Module Stdlib.BytesLabels

Byte sequence operations.

A byte sequence is a mutable data structure that contains a fixed-length sequence of bytes. Each byte can be indexed in constant time for reading or writing.

Given a byte sequence s of length l, we can access each of the l bytes of s via its index in the sequence. Indexes start at 0, and we will 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 beginning or end of the sequence. We call a position valid in s if it falls within the range [0...l] (inclusive). Note that the byte at index n is between positions n and n+1.

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.

Byte sequences can be modified in place, for instance via the set and blit functions described below. See also strings (module String), which are almost the same data structure, but cannot be modified in place.

Bytes are represented by the OCaml type char.

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

let length: bytes => int;

Return the length (number of bytes) of the argument.

let get: bytes => int => char;

get s n returns the byte at index n in argument s.

let set: bytes => int => char => unit;

set s n c modifies s in place, replacing the byte at index n with c.

let create: int => bytes;

create n returns a new byte sequence of length n. The sequence is uninitialized and contains arbitrary bytes.

let make: int => char => bytes;

make n c returns a new byte sequence of length n, filled with the byte c.

let 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).

let empty: bytes;

A byte sequence of size 0.

let copy: bytes => bytes;

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

let of_string: string => bytes;

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

let to_string: bytes => string;

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

let 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.

let sub_string: bytes => pos:int => len:int => string;

Same as sub but return a string instead of a byte sequence.

let 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. If left or right is negative, then bytes are removed (instead of appended) from the corresponding side of s.

  • since 4.05 in BytesLabels
let 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.

let 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.

  • 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.

let 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.

  • 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.

  • since 4.05 in BytesLabels
let concat: sep:bytes => list(bytes) => 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.

let 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
let 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)); ().

let iteri: f:(int => char => unit) => bytes => unit;

Same as iter, but the function is applied to the index of the byte as first argument and the byte itself as second argument.

let 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.

let 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.

let 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
let 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
let for_all: f:(char => bool) => bytes => bool;

for_all p s checks if all characters in s satisfy the predicate p.

  • since 4.13
let exists: f:(char => bool) => bytes => bool;

exists p s checks if at least one character of s satisfies the predicate p.

  • since 4.13
let 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'.

let 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.

let index: bytes => char => int;

index s c returns the index of the first occurrence of byte c in s.

let index_opt: bytes => char => option(int);

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.

  • since 4.05
let rindex: bytes => char => int;

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

let rindex_opt: bytes => char => option(int);

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
let 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 Not_found

    if c does not occur in s after position i.

let index_from_opt: bytes => int => char => option(int);

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
let 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 Not_found

    if c does not occur in s before position i+1.

let rindex_from_opt: bytes => int => char => option(int);

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
let contains: bytes => char => bool;

contains s c tests if byte c appears in s.

let 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.

let rcontains_from: bytes => int => char => bool;

rcontains_from s stop c tests if byte c appears in s before position stop+1.

let uppercase_ascii: bytes => bytes;

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

  • since 4.03 (4.05 in BytesLabels)
let lowercase_ascii: bytes => bytes;

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

  • since 4.03 (4.05 in BytesLabels)
let capitalize_ascii: bytes => bytes;

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

  • since 4.03 (4.05 in BytesLabels)
let uncapitalize_ascii: bytes => bytes;

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

  • since 4.03 (4.05 in BytesLabels)
type t = bytes;

An alias for the type of byte sequences.

let compare: t => t => int;

The comparison function for byte sequences, with the same specification as Stdlib.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.

let equal: t => t => bool;

The equality function for byte sequences.

  • since 4.03 (4.05 in BytesLabels)
let starts_with: prefix:bytes => bytes => bool;

starts_with ~prefix s is true if and only if s starts with prefix.

  • since 4.13
let ends_with: suffix:bytes => bytes => bool;

ends_with ~suffix s is true if and only if s ends with suffix.

  • since 4.13

Unsafe conversions (for advanced users)

This section 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 provided by the -safe-string option. They are available for expert library authors, but for most purposes you should use the always-correct to_string and of_string instead.

let 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.

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.

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.

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.

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.

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.

let 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.

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

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.

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.

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.

let split_on_char: sep:char => bytes => list(bytes);

split_on_char sep s returns the list of all (possibly empty) subsequences of s that are delimited by the sep character.

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

let to_seq: t => Seq.t(char);

Iterate on the string, in increasing index order. Modifications of the string during iteration will be reflected in the sequence.

  • since 4.07
let to_seqi: t => Seq.t((int, char));

Iterate on the string, in increasing order, yielding indices along chars

  • since 4.07
let of_seq: Seq.t(char) => t;

Create a string from the generator

  • since 4.07

UTF codecs and validations

UTF-8

let 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.

let 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.

let is_valid_utf_8: t => bool;

is_valid_utf_8 b is true if and only if b contains valid UTF-8 data.

UTF-16BE

let 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.

let 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.

let 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

let 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.

let 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.

let is_valid_utf_16le: t => bool;

is_valid_utf_16le b is true if and only if b contains valid UTF-16LE data.

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.

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:

let get_uint8: bytes => int => int;

get_uint8 b i is b's unsigned 8-bit integer starting at byte index i.

  • since 4.08
let get_int8: bytes => int => int;

get_int8 b i is b's signed 8-bit integer starting at byte index i.

  • since 4.08
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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
let 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

Care must be taken when concurrently accessing byte sequences from multiple domains: accessing a byte sequence will never crash a program, but unsynchronized accesses might yield surprising (non-sequentially-consistent) results.

Atomicity

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

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.