3. Event stream
An event stream can be divided into 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. The rationale for the
-event stream design choices is explained in Appendix B. Stream Header
-Rationale.
+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.
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
include all event timestamps assigned to events contained within the packet.
- Events discarded count
- Snapshot of a per-stream free-running counter, counting the number of
- events discarded that were supposed to be written in the stream prior to
- the first event in the event packet.
- * Note: producer-consumer buffer full condition should fill the current
+ events discarded that were supposed to be written in the stream after
+ the last event in the event packet.
+ * Note: producer-consumer buffer full condition can fill the current
event packet with padding so we know exactly where events have been
- discarded.
+ discarded. However, if the buffer full condition chooses not
+ to fill the current event packet with padding, all we know
+ about the timestamp range in which the events have been
+ discarded is that it is somewhere between the beginning and
+ the end of the packet.
- Lossless compression scheme used for the event packet content. Applied
directly to raw data. New types of compression can be added in following
versions of the format.
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 */
};
The packet-based meta-data can be converted to a text-only meta-data by
-concatenating all the strings in contains.
+concatenating all the strings it contains.
In the textual representation of the meta-data, the text contained
within "/*" and "*/", as well as within "//" and end of line, are
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