Add model.emf.uri example to spec

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

diff --git a/common-trace-format-specification.txt b/common-trace-format-specification.txt
index 3130d98014ea9639a8ebe302d968abc48ee20ebf..4c5e35603c94eaf60855d5d49006fbf54652aa7a 100644 (file)
--- a/common-trace-format-specification.txt
+++ b/common-trace-format-specification.txt
@@ -454,11 +454,28 @@ type selector. The field to use as tag is specified by the "tag_field",
 specified between "< >" after the "variant" keyword for unnamed
 variants, and after "variant name" for named variants.
 
-The alignment of the variant is the alignment of the type as selected by the tag
-value for the specific instance of the variant. The alignment of the type
-containing the variant is independent of the variant alignment.  The size of the
-variant is the size as selected by the tag value for the specific instance of
-the variant.
+The alignment of the variant is the alignment of the type as selected by
+the tag value for the specific instance of the variant.  The size of the
+variant is the size as selected by the tag value for the specific
+instance of the variant.
+
+The alignment of the type containing the variant is independent of the
+variant alignment.  For instance, if a structure contains two fields, a
+32-bit integer, aligned on 32 bits, and a variant, which contains two
+choices: either a 32-bit field, aligned on 32 bits, or a 64-bit field,
+aligned on 64 bits, the alignment of the outmost structure will be
+32-bit (the alignment of its largest field, disregarding the alignment
+of the variant). The alignment of the variant will depend on the
+selector: if the variant's 32-bit field is selected, its alignment will
+be 32-bit, or 64-bit otherwise. It is important to note that variants
+are specifically tailored for compactness in a stream. Therefore, the
+relative offsets of compound type fields can vary depending on
+the offset at which the compound type starts if it contains a variant
+that itself contains a type with alignment larger than the largest field
+contained within the compound type. This is caused by the fact that the
+compound type may contain the enumeration that select the variant's
+choice, and therefore the alignment to be applied to the compound type
+cannot be determined before encountering the enumeration.
 
 Each variant type selector possess a field name, which is a unique
 identifier within the variant. The identifier is not allowed to use any
@@ -973,12 +990,13 @@ beginning of the file. This magic number is also used to detect the
 endianness of the architecture by trying to read the CTF magic number
 and its counterpart in reversed endianness. The events within the
 meta-data stream have no event header nor event context. Each event only
-contains a "sequence" payload, which is a sequence of bits using the
-"trace.packet.header.content_size" field as a placeholder for its length
-(the packet header size should be substracted). The formatting of this
-sequence of bits is a plain-text representation of the TSDL description.
-Each meta-data packet start with a special packet header, specific to
-the meta-data stream, which contains, exactly:
+contains a special "sequence" payload, which is a sequence of bits which
+length is implicitly calculated by using the
+"trace.packet.header.content_size" field, minus the packet header size.
+The formatting of this sequence of bits is a plain-text representation
+of the TSDL description.  Each meta-data packet start with a special
+packet header, specific to the meta-data stream, which contains,
+exactly:
 
 struct metadata_packet_header {
   uint32_t magic;                      /* 0x75D11D57 */
@@ -1155,6 +1173,7 @@ event {
   id = value;                  /* Numeric identifier within the stream */
   stream_id = stream_id;
   loglevel = value;
+  model.emf.uri = "string";
   context := struct {
     ...
   };