This directory demonstrates the ability of flatc to annotate binary flatbuffers with helpful annotations. The resulting annotated flatbuffer binary (afb) contains all the binary data with line-by-line annotations.
Given a schema in either plain-text (.fbs) or already compiled to a binary schema (.bfbs) and binary file(s) that was created by the schema.
flatc --annotate {schema_file} -- {binary_file}...
The following command should produce annotated_binary.afb in this directory:
cd tests\annotated_binary ..\..\flatc --annotate annotated_binary.fbs -- annotated_binary.bin
The annotated_binary.bin is the flatbufer binary of the data contained within annotated_binary.json, which was made by the following command:
..\..\flatc -b annotated_binary.fbs annotated_binary.json
Currently there is a built-in text-based format for outputting the annotations. The annotated_binary.afb is an example of the text format of a binary annotated_binary.bin and the annotated_binary.fbs (or annotated_binary.bfbs) schema.
The file is ordered in increasing the offsets from the beginning of the binary. The offset is the 1st column, expressed in hexadecimal format (e.g. +0x003c).
Binary sections are comprised of contigious binary regions that are logically grouped together. For example, a binary section may be a single instance of a flatbuffer Table or its vtable. The sections may be labelled with the name of the associated type, as defined in the input schema.
Example of a vtable Binary Section that is associated with the user-defined AnnotateBinary.Bar table.
vtable (AnnotatedBinary.Bar): +0x00A0 | 08 00 | uint16_t | 0x0008 (8) | size of this vtable +0x00A2 | 13 00 | uint16_t | 0x0013 (19) | size of referring table +0x00A4 | 08 00 | VOffset16 | 0x0008 (8) | offset to field `a` (id: 0) +0x00A6 | 04 00 | VOffset16 | 0x0004 (4) | offset to field `b` (id: 1)
Binary regions are contigious bytes regions that are grouped together to form some sort of value, e.g. a scalar or an array of scalars. A binary region may be split up over multiple text lines, if the size of the region is large.
Looking at an example binary region:
vtable (AnnotatedBinary.Bar): +0x00A0 | 08 00 | uint16_t | 0x0008 (8) | size of this vtable
The first column (+0x00A0) is the offset to this region from the beginning of the buffer.
The second column are the raw bytes (hexadecimal) that make up this region. These are expressed in the little-endian format that flatbuffers uses for the wire format.
The third column is the type to interpret the bytes as. Some types are special to flatbuffer internals (e.g. SOffet32, Offset32, and VOffset16) which are used by flatbuffers to point to various offsetes. The other types are specified as C++-like types which are the standard fix-width scalars. For the above example, the type is uint16_t which is a 16-bit unsigned integer type.
The fourth column shows the raw bytes as a compacted, big-endian value. The raw bytes are duplicated in this fashion since it is more intutive to read the data in the big-endian format (e.g., 0x0008). This value is followed by the decimal representation of the value (e.g., (8)). (For strings, the raw string value is shown instead).
The fifth column is a textual comment on what the value is. As much metadata as known is provided.
If the type in the 3rd column is of an absolute offset (SOffet32 or Offset32), the fourth column also shows an Loc: +0x025A value which shows where in the binary this region is pointing to. These values are absolute from the beginning of the file, their calculation from the raw value in the 4th column depends on the context.