 | ||
94 | A basic type is a scalar type, as described in this section. | |
95 | ||
96 | 4.1.1 Type inheritance | |
97 | ||
98 | Type specifications can be inherited to allow deriving concrete types from an | |
99 | abstract type. For example, see the uint32_t type derived from the "integer" | |
100 | abstract type below ("Integers" section). Concrete types have a precise binary | |
101 | representation in the trace. Abstract types have methods to read and write these | |
102 | types, but must be derived into a concrete type to be usable in an event field. | |
103 | ||
104 | Concrete types inherit from abstract types. Abstract types can inherit from | |
105 | other abstract types. | |
106 | ||
107 | 4.1.2 Alignment | |
108 | ||
109 | We define "byte-packed" types as aligned on the byte size, namely 8-bit. | |
110 | We define "bit-packed" types as following on the next bit, as defined by the | |
111 | "bitfields" section. | |
112 | We define "natural alignment" of a basic type as the lesser value between the | |
113 | type size and the architecture word size. | |
114 | ||
115 | All basic types, except bitfields, are either aligned on their "natural" | |
116 | alignment or byte-packed, depending on the architecture preference. | |
117 | Architectures providing fast unaligned writes byte-packed basic types to save | |
118 | space, aligning each type on byte boundaries (8-bit). Architectures with slow | |
119 | unaligned writes align types on the lesser value between their size and the | |
120 | architecture word size (the type "natural" alignment on the architecture). | |
121 | ||
122 | Note that the natural alignment for 64-bit integers and double-precision | |
123 | floating point values is fixed to 32-bit on a 32-bit architecture, but to 64-bit | |
124 | for a 64-bit architecture. | |
125 | ||
126 | Metadata attribute representation: | |
127 | ||
128 | align = value; /* value in bits */ | |
129 | ||
130 | 4.1.3 Byte order | |
131 | ||
132 | By default, target architecture endianness is used. Byte order can be overridden | |
133 | for a basic type by specifying a "byte_order" attribute. Typical use-case is to | |
134 | specify the network byte order (big endian: "be") to save data captured from the | |
135 | network into the trace without conversion. If not specified, the byte order is | |
136 | native. | |
137 | ||
138 | Metadata representation: | |
139 | ||
140 | byte_order = native OR network OR be OR le; /* network and be are aliases */ | |
141 | ||
142 | 4.1.4 Size | |
143 | ||
144 | Type size, in bits, for integers and floats is that returned by "sizeof()" in C | |
145 | multiplied by CHAR_BIT. | |
146 | We require the size of "char" and "unsigned char" types (CHAR_BIT) to be fixed | |
147 | to 8 bits for cross-endianness compatibility. | |
148 | ||
149 | Metadata representation: | |
150 | ||
151 | size = value; (value is in bits) | |
152 | ||
153 | 4.1.5 Integers | |
154 | ||
155 | Signed integers are represented in two-complement. Integer alignment, size, | |
156 | signedness and byte ordering are defined in the metadata. Integers aligned on | |
157 | byte size (8-bit) and with length multiple of byte size (8-bit) correspond to | |
158 | the C99 standard integers. In addition, integers with alignment and/or size that | |
159 | are _not_ a multiple of the byte size are permitted; these correspond to the C99 | |
160 | standard bitfields, with the added specification that the CTF integer bitfields | |
161 | have a fixed binary representation. A MIT-licensed reference implementation of | |
162 | the CTF portable bitfields is available at: | |
163 | ||
164 | http://git.efficios.com/?p=babeltrace.git;a=blob;f=include/babeltrace/bitfield.h | |
165 | ||
166 | Binary representation of integers: | |
167 | ||
168 | - On little and big endian: | |
169 | - Within a byte, high bits correspond to an integer high bits, and low bits | |
170 | correspond to low bits. | |
171 | - On little endian: | |
172 | - Integer across multiple bytes are placed from the less significant to the | |
173 | most significant. | |
174 | - Consecutive integers are placed from lower bits to higher bits (even within | |
175 | a byte). | |
176 | - On big endian: | |
177 | - Integer across multiple bytes are placed from the most significant to the | |
178 | less significant. | |
179 | - Consecutive integers are placed from higher bits to lower bits (even within | |
180 | a byte). | |
181 | ||
182 | This binary representation is derived from the bitfield implementation in GCC | |
183 | for little and big endian. However, contrary to what GCC does, integers can | |
184 | cross units boundaries (no padding is required). Padding can be explicitely | |
185 | added (see 4.1.6 GNU/C bitfields) to follow the GCC layout if needed. | |
186 | ||
187 | Metadata representation: | |
188 | ||
189 | abstract_type integer { | |
190 | signed = true OR false; /* default false */ | |
191 | byte_order = native OR network OR be OR le; /* default native */ | |
192 | size = value; /* value in bits, no default */ | |
193 | align = value; /* value in bits */ | |
194 | } | |
195 | ||
196 | Example of type inheritance (creation of a concrete type uint32_t): | |
197 | ||
198 | type uint32_t { | |
199 | parent = integer; | |
200 | size = 8; | |
201 | signed = false; | |
202 | align = 32; | |
203 | } | |
204 | ||
205 | Definition of a 5-bit signed bitfield: | |
206 | ||
207 | type int5_t { | |
208 | parent = integer; | |
209 | size = 5; | |
210 | signed = true; | |
211 | align = 1; | |
212 | } | |
213 | ||
214 | 4.1.6 GNU/C bitfields | |
215 | ||
216 | The GNU/C bitfields follow closely the integer representation, with a | |
217 | particularity on alignment: if a bitfield cannot fit in the current unit, the | |
218 | unit is padded and the bitfield starts at the following unit. We therefore need | |
219 | to express the extra "unit size" information. | |
220 | ||
221 | Metadata representation: | |
222 | ||
223 | abstract_type gcc_bitfield { | |
224 | parent = integer; | |
225 | unit_size = value; | |
226 | } | |
227 | ||
228 | As an example, the following structure declared in C compiled by GCC: | |
229 | ||
230 | struct example { | |
231 | short a:12; | |
232 | short b:5; | |
233 | }; | |
234 | ||
235 | Would correspond to the following structure, aligned on the largest element | |
236 | (short). The second bitfield would be aligned on the next unit boundary, because | |
237 | it would not fit in the current unit. | |
238 | ||
239 | type struct_example { | |
240 | parent = struct; | |
241 | fields = { | |
242 | { | |
243 | type { | |
244 | parent = gcc_bitfield; | |
245 | unit_size = 16; /* sizeof(short) */ | |
246 | size = 12; | |
247 | signed = true; | |
248 | align = 1; | |
249 | }, | |
250 | a, | |
251 | }, | |
252 | { | |
253 | type { | |
254 | parent = gcc_bitfield; | |
255 | unit_size = 16; /* sizeof(short) */ | |
256 | size = 5; | |
257 | signed = true; | |
258 | align = 1; | |
259 | }, | |
260 | b, | |
261 | }, | |
262 | }; | |
263 | } | |
264 | ||
265 | 4.1.7 Floating point | |
266 | ||
267 | The floating point values byte ordering is defined in the metadata. | |
268 | ||
269 | Floating point values follow the IEEE 754-2008 standard interchange formats. | |
270 | Description of the floating point values include the exponent and mantissa size | |
271 | in bits. Some requirements are imposed on the floating point values: | |
272 | ||
273 | - FLT_RADIX must be 2. | |
274 | - mant_dig is the number of digits represented in the mantissa. It is specified | |
275 | by the ISO C99 standard, section 5.2.4, as FLT_MANT_DIG, DBL_MANT_DIG and | |
276 | LDBL_MANT_DIG as defined by <float.h>. | |
277 | - exp_dig is the number of digits represented in the exponent. Given that | |
278 | mant_dig is one bit more than its actual size in bits (leading 1 is not | |
279 | needed) and also given that the sign bit always takes one bit, exp_dig can be | |
280 | specified as: | |
281 | ||
282 | - sizeof(float) * CHAR_BIT - FLT_MANT_DIG | |
283 | - sizeof(double) * CHAR_BIT - DBL_MANT_DIG | |
284 | - sizeof(long double) * CHAR_BIT - LDBL_MANT_DIG | |
285 | ||
286 | Metadata representation: | |
287 | ||
288 | abstract_type floating_point { | |
289 | exp_dig = value; | |
290 | mant_dig = value; | |
291 | byte_order = native OR network OR be OR le; | |
292 | } | |
293 | ||
294 | Example of type inheritance: | |
295 | ||
296 | type float { | |
297 | exp_dig = 8; /* sizeof(float) * CHAR_BIT - FLT_MANT_DIG */ | |
298 | mant_dig = 24; /* FLT_MANT_DIG */ | |
299 | byte_order = native; | |
300 | } | |
301 | ||
302 | TODO: define NaN, +inf, -inf behavior. | |
303 | ||
304 | 4.1.8 Enumerations | |
305 | ||
306 | Enumerations are a mapping between an integer type and a table of strings. The | |
307 | numerical representation of the enumeration follows the integer type specified | |
308 | by the metadata. The enumeration mapping table is detailed in the enumeration | |
309 | description within the metadata. | |
310 | ||
311 | abstract_type enum { | |
312 | .parent = integer; | |
313 | .map = { | |
314 | { value , string }, | |
315 | { value , string }, | |
316 | { value , string }, | |
317 | ... | |
318 | }; | |
319 | } | |
320 | ||
321 | ||
322 | 4.2 Compound types | |
323 | ||
324 | 4.2.1 Structures | |
325 | ||
326 | Structures are aligned on the largest alignment required by basic types | |
327 | contained within the structure. (This follows the ISO/C standard for structures) | |
328 | ||
329 | Metadata representation: | |
330 | ||
331 | abstract_type struct { | |
332 | fields = { | |
333 | { field_type, field_name }, | |
334 | { field_type, field_name }, | |
335 | ... | |
336 | }; | |
337 | } | |
338 | ||
339 | Example: | |
340 | ||
341 | type struct_example { | |
342 | parent = struct; | |
343 | fields = { | |
344 | { | |
345 | type { /* Nameless type */ | |
346 | parent = integer; | |
347 | size = 16; | |
348 | signed = true; | |
349 | align = 16; | |
350 | }, | |
351 | first_field_name, | |
352 | }, | |
353 | { | |
354 | uint64_t, /* Named type declared in the metadata */ | |
355 | second_field_name, | |
356 | } | |
357 | }; | |
358 | } | |
359 | ||
360 | The fields are placed in a sequence next to each other. They each possess a | |
361 | field name, which is a unique identifier within the structure. | |
362 | ||
363 | 4.2.2 Arrays | |
364 | ||
365 | Arrays are fixed-length. Their length is declared in the type declaration within | |
366 | the metadata. They contain an array of "inner type" elements, which can refer to | |
367 | any type not containing the type of the array being declared (no circular | |
368 | dependency). | |
369 | ||
370 | Metadata representation: | |
371 | ||
372 | abstract_type array { | |
373 | length = value; | |
374 | elem_type = type; | |
375 | } | |
376 | ||
377 | E.g.: | |
378 | ||
379 | type example_array { | |
380 | parent = array; | |
381 | length = 10; | |
382 | elem_type = uint32_t; | |
383 | } | |
384 | ||
385 | 4.2.3 Sequences | |
386 | ||
387 | Sequences are dynamically-sized arrays. They start with an integer that specify | |
388 | the length of the sequence, followed by an array of "inner type" elements. | |
389 | ||
390 | abstract_type sequence { | |
391 | length_type = type; /* Inheriting from integer */ | |
392 | elem_type = type; | |
393 | } | |
394 | ||
395 | The integer type follows the integer types specifications, and the sequence | |
396 | elements follow the "array" specifications. | |
397 | ||
398 | 4.2.4 Strings | |
399 | ||
400 | Strings are an array of bytes of variable size and are terminated by a '\0' | |
401 | "NULL" character. Their encoding is described in the metadata. In absence of | |
402 | encoding attribute information, the default encoding is UTF-8. | |
403 | ||
404 | abstract_type string { | |
405 | encoding = UTF8 OR ASCII; | |
406 | } | |
407 | ||
408 | ||
409 | 5. Trace Packet Header | |
410 | ||
411 | - Aligned on page size. Fixed size. Fields aligned on their natural size or | |
412 | packed (depending on the architecture preference). | |
413 | No padding at the end of the trace packet header. Native architecture byte | |
414 | ordering. | |
415 | - Magic number (CTF magic numbers: 0xC1FC1FC1 and its reverse endianness | |
416 | representation: 0xC11FFCC1) It needs to have a non-symmetric bytewise | |
417 | representation. Used to distinguish between big and little endian traces (this | |
418 | information is determined by knowing the endianness of the architecture | |
419 | reading the trace and comparing the magic number against its value and the | |
420 | reverse, 0xC11FFCC1). This magic number specifies that we use the CTF metadata | |
421 | description language described in this document. Different magic numbers | |
422 | should be used for other metadata description languages. | |
423 | - Session ID, used to ensure the packet match the metadata used. | |
424 | (note: we cannot use a metadata checksum because metadata can be appended to | |
425 | while tracing is active) | |
426 | - Packet content size (in bytes). | |
427 | - Packet size (in bytes, includes padding). | |
428 | - Packet content checksum (optional). Checksum excludes the packet header. | |
429 | - Per-section packet sequence count (to deal with UDP packet loss). The number | |
430 | of significant sequence counter bits should also be present, so wrap-arounds | |
431 | are deal with correctly. | |
432 | - Timestamp at the beginning and end of the packet. Should include all | |
433 | event timestamps contained therein. | |
434 | - Events discarded count | |
435 | - Snapshot of a per-section free-running counter, counting the number of | |
436 | events discarded that were supposed to be written in the section prior to | |
437 | the first event in the packet. | |
438 | * Note: producer-consumer buffer full condition should fill the current | |
439 | packet with padding so we know exactly where events have been | |
440 | discarded. | |
441 | - Lossless compression scheme used for the packet content. Applied directly to | |
442 | raw data. | |
443 | 0: no compression scheme | |
444 | 1: bzip2 | |
445 | 2: gzip | |
446 | - Cypher used for the packet content. Applied after compression. | |
447 | 0: no encryption | |
448 | 1: AES | |
449 | - Checksum scheme used for the packet content. Applied after encryption. | |
450 | 0: no checksum | |
451 | 1: md5 | |
452 | 2: sha1 | |
453 | 3: crc32 | |
454 | ||
455 | type packet_header { | |
456 | parent = struct; | |
457 | fields = { | |
458 | { uint32_t, magic }, | |
459 | { uint32_t, session_id }, | |
460 | { uint32_t, content_size }, | |
461 | { uint32_t, packet_size }, | |
462 | { uint32_t, checksum }, | |
463 | { uint32_t, section_packet_count }, | |
464 | { uint64_t, timestamp_begin } | |
465 | { uint64_t, timestamp_end } | |
466 | [ uint32_t, events_discarded }, | |
467 | { uint8_t, section_packet_count_bits }, /* Significant counter bits */ | |
468 | { uint8_t, compression_scheme }, | |
469 | { uint8_t, encryption_scheme }, | |
470 | { uint8_t, checksum }, | |
471 | }; | |
472 | }; | |
473 | ||
474 | ||
475 | 6. Event Structure | |
476 | ||
477 | The overall structure of an event is: | |
478 | ||
479 | - Event Header (as specifed by the section metadata) | |
480 | - Extended Event Header (as specified by the event header) | |
481 | - Event Context (as specified by the section metadata) | |
482 | - Event Payload (as specified by the event metadata) | |
483 | ||
484 | ||
485 | 6.1 Event Header | |
486 | ||
487 | One major factor can vary between sections: the number of event IDs assigned to | |
488 | a section. Luckily, this information tends to stay relatively constant (modulo | |
489 | event registration while trace is being recorded), so we can specify different | |
490 | representations for sections containing few event IDs and sections containing | |
491 | many event IDs, so we end up representing the event ID and timestamp as densely | |
492 | as possible in each case. | |
493 | ||
494 | We therefore provide two types of events headers. Type 1 accommodates sections | |
495 | with less than 31 event IDs. Type 2 accommodates sections with 31 or more event | |
496 | IDs. | |
497 | ||
498 | The "extended headers" are used in the rare occasions where the information | |
499 | cannot be represented in the ranges available in the event header. | |
500 | ||
501 | Types uintX_t represent an X-bit unsigned integer. | |
502 | ||
503 | ||
504 | 6.1.1 Type 1 - Few event IDs | |
505 | ||
506 | - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture | |
507 | preference). | |
508 | - Fixed size: 32 bits. | |
509 | - Native architecture byte ordering. | |
510 | ||
511 | type event_header_1 { | |
512 | parent = struct; | |
513 | fields = { | |
514 | { uint5_t, id }, /* | |
515 | * id: range: 0 - 30. | |
516 | * id 31 is reserved to indicate a following | |
517 | * extended header. | |
518 | */ | |
519 | { uint27_t, timestamp }, | |
520 | }; | |
521 | }; | |
522 | ||
523 | The end of a type 1 header is aligned on a 32-bit boundary (or packed). | |
524 | ||
525 | ||
526 | 6.1.2 Extended Type 1 Event Header | |
527 | ||
528 | - Follows struct event_header_1, which is aligned on 32-bit, so no need to | |
529 | realign. | |
530 | - Fixed size: 96 bits. | |
531 | - Native architecture byte ordering. | |
532 | ||
533 | type event_header_1_ext { | |
534 | parent = struct; | |
535 | fields = { | |
536 | { uint32_t, id }, /* 32-bit event IDs */ | |
537 | { uint64_t, timestamp }, /* 64-bit timestamps */ | |
538 | }; | |
539 | }; | |
540 | ||
541 | The end of a type 1 extended header is aligned on the natural alignment of a | |
542 | 64-bit integer (or 8-bit if byte-packed). | |
543 | ||
544 | ||
545 | 6.1.3 Type 2 - Many event IDs | |
546 | ||
547 | - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture | |
548 | preference). | |
549 | - Fixed size: 48 bits. | |
550 | - Native architecture byte ordering. | |
551 | ||
552 | type event_header_2 { | |
553 | parent = struct; | |
554 | fields = { | |
555 | { uint32_t, timestamp }, | |
556 | { uint16_t, id }, /* | |
557 | * id: range: 0 - 65534. | |
558 | * id 65535 is reserved to indicate a following | |
559 | * extended header. | |
560 | */ | |
561 | }; | |
562 | }; | |
563 | ||
564 | The end of a type 2 header is aligned on a 16-bit boundary (or 8-bit if | |
565 | byte-packed). | |
566 | ||
567 | ||
568 | 6.1.4 Extended Type 2 Event Header | |
569 | ||
570 | - Follows struct event_header_2, which alignment end on a 16-bit boundary, so | |
571 | we need to align on 64-bit integer natural alignment (or 8-bit if | |
572 | byte-packed). | |
573 | - Fixed size: 96 bits. | |
574 | - Native architecture byte ordering. | |
575 | ||
576 | type event_header_2_ext { | |
577 | parent = struct; | |
578 | fields = { | |
579 | { uint64_t, timestamp }, /* 64-bit timestamps */ | |
580 | { uint32_t, id }, /* 32-bit event IDs */ | |
581 | }; | |
582 | }; | |
583 | ||
584 | The end of a type 2 extended header is aligned on the natural alignment of a | |
585 | 32-bit integer (or 8-bit if byte-packed). | |
586 | ||
587 | ||
588 | 6.2 Event Context | |
589 | ||
590 | The event context contains information relative to the current event. The choice | |
591 | and meaning of this information is specified by the metadata "section" | |
592 | information. For this trace format, event context is usually empty, except when | |
593 | the metadata "section" information specifies otherwise by declaring a non-empty | |
594 | structure for the event context. An example of event context is to save the | |
595 | event payload size with each event, or to save the current PID with each event. | |
596 | ||
597 | 6.2.1 Event Context Description | |
598 | ||
599 | Event context example. These are declared within the section declaration within | |
600 | the metadata. | |
601 | ||
602 | type per_section_event_ctx { | |
603 | parent = struct; | |
604 | fields = { | |
605 | { uint, pid }, | |
606 | { uint16_t, payload_size }, | |
607 | }; | |
608 | }; | |
609 | ||
610 | ||
611 | 6.3 Event Payload | |
612 | ||
613 | An event payload contains fields specific to a given event type. The fields | |
614 | belonging to an event type are described in the event-specific metadata | |
615 | within a structure type. | |
616 | ||
617 | 6.3.1 Padding | |
618 | ||
619 | No padding at the end of the event payload. This differs from the ISO/C standard | |
620 | for structures, but follows the CTF standard for structures. In a trace, even | |
621 | though it makes sense to align the beginning of a structure, it really makes no | |
622 | sense to add padding at the end of the structure, because structures are usually | |
623 | not followed by a structure of the same type. | |
624 | ||
625 | This trick can be done by adding a zero-length "end" field at the end of the C | |
626 | structures, and by using the offset of this field rather than using sizeof() | |
627 | when calculating the size of a structure (see section "A.1 Helper macros"). | |
628 | ||
629 | 6.3.2 Alignment | |
630 | ||
631 | The event payload is aligned on the largest alignment required by types | |
632 | contained within the payload. (This follows the ISO/C standard for structures) | |
633 | ||
634 | ||
635 | ||
636 | 7. Metadata | |
637 | ||
638 | The meta-data is located in a tracefile section named "metadata". It is made of | |
639 | "packets", which each start with a packet header. The event type within the | |
640 | metadata section have no event header nor event context. Each event only | |
641 | contains a null-terminated "string" payload, which is a metadata description | |
642 | entry. The events are packed one next to another. Each packet start with a | |
643 | packet header, which contains, amongst other fields, the session ID and magic | |
644 | number. | |
645 | ||
646 | The metadata can be parsed by reading through the metadata strings, skipping | |
647 | spaces, newlines and null-characters. | |
648 | ||
649 | trace { | |
650 | major = value; /* Trace format version */ | |
651 | minor = value; | |
652 | } | |
653 | ||
654 | section { | |
655 | name = section_name; | |
656 | event { | |
657 | /* Type 1 - Few event IDs; Type 2 - Many event IDs */ | |
658 | header_type = type1 OR type2; | |
659 | context { | |
660 | event_size = true OR false; /* Includes event size field or not */ | |
661 | } | |
662 | } | |
663 | } | |
664 | ||
665 | event { | |
666 | name = event_name; | |
667 | id = value; /* Numeric identifier within the section */ | |
668 | section = section_name; | |
669 | fields = type inheriting from "struct" abstract type. | |
670 | } | |
671 | ||
672 | /* More detail on types in section 4. Types */ | |
673 | ||
674 | /* Named types */ | |
675 | type typename { | |
676 | ... | |
677 | } | |
678 | ||
679 | /* Unnamed types, contained within compound type fields */ | |
680 | type { | |
681 | ... | |
682 | } | |
683 | ||
684 | A.1 Helper macros | |
685 | ||
686 | The two following macros keep track of the size of a GNU/C structure without | |
687 | padding at the end by placing HEADER_END as the last field. A one byte end field | |
688 | is used for C90 compatibility (C99 flexible arrays could be used here). Note | |
689 | that this does not affect the effective structure size, which should always be | |
690 | calculated with the header_sizeof() helper. | |
691 | ||
692 | #define HEADER_END char end_field | |
693 | #define header_sizeof(type) offsetof(typeof(type), end_field) |