[Bug libfortran/48587] New: Avoid exhausting unit number with NEWUNIT=

["jb at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48587

           Summary: Avoid exhausting unit number with NEWUNIT=
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libfortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jb@gcc.gnu.org


As was recently pointed out by Tobias Burnus in a thread on comp.lang.fortran,
the current implementation of NEWUNIT= doesn't reuse unit numbers. Hence it's
possible that a program might exhaust the available unit numbers (this requires
that the program repeatedly closes and reopens files, as OS's have limits on
the number of file descriptors a process may have concurrently opened,
typically 1024 or something like that)

Currently there is just a (mutex protected) static variable which is
decremented for each time an OPEN statement with NEWUNIT= is issued, with a
wraparound check that generates an error if wraparound is detected.

IMHO an elegant solution would be to just reuse the kernel provided file
descriptor. E.g. 

int fd = open(...);
if (fd == -1)
{
    /* Handle error... */
}
new_unit_number = -fd;

This should work because a successful open() will always return a positive fd
number (see
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html ), and
the kernel takes care of reusing file descriptor numbers of closed files.