StdLabels.Bytes(3)

💅 StdLabels.Bytes(3)

okay bestie, this one's from the OCaml library manual — it's the StdLabels.Bytes page, and it's a long one, so buckle up 🎀 (this is part 1 of 3, fr fr).

---

📛 NAME

StdLabels.Bytes — and the official description is literally no description 😭 the manpage said "figure it out yourself" and gave us nothing. iconic of it, honestly.

---

📦 Module

Module StdLabels.Bytes

---

📖 Documentation

Module Bytes : (module BytesLabels)

---

🌸 the actual functions, let's go

• val length : bytes -> int — hands you the length (number of bytes) of the argument. that's it, that's the whole vibe. 💖
• val get : bytes -> int -> char — get s n returns the byte sitting at index n in argument s.
  • Raises Invalid_argument if n is not a valid index in s. 🚨
• val set : bytes -> int -> char -> unit — set s n c modifies s in place, swapping whatever byte was at index n for c. main character move: it mutates.
  • Raises Invalid_argument if n is not a valid index in s. 🚨
• val create : int -> bytes — create n returns a brand-new byte sequence of length n. heads up though: the sequence is uninitialized and contains arbitrary bytes — it's giving "just vibes, no data" 💀
  • Raises Invalid_argument if n < 0 or n > Sys.max_string_length. 🚨
• val make : int -> char -> bytes — make n c returns a new byte sequence of length n, filled with the byte c. same energy as create but it actually commits to a value ✨
  • Raises Invalid_argument if n < 0 or n > Sys.max_string_length. 🚨
• val init : int -> f:(int -> char) -> bytes — init n f returns a fresh byte sequence of length n, with character i initialized to the result of f i (in increasing index order). lowkey the most useful one here 🫶
  • Raises Invalid_argument if n < 0 or n > Sys.max_string_length. 🚨
• val empty : bytes — a byte sequence of size 0. the void. the nada. she's empty but she's valid 🦋
• val copy : bytes -> bytes — returns a new byte sequence containing the same bytes as the argument. a true duplicate, no sharing 🎀
• val of_string : string -> bytes — returns a new byte sequence that contains the same bytes as the given string. string ➡️ bytes, slay.
• val to_string : bytes -> string — returns a new string that contains the same bytes as the given byte sequence. bytes ➡️ string, also slay.
• val sub : bytes -> pos:int -> len:int -> bytes — sub s ~pos ~len returns a new byte sequence of length len, containing the subsequence of s that starts at position pos and has length len. little slice of the action 🍓
  • Raises Invalid_argument if pos and len do not designate a valid range of s. 🚨
• val sub_string : bytes -> pos:int -> len:int -> string — same as BytesLabels.sub but returns a string instead of a byte sequence. plot twist: it's a string now 😭
• val extend : bytes -> left:int -> right:int -> bytes — extend s ~left ~right returns a new byte sequence with the bytes of s, with left uninitialized bytes prepended and right uninitialized bytes appended. if left or right is negative, then bytes are removed (instead of appended) from the corresponding side of s. negative numbers = we're taking bytes away, not adding 💀
  • Since 4.05 in BytesLabels
  • Raises Invalid_argument if the result length is negative or longer than Sys.max_string_length bytes. 🚨
• val fill : bytes -> pos:int -> len:int -> char -> unit — fill s ~pos ~len c modifies s in place, replacing len characters with c, starting at pos. in-place glow-up ✨
  • Raises Invalid_argument if pos and len do not designate a valid range of s. 🚨
• val blit : src:bytes -> src_pos:int -> dst:bytes -> dst_pos:int -> len:int -> unit — blit ~src ~src_pos ~dst ~dst_pos ~len copies len bytes from byte sequence src, starting at index src_pos, to byte sequence dst, starting at index dst_pos. it works correctly even if src and dst are the same byte sequence and the source and destination intervals overlap — overlapping ranges? handled. no cap. 👑
  • Raises Invalid_argument if src_pos and len do not designate a valid range of src, or if dst_pos and len do not designate a valid range of dst. 🚨
• val blit_string : src:string -> src_pos:int -> dst:bytes -> dst_pos:int -> len:int -> unit — blit_string ~src ~src_pos ~dst ~dst_pos ~len copies len bytes from string src, starting at index src_pos, to byte sequence dst, starting at index dst_pos. string in, bytes out 🌀
  • Since 4.05 in BytesLabels
  • Raises Invalid_argument if src_pos and len do not designate a valid range of src, or if dst_pos and len do not designate a valid range of dst. 🚨
• val concat : sep:bytes -> bytes list -> bytes — concat ~sep sl concatenates the list of byte sequences sl, inserting the separator byte sequence sep between each, and returns the result as a new byte sequence. the classic join-with-a-separator moment 💅
  • Raises Invalid_argument if the result is longer than Sys.max_string_length bytes. 🚨
• val cat : bytes -> bytes -> bytes — cat s1 s2 concatenates s1 and s2 and returns the result as a new byte sequence.
  • Since 4.05 in BytesLabels
  • Raises Invalid_argument if the result is longer than Sys.max_string_length bytes. 🚨
• val iter : f:(char -> unit) -> bytes -> unit — iter ~f s applies function f in turn to all the bytes of s. it's equivalent to f (get s 0); f (get s 1); ...; f (get s (length s - 1)); (). a little for-each loop but make it functional 🧚
  • (note: the original writes that equivalence across a line break like f (get s / (length s - 1)); () — same thing, just wrapped 💖)
• val iteri : f:(int -> char -> unit) -> bytes -> unit — same as BytesLabels.iter, but the function gets the index of the byte as first argument and the byte itself as second argument. indices included, we love an index 🌈
• val map : f:(char -> char) -> bytes -> bytes — map ~f s applies function f in turn to all the bytes of s (in increasing index order) and stores the resulting bytes in a new sequence that is returned as the result. glow-up: byte edition ✨
  • (the original literally leaves off the trailing ~ on map's first call in the synopsis — we're keeping the name, not fixing anything, just so you know what's what 🫶)
• val mapi : f:(int -> char -> char) -> bytes -> bytes — mapi ~f s calls f with each character of s and its index (in increasing index order) and stores the resulting bytes in a new sequence that is returned as the result. same as map but the index is invited to the party 🎉
• val fold_left : f:('acc -> char -> 'acc) -> init:'acc -> bytes -> 'acc — fold_left f x s computes f (... (f (f x (get s 0)) (get s 1)) ...) (get s (n-1)), where n is the length of s. left-to-right accumulator realness 🔥
  • Since 4.13
• val fold_right : f:(char -> 'acc -> 'acc) -> bytes -> init:'acc -> 'acc — fold_right f s x computes f (get s 0) (f (get s 1) ( ... (f (get s (n-1)) x) ...)), where n is the length of s. the other direction, still iconic 💜
  • Since 4.13
• val for_all : f:(char -> bool) -> bytes -> bool — for_all p s checks if all characters in s satisfy the predicate p. everyone gotta pass the vibe check ✅
  • Since 4.13
• val exists : f:(char -> bool) -> bytes -> bool — exists p s checks if at least one character of s satisfies the predicate p. just one is enough, we're not picky 💫
  • Since 4.13
• val trim : bytes -> bytes — returns a copy of the argument, minus leading and trailing whitespace. the bytes regarded as whitespace are the ASCII characters ' ', '\012', '\n', '\r', and '\t'. bye bye, extra space 🧹
  • (exact list, bestie: ' ', '\012', '\n', '\r', '\t' — that's five 🖐️)
• val escaped : bytes -> bytes — returns a copy of the argument, with special characters represented by escape sequences, following the lexical conventions of OCaml. all characters outside the ASCII printable range (32..126) are escaped, as well as backslash and double-quote. safety first, but make it escape sequences 🛡️
  • Raises Invalid_argument if the result is longer than Sys.max_string_length bytes. 🚨
• val index : bytes -> char -> int — index s c returns the index of the first occurrence of byte c in s. first come, first served 🔍
  • Raises Not_found if c does not occur in s. 😭
• val index_opt : bytes -> char -> int option — index_opt s c returns the index of the first occurrence of byte c in s, or None if c does not occur in s. no exception, just option energy 🦋
  • Since 4.05
• val rindex : bytes -> char -> int — rindex s c returns the index of the last occurrence of byte c in s. we're searching from the back now 💅
  • Raises Not_found if c does not occur in s. 😭
• val rindex_opt : bytes -> char -> int option — rindex_opt s c returns the index of the last occurrence of byte c in s, or None if c does not occur in s. reverse search, but polite 🌸
  • Since 4.05
• val index_from : bytes -> int -> char -> int — index_from s i c returns the index of the first occurrence of byte c in s after position i. index s c is equivalent to index_from s 0 c. start where I tell you to start ⭐
  • Raises Invalid_argument if i is not a valid position in s. 🚨
  • Raises Not_found if c does not occur in s after position i. 😭
• val index_from_opt : bytes -> int -> char -> int option — index_from_opt s i c returns the index of the first occurrence of byte c in s after position i, or None if c does not occur in s after position i. index_opt s c is equivalent to index_from_opt s 0 c. same search, but return option instead of throwing hands 🙈
  • Since 4.05
  • Raises Invalid_argument if i is not a valid position in s. 🚨
• val rindex_from : bytes -> int -> char -> int — rindex_from s i c returns the index of the last occurrence of byte c in s before position i+1. rindex s c is equivalent to rindex_from s (length s - 1) c. searching backwards from a specific spot 🌀
  • Raises Invalid_argument if i+1 is not a valid position in s. 🚨
  • Raises Not_found if c does not occur in s before position i+1. 😭
• val rindex_from_opt : bytes -> int -> char -> int option — rindex_from_opt s i c returns the index of the last occurrence of byte c in s before position i+1, or None if c does not occur in s before position i+1. rindex_opt s c is equivalent to rindex_from s (length s - 1) c. yes, the docs really write rindex_from there and not rindex_from_opt — we're keeping it character for character, bestie, don't come for me 💀
  • Since 4.05
  • Raises Invalid_argument if i+1 is not a valid position in s. 🚨
• val contains : bytes -> char -> bool — contains s c tests if byte c appears in s. is she there or not, a simple yes/no 🎀
• val contains_from : bytes -> int -> char -> bool — contains_from s start c tests if byte c appears in s after position start. contains s c is equivalent to contains_from s 0 c. start-position-aware presence check 🔎
  • Raises Invalid_argument if start is not a valid position in s. 🚨
• val rcontains_from : bytes -> int -> char -> bool — rcontains_from s stop c tests if byte c appears in s before position stop+1. backwards presence check, she's thorough 💜
  • Raises Invalid_argument if stop < 0 or stop+1 is not a valid position in s. 🚨
• val uppercase_ascii : bytes -> bytes — returns a copy of the argument, with all lowercase letters translated to uppercase, using the US-ASCII character set. uppercase era 🔠
  • Since 4.05
• val lowercase_ascii : bytes -> bytes — returns a copy of the argument, with all uppercase letters translated to lowercase, using the US-ASCII character set. lowercase era 🔡
  • Since 4.05
• val capitalize_ascii : bytes -> bytes — returns a copy of the argument, with the first character set to uppercase, using the US-ASCII character set. just the first one gets the crown 👑
  • Since 4.05
• val uncapitalize_ascii : bytes -> bytes — returns a copy of the argument, with the first character set to lowercase, using the US-ASCII character set. crown removed, respectfully 👑➡️🙈
  • Since 4.05
• type t = bytes — an alias for the type of byte sequences. tiny type, big responsibilities 💖
• val compare : t -> t -> int — the comparison function for byte sequences, with the same specification as compare. along with the type t, this function compare allows the module Bytes to be passed as argument to the functors Set.Make and Map.Make. functor-ready, we love to see it 🧚
  • (yes, the original says "the functors Set.Make and Map.Make" — those exact names, no edits 🌈)
• val equal : t -> t -> bool — the equality function for byte sequences. are they the same? a yes/no moment ✨
  • Since 4.05
• val starts_with : prefix:bytes -> bytes -> bool — starts_with ~prefix s is true if and only if s starts with prefix. beginning-of-the-string check, periodt 💅
  • Since 4.13
• val ends_with : suffix:bytes -> bytes -> bool — ends_with ~suffix s is true if and only if s ends with suffix. end-of-the-string check, also periodt 💅
  • Since 4.13

---

⚠️ Unsafe conversions (for advanced users)

okay so this section is the danger zone 🚨 it describes unsafe, low-level conversion functions between bytes and string. they do not copy the internal data — and used improperly, they can break the immutability invariant on strings. they're available for expert library authors, but for most purposes you should use the always-correct BytesLabels.to_string and BytesLabels.of_string instead. fr fr, this is the "you better know what you're doing" corner of the page 💀

• val unsafe_to_string : bytes -> string — unsafely convert a byte sequence into a string. the word "unsafely" is right there in the name, bestie 🙈

---

✨ that's the whole first chunk — there's more where that came from, so don't go anywhere 🫶

Okay so to reason about the use of unsafe_to_string, it is convenient to consider an “ownership” discipline. A piece of code that manipulates some data “owns” it; there are several disjoint ownership modes, including:

-Unique ownership: the data may be accessed and mutated.

-Shared ownership: the data has several owners, that may only access it, not mutate it.

Unique ownership is linear: passing the data to another piece of code means giving up ownership (we cannot write the data again). A unique owner may decide to make the data shared (giving up mutation rights on it), but shared data may not become uniquely-owned again. Periodt. 💅

unsafe_to_string s can only be used when the caller owns the byte sequence s -- either uniquely or as shared immutable data. The caller gives up ownership of s, and gains ownership of the returned string. That trade is the whole vibe. ✨

There are two valid use-cases that respect this ownership discipline:

1. Creating a string by initializing and mutating a byte sequence that is never changed after initialization is performed.

let string_init len f : string =
  let s = Bytes.create len in
  for i = 0 to len - 1 do Bytes.set s i (f i) done;
  Bytes.unsafe_to_string s

This function is safe because the byte sequence s will never be accessed or mutated after unsafe_to_string is called. The string_init code gives up ownership of s, and returns the ownership of the resulting string to its caller. It’s giving clean handoff energy. 🫶

Note that it would be unsafe if s was passed as an additional parameter to the function f as it could escape this way and be mutated in the future -- string_init would give up ownership of s to pass it to f, and could not call unsafe_to_string safely.

We have provided the String.init, String.map and String.mapi functions to cover most cases of building new strings. You should prefer those over to_string or unsafe_to_string whenever applicable. Lowkey use the safe ones first, bestie. 💖

2. Temporarily giving ownership of a byte sequence to a function that expects a uniquely owned string and returns ownership back, so that we can mutate the sequence again after the call ended.

let bytes_length (s : bytes) =
  String.length (Bytes.unsafe_to_string s)

In this use-case, we do not promise that s will never be mutated after the call to bytes_length s. The String.length function temporarily borrows unique ownership of the byte sequence (and sees it as a string), but returns this ownership back to the caller, which may assume that s is still a valid byte sequence after the call. Note that this is only correct because we know that String.length does not capture its argument -- it could escape by a side-channel such as a memoization combinator. Borrowing arc, not a forever-gift arc. 🦋

The caller may not mutate s while the string is borrowed (it has temporarily given up ownership). This affects concurrent programs, but also higher-order functions: if String.length returned a closure to be called later, s should not be mutated until this closure is fully applied and returns ownership. Basically: don’t touch while it’s on loan. 🎀

• val unsafe_of_string : string -> bytes — Unsafely convert a shared string to a byte sequence that should not be mutated. Yes, “unsafely” is right there in the name, so respect it. 💀

The same ownership discipline that makes unsafe_to_string correct applies to unsafe_of_string: you may use it if you were the owner of the string value, and you will own the return bytes in the same mode.

In practice, unique ownership of string values is extremely difficult to reason about correctly. You should always assume strings are shared, never uniquely owned. Fr fr, just assume shared. 🌸

For example, string literals are implicitly shared by the compiler, so you never uniquely own them.

let incorrect = Bytes.unsafe_of_string "hello"
let s = Bytes.of_string "hello"

The first declaration is incorrect, because the string literal "hello" could be shared by the compiler with other parts of the program, and mutating incorrect is a bug. You must always use the second version, which performs a copy and is thus correct. Copying ate; aliasing did not. 👑

Assuming unique ownership of strings that are not string literals, but are (partly) built from string literals, is also incorrect. For example, mutating unsafe_of_string ("foo" ^ s) could mutate the shared string "foo" -- assuming a rope-like representation of strings. More generally, functions operating on strings will assume shared ownership, they do not preserve unique ownership. It is thus incorrect to assume unique ownership of the result of unsafe_of_string. No cap, this is the dangerous part. ⚠️

The only case we have reasonable confidence is safe is if the produced bytes is shared -- used as an immutable byte sequence. This is possibly useful for incremental migration of low-level programs that manipulate immutable sequences of bytes (for example Marshal.from_bytes) and previously used the string type for this purpose. Niche migration vibe, but valid. 🌈

• val split_on_char : sep:char -> bytes -> bytes list — split_on_char sep s returns the list of all (possibly empty) subsequences of s that are delimited by the sep character. If s is empty, the result is the singleton list [empty]. Split era. ✂️

The function's output is specified by the following invariants:

-The list is not empty.

-Concatenating its elements using sep as a separator returns a byte sequence equal to the input (Bytes.concat (Bytes.make 1 sep) (Bytes.split_on_char sep s) = s).

-No byte sequence in the result contains the sep character.

Since 4.13. New-ish but iconic. ✅

✨ Iterators ✨

• val to_seq : t -> char Seq.t — Iterate on the string, in increasing index order. Modifications of the string during iteration will be reflected in the sequence. Since 4.07.
• val to_seqi : t -> (int * char) Seq.t — Iterate on the string, in increasing order, yielding indices along chars. Since 4.07.
• val of_seq : char Seq.t -> t — Create a string from the generator. Since 4.07.

💜 UTF codecs and validations 💜

UTF-8

• val get_utf_8_uchar : t -> int -> Uchar.utf_decode — get_utf_8_uchar b i decodes an UTF-8 character at index i in b.
• val set_utf_8_uchar : t -> int -> Uchar.t -> int — set_utf_8_uchar b i u UTF-8 encodes u at index i in b and returns the number of bytes n that were written starting at i. If n is 0 there was not enough space to encode u at i and b was left untouched. Otherwise a new character can be encoded at i + n.
• val is_valid_utf_8 : t -> bool — is_valid_utf_8 b is true if and only if b contains valid UTF-8 data. Validation slay. 🧚

UTF-16BE

• val get_utf_16be_uchar : t -> int -> Uchar.utf_decode — get_utf_16be_uchar b i decodes an UTF-16BE character at index i in b.
• val set_utf_16be_uchar : t -> int -> Uchar.t -> int — set_utf_16be_uchar b i u UTF-16BE encodes u at index i in b and returns the number of bytes n that were written starting at i. If n is 0 there was not enough space to encode u at i and b was left untouched. Otherwise a new character can be encoded at i + n.
• val is_valid_utf_16be : t -> bool — is_valid_utf_16be b is true if and only if b contains valid UTF-16BE data.

UTF-16LE

• val get_utf_16le_uchar : t -> int -> Uchar.utf_decode — get_utf_16le_uchar b i decodes an UTF-16LE character at index i in b.
• val set_utf_16le_uchar : t -> int -> Uchar.t -> int — set_utf_16le_uchar b i u UTF-16LE encodes u at index i in b and returns the number of bytes n that were written starting at i. If n is 0 there was not enough space to encode u at i and b was left untouched. Otherwise a new character can be encoded at i + n.
• val is_valid_utf_16le : t -> bool — is_valid_utf_16le b is true if and only if b contains valid UTF-16LE data. Endian bestie representation! 🌈

🔥 Binary encoding/decoding of integers 🔥

The functions in this section binary encode and decode integers to and from byte sequences.

All following functions raise Invalid_argument if the space needed at index i to decode or encode the integer is not available. Do not ignore this one, bestie. 🚨

Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are stored first. Big-endian is also known as network byte order. Native-endian encoding is either little-endian or big-endian depending on Sys.big_endian.

32-bit and 64-bit integers are represented by the int32 and int64 types, which can be interpreted either as signed or unsigned numbers.

8-bit and 16-bit integers are represented by the int type, which has more bits than the binary encoding. These extra bits are handled as follows:

-Functions that decode signed (resp. unsigned) 8-bit or 16-bit integers represented by int values sign-extend (resp. zero-extend) their result.

-Functions that encode 8-bit or 16-bit integers represented by int values truncate their input to their least significant bytes.

• val get_uint8 : bytes -> int -> int — get_uint8 b i is b's unsigned 8-bit integer starting at byte index i. Since 4.08.
• val get_int8 : bytes -> int -> int — get_int8 b i is b's signed 8-bit integer starting at byte index i. Since 4.08.
• val get_uint16_ne : bytes -> int -> int — get_uint16_ne b i is b's native-endian unsigned 16-bit integer starting at byte index i. Since 4.08.
• val get_uint16_be : bytes -> int -> int — get_uint16_be b i is b's big-endian unsigned 16-bit integer starting at byte index i. Since 4.08.
• val get_uint16_le : bytes -> int -> int — get_uint16_le b i is b's little-endian unsigned 16-bit integer starting at byte index i. Since 4.08.
• val get_int16_ne : bytes -> int -> int — get_int16_ne b i is b's native-endian signed 16-bit integer starting at byte index i. Since 4.08.
• val get_int16_be : bytes -> int -> int — get_int16_be b i is b's big-endian signed 16-bit integer starting at byte index i. Since 4.08.
• val get_int16_le : bytes -> int -> int — get_int16_le b i is b's little-endian signed 16-bit integer starting at byte index i. Since 4.08.
• val get_int32_ne : bytes -> int -> int32 — get_int32_ne b i is b's native-endian 32-bit integer starting at byte index i. Since 4.08.
• val get_int32_be : bytes -> int -> int32 — get_int32_be b i is b's big-endian 32-bit integer starting at byte index i. Since 4.08.
• val get_int32_le : bytes -> int -> int32 — get_int32_le b i is b's little-endian 32-bit integer starting at byte index i. Since 4.08.
• val get_int64_ne : bytes -> int -> int64 — get_int64_ne b i is b's native-endian 64-bit integer starting at byte index i. Since 4.08.
• val get_int64_be : bytes -> int -> int64 — get_int64_be b i is b's big-endian 64-bit integer starting at byte index i. Since 4.08.
• val get_int64_le : bytes -> int -> int64 — get_int64_le b i is b's little-endian 64-bit integer starting at byte index i. Since 4.08.
• val set_uint8 : bytes -> int -> int -> unit — set_uint8 b i v sets b's unsigned 8-bit integer starting at byte index i to v. Since 4.08.
• val set_int8 : bytes -> int -> int -> unit — set_int8 b i v sets b's signed 8-bit integer starting at byte index i to v. Since 4.08.
• val set_uint16_ne : bytes -> int -> int -> unit — set_uint16_ne b i v sets b's native-endian unsigned 16-bit integer starting at byte index i to v. ✨ Since 4.08 💖
• val set_uint16_be : bytes -> int -> int -> unit — set_uint16_be b i v sets b's big-endian unsigned 16-bit integer starting at byte index i to v. 🌸 Since 4.08 💖
• val set_uint16_le : bytes -> int -> int -> unit — set_uint16_le b i v sets b's little-endian unsigned 16-bit integer starting at byte index i to v. 🎀 Since 4.08 💖
• val set_int16_ne : bytes -> int -> int -> unit — set_int16_ne b i v sets b's native-endian signed 16-bit integer starting at byte index i to v. 🦋 Since 4.08 💖
• val set_int16_be : bytes -> int -> int -> unit — set_int16_be b i v sets b's big-endian signed 16-bit integer starting at byte index i to v. 😭 Since 4.08 💖
• val set_int16_le : bytes -> int -> int -> unit — set_int16_le b i v sets b's little-endian signed 16-bit integer starting at byte index i to v. 🔥 Since 4.08 💖
• val set_int32_ne : bytes -> int -> int32 -> unit — set_int32_ne b i v sets b's native-endian 32-bit integer starting at byte index i to v. 👑 Since 4.08 💖
• val set_int32_be : bytes -> int -> int32 -> unit — set_int32_be b i v sets b's big-endian 32-bit integer starting at byte index i to v. 🌈 Since 4.08 💖
• val set_int32_le : bytes -> int -> int32 -> unit — set_int32_le b i v sets b's little-endian 32-bit integer starting at byte index i to v. ⭐ Since 4.08 💖
• val set_int64_ne : bytes -> int -> int64 -> unit — set_int64_ne b i v sets b's native-endian 64-bit integer starting at byte index i to v. 💀 Since 4.08 💖
• val set_int64_be : bytes -> int -> int64 -> unit — set_int64_be b i v sets b's big-endian 64-bit integer starting at byte index i to v. 🙈 Since 4.08 💖
• val set_int64_le : bytes -> int -> int64 -> unit — set_int64_le b i v sets b's little-endian 64-bit integer starting at byte index i to v. 🍓 Since 4.08 💖

Byte sequences and concurrency safety 🌈

Okay so bestie, when you're touching byte sequences from multiple domains at the same time, you gotta be careful. Accessing a byte sequence will never crash your program — no cap — but if your accesses aren't synchronized, you might get some surprising (non-sequentially-consistent) results. 😭💅

Atomicity ✨

Every byte sequence operation that accesses more than one byte is not atomic. That includes iteration and scanning. Periodt. 💀

For example, consider the following program:

let size = 100_000_000
let b = Bytes.make size  ' '
let update b f ()  =
  Bytes.iteri (fun i x -> Bytes.set b i (Char.chr (f (Char.code x)))) b
let d1 = Domain.spawn (update b (fun x -> x + 1))
let d2 = Domain.spawn (update b (fun x -> 2 * x + 1))
let () = Domain.join d1; Domain.join d2

the bytes sequence b may contain a non-deterministic mixture of '!', 'A', 'B', and 'C' values. 🎀

After executing this code, each byte of the sequence b is either '!', 'A', 'B', or 'C'. If atomicity is required, then the user must implement their own synchronization (for example, using Mutex.t). 🔥

Data races 🦋

If two domains only access disjoint parts of a byte sequence, then the observed behaviour is the equivalent to some sequential interleaving of the operations from the two domains. ✨

A data race is said to occur when two domains access the same byte without synchronization and at least one of the accesses is a write. In the absence of data races, the observed behaviour is equivalent to some sequential interleaving of the operations from different domains. 👑

Whenever possible, data races should be avoided by using synchronization to mediate the accesses to the elements of the sequence. 🫶

Indeed, in the presence of data races, programs will not crash but the observed behaviour may not be equivalent to any sequential interleaving of operations from different domains. Nevertheless, even in the presence of data races, a read operation will return the value of some prior write to that location. 💜

Mixed-size accesses 🌸

Another subtle point is that if a data race involves mixed-size writes and reads to the same location, the order in which those writes and reads are observed by domains is not specified. 😱 For instance, the following code write sequentially a 32-bit integer and a char to the same index:

let b = Bytes.make 10 '\000'
let d1 = Domain.spawn (fun () -> Bytes.set_int32_ne b 0 100; b.[0] <- 'd' )

In this situation, a domain that observes the write of 'd' to b. 0 is not guaranteed to also observe the write to indices 1, 2, or 3. 🙈

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✧･ﾟ: ✧･ﾟ: And that's a wrap, bestie! This iconic page was brought to you by OCamldoc on 2026-06-22 for StdLabels.Bytes(3). Stay slay, stay synchronized, and may your byte sequences always vibe. 💅💖✨