1 (** Bitmatch library. *)
2 (* Copyright (C) 2008 Red Hat Inc., Richard W.M. Jones
4 * This library is free software; you can redistribute it and/or
5 * modify it under the terms of the GNU Lesser General Public
6 * License as published by the Free Software Foundation; either
7 * version 2 of the License, or (at your option) any later version.
9 * This library is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 * Lesser General Public License for more details.
14 * You should have received a copy of the GNU Lesser General Public
15 * License along with this library; if not, write to the Free Software
16 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
22 {{:#reference}Jump straight to the reference section for
23 documentation on types and functions}.
27 Bitmatch adds Erlang-style bitstrings and matching over bitstrings
28 as a syntax extension and library for OCaml. You can use
29 this module to both parse and generate binary formats, for
30 example, communications protocols, disk formats and binary files.
32 {{:http://code.google.com/p/bitmatch/}OCaml bitmatch website}
36 A function which can parse IPv4 packets:
43 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
44 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
45 | 4 | IHL |Type of Service| Total Length |
46 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
47 | Identification |Flags| Fragment Offset |
48 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
49 | Time to Live | Protocol | Header Checksum |
50 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
52 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
53 | Destination Address |
54 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
56 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
58 | { 4 : 4; hdrlen : 4; tos : 8; length : 16;
59 identification : 16; flags : 3; fragoffset : 13;
60 ttl : 8; protocol : 8; checksum : 16;
63 options : (hdrlen-5)*32 : bitstring;
64 payload : -1 : bitstring } ->
67 printf " header length: %d * 32 bit words\n" hdrlen;
68 printf " type of service: %d\n" tos;
69 printf " packet length: %d bytes\n" length;
70 printf " identification: %d\n" identification;
71 printf " flags: %d\n" flags;
72 printf " fragment offset: %d\n" fragoffset;
73 printf " ttl: %d\n" ttl;
74 printf " protocol: %d\n" protocol;
75 printf " checksum: %d\n" checksum;
76 printf " source: %lx dest: %lx\n" source dest;
77 printf " header options + padding:\n";
78 Bitmatch.hexdump_bitstring stdout options;
79 printf " packet payload:\n";
80 Bitmatch.hexdump_bitstring stdout payload
83 eprintf "unknown IP version %d\n" version;
87 eprintf "data is smaller than one nibble:\n";
88 Bitmatch.hexdump_bitstring stderr pkt;
92 A program which can parse
93 {{:http://lxr.linux.no/linux/include/linux/ext3_fs.h}Linux EXT3 filesystem superblocks}:
96 let bits = Bitmatch.bitstring_of_file "tests/ext3_sb"
100 | { s_inodes_count : 32 : littleendian; (* Inodes count *)
101 s_blocks_count : 32 : littleendian; (* Blocks count *)
102 s_r_blocks_count : 32 : littleendian; (* Reserved blocks count *)
103 s_free_blocks_count : 32 : littleendian; (* Free blocks count *)
104 s_free_inodes_count : 32 : littleendian; (* Free inodes count *)
105 s_first_data_block : 32 : littleendian; (* First Data Block *)
106 s_log_block_size : 32 : littleendian; (* Block size *)
107 s_log_frag_size : 32 : littleendian; (* Fragment size *)
108 s_blocks_per_group : 32 : littleendian; (* # Blocks per group *)
109 s_frags_per_group : 32 : littleendian; (* # Fragments per group *)
110 s_inodes_per_group : 32 : littleendian; (* # Inodes per group *)
111 s_mtime : 32 : littleendian; (* Mount time *)
112 s_wtime : 32 : littleendian; (* Write time *)
113 s_mnt_count : 16 : littleendian; (* Mount count *)
114 s_max_mnt_count : 16 : littleendian; (* Maximal mount count *)
115 0xef53 : 16 : littleendian } -> (* Magic signature *)
117 printf "ext3 superblock:\n";
118 printf " s_inodes_count = %ld\n" s_inodes_count;
119 printf " s_blocks_count = %ld\n" s_blocks_count;
120 printf " s_free_inodes_count = %ld\n" s_free_inodes_count;
121 printf " s_free_blocks_count = %ld\n" s_free_blocks_count
124 eprintf "not an ext3 superblock!\n%!";
128 Constructing packets for a simple binary message
133 +---------------+---------------+--------------------------+
134 | type | subtype | parameter |
135 +---------------+---------------+--------------------------+
136 <-- 16 bits --> <-- 16 bits --> <------- 32 bits -------->
138 All fields are in network byte order.
141 let make_message typ subtype param =
149 {2 Loading, creating bitstrings}
151 The basic data type is the {!bitstring}, a string of bits of
152 arbitrary length. Bitstrings can be any length in bits and
153 operations do not need to be byte-aligned (although they will
154 generally be more efficient if they are byte-aligned).
156 Internally a bitstring is stored as a normal OCaml [string]
157 together with an offset and length, where the offset and length are
158 measured in bits. Thus one can efficiently form substrings of
159 bitstrings, overlay a bitstring on existing data, and load and save
160 bitstrings from files or other external sources.
162 To load a bitstring from a file use {!bitstring_of_file} or
163 {!bitstring_of_chan}.
165 There are also functions to create bitstrings from arbitrary data.
166 See the {{:#reference}reference} below.
168 {2 Matching bitstrings with patterns}
170 Use the [bitmatch] operator (part of the syntax extension) to break
171 apart a bitstring into its fields. [bitmatch] works a lot like the
172 OCaml [match] operator.
174 The general form of [bitmatch] is:
176 [bitmatch {] {i bitstring-expression} [} with]
178 [| {] {i pattern} [} ->] {i code}
180 [| {] {i pattern} [} ->] {i code}
184 As with normal match, the statement attempts to match the
185 bitstring against each pattern in turn. If none of the patterns
186 match then the standard library [Match_failure] exception is
189 Patterns look a bit different from normal match patterns. They
190 consist of a list of bitfields separated by [;] where each bitfield
191 contains a bind variable, the width (in bits) of the field, and
192 other information. Some example patterns:
197 | { version : 8; name : 8; param : 8 } -> ...
199 (* Bitstring of at least 3 bytes. First byte is the version
200 number, second byte is a field called name, third byte is
201 a field called parameter. *)
204 printf "flag is %b\n" flag
206 (* A single flag bit (mapped into an OCaml boolean). *)
208 | { len : 4; data : 1+len } ->
209 printf "len = %d, data = 0x%Lx\n" len data
211 (* A 4-bit length, followed by 1-16 bits of data, where the
212 length of the data is computed from len. *)
214 | { ipv6_source : 128 : bitstring;
215 ipv6_dest : 128 : bitstring } -> ...
217 (* IPv6 source and destination addresses. Each is 128 bits
218 and is mapped into a bitstring type which will be a substring
219 of the main bitstring expression. *)
222 You can also add conditional when-clauses:
226 when version = 4 || version = 6 -> ...
228 (* Only match and run the code when version is 4 or 6. If
229 it isn't we will drop through to the next case. *)
232 Note that the pattern is only compared against the first part of
233 the bitstring (there may be more data in the bitstring following
234 the pattern, which is not matched). In terms of regular
235 expressions you might say that the pattern matches [^pattern], not
236 [^pattern$]. To ensure that the bitstring contains only the
237 pattern, add a length -1 bitstring to the end and test that its
238 length is zero in the when-clause:
242 rest : -1 : bitstring }
243 when Bitmatch.bitstring_length rest = 0 -> ...
245 (* Only matches exactly 4 bits. *)
248 Normally the first part of each field is a binding variable,
249 but you can also match a constant, as in:
252 | { (4|6) : 4 } -> ...
254 (* Only matches if the first 4 bits contain either
255 the integer 4 or the integer 6. *)
258 One may also match on strings:
261 | { "MAGIC" : 5*8 : string } -> ...
263 (* Only matches if the string "MAGIC" appears at the start
267 {3:patternfieldreference Pattern field reference}
269 The exact format of each pattern field is:
271 [pattern : length [: qualifier [,qualifier ...]]]
273 [pattern] is the pattern, binding variable name, or constant to
274 match. [length] is the length in bits which may be either a
275 constant or an expression. The length expression is just an OCaml
276 expression and can use any values defined in the program, and refer
277 back to earlier fields (but not to later fields).
279 Integers can only have lengths in the range \[1..64\] bits. See the
280 {{:#integertypes}integer types} section below for how these are
281 mapped to the OCaml int/int32/int64 types. This is checked
282 at compile time if the length expression is constant, otherwise it is
283 checked at runtime and you will get a runtime exception eg. in
284 the case of a computed length expression.
286 A bitstring field of length -1 matches all the rest of the
287 bitstring (thus this is only useful as the last field in a
290 A bitstring field of length 0 matches an empty bitstring
291 (occasionally useful when matching optional subfields).
293 Qualifiers are a list of identifiers/expressions which control the type,
294 signedness and endianness of the field. Permissible qualifiers are:
296 - [int]: field has an integer type
297 - [string]: field is a string type
298 - [bitstring]: field is a bitstring type
299 - [signed]: field is signed
300 - [unsigned]: field is unsigned
301 - [bigendian]: field is big endian - a.k.a network byte order
302 - [littleendian]: field is little endian - a.k.a Intel byte order
303 - [nativeendian]: field is same endianness as the machine
304 - [endian (expr)]: [expr] should be an expression which evaluates to
305 a {!endian} type, ie. [LittleEndian], [BigEndian] or [NativeEndian].
306 The expression is an arbitrary OCaml expression and can use the
307 value of earlier fields in the bitmatch.
309 The default settings are [int], [unsigned], [bigendian].
311 Note that many of these qualifiers cannot be used together,
312 eg. bitstrings do not have endianness. The syntax extension should
313 give you a compile-time error if you use incompatible qualifiers.
315 {3 Other cases in bitmatch}
317 As well as a list of fields, it is possible to name the
318 bitstring and/or have a default match case:
323 (* Default match case. *)
325 | { _ } as pkt -> ...
327 (* Default match case, with 'pkt' bound to the whole bitstring. *)
330 {2 Constructing bitstrings}
332 Bitstrings may be constructed using the [BITSTRING] operator (as an
333 expression). The [BITSTRING] operator takes a list of fields,
334 similar to the list of fields for matching:
345 (* Constructs a 16-bit bitstring with the first four bits containing
346 the integer 1, and the following 12 bits containing the integer 10,
347 arranged in network byte order. *)
349 Bitmatch.hexdump_bitstring stdout bits ;;
357 The format of each field is the same as for pattern fields (see
358 {{:#patternfieldreference}Pattern field reference section}), and
359 things like computed length fields, fixed value fields, insertion
360 of bitstrings within bitstrings, etc. are all supported.
362 {3 Construction exception}
364 The [BITSTRING] operator may throw a {!Construct_failure}
365 exception at runtime.
367 Runtime errors include:
369 - int field length not in the range \[1..64\]
370 - a bitstring with a length declared which doesn't have the
371 same length at runtime
372 - trying to insert an out of range value into an int field
373 (eg. an unsigned int field which is 2 bits wide can only
374 take values in the range \[0..3\]).
376 {2:integertypes Integer types}
378 Integer types are mapped to OCaml types [bool], [int], [int32] or
379 [int64] using a system which tries to ensure that (a) the types are
380 reasonably predictable and (b) the most efficient type is
383 The rules are slightly different depending on whether the bit
384 length expression in the field is a compile-time constant or a
387 Detection of compile-time constants is quite simplistic so only an
388 simple integer literals and simple expressions (eg. [5*8]) are
389 recognized as constants.
391 In any case the bit size of an integer is limited to the range
392 \[1..64\]. This is detected as a compile-time error if that is
393 possible, otherwise a runtime check is added which can throw an
394 [Invalid_argument] exception.
399 Bit size ---- OCaml type ----
400 Constant Computed expression
408 A possible future extension may allow people with 64 bit computers
409 to specify a more optimal [int] type for bit sizes in the range
410 [32..63]. If this was implemented then such code {i could not even
411 be compiled} on 32 bit platforms, so it would limit portability.
413 Another future extension may be to allow computed
414 expressions to assert min/max range for the bit size,
415 allowing a more efficient data type than int64 to be
416 used. (Of course under such circumstances there would
417 still need to be a runtime check to enforce the
422 Using the compiler directly you can do:
425 ocamlc -I +bitmatch \
426 -pp "camlp4o `ocamlc -where`/bitmatch/pa_bitmatch.cmo" \
427 bitmatch.cma test.ml -o test
430 Simpler method using findlib:
434 -package bitmatch.syntax -syntax bitmatch.syntax \
435 -linkpkg test.ml -o test
438 {2 Security and type safety}
440 {3 Security on input}
442 The main concerns for input are buffer overflows and denial
445 It is believed that this library is robust against attempted buffer
446 overflows. In addition to OCaml's normal bounds checks, we check
447 that field lengths are >= 0, and many additional checks.
449 Denial of service attacks are more problematic. We only work
450 forwards through the bitstring, thus computation will eventually
451 terminate. As for computed lengths, code such as this is thought
457 buffer : Int64.to_int len : bitstring } ->
460 The [len] field can be set arbitrarily large by an attacker, but
461 when pattern-matching against the [buffer] field this merely causes
462 a test such as [if len <= remaining_size] to fail. Even if the
463 length is chosen so that [buffer] bitstring is allocated, the
464 allocation of sub-bitstrings is efficient and doesn't involve an
465 arbitary-sized allocation or any copying.
467 However the above does not necessarily apply to strings used in
468 matching, since they may cause the library to use the
469 {!Bitmatch.string_of_bitstring} function, which allocates a string.
470 So you should take care if you use the [string] type particularly
471 with a computed length that is derived from external input.
473 The main protection against attackers should be to ensure that the
474 main program will only read input bitstrings up to a certain
475 length, which is outside the scope of this library.
477 {3 Security on output}
479 As with the input side, computed lengths are believed to be
483 let len = read_untrusted_source () in
484 let buffer = allocate_bitstring () in
486 buffer : len : bitstring
490 This code merely causes a check that buffer's length is the same as
491 [len]. However the program function [allocate_bitstring] must
492 refuse to allocate an oversized buffer (but that is outside the
493 scope of this library).
495 {3 Order of evaluation}
497 In [bitmatch] statements, fields are evaluated left to right.
499 Note that the when-clause is evaluated {i last}, so if you are
500 relying on the when-clause to filter cases then your code may do a
501 lot of extra and unncessary pattern-matching work on fields which
502 may never be needed just to evaluate the when-clause. You can
503 usually rearrange the code to do only the first part of the match,
504 followed by the when-clause, followed by a second inner bitmatch.
508 The current implementation is believed to be fully type-safe,
509 and makes compile and run-time checks where appropriate. If
510 you find a case where a check is missing please submit a
511 bug report or a patch.
515 These are thought to be the current limits:
517 Integers: \[1..64\] bits.
519 Bitstrings (32 bit platforms): maximum length is limited
520 by the string size, ie. 16 MBytes.
522 Bitstrings (64 bit platforms): maximum length is thought to be
523 limited by the string size, ie. effectively unlimited.
525 Bitstrings must be loaded into memory before we can match against
526 them. Thus available memory may be considered a limit for some
529 {2:reference Reference}
533 type endian = BigEndian | LittleEndian | NativeEndian
535 val string_of_endian : endian -> string
538 type bitstring = string * int * int
539 (** [bitstring] is the basic type used to store bitstrings.
541 The type contains the underlying data (a string),
542 the current bit offset within the string and the
543 current bit length of the string (counting from the
544 bit offset). Note that the offset and length are
545 in {b bits}, not bytes.
547 Normally you don't need to use the bitstring type
548 directly, since there are functions and syntax
549 extensions which hide the details.
551 See also {!bitstring_of_string}, {!bitstring_of_file},
552 {!hexdump_bitstring}, {!bitstring_length}.
555 (** {3 Exceptions} *)
557 exception Construct_failure of string * string * int * int
558 (** [Construct_failure (message, file, line, char)] may be
559 raised by the [BITSTRING] constructor.
561 Common reasons are that values are out of range of
562 the fields that contain them, or that computed lengths
563 are impossible (eg. negative length bitfields).
565 [message] is the error message.
567 [file], [line] and [char] point to the original source
568 location of the [BITSTRING] constructor that failed.
571 (** {3 Bitstrings} *)
573 val empty_bitstring : bitstring
574 (** [empty_bitstring] is the empty, zero-length bitstring. *)
576 val create_bitstring : int -> bitstring
577 (** [create_bitstring n] creates an [n] bit bitstring
578 containing all zeroes. *)
580 val make_bitstring : int -> char -> bitstring
581 (** [make_bitstring n c] creates an [n] bit bitstring
582 containing the repeated 8 bit pattern in [c].
584 For example, [make_bitstring 16 '\x5a'] will create
585 the bitstring [0x5a5a] or in binary [0101 1010 0101 1010].
587 Note that the length is in bits, not bytes. *)
589 val bitstring_of_string : string -> bitstring
590 (** [bitstring_of_string str] creates a bitstring
591 of length [String.length str * 8] (bits) containing the
594 Note that the bitstring uses [str] as the underlying
595 string (see the representation of {!bitstring}) so you
596 should not change [str] after calling this. *)
598 val bitstring_of_file : string -> bitstring
599 (** [bitstring_of_file filename] loads the named file
602 val bitstring_of_chan : in_channel -> bitstring
603 (** [bitstring_of_chan chan] loads the contents of
604 the input channel [chan] as a bitstring.
606 The length of the final bitstring is determined
607 by the remaining input in [chan], but will always
608 be a multiple of 8 bits.
610 See also {!bitstring_of_chan_max}. *)
612 val bitstring_of_chan_max : in_channel -> int -> bitstring
613 (** [bitstring_of_chan_max chan max] works like
614 {!bitstring_of_chan} but will only read up to
615 [max] bytes from the channel (or fewer if the end of input
616 occurs before that). *)
618 val bitstring_of_file_descr : Unix.file_descr -> bitstring
619 (** [bitstring_of_file_descr fd] loads the contents of
620 the file descriptor [fd] as a bitstring.
622 See also {!bitstring_of_chan}, {!bitstring_of_file_descr_max}. *)
624 val bitstring_of_file_descr_max : Unix.file_descr -> int -> bitstring
625 (** [bitstring_of_file_descr_max fd max] works like
626 {!bitstring_of_file_descr} but will only read up to
627 [max] bytes from the channel (or fewer if the end of input
628 occurs before that). *)
630 val bitstring_length : bitstring -> int
631 (** [bitstring_length bitstring] returns the length of
632 the bitstring in bits. *)
634 val string_of_bitstring : bitstring -> string
635 (** [string_of_bitstring bitstring] converts a bitstring to a string
636 (eg. to allow comparison).
638 This function is inefficient. In the best case when the bitstring
639 is nicely byte-aligned we do a [String.sub] operation. If the
640 bitstring isn't aligned then this involves a lot of bit twiddling
641 and is particularly inefficient.
643 If the bitstring is not a multiple of 8 bits wide then the
644 final byte of the string contains the high bits set to the
645 remaining bits and the low bits set to 0. *)
647 val bitstring_to_file : bitstring -> string -> unit
648 (** [bitstring_to_file bits filename] writes the bitstring [bits]
649 to the file [filename]. It overwrites the output file.
651 Some restrictions apply, see {!bitstring_to_chan}. *)
653 val bitstring_to_chan : bitstring -> out_channel -> unit
654 (** [bitstring_to_file bits filename] writes the bitstring [bits]
655 to the channel [chan].
657 Channels are made up of bytes, bitstrings can be any bit length
658 including fractions of bytes. So this function only works
659 if the length of the bitstring is an exact multiple of 8 bits
660 (otherwise it raises [Invalid_argument "bitstring_to_chan"]).
662 Furthermore the function is efficient only in the case where
663 the bitstring is stored fully aligned, otherwise it has to
664 do inefficient bit twiddling like {!string_of_bitstring}.
666 In the common case where the bitstring was generated by the
667 [BITSTRING] operator and is an exact multiple of 8 bits wide,
668 then this function will always work efficiently.
671 (** {3 Printing bitstrings} *)
673 val hexdump_bitstring : out_channel -> bitstring -> unit
674 (** [hexdump_bitstring chan bitstring] prints the bitstring
675 to the output channel in a format similar to the
676 Unix command [hexdump -C]. *)
678 (** {3 Bitstring buffer} *)
682 val create : unit -> t
683 val contents : t -> bitstring
684 val add_bits : t -> string -> int -> unit
685 val add_bit : t -> bool -> unit
686 val add_byte : t -> int -> unit
688 (** Buffers are mainly used by the [BITSTRING] constructor, but
689 may also be useful for end users. They work much like the
690 standard library [Buffer] module. *)
692 (** {3 Miscellaneous} *)
695 (** The package name, always ["ocaml-bitmatch"] *)
698 (** The package version as a string. *)
701 (** Set this variable to true to enable extended debugging.
702 This only works if debugging was also enabled in the
703 [pa_bitmatch.ml] file at compile time, otherwise it
708 (* Private functions, called from generated code. Do not use
709 * these directly - they are not safe.
712 val extract_bitstring : string -> int -> int -> int -> bitstring * int * int
714 val extract_remainder : string -> int -> int -> bitstring * int * int
716 val extract_bit : string -> int -> int -> int -> bool * int * int
718 val extract_char_unsigned : string -> int -> int -> int -> int * int * int
720 val extract_int_be_unsigned : string -> int -> int -> int -> int * int * int
722 val extract_int_le_unsigned : string -> int -> int -> int -> int * int * int
724 val extract_int_ne_unsigned : string -> int -> int -> int -> int * int * int
726 val extract_int_ee_unsigned : endian -> string -> int -> int -> int -> int * int * int
728 val extract_int32_be_unsigned : string -> int -> int -> int -> int32 * int * int
730 val extract_int32_le_unsigned : string -> int -> int -> int -> int32 * int * int
732 val extract_int32_ne_unsigned : string -> int -> int -> int -> int32 * int * int
734 val extract_int32_ee_unsigned : endian -> string -> int -> int -> int -> int32 * int * int
736 val extract_int64_be_unsigned : string -> int -> int -> int -> int64 * int * int
738 val extract_int64_le_unsigned : string -> int -> int -> int -> int64 * int * int
740 val extract_int64_ne_unsigned : string -> int -> int -> int -> int64 * int * int
742 val extract_int64_ee_unsigned : endian -> string -> int -> int -> int -> int64 * int * int
744 val construct_bit : Buffer.t -> bool -> int -> exn -> unit
746 val construct_char_unsigned : Buffer.t -> int -> int -> exn -> unit
748 val construct_int_be_unsigned : Buffer.t -> int -> int -> exn -> unit
750 val construct_int_ne_unsigned : Buffer.t -> int -> int -> exn -> unit
752 val construct_int_ee_unsigned : endian -> Buffer.t -> int -> int -> exn -> unit
754 val construct_int32_be_unsigned : Buffer.t -> int32 -> int -> exn -> unit
756 val construct_int32_ne_unsigned : Buffer.t -> int32 -> int -> exn -> unit
758 val construct_int32_ee_unsigned : endian -> Buffer.t -> int32 -> int -> exn -> unit
760 val construct_int64_be_unsigned : Buffer.t -> int64 -> int -> exn -> unit
762 val construct_int64_ne_unsigned : Buffer.t -> int64 -> int -> exn -> unit
764 val construct_int64_ee_unsigned : endian -> Buffer.t -> int64 -> int -> exn -> unit
766 val construct_string : Buffer.t -> string -> unit