Explain that ids can be left out if single stream/event

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

diff --git a/common-trace-format-proposal.txt b/common-trace-format-proposal.txt
index 6e234350ad4bd779e286532df454864ae32e3009..71d9140704eb049dba1032c11c392f209f802b21 100644 (file)
--- a/common-trace-format-proposal.txt
+++ b/common-trace-format-proposal.txt
@@ -582,12 +582,32 @@ Metadata-defined layout (event packet context):
 
 5.1 Event Packet Header Fixed Layout Description
 
+The event packet header layout is indicated by the trace packet.header
+field. Here is an example structure type for the packet header with the
+fields typically expected:
+
 struct event_packet_header {
   uint32_t magic;
   uint8_t  trace_uuid[16];
   uint32_t stream_id;
 };
 
+trace {
+  ...
+  packet.header := struct event_packet_header;
+};
+
+If a packet header lacks the "magic" number field, the trace endianness
+needs to be specified in the trace "endian" metadata attribute.
+
+If the trace_uuid is not present, no validation that the metadata
+actually corresponds to the stream is performed.
+
+If the stream_id packet header field is missing, the trace can only
+contain a single stream. Its "id" field can be left out, and its events
+don't need to declare a "stream_id" field.
+
+
 5.2 Event Packet Context Description
 
 Event packet context example. These are declared within the stream declaration
@@ -879,12 +899,26 @@ readable by accessing the upper dynamic scopes.
 
 The grammar representing the TSDL metadata is presented in Appendix C.
 TSDL Grammar. This section presents a rather ligher reading that
-consists in examples of TSDL metadata, with template values:
+consists in examples of TSDL metadata, with template values.
+
+Note that the trace endianness is usually extracted from the packet
+header magic number. The "endian" field is only needed when the magic
+number packet header field is not present.
+
+The stream "id" can be left out if there is only one stream in the
+trace. The event "id" field can be left out if there is only one event
+in a stream.
 
 trace {
   major = value;                               /* Trace format version */
   minor = value;
   uuid = "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa";       /* Trace UUID */
+  endian = big OR little;                      /* Endianness (optional) */
+  packet.header := struct {
+    uint32_t magic;
+    uint8_t  trace_uuid[16];
+    uint32_t stream_id;
+  };
 };
 
 stream {