BytesLabels(3)

📦
version 5.5.0 · 19 versions
, in arch rolling

✨ NAME ✨

BytesLabels — byte sequence operations. That's it. That's the name. (This page lives in the OCaml library manual, section 3 — so like, BytesLabels(3), iykyk 💅)

🦋 Module 🦋

Module BytesLabels

📖 Documentation 📖

💖 Module BytesLabels 💖

: sig end

Byte sequence operations. Okay so listen — a byte sequence is a mutable data structure holding a fixed-length sequence of bytes, and every single byte can be indexed in constant time for reading or writing. Constant time, bestie. We love efficiency in this house. 🫶

Say you've got a byte sequence s of length l. You can reach each of those l bytes through its index. Indexes start at 0, and we call an index valid in s if it falls within the range [0...l-1] (inclusive). A position is the point between two bytes, or at the very beginning or the very end of the sequence. We call a position valid in s if it falls within [0...l] (inclusive). And heads up: the byte at index n is between positions n and n+1. Yes, that's the whole vibe — positions are the gaps, indexes are the things. 🌸

Two parameters, start and len, are said to designate a valid range of s if len >= 0 and start and start+len are valid positions in s. Not indexes. Positions. Don't mix them up or you're gonna have a bad time. 💀

Byte sequences can be modified in place — for instance via the set and blit functions down below. 👑 See also strings (module String), which are almost the same data structure, but cannot be modified in place. So like, String is the read-only cousin. Same energy, different rules.

Bytes are represented by the OCaml type char.

The labeled version of this module can be used as described in the StdLabels module.

Since 4.02

⭐ The functions, in order, no skipping ⭐

🎀 val length : bytes -> int

Return the length (number of bytes) of the argument. That's it. Simple queen. ✨

🎀 val get : bytes -> int -> char

get s n returns the byte 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, replacing the byte at index n with c.

Raises Invalid_argument if n is not a valid index in s.

🎀 val create : int -> bytes

create n returns a new byte sequence of length n. The sequence is uninitialized and contains arbitrary bytes — so like, don't trust what's in there, it's giving chaos. 🙈

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. Predictable. Reliable. Main character energy. 💜

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).

Raises Invalid_argument if n < 0 or n > Sys.max_string_length.

🎀 val empty : bytes

A byte sequence of size 0. The void. The little nothing. We stan. 🕳️

🎀 val copy : bytes -> bytes

Return a new byte sequence that contains the same bytes as the argument.

🎀 val of_string : string -> bytes

Return a new byte sequence that contains the same bytes as the given string.

🎀 val to_string : bytes -> string

Return a new string that contains the same bytes as the given byte sequence.

🎀 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.

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 return a string instead of a byte sequence. Same vibes, different return type. 💫

🎀 val extend : bytes -> left:int -> right:int -> bytes

extend s ~left ~right returns a new byte sequence that contains the bytes of s, with left uninitialized bytes prepended and right uninitialized bytes appended to it. And plot twist: if left or right is negative, then bytes are removed (instead of appended) from the corresponding side of s. 😭 Not me finding out it can shrink.

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.

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. Overlap? Handled. Slay. 🔥

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.

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.

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 is equivalent to f (get s 0); f (get s 1); ...; f (get s (length s - 1)); ().

🎀 val iteri : f:(int -> char -> unit) -> bytes -> unit

Same as BytesLabels.iter, but the function is applied to the index of the byte as first argument and the byte itself as second argument. So like, now you know where you are. 📍

🎀 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.

🎀 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.

🎀 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.

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.

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.

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.

Since 4.13

🎀 val trim : bytes -> bytes

Return a copy of the argument, without leading and trailing whitespace. The bytes regarded as whitespace are the ASCII characters ' ', '\012', '\n', '\r', and '\t'. That's the whole list, no extras, don't come for me. 🧚

🎀 val escaped : bytes -> bytes

Return 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.

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.

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 drama, just an option type. 🎀

Since 4.05

🎀 val rindex : bytes -> char -> int

rindex s c returns the index of the last occurrence of byte c in s.

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.

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.

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.

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.

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.

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.

🎀 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.

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.

Raises Invalid_argument if stop < 0 or stop+1 is not a valid position in s.

🎀 val uppercase_ascii : bytes -> bytes

Return a copy of the argument, with all lowercase letters translated to uppercase, using the US-ASCII character set.

Since 4.05

🎀 val lowercase_ascii : bytes -> bytes

Return a copy of the argument, with all uppercase letters translated to lowercase, using the US-ASCII character set.

Since 4.05

🎀 val capitalize_ascii : bytes -> bytes

Return a copy of the argument, with the first character set to uppercase, using the US-ASCII character set.

Since 4.05

🎀 val uncapitalize_ascii : bytes -> bytes

Return a copy of the argument, with the first character set to lowercase, using the US-ASCII character set.

Since 4.05

🌈 The type 🌈

type t = bytes

okay so t is literally just an alias for the type of byte sequences. iconic, no cap ✨

  • val compare : t -> t -> int — bestie, this is the comparison function for byte sequences, with the same specification as compare. along with the type t, this function compare lets the module Bytes be passed as an argument to the functors Set.Make and Map.Make. periodt 💅
  • val equal : t -> t -> bool — the equality function for byte sequences. simple, clean, slay.

Since 4.05 🌸

  • val starts_with : prefix:bytes -> bytes -> bool — starts_with ~prefix s is true if and only if s starts with prefix. vibe check passed ✅

Since 4.13 🦋

  • val ends_with : suffix:bytes -> bytes -> bool — ends_with ~suffix s is true if and only if s ends with suffix. periodt, ending on a good note 💖

Since 4.13 🎀

😭 Unsafe conversions (for advanced users)

okay so this section is giving danger, lowkey. it describes unsafe, low-level conversion functions between bytes and string. they do not copy the internal data; used improperly, they can break the immutability invariant on strings. they are 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 🫶

  • val unsafe_to_string : bytes -> string — unsafely convert a byte sequence into a string.

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. the way that ownership works is literally so serious 😭

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. no take-backs, bestie.

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. honestly ate 💅

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. do not do this 🙈

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. iykyk ✨

  1. 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. the details are giving "trust me bro" but actually correct 😌

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. respect the borrow or else 💀

  • val unsafe_of_string : string -> bytes — unsafely convert a shared string to a byte sequence that should not be mutated.

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. sounds cute until you mess it up 😭

in practice, unique ownership of string values is extremely difficult to reason about correctly. you should always assume strings are shared, never uniquely owned. periodt.

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. bestie please use the second one 🌸

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. this is not me being dramatic, this is fr fr 🔥

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 but iconic 🎀

  • 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]. slay, splitting is free.

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 🦋

🌈 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. yes bestie, it's live 👀

Since 4.07 ✨

  • val to_seqi : t -> (int * char) Seq.t — iterate on the string, in increasing order, yielding indices along chars. now with index labels, we love context 💅

Since 4.07 🌸

  • val of_seq : char Seq.t -> t — create a string from the generator. generator to bytes pipeline, we love to see it 🫶

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. cute.
  • 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. no room? no problem, it just didn't write. respectfully 😌
  • val is_valid_utf_8 : t -> bool — is_valid_utf_8 b is true if and only if b contains valid UTF-8 data. vibe check but for bytes ✨

🦋 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. big-endian bestie.
  • 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. exact same energy, different endian 💅
  • val is_valid_utf_16be : t -> bool — is_valid_utf_16be b is true if and only if b contains valid UTF-16BE data. good data only 🎀

🌸 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. little-endian legend.
  • 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. yes queen, encode responsibly 🫶
  • val is_valid_utf_16le : t -> bool — is_valid_utf_16le b is true if and only if b contains valid UTF-16LE data. validation nation 💖

🔥 Binary encoding/decoding of integers

the functions in this section binary encode and decode integers to and from byte sequences. we're getting into the byte-level math and honestly it's giving main character energy 🔥

all following functions raise Invalid_argument if the space needed at index i to decode or encode the integer is not available. bestie, check your space 😭

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. the way endianness flips is so real 🌈

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

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. tiny but mighty 🍓

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. signed, sealed, delivered 💅

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. native-endian no cap 🫶

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. big-endian bestie 💖

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. little-endian legend 🌸

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. native and signed, versatile queen 👑

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. big-endian signed, we stan 💜

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. little-endian signed, periodt 💅

Since 4.08 🎀

okay picking right back up with the integer getters, no context switching, we're locked in 💅

  • 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
  • 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
  • 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
  • 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
  • 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
  • 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

now the setters, aka the girlies that actually write stuff down 🎀 same energy, mirrored vibes:

  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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 bestie, this is the part where you have to actually be careful. when multiple domains touch byte sequences at the same time, you gotta take care: accessing a byte sequence will never crash your program, periodt — but if the accesses aren't synchronized, you might get some genuinely surprising (read: not sequentially consistent) results. 😭

💀 Atomicity

heads up: every single byte sequence operation that touches more than one byte is not atomic. and yes, that includes iteration and scanning. rotting.

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 byte sequence b may contain a non-deterministic mixture of '!', 'A', 'B', and 'C' values. like, a literal salad. 🥗

after running this code, each byte of the sequence b is either '!', 'A', 'B', or 'C'. if atomicity is a thing you actually need, then you have to implement your own synchronization (for example, using Mutex.t). no one's gonna do it for you, queen. 👑

🌈 Data races

if two domains only touch disjoint parts of a byte sequence, then what you observe is equivalent to some sequential interleaving of the operations from the two domains. cute, clean, fine.

a data race happens when two domains access the same byte without synchronization and at least one of those accesses is a write. in the absence of data races, the observed behaviour is equivalent to some sequential interleaving of the operations from the different domains. ✨

whenever possible, avoid data races by using synchronization to mediate the accesses to the elements of the sequence. fr fr, just do it.

so like — in the presence of data races, programs won't crash, but the observed behaviour may not be equivalent to any sequential interleaving of operations from different domains. nevertheless, even with data races, a read operation will return the value of some prior write to that location. so it's garbage but it's consistent garbage, iykyk. 🙈

🫶 Mixed-size accesses

another subtle little point: 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. not defined. do not count on it. 💀

for instance, the following code writes 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. so like... yeah. don't assume. 🎀

okay that's the whole page, you made it to the end, icon behaviour honestly 🌸 this one's from OCamldoc, dated 2026-06-22, and it's the BytesLabels(3) manual page — go forth and byte responsibly, bestie 💖👑
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