cstruct 6.0.1 · OCaml Package

Manipulate external memory buffers as C-like structures.

Cstruct is a library and ppx rewriter to make it easier to access C-like structures directly from OCaml. It supports both reading and writing to these memory buffers, and they are accessed via the Bigarray module.

The library interface below is intended to be used in conjunction with the ppx rewriter that is also supplied with this library (in the cstruct-ppx ocamlfind package).

An example description for the pcap packet format is:

[%%cstruct
type pcap_header = {
  magic_number:  uint32_t; (* magic number *)
  version_major: uint16_t; (* major version number *)
  version_minor: uint16_t; (* minor version number *)
  thiszone:      uint32_t; (* GMT to local correction *)
  sigfigs:       uint32_t; (* accuracy of timestamps *)
  snaplen:       uint32_t; (* max length of captured packets, in octets *)
  network:       uint32_t; (* data link type *)
} [@@little_endian]
]
[%%cstruct
type pcap_packet = {
  ts_sec:   uint32_t; (* timestamp seconds *)
  ts_usec:  uint32_t; (* timestamp microseconds *)
  incl_len: uint32_t; (* number of octets of packet saved in file *)
  orig_len: uint32_t; (* actual length of packet *)
} [@@little_endian]
]
[%%cstruct
type ethernet = {
  dst:       uint8_t;  [@len 6];
  src:       uint8_t;  [@len 6];
  ethertype: uint16_t;
} [@@big_endian]
]
[%%cstruct
type ipv4 = {
  hlen_version: uint8_t;
  tos:          uint8_t;
  len:          uint16_t;
  id:           uint16_t;
  off:          uint16_t;
  ttl:          uint8_t;
  proto:        uint8_t;
  csum:         uint16_t;
  src:          uint8_t;  [@len 4];
  dst:          uint8_t;  [@len 4]
} [@@big_endian]
]

These will expand to get and set functions for every field, with types appropriate to the particular definition. For instance:

val get_pcap_packet_ts_sec : Cstruct.t -> Cstruct.uint32
val set_pcap_packet_ts_sec : Cstruct.t -> Cstruct.uint32 -> unit
val get_pcap_packet_ts_usec : Cstruct.t -> Cstruct.uint32
val set_pcap_packet_ts_usec : Cstruct.t -> Cstruct.uint32 -> unit
val get_pcap_packet_incl_len : Cstruct.t -> Cstruct.uint32
val set_pcap_packet_incl_len : Cstruct.t -> Cstruct.uint32 -> unit
val get_pcap_packet_orig_len : Cstruct.t -> Cstruct.uint32
val set_pcap_packet_orig_len : Cstruct.t -> Cstruct.uint32 -> unit
val hexdump_pcap_packet_to_buffer : Buffer.t -> Cstruct.t -> unit

The buffers generate a different set of functions. For the ethernet definitions, we have:

val sizeof_ethernet : int
val get_ethernet_dst : Cstruct.t -> Cstruct.t
val copy_ethernet_dst : Cstruct.t -> string
val set_ethernet_dst : string -> int -> Cstruct.t -> unit
val blit_ethernet_dst : Cstruct.t -> int -> Cstruct.t -> unit
val get_ethernet_src : Cstruct.t -> Cstruct.t
val copy_ethernet_src : Cstruct.t -> string

You can also declare C-like enums:

[%%cenum
type foo32 =
  | ONE32
  | TWO32     [@id 0xfffffffel]
  | THREE32
[@@uint32_t]
]
[%%cenum
type bar16 =
  | ONE  [@id 1]
  | TWO
  | FOUR [@id 4
  | FIVE
[@@uint16_t]
]

This generates signatures of the form:

type foo32 = | ONE32 | TWO32 | THREE32
val int_to_foo32 : int32 -> foo32 option
val foo32_to_int : foo32 -> int32
val foo32_to_string : foo32 -> string
val string_to_foo32 : string -> foo32 option
type bar16 = | ONE | TWO | FOUR | FIVE
val int_to_bar16 : int -> bar16 option
val bar16_to_int : bar16 -> int
val bar16_to_string : bar16 -> string
val string_to_bar16 : string -> bar16 option

Base types

type buffer =
  (char, Bigarray_compat.int8_unsigned_elt, Bigarray_compat.c_layout)
    Bigarray_compat.Array1.t

Type of a buffer. A cstruct is composed of an underlying buffer and position/length within this buffer.

type t = private {

buffer : buffer;
off : int;
len : int;

}

Type of a cstruct.

type byte = char

A single byte type

val byte : int -> byte

byte v convert v to a single byte.

raises Invalid_argument
if v is negative or greater than 255.

type uint8 = int

8-bit unsigned integer. The representation is currently an unboxed OCaml integer.

type uint16 = int

16-bit unsigned integer. The representation is currently an unboxed OCaml integer.

type uint32 = int32

32-bit unsigned integer. The representation is currently a boxed OCaml int32.

type uint64 = int64

64-bit unsigned integer. The representation is currently a boxed OCaml int64.

Creation and conversion

val empty : t

empty is the cstruct of length 0.

val of_bigarray : ?off:int -> ?len:int -> buffer -> t

of_bigarray ~off ~len b is the cstruct contained in b starting at offset off (default 0) of length len (default Bigarray.Array1.dim b - off).

val to_bigarray : t -> buffer

to_bigarray t converts a t into a buffer Bigarray, using the Bigarray slicing to allocate a fresh array that preserves sharing of the underlying buffer.

val create : int -> t

create len is a fresh cstruct of size len with an offset of 0, filled with zero bytes.

val create_unsafe : int -> t

create_unsafe len is a cstruct of size len with an offset of 0.

Note that the returned cstruct will contain arbitrary data, likely including the contents of previously-deallocated cstructs.

Beware!

Forgetting to replace this data could cause your application to leak sensitive information.

val of_string : ?allocator:(int -> t) -> ?off:int -> ?len:int -> string -> t

of_string ~allocator ~off ~len str is the cstruct representation of str slice located at offset off (default 0) and of length len (default String.length str - off), with the underlying buffer allocated by alloc. If allocator is not provided, create is used.

raises Invalid_argument
if off or len is negative, or String.length str - off < len.

val of_bytes : ?allocator:(int -> t) -> ?off:int -> ?len:int -> bytes -> t

of_bytes ~allocator byt is the cstruct representation of byt slice located at offset off (default 0) and of length len (default Bytes.length byt - off), with the underlying buffer allocated by alloc. If allocator is not provided, create is used.

raises Invalid_argument
if off or len is negative, or Bytes.length str - off < len.

val of_hex : ?off:int -> ?len:int -> string -> t

of_hex ~off ~len str is the cstruct cs. Every pair of hex-encoded characters in str starting at offset off (default 0) of length len (default String.length str - off) are converted to one byte in cs. Whitespaces (space, newline, tab, carriage return) in str are skipped.

raises Invalid_argument
if the input string contains invalid characters or has an odd numbers of non-whitespace characters, or if off or len are negative, or String.length str - off < len.

Comparison

val equal : t -> t -> bool

equal t1 t2 is true iff t1 and t2 correspond to the same sequence of bytes.

val compare : t -> t -> int

compare t1 t2 gives an unspecified total ordering over t.

Getters and Setters

val byte_to_int : byte -> int

Convert a byte to an integer

val check_bounds : t -> int -> bool

check_bounds cstr len is true if len is a non-negative integer and cstr.buffer's size is greater or equal than len false otherwise.

val check_alignment : t -> int -> bool

check_alignment cstr alignment is true if the first byte stored within cstr is at a memory address where address mod alignment = 0, false otherwise. Typical uses are to check a buffer is aligned to a page or disk sector boundary.

raises Invalid_argument
if alignment is not a positive integer.

val get_char : t -> int -> char

get_char t off returns the character contained in the cstruct at offset off.

raises Invalid_argument
if the offset exceeds cstruct length.

val get_uint8 : t -> int -> uint8

get_uint8 t off returns the byte contained in the cstruct at offset off.

raises Invalid_argument
if the offset exceeds cstruct length.

val set_char : t -> int -> char -> unit

set_char t off c sets the byte contained in the cstruct at offset off to character c.

raises Invalid_argument
if the offset exceeds cstruct length.

val set_uint8 : t -> int -> uint8 -> unit

set_uint8 t off c sets the byte contained in the cstruct at offset off to byte c.

raises Invalid_argument
if the offset exceeds cstruct length.

val sub : t -> int -> int -> t

sub cstr off len is { t with off = t.off + off; len }

raises Invalid_argument
if the offset exceeds cstruct length.

val shift : t -> int -> t

shift cstr len is { cstr with off=t.off+len; len=t.len-len }

raises Invalid_argument
if the offset exceeds cstruct length.

val copy : t -> int -> int -> string

copy cstr off len is the string representation of the segment of t starting at off of size len.

raises Invalid_argument
if off and len do not designate a valid segment of t.

val blit : t -> int -> t -> int -> int -> unit

blit src srcoff dst dstoff len copies len characters from cstruct src, starting at index srcoff, to cstruct dst, starting at index dstoff. It works correctly even if src and dst are the same string, and the source and destination intervals overlap.

raises Invalid_argument
if srcoff and len do not designate a valid segment of src, or if dstoff and len do not designate a valid segment of dst.

val blit_from_string : string -> int -> t -> int -> int -> unit

blit_from_string src srcoff dst dstoff len copies len characters from string src, starting at index srcoff, to cstruct dst, starting at index dstoff.

raises Invalid_argument
if srcoff and len do not designate a valid substring of src, or if dstoff and len do not designate a valid segment of dst.

val blit_from_bytes : bytes -> int -> t -> int -> int -> unit

blit_from_bytes src srcoff dst dstoff len copies len characters from bytes src, starting at index srcoff, to cstruct dst, starting at index dstoff.

raises Invalid_argument
if srcoff and len do not designate a valid subsequence of src, or if dstoff and len do not designate a valid segment of dst.

val blit_to_bytes : t -> int -> bytes -> int -> int -> unit

blit_to_bytes src srcoff dst dstoff len copies len characters from cstruct src, starting at index srcoff, to the dst buffer, starting at index dstoff.

raises Invalid_argument
if srcoff and len do not designate a valid segment of src, or if dstoff and len do not designate a valid segment of dst.

val blit_to_string : t -> int -> bytes -> int -> int -> unit

blit_to_string is a deprecated alias of blit_to_bytes.

deprecated Use blit_to_bytes instead, blit_to_string will be removed in cstruct 5.0.0

val memset : t -> int -> unit

memset t x sets all the bytes of t to x land 0xff.

val len : t -> int

Returns the length of the current cstruct view. Note that this length is potentially smaller than the actual size of the underlying buffer, as the sub or set_len functions can construct a smaller view.

deprecated len is deprecated, you should use length instead.

val set_len : t -> int -> t

set_len t len sets the length of the cstruct t to a new absolute value, and returns a fresh cstruct with these settings.

raises Invalid_argument
if len exceeds the size of the buffer.

deprecated This function will be removed in cstruct 5.0.0. If you need this function, discuss other ways in the issue tracker https://github.com/mirage/ocaml-cstruct.

val add_len : t -> int -> t

add_len t l will add l bytes to the length of the buffer, and return a fresh cstruct with these settings.

raises Invalid_argument
if len exceeds the size of the buffer.

deprecated This function will be removed in cstruct 5.0.0. If you need this function, discuss other ways in the issue tracker https://github.com/mirage/ocaml-cstruct.

val split : ?start:int -> t -> int -> t * t

split ~start cstr len is a tuple containing the cstruct extracted from cstr at offset start (default: 0) of length len as first element, and the rest of cstr as second element.

raises Invalid_argument
if start exceeds the cstruct length, or if there is a bounds violation of the cstruct via len+start.

val to_string : ?off:int -> ?len:int -> t -> string

to_string ~off ~len t will allocate a fresh OCaml string and copy the contents of the cstruct starting at offset off (default 0) of length len (default Cstruct.len t - off) into it, and return that string.

raises Invalid_argument
if off or len is negative, or Cstruct.len str - off < len.

val to_bytes : ?off:int -> ?len:int -> t -> bytes

to_bytes ~off ~len t will allocate a fresh OCaml bytes and copy the contents of the cstruct starting at offset off (default 0) of length len (default Cstruct.len t - off) into it, and return that bytes.

raises Invalid_argument
if off or len is negative, or Cstruct.len str - off < len.

Debugging

val hexdump : t -> unit

When the going gets tough, the tough hexdump their cstructs and peer at it until the bug disappears. This will directly prettyprint the contents of the cstruct to the standard output.

val hexdump_to_buffer : Buffer.t -> t -> unit

hexdump_to_buffer buf c will append the pretty-printed hexdump of the cstruct c to the buffer buf.

val hexdump_pp : Format.formatter -> t -> unit

hexdump_pp f c pretty-prints a hexdump of c to f.

val debug : t -> string

debug t will print out the internal details of a cstruct such as its base offset and the length, and raise an assertion failure if invariants have been violated. Not intended for casual use.

module BE : sig ... end

Get/set big-endian integers of various sizes. The second argument of those functions is the position relative to the current offset of the cstruct.

module LE : sig ... end

Get/set little-endian integers of various sizes. The second argument of those functions is the position relative to the current offset of the cstruct.

List of buffers

val lenv : t list -> int

lenv cstrs is the combined length of all cstructs in cstrs.

raises Invalid_argument
if computing the sum overflows.

val copyv : t list -> string

copyv cstrs is the string representation of the concatenation of all cstructs in cstrs.

raises Invalid_argument
if the length of the result would exceed Sys.max_string_length.

val fillv : src:t list -> dst:t -> int * t list

fillv ~src ~dst copies from src to dst until src is exhausted or dst is full. Returns the number of bytes copied and the remaining data from src, if any. This is useful if you want buffer data into fixed-sized chunks.

val shiftv : t list -> int -> t list

shiftv ts n is ts without the first n bytes. It has the property that equal (concat (shiftv ts n)) (shift (concat ts) n). This operation is fairly fast, as it will share the tail of the list. The first item in the returned list is never an empty cstruct, so you'll get [] if and only if lenv ts = n.

Iterations

type 'a iter = unit -> 'a option

Type of an iterator.

val iter : (t -> int option) -> (t -> 'a) -> t -> 'a iter

iter lenf of_cstr cstr is an iterator over cstr that returns elements of size lenf cstr and type of_cstr cstr.

val fold : ('b -> 'a -> 'b) -> 'a iter -> 'b -> 'b

fold f iter acc is (f iterN accN ... (f iter acc)...).

val append : t -> t -> t

append t1 t2 is the concatenation t1 || t2.

val concat : t list -> t

concat ts is the concatenation of all the ts. It is not guaranteed that * the result is a newly created t in the zero- and one-element cases.

val rev : t -> t

rev t is t in reverse order. The return value is a freshly allocated cstruct, and the argument is not modified.

Helpers to parse.

Cstruct is used to manipulate payloads which can be formatted according an RFC or an user-defined format. In such context, this module provides utilities to be able to easily parse payloads.

Due to the type Cstruct.t, no copy are done when you use these utilities and you are able to extract your information without a big performance cost.

More precisely, each values returned by these utilities will be located into the minor-heap where the base buffer will never be copied or relocated.

For instance, to parse a Git tree object:

  entry := perm ' ' name '\000' 20byte
  tree  := entry *

open Cstruct

let ( >>= ) = Option.bind

let rec hash_of_name ~name payload =
  if is_empty payload then raise Not_found
  else
    cut ~sep:(v " ") payload >>= fun (_, payload) ->
    cut ~sep:(v "\000") payload >>= fun (name', payload) ->
    if name = name' then with_range ~len:20 payload
    else hash_of_name ~name (shift payload 20)

A Cstruct defines a possibly empty subsequence of bytes in a base buffer (a Bigarray.Array1.t).

The positions of a buffer b of length l are the slits found before each byte and after the last byte of the buffer. They are labelled from left to right by increasing number in the range [0;l].

positions  0   1   2   3   4    l-1    l
           +---+---+---+---+     +-----+
  indices  | 0 | 1 | 2 | 3 | ... | l-1 |
           +---+---+---+---+     +-----+

The ith byte index is between positions i and i+1.

Formally we define a subbuffer of b as being a subsequence of bytes defined by a off position and a len number. When len is 0 the subbuffer is empty. Note that for a given base buffer there are as many empty subbuffers as there are positions in the buffer.

Like in strings, we index the bytes of a subbuffer using zero-based indices.

val get : t -> int -> char

get cs zidx is the byte of cs at its zero-based index zidx. It's an alias of get_char.

raises Invalid_argument
if zidx is not an index of cs.

val get_byte : t -> int -> int

get_byte cs zidx is Char.code (get cs zidx). It's an alias of get_uint8.

val string : ?off:int -> ?len:int -> string -> t

string ~off ~len str is the subbuffer of str that starts at position off (defaults to 0) and stops at position off + len (defaults to String.length str). str is fully-replaced by an fresh allocated Cstruct.buffer.

raises Invalid_argument
if off or off + len are not positions of str.

val buffer : ?off:int -> ?len:int -> buffer -> t

buffer ~off ~len buffer is the sub-part of buffer that starts at position off (default to 0) and stops at position off + len (default to Bigarray.Array1.dim buffer). buffer is used as the base buffer of the returned value (no major-heap allocation are performed).

raises Invalid_argument
if off or off + len are not positions of buffer.

val start_pos : t -> int

start_pos cs is cs's start position in the base Cstruct.buffer.

val stop_pos : t -> int

stop_pos cs is cs's stop position in the base Cstruct.buffer.

val length : t -> int

val head : ?rev:bool -> t -> char option

head cs is Some (get cs h) with h = 0 if rev = false (default) or h = length cs - 1 if rev = true. None is returned if cs is empty.

val tail : ?rev:bool -> t -> t

tail cs is cs without its first (rev is false, default) or last (rev is true) byte or cs is empty.

val is_empty : t -> bool

is_empty cs is length cs = 0.

val is_prefix : affix:t -> t -> bool

is_prefix ~affix cs is true iff affix.[zidx] = cs.[zidx] for all indices zidx of affix.

val is_suffix : affix:t -> t -> bool

is_suffix ~affix cs is true iff affix.[n - zidx] = cs.[m - zidx] for all indices zidx of affix with n = length affix - 1 and m = length cs - 1.

val is_infix : affix:t -> t -> bool

is_infix ~affix cs is true iff there exists an index z in cs such that for all indices zidx of affix we have affix.[zidx] = cs.[z + zidx].

val for_all : (char -> bool) -> t -> bool

for_all p cs is true iff for all indices zidx of cs, p cs.[zidx] = true.

val exists : (char -> bool) -> t -> bool

exists p cs is true iff there exists an index zidx of cs with p cs.[zidx] = true.

val start : t -> t

start cs is the empty sub-part at the start position of cs.

val stop : t -> t

stop cs is the empty sub-part at the stop position of cs.

val trim : ?drop:(char -> bool) -> t -> t

trim ~drop cs is cs with prefix and suffix bytes satisfying drop in cs removed. drop defaults to function ' ' | '\r' .. '\t' -> true | _ -> false.

val span : 
  ?rev:bool ->
  ?min:int ->
  ?max:int ->
  ?sat:(char -> bool) ->
  t ->
  t * t

span ~rev ~min ~max ~sat cs is (l, r) where:

if rev is false (default), l is at least min and at most max consecutive sat satisfying initial bytes of cs or empty if there are no such bytes. r are the remaining bytes of cs.
if rev is true, r is at least min and at most max consecutive sat satisfying final bytes of cs or empty if there are no such bytes. l are the remaining bytes of cs.

If max is unspecified the span is unlimited. If min is unspecified it defaults to 0. If min > max the condition can't be satisfied and the left or right span, depending on rev, is always empty. sat defaults to (fun _ -> true).

The invariant l ^ r = s holds.

For instance, the ABNF expression:

  time := 1*10DIGIT

can be translated to:

let (time, _) = span ~min:1 ~max:10 is_digit cs in

raises Invalid_argument
if max or min is negative.

val take : ?rev:bool -> ?min:int -> ?max:int -> ?sat:(char -> bool) -> t -> t

take ~rev ~min ~max ~sat cs is the matching span of span without the remaining one. In other words:

(if rev then snd else fst) @@ span ~rev ~min ~max ~sat cs

val drop : ?rev:bool -> ?min:int -> ?max:int -> ?sat:(char -> bool) -> t -> t

drop ~rev ~min ~max ~sat cs is the remaining span of span without the matching one. In other words:

(if rev then fst else snd) @@ span ~rev ~min ~max ~sat cs

val cut : ?rev:bool -> sep:t -> t -> (t * t) option

cut ~sep cs is either the pair Some (l, r) of the two (possibly empty) sub-buffers of cs that are delimited by the first match of the non empty separator string sep or None if sep can't be matched in cs. Matching starts from the beginning of cs (rev is false, default) or the end (rev is true).

The invariant l ^ sep ^ r = s holds.

For instance, the ABNF expression:

  field_name := *PRINT
  field_value := *ASCII
  field := field_name ":" field_value

can be translated to:

match cut ~sep:":" value with
| Some (field_name, field_value) -> ...
| None -> invalid_arg "invalid field"

raises Invalid_argument
if sep is the empty buffer.

val cuts : ?rev:bool -> ?empty:bool -> sep:t -> t -> t list

cuts ~sep cs is the list of all sub-buffers of cs that are delimited by matches of the non empty separator sep. Empty sub-buffers are omitted in the list if empty is false (default to true).

Matching separators in cs starts from the beginning of cs (rev is false, default) or the end (rev is true). Once one is found, the separator is skipped and matching starts again, that is separator matches can't overlap. If there is no separator match in cs, the list [cs] is returned.

The following invariants hold:

concat ~sep (cuts ~empty:true ~sep cs) = cs
cuts ~empty:true ~sep cs <> []

For instance, the ABNF expression:

  arg := *(ASCII / ",") ; any characters exclude ","
  args := arg *("," arg)

can be translated to:

let args = cuts ~sep:"," buffer in

raises Invalid_argument
if sep is the empty buffer.

val fields : ?empty:bool -> ?is_sep:(char -> bool) -> t -> t list

fields ~empty ~is_sep cs is the list of (possibly empty) sub-buffers that are delimited by bytes for which is_sep is true. Empty sub-buffers are omitted in the list if empty is false (defaults to true). is_sep c if it's not define by the user is true iff c is an US-ASCII white space character, that is one of space ' ' (0x20), tab '\t' (0x09), newline '\n' (0x0a), vertical tab (0x0b), form feed (0x0c), carriage return '\r' (0x0d).

val find : ?rev:bool -> (char -> bool) -> t -> t option

find ~rev sat cs is the sub-buffer of cs (if any) that spans the first byte that satisfies sat in cs after position start cs (rev is false, default) or before stop cs (rev is true). None is returned if there is no matching byte in s.

val find_sub : ?rev:bool -> sub:t -> t -> t option

find_sub ~rev ~sub cs is the sub-buffer of cs (if any) that spans the first match of sub in cs after position start cs (rev is false, default) or before stop cs (rev is true). Only bytes are compared and sub can be on a different base buffer. None is returned if there is no match of sub in s.

val filter : (char -> bool) -> t -> t

filter sat cs is the buffer made of the bytes of cs that satisfy sat, in the same order.

val filter_map : (char -> char option) -> t -> t

filter_map f cs is the buffer made of the bytes of cs as mapped by f, in the same order.

val map : (char -> char) -> t -> t

map f cs is cs' with cs'.[i] = f cs.[i] for all indices i of cs. f is invoked in increasing index order.

val mapi : (int -> char -> char) -> t -> t

map f cs is cs' with cs'.[i] = f i cs.[i] for all indices i of cs. f is invoked in increasing index order.

package cstruct

Base types

Creation and conversion

Comparison

Getters and Setters

Debugging

List of buffers

Iterations

Helpers to parse.