module Bytes:sig..end
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.
Since 4.02.0
val length : bytes -> intval get : bytes -> int -> charget s n returns the byte at index n in argument s.
Raise Invalid_argument if n not a valid index in s.
val set : bytes -> int -> char -> unitset s n c modifies s in place, replacing the byte at index n
with c.
Raise Invalid_argument if n is not a valid index in s.
val create : int -> bytescreate n returns a new byte sequence of length n. The
sequence is uninitialized and contains arbitrary bytes.
Raise Invalid_argument if n < 0 or n > Sys.max_string_length.
val make : int -> char -> bytesmake n c returns a new byte sequence of length n, filled with
the byte c.
Raise Invalid_argument if n < 0 or n > Sys.max_string_length.
val init : int -> (int -> char) -> bytesBytes.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).
Raise Invalid_argument if n < 0 or n > Sys.max_string_length.
val empty : bytesval copy : bytes -> bytesval of_string : string -> bytesval to_string : bytes -> stringval sub : bytes -> int -> int -> bytessub s start len returns a new byte sequence of length len,
containing the subsequence of s that starts at position start
and has length len.
Raise Invalid_argument if start and len do not designate a
valid range of s.
val sub_string : bytes -> int -> int -> stringsub but return a string instead of a byte sequence.val extend : bytes -> int -> int -> bytesextend 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.
Raise Invalid_argument if the result length is negative or
longer than Sys.max_string_length bytes.
val fill : bytes -> int -> int -> char -> unitfill s start len c modifies s in place, replacing len
characters with c, starting at start.
Raise Invalid_argument if start and len do not designate a
valid range of s.
val blit : bytes -> int -> bytes -> int -> int -> unitblit src srcoff dst dstoff len copies len bytes from sequence
src, starting at index srcoff, to sequence dst, starting at
index dstoff. It works correctly even if src and dst are the
same byte sequence, and the source and destination intervals
overlap.
Raise Invalid_argument if srcoff and len do not
designate a valid range of src, or if dstoff and len
do not designate a valid range of dst.
val blit_string : string -> int -> bytes -> int -> int -> unitblit src srcoff dst dstoff len copies len bytes from string
src, starting at index srcoff, to byte sequence dst,
starting at index dstoff.
Raise Invalid_argument if srcoff and len do not
designate a valid range of src, or if dstoff and len
do not designate a valid range of dst.
val concat : bytes -> bytes list -> bytesconcat 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.
Raise Invalid_argument if the result is longer than
Sys.max_string_length bytes.
val cat : bytes -> bytes -> bytescat s1 s2 concatenates s1 and s2 and returns the result
as new byte sequence.
Raise Invalid_argument if the result is longer than
Sys.max_string_length bytes.
val iter : (char -> unit) -> bytes -> unititer 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 : (int -> char -> unit) -> bytes -> unitBytes.iter, but the function is applied to the index of
the byte as first argument and the byte itself as second
argument.val map : (char -> char) -> bytes -> bytesmap 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 : (int -> char -> char) -> bytes -> bytesmapi 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 trim : bytes -> bytes' ', '\012', '\n', '\r', and '\t'.val escaped : bytes -> bytes
Raise Invalid_argument if the result is longer than
Sys.max_string_length bytes.
val index : bytes -> char -> intindex s c returns the index of the first occurrence of byte c
in s.
Raise Not_found if c does not occur in s.
val rindex : bytes -> char -> intrindex s c returns the index of the last occurrence of byte c
in s.
Raise Not_found if c does not occur in s.
val index_from : bytes -> int -> char -> intindex_from s i c returns the index of the first occurrence of
byte c in s after position i. Bytes.index s c is
equivalent to Bytes.index_from s 0 c.
Raise Invalid_argument if i is not a valid position in s.
Raise Not_found if c does not occur in s after position i.
val rindex_from : bytes -> int -> char -> intrindex_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 (Bytes.length s - 1) c.
Raise Invalid_argument if i+1 is not a valid position in s.
Raise Not_found if c does not occur in s before position i+1.
val contains : bytes -> char -> boolcontains s c tests if byte c appears in s.val contains_from : bytes -> int -> char -> boolcontains_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.
Raise Invalid_argument if start is not a valid position in s.
val rcontains_from : bytes -> int -> char -> boolrcontains_from s stop c tests if byte c appears in s before
position stop+1.
Raise Invalid_argument if stop < 0 or stop+1 is not a valid
position in s.
val uppercase : bytes -> bytesval lowercase : bytes -> bytesval capitalize : bytes -> bytesval uncapitalize : bytes -> bytesval uppercase_ascii : bytes -> bytesval lowercase_ascii : bytes -> bytesval capitalize_ascii : bytes -> bytesval uncapitalize_ascii : bytes -> bytestypet =bytes
val compare : t -> t -> intcompare. 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.val equal : t -> t -> bool
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 Bytes.to_string and Bytes.of_string instead.
val unsafe_to_string : bytes -> 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:
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.
val unsafe_of_string : string -> bytes
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.