Module Cstruct

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.int8_unsigned_eltBigarray.c_layoutBigarray.Array1.t

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

val sexp_of_buffer : buffer ‑> Sexplib.Sexp.t

sexp_of_buffer b returns the s-expression representation of the raw memory buffer b

val buffer_of_sexp : Sexplib.Sexp.t ‑> buffer

buffer_of_sexp s returns a fresh memory buffer from the s-expression s. s should have been constructed using sexp_of_buffer.

type t = private {
buffer : buffer;
off : int;
len : int;
}

Type of a cstruct.

val sexp_of_t : t ‑> Sexplib.Sexp.t

sexp_of_t t returns the s-expression representation of the Cstruct t

val t_of_sexp : Sexplib.Sexp.t ‑> t

t_of_sexp s returns a fresh Cstruct.t that represents the s-expression previously serialised by sexp_of_t.

type byte = char

A single byte type

val byte : int ‑> byte

byte v convert v to a single byte.

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 of_bigarray : ?⁠off:int ‑> ?⁠len:int ‑> buffer ‑> t

of_bigarray ~off ~len b is the cstruct contained in b starting at off, of length len.

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 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) ‑> string ‑> t

of_string ~allocator str is the cstruct representation of str, with the underlying buffer allocated by alloc. If allocator is not provided, create is used.

val of_bytes : ?⁠allocator:(int ‑> t) ‑> Bytes.t ‑> t

of_bytes ~allocator byt is the cstruct representation of byt, with the underlying buffer allocated by alloc. If allocator is not provided, create is used.

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 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. alignment must a positive integer or else Invalid_argument is raised.

val get_char : t ‑> int ‑> char

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

val get_uint8 : t ‑> int ‑> uint8

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

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.

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.

val sub : t ‑> int ‑> int ‑> t

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

val shift : t ‑> int ‑> t

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

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.

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.

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.

val blit_from_bytes : Bytes.t ‑> 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.

val blit_to_bytes : t ‑> int ‑> Bytes.t ‑> int ‑> int ‑> unit

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

val blit_to_string : t ‑> int ‑> Bytes.t ‑> int ‑> int ‑> unit

blit_to_string is a deprecated alias of blit_to_bytes.

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.

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.

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.

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.

val to_string : t ‑> string

to_string t will allocate a fresh OCaml string and copy the contents of the cstruct into it, and return that string copy.

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
module LE : sig ... end

List of buffers

val lenv : t list ‑> int

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

val copyv : t list ‑> string

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

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.

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.