In the finest tradition of version 0.1 announcements, this is the
first announcement of a highly experimental camlp4 syntax extension
which implements Erlang-style bitstrings, matching over bitstrings,
and construction of bitstrings.
Source: http://www.annexia.org/tmp/ocaml-bitmatch-0.1.tar.gz
License: LGPLv2+ with OCaml linking exception
Erlang has a "byte-oriented" data type which can be treated as a
stream of bits, and provides rather elegant features for creating and
matching over such streams. This is a key feature of Erlang and was
developed because of its history in telecommunications. (More about
the feature in this paper:
http://user.it.uu.se/~kostis/Papers/padl07.pdf)
I have written a camlp4 syntax extension which does much the same in
OCaml. For example, you can now effortlessly parse IP packets:
let display pkt =
bitmatch pkt with
(* IPv4 packet header from RFC 791:
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
*)
| 4 : 4; hdrlen : 4; tos : 8; length : 16; (* same as above in OCaml *)
identification : 16; flags : 3; fragoffset : 13;
ttl : 8; protocol : 8; checksum : 16;
source : 32;
dest : 32;
options : (hdrlen-5)*32 : bitstring; (* NB computed length *)
payload : -1 : bitstring ->
printf "IPv4:\n";
printf " header length: %d * 32 bit words\n" hdrlen;
printf " type of service: %d\n" tos;
printf " packet length: %d bytes\n" length;
(* etc *)
(* IPv6 packet header *)
| 6 : 4; tclass : 8; flow : 20;
length : 16; nexthdr : 8; ttl : 8;
source : 128 : bitstring;
dest : 128 : bitstring;
payload : -1 : bitstring ->
printf "IPv6:\n";
printf " traffic class: %d\n" tclass;
printf " flow label: %d\n" flow;
printf " packet (payload) length: %d bytes\n" length;
printf " next header: %d\n" nexthdr;
printf " ttl: %d\n" ttl;
(* etc *)
| version : 4 ->
eprintf "unknown IP version %d\n" version;
exit 1
| _ as pkt ->
eprintf "data is smaller than one nibble:\n";
Bitmatch.hexdump_bitstring stderr pkt;
exit 1
Or filesystems, as in this parser for Linux EXT3 superblocks:
let bits = Bitmatch.bitstring_of_file "tests/ext3_sb"
let () =
bitmatch bits with
| s_inodes_count : 32 : littleendian; (* Inodes count *)
s_blocks_count : 32 : littleendian; (* Blocks count *)
s_r_blocks_count : 32 : littleendian; (* Reserved blocks count *)
s_free_blocks_count : 32 : littleendian; (* Free blocks count *)
s_free_inodes_count : 32 : littleendian; (* Free inodes count *)
s_first_data_block : 32 : littleendian; (* First Data Block *)
s_log_block_size : 32 : littleendian; (* Block size *)
s_log_frag_size : 32 : littleendian; (* Fragment size *)
s_blocks_per_group : 32 : littleendian; (* # Blocks per group *)
s_frags_per_group : 32 : littleendian; (* # Fragments per group *)
s_inodes_per_group : 32 : littleendian; (* # Inodes per group *)
s_mtime : 32 : littleendian; (* Mount time *)
s_wtime : 32 : littleendian; (* Write time *)
s_mnt_count : 16 : littleendian; (* Mount count *)
s_max_mnt_count : 16 : littleendian; (* Maximal mount count *)
0xef53 : 16 : littleendian -> (* Magic signature *)
printf "ext3 superblock:\n";
printf " s_inodes_count = %ld\n" s_inodes_count;
printf " s_blocks_count = %ld\n" s_blocks_count;
printf " s_free_inodes_count = %ld\n" s_free_inodes_count;
printf " s_free_blocks_count = %ld\n" s_free_blocks_count
| _ ->
eprintf "not an ext3 superblock!\n%!";
exit 2
There is also a similar syntax for contructing bitstrings.
Please let me know if you are interested in using this. I may change
the syntax a little before the next release.
Thanks to several people on #ocaml for answering my questions when I
was writing this.
Rich.
--
Richard Jones
Red Hat
_______________________________________________
Caml-list mailing list. Subscription management:
http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list
Archives: http://caml.inria.fr
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
Bug reports: http://caml.inria.fr/bin/caml-bugs
