> The following illustrates my problem:
> -------------------------------
> $ocaml
>         Objective Caml version 3.00
> 
> # let l = open_out_gen [Open_creat;Open_append] 0o666 "/tmp/xx";;
> val l : out_channel = <abstr>
> # close_out l;;
> - : unit = ()
> # let l = open_out_gen [Open_creat;Open_append] 0o666 "/tmp/xx";;
> val l : out_channel = <abstr>
> # output_string l "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
>   bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
>   ccccccccccccccccccccccccccccccccc";;
> - : unit = ()
> # close_out l;;
> Uncaught exception: Sys_error "Bad file descriptor".

The Open_append flag does not imply write access, it's just a modifier
saying "whatever writes you do, do them at the end of the file".  So
you need to specify Open_wronly as well.  (Agreed, the documentation
should state this.)

Otherwise, as you found out, actual writes to the file will fail.
(Since output is buffered, the "output_string" didn't perform an
actual write to the file, hence no error, but "close_out" flushes the
buffer to the file, hence the error pops up at that time.)

- Xavier Leroy
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