FIX: Binary (or Unformatted) I/O Fails, Array Size > 64K

The information in this article applies to:

• Microsoft FORTRAN PowerStation for MS-DOS, version 1.0

SYMPTOMS

An application developed in Microsoft FORTRAN PowerStation that reads or writes an array larger than 64K in size to a binary or unformatted file may read or write an incorrect quantity of data.

CAUSE

The application specifies the name of an array without a subscript or specifies a simple variable for the subscript. The compiler does not generate the correct code to access the low-level functions that perform binary or unformatted I/O. Consequently, the application reads or writes the amount of data specified by the following formula: (size of array) modulus 64K.

RESOLUTION

To work around this problem, use an explicit implied DO loop in the I/O statement. If the array has only one dimension, the array subscript must be an expression and not a simple variable for the compiler to generate the correct code. If the array is a multi-dimensional array, then the normal, multi-dimensional implied DO loop is sufficient to avoid the problem.

STATUS

Microsoft has confirmed this to be a problem in Microsoft PowerStation version 1.0. This problem has been resolved with FORTRAN PowerStation maintenance release version 1.0a for MS-DOS.

FORTRAN PowerStation version 1.0 can be differentiated from the maintenance release version 1.0a by invoking the linker. Typing "link32 | more" from \F32\BIN directory will show version 2.8 for FORTRAN PowerStation version 1.0, and it will show version 1.0f for the maintenance release version 1.0a.

MORE INFORMATION

Sample Code #1

C Compile options needed: none C This program generates the following output C

C     Program failed, number of bytes read:           1

      parameter(n = 64 * 1024 + 1)  ! 64K + 1 size
      character*1 arr(n), chr
      integer*4 i

      open(1, file = 'test.dat', form = 'binary')
      arr = 'x'      ! initialize entire array to 'x'
      write(1) arr   ! write entire array
      rewind(1)      ! rewind to read file
      do i = 1, n
        read(1, iostat = ierr) chr ! read a byte at a time
        if (ierr .eq. -1) exit     ! exit at end of file
      end do
      if (i - 1 .ne. n) then
        print *, 'Program failed, number of bytes read: ', i - 1
      else
        print *, 'Program OK, number of bytes read: ', i - 1
      end if
      end

Sample Code #2

C Compile options needed: none C This program generates the following output C

C     Program OK, number of bytes read:       65537

      parameter(n = 64 * 1024 + 1)  ! 64K + 1 size
      character*1 arr(n), chr
      integer*4 i

      open(1, file = 'test.dat', form = 'binary')
      arr = 'x'      ! initialize entire array to 'x'
      write(1) (arr(i + 0), i = 1, n)   ! note the addition of zero
      rewind(1)      ! rewind to read file
      do i = 1, n
        read(1, iostat = ierr) chr ! read a byte at a time
        if (ierr .eq. -1) exit     ! exit at end of file
      end do
      if (i - 1 .ne. n) then
        print *, 'Program failed, number of bytes read: ', i - 1
      else
        print *, 'Program OK, number of bytes read: ', i - 1
      end if
      end