John Prevost wrote:
>  As far as I can tell, there's no way to find out whether a given fd is
>  open read only or read-write or even write-only without actually
>  trying to read or write it.  This means I can't detect at mmap time
>  that something's wrong.  So my options are:

POSIX has functions stat and fstat. They write info about a file
in a struct stat. One member of the struct is st_mode. You can
use macros like S_IWUSR to check if the owner of the file has
write permission. See   man 2 stat   for details.

Oooh, this means using C code and not OCaml, but that should be
no problem. You are already using C for your mmap stuff.

I would really like to see some work on using/writing a DB with OCaml
(anyone with an interface for DB/2 out of the box ?) so I look forward
to your project.

pfitzen