3. Event stream
An event stream is divided in contiguous event packets of variable size. These
-subdivisions have a variable size. An event packet can contain a certain amount
-of padding at the end. The rationale for the event stream design choices is
-explained in Appendix B. Stream Header Rationale.
-
-An event stream is divided in contiguous event packets of variable size. These
-subdivisions have a variable size. An event packet can contain a certain amount
-of padding at the end. The stream header is repeated at the beginning of each
-event packet.
+subdivisions have a variable size. An event packet can contain a certain
+amount of padding at the end. The stream header is repeated at the
+beginning of each event packet. The rationale for the event stream
+design choices is explained in Appendix B. Stream Header Rationale.
The event stream header will therefore be referred to as the "event packet
header" throughout the rest of this document.
We define "byte-packed" types as aligned on the byte size, namely 8-bit.
We define "bit-packed" types as following on the next bit, as defined by the
-"bitfields" section.
+"Integers" section.
All basic types, except bitfields, are either aligned on an architecture-defined
specific alignment or byte-packed, depending on the architecture preference.
Architectures providing fast unaligned write byte-packed basic types to save
space, aligning each type on byte boundaries (8-bit). Architectures with slow
unaligned writes align types on specific alignment values. If no specific
-alignment is declared for a type nor its parents, it is assumed to be bit-packed
-for bitfields and byte-packed for other types.
+alignment is declared for a type, it is assumed to be bit-packed for
+integers with size not multiple of 8 bits and for gcc bitfields. All
+other types are byte-packed.
Metadata attribute representation of a specific alignment:
5 - Event Payload (as specified by the event metadata)
This structure defines an implicit dynamic scoping, where variants
-located in structures with higher number can refer to the fields of
-structures with lower number. See Section 7.2 Metadata Scopes for more
-detail.
+located in inner structures (those with a higher number in the listing
+above) can refer to the fields of outer structures (with lower number in
+the listing above). See Section 7.2 Metadata Scopes for more detail.
6.1 Event Header
7. Metadata
-The meta-data is located in a stream named "metadata". It is made of "event
-packets", which each start with an event packet header. The event type within
-the metadata stream have no event header nor event context. Each event only
-contains a null-terminated "string" payload, which is a metadata description
-entry. The events are packed one next to another. Each event packet start with
-an event packet header, which contains, amongst other fields, the magic number
-and trace UUID. The trace UUID is represented as a string of hexadecimal digits
-and dashes "-".
-
-The metadata can be parsed by reading through the metadata strings, skipping
-newlines and null-characters. Type names are made of a single identifier, and
-can be surrounded by prefix/postfix. Text contained within "/*" and "*/", as
-well as within "//" and end of line, are treated as comments. Boolean values can
-be represented as true, TRUE, or 1 for true, and false, FALSE, or 0 for false.
+The meta-data is located in a stream identified by its name: "metadata".
+It is made of "event packets", which each start with an event packet
+header. The event type within the metadata stream have no event header
+nor event context. Each event only contains a null-terminated "string"
+payload, which is a metadata description entry. The events are packed
+one next to another. Each event packet start with an event packet
+header, which contains, amongst other fields, the magic number and trace
+UUID. In the event packet header, the trace UUID is represented as an
+array of bytes. Within the string-based metadata description, the trace
+UUID is represented as a string of hexadecimal digits and dashes "-".
+
+The metadata can be parsed by reading through the metadata strings,
+skipping null-characters. Type names are made of a single identifier,
+and can be surrounded by prefix/postfix. Text contained within "/*" and
+"*/", as well as within "//" and end of line, are treated as comments.
+Boolean values can be represented as true, TRUE, or 1 for true, and
+false, FALSE, or 0 for false.
7.1 Declaration vs Definition
the newly defined type name (for typedef), or the field name (for
declarations located within structure and variants). Array and sequence,
declared with square brackets ("[" "]"), are part of the declarator,
-similarly to C99.
+similarly to C99. The enumeration type specifier and variant tag name
+(both specified with "<" ">") are part of the type specifier.
A definition associates a type to a location in the event structure
-hierarchy (see Section 6).
+hierarchy (see Section 6). This association is denoted by ":=", as shown
+in Section 7.3.
7.2 Metadata Scopes
Each of "trace", "stream", "event", "struct" and "variant" have their own
nestable declaration scope, within which types can be declared using "typedef"
and "typealias". A root declaration scope also contains all declarations
-located outside of any of the aforementioned declarations. An innermost
+located outside of any of the aforementioned declarations. An inner
declaration scope can refer to type declared within its container
-lexical scope prior to the innermost declaration scope. Redefinition of
-a typedef or typealias is not valid, although hiding an uppermost scope
+lexical scope prior to the inner declaration scope. Redefinition of a
+typedef or typealias is not valid, although hiding an upper scope
typedef or typealias is allowed within a sub-scope.
7.2.2 Dynamic Scope
-For variant tag definition only, the dynamic scope used to look up the
-location of the associated tag field consists in the lexical scope of
-the structures where the variant is declared, extended with the implicit
-dynamic scope specified by the event structure hierarchy presented at
-the beginning of Section 6. Therefore, lower levels in the dynamic scope
-(e.g. event context) can refer to a tag field located in upper levels
-(e.g. in the event header) by specifying, in this case,
-"header.field_name" as tag identifier. This allows, for instance, the
-event context to define a variant referring to the "id" field of the
-event header as selector.
+A dynamic scope consists in the lexical scope augmented with the
+implicit event structure definition hierarchy presented at Section 6.
+The dynamic scope is only used for variant tag definitions. It is used
+at definition time to look up the location of the tag field associated
+with a variant.
+
+Therefore, variants in lower levels in the dynamic scope (e.g. event
+context) can refer to a tag field located in upper levels (e.g. in the
+event header) by specifying, in this case, the associated tag with
+<header.field_name>. This allows, for instance, the event context to
+define a variant referring to the "id" field of the event header as
+selector.
The target dynamic scope must be specified explicitly when referring to
a field outside of the local static scope. The dynamic scope prefixes
are thus:
- - Stream Packet Context: "stream.packet.context.",
- - Event Header: "stream.event.header.",
- - Stream Event Context: "stream.event.context.",
- - Event Context: "event.context.",
- - Event Payload: "event.fields.".
+ - Stream Packet Context: <stream.packet.context. >,
+ - Event Header: <stream.event.header. >,
+ - Stream Event Context: <stream.event.context. >,
+ - Event Context: <event.context. >,
+ - Event Payload: <event.fields. >.
Multiple declarations of the same field name within a single scope is
not valid. It is however valid to re-use the same field name in
scope must be specified when a variant refers to a tag field located in
a different dynamic scope.
+The information available in the dynamic scopes can be thought of as the
+current tracing context. At trace production, information about the
+current context is saved into the specified scope field levels. At trace
+consumption, for each event, the current trace context is therefore
+readable by accessing the upper dynamic scopes.
-7.2 Metadata Examples
+
+7.3 Metadata Examples
The grammar representing the CTF metadata is presented in
Appendix C. CTF Metadata Grammar. This section presents a rather ligher
assignment-operator:
=
+type-assignment-operator:
+ :=
+
constant-expression:
unary-expression
2.2) Declarations:
declaration:
- declaration-specifiers ;
- declaration-specifiers storage-class-specifier declaration-specifiers declarator-list ;
+ declaration-specifiers declarator-list-opt ;
ctf-specifier ;
declaration-specifiers:
+ storage-class-specifier declaration-specifiers-opt
type-specifier declaration-specifiers-opt
type-qualifier declaration-specifiers-opt
unsigned
_Bool
_Complex
+ _Imaginary
struct-specifier
variant-specifier
enum-specifier
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