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Buffer.add_channel hangs
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Date: 2010-03-17 (23:20)
From: Martin Jambon <martin.jambon@e...>
Subject: Re: [Caml-list] Buffer.add_channel hangs
Stas Miasnikou wrote:
> Hi,
> OCaml 3.11.1, OpenBSD 4.6, i386.
> I am trying to read whole file by doing:
> let read_file_bin name =
>   let ic = open_in_bin name in
>   let b = Buffer.create 1024 in
>   (try Buffer.add_channel b ic max_int with _ -> ()); (* <-- HERE *)
>   close_in ic;
>   Array.init (Buffer.length b) (fun i -> int_of_char (Buffer.nth b i))
> but it hangs on the line marked. Am I doing something wrong?

The problem is max_int and the fact that Buffer.add_channel and Buffer.resize
 do not check for this possibility:

let add_channel b ic len =
  if b.position + len > b.length then resize b len;
  really_input ic b.buffer b.position len;
  b.position <- b.position + len

Something like the following would be better:

let add_channel b ic len =
  if len < 0 || len > Sys.max_string_length then
    invalid_arg "Buffer.add_channel";

Since you uncovered this problem, please kindly submit a proper bug report at

(and figure what to do if the file is larger than 16MB on 32-bit systems)

Of course, you can see from the implementation of the Buffer module that a
string of your maximum length is created no matter what, which you surely want
to avoid especially on 64-bit systems where Sys.max_string_length is very large.