Explain dynamic scope vs current context

[ctf.git] / common-trace-format-proposal.txt

diff --git a/common-trace-format-proposal.txt b/common-trace-format-proposal.txt
index a036b30625c7c3db484e879254146ae424ff2af9..4cb69429e9c0256ee8d46d261e34f924fe747ce4 100644 (file)
--- a/common-trace-format-proposal.txt
+++ b/common-trace-format-proposal.txt
@@ -616,9 +616,9 @@ The overall structure of an event is:
     5 - Event Payload (as specified by the event metadata)
 
 This structure defines an implicit dynamic scoping, where variants
     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
 
 
 6.1 Event Header
 


@@ -791,7 +791,8 @@ variant field), a declaration is followed by a declarator, which specify
 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,
 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).
 
 A definition associates a type to a location in the event structure
 hierarchy (see Section 6).


@@ -808,34 +809,36 @@ for variants references to tag fields.
 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
 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
 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
 
 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:
 
 
 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
 
 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


@@ -843,6 +846,12 @@ different scopes. There is no possible conflict, because the dynamic
 scope must be specified when a variant refers to a tag field located in
 a different dynamic scope.
 
 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
 
 
 7.2 Metadata Examples