[Bug libfortran/83649] New: Large reads fail

[jb at gcc dot gnu.org]

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

            Bug ID: 83649
           Summary: Large reads fail
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libfortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jb at gcc dot gnu.org
  Target Milestone: ---

Consider the testcase

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

This fails on x86_64-pc-linux-gnu with

./a.out
At line 10 of file largewr.f90 (unit = 10, file = 'largewr.dat')
Fortran runtime error: End of file
[snip]

The reason is that Linux never reads or writes more than 2,147,479,552 bytes in
a single syscall. For write libgfortran does the writing in a loop, and it's
handled correctly, but for read we cannot do that loop since then it will hang
when reading from the terminal where short reads are perfectly Ok.