Question

Given a protobuf serialization is it possible to get a list of all tag numbers that are in the message? Generally is it possible to view the structure of the message without the defining .proto files?

Answer 1

Most APIs will indeed have some form of reader-based API that allows you to enumerate a raw protobuf stream. However, that by itself is not enough to fully understand the data, since without the schema the interpretation is ambiguous:

• a varint could be zig-zag encoded (sint32/sint64), or not (int32/int64/uint32/uint64) – radically changing the meaning, or a boolean, or an enum
• a fixed-32/fixed-64 could be a signed or unsigned integer, or could be an IEEE754 float/double
• a length-prefixed chunk could be a UTF-8 string, a BLOB, a sub-message, or a “packed” repeated set of primitives; if it is a sub-message, you’ll have to repeat recursively

So… yes and no. Certainly you can get the field numbers of the outermost message.

Another approach would be to use the regular API against a type with no members (message Naked {}), and then query the unexpected data (i.e. all of it) via the “extension” API that many implementations provide.