cbor_decode_prefix: Decode the CBOR item at the start of a raw vector

In zucbor: Deterministic and Secure CBOR Encoding and Decoding

Decode the CBOR item at the start of a raw vector

Description

Decodes the one CBOR data item that x starts with and reports how many bytes it used, for CBOR inside binary framing: a COSE key in the middle of WebAuthn authData, or a payload after a fixed header. The item is checked and decoded exactly as by cbor_decode(), with the same arguments; what follows it is not read at all.

Usage

cbor_decode_prefix(
  x,
  simplify = c("preserve", "none"),
  map_keys = c("auto", "map", "string"),
  tags = c("convert", "keep"),
  big_integers = c("bigint", "double", "error"),
  duplicate_keys = FALSE,
  deterministic = FALSE,
  max_depth = 256L,
  max_size = 64 * 1024^2,
  max_items = 1e+06,
  tag_handlers = NULL,
  data_frame = FALSE,
  max_cells = 1e+07
)

Arguments

x	A raw vector starting with a CBOR data item.
simplify	"preserve" simplifies arrays whose elements agree to atomic vectors; "none" makes every array a list.
map_keys	"auto" gives a named list when every key is a non-empty, unique text string, and a cbor_map otherwise. "map" always gives a cbor_map. "string" always gives a named list, naming each entry by its text key, or by the key's diagnostic notation when it is not text; it is lossy, and refuses a map whose keys collide once stringified.
tags	"convert" turns the tags in the table into R values and keeps the rest as cbor_tag; "keep" makes every tag a cbor_tag.
big_integers	What to do with an integer beyond 2^53, which a double cannot hold exactly: "bigint" returns a cbor_bigint, "double" the nearest double, and "error" refuses the input.
duplicate_keys	If FALSE (the default), a map with the same key twice is invalid.
deterministic	If TRUE, also require RFC 8949 section 4.2.1 core deterministic encoding: shortest integers, lengths and floats, definite lengths, map keys in bytewise order, and bignums in preferred form.
max_depth	Deepest nesting allowed, counting arrays, maps and tags. At most zucbor_info()$max_depth_cap.
max_size	Largest input allowed, in bytes, or Inf.
max_items	Most data items allowed, counting every tag and every chunk of an indefinite-length string, or Inf.
tag_handlers	NULL, or a list of functions of one argument, named by tag number, such as list("37" = function(value) ...). See "Tag handlers".
data_frame	If TRUE, an array whose elements are all maps with text keys becomes a data frame. See "Data frames".
max_cells	With data_frame = TRUE, the most cells (rows times columns) one data frame may have, or Inf. Checked before the frame is allocated: rows that share no keys make a frame quadratic in the input.

Details

That also means nothing about the rest is known: it may be more CBOR, or the next field of the framing, or garbage. It is the caller's to make sense of, as x[-seq_len(consumed)].

max_size applies to x as a whole, since all of it is in memory already. An empty x is zucbor_parse_error, as for cbor_decode(); so is an item cut short by the end of x.

Value

A list: value, the decoded item, and consumed, the number of bytes it took, a double.

See Also

cbor_decode() for one item and nothing else, and cbor_decode_seq() for items all the way to the end.

Examples

# [1, 2] followed by four bytes of something else.
x <- as.raw(c(0x82, 0x01, 0x02, 0xde, 0xad, 0xbe, 0xef))
r <- cbor_decode_prefix(x)
r$value
x[-seq_len(r$consumed)]

zucbor documentation built on Oct. 21, 2026, 5:14 p.m.