Issue #18254 has been updated by nobu (Nobuyoshi Nakada). I wasn't aware that negative offset is not allowed. Shouldn't `unpack`/`unpack1` also it as well as the other methods, `String#slice`, `String#[]` and so on? ---------------------------------------- Feature #18254: Add an `offset` parameter to String#unpack and String#unpack1 https://bugs.ruby-lang.org/issues/18254#change-116911 * Author: byroot (Jean Boussier) * Status: Closed ---------------------------------------- When working with binary protocols it's common to have to first unpack some kind of header or type prefix, and then based on that unpack another part of the string. For instance here's [a code snippet from Dalli, the most common Memcached client](https://github.com/petergoldstein/dalli/blob/76b79d78cda13562da17bc99f92edce...): ```ruby while buf.bytesize - pos >= 24 header = buf.slice(pos, 24) (key_length, _, body_length, cas) = header.unpack(KV_HEADER) if key_length == 0 # all done! @multi_buffer = nil @position = nil @inprogress = false break elsif buf.bytesize - pos >= 24 + body_length flags = buf.slice(pos + 24, 4).unpack1("N") key = buf.slice(pos + 24 + 4, key_length) value = buf.slice(pos + 24 + 4 + key_length, body_length - key_length - 4) if body_length - key_length - 4 > 0 pos = pos + 24 + body_length begin values[key] = [deserialize(value, flags), cas] rescue DalliError end else # not enough data yet, wait for more break end end @position = pos ``` ### Proposal If `unpack` and `unpack1` had an `offset:` parameter, it would allow this kind of code to extract the fields it needs without allocating and copying as much strings, e.g.: ```ruby flags = buf.slice(pos + 24, 4).unpack1("N") ``` could be: ```ruby buf.unpack1("N", offset: pos + 24) ``` -- https://bugs.ruby-lang.org/