[Bug libfortran/83649] Large reads fail

[jb at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83649

--- Comment #2 from Janne Blomqvist <jb at gcc dot gnu.org> ---
Author: jb
Date: Tue Jan  2 13:25:10 2018
New Revision: 256074

URL: https://gcc.gnu.org/viewcvs?rev=256074&root=gcc&view=rev
Log:
PR libgfortran/83649 Chunk large reads and writes

It turns out that Linux never reads or writes more than 2147479552
bytes in a single syscall. For writes this is not a problem as
libgfortran already contains a loop around write() to handle short
writes. But for reads we cannot do this, since then read will hang if
we have a short read when reading from the terminal.  Also, there are
reports that macOS fails I/O's larger than 2 GB.  Thus, to work around
these issues do large reads/writes in chunks.

The testcase from the PR

program largewr
  integer(kind=1) :: a(2_8**31+1)
  a = 0
  a(size(a, kind=8)) = 1
  open(10, file="largewr.dat", access="stream", form="unformatted")
  write (10) a
  close(10)
  a(size(a, kind=8)) = 2
  open(10, file="largewr.dat", access="stream", form="unformatted")
  read (10) a
  if (a(size(a, kind=8)) == 1) then
    print *, "All is well"
  else
    print *, "Oh no"
  end if
end program largewr

fails on trunk but works with the patch.

Regtested on x86_64-pc-linux-gnu, committed to trunk.

libgfortran/ChangeLog:

2018-01-02  Janne Blomqvist  <jb@gcc.gnu.org>

        PR libgfortran/83649
        * io/unix.c (MAX_CHUNK): New define.
        (raw_read): For reads larger than MAX_CHUNK, loop.
        (raw_write): Write no more than MAX_CHUNK bytes per iteration.

Modified:
    trunk/libgfortran/ChangeLog
    trunk/libgfortran/io/unix.c