The GNU Fortran Compiler: File format of unformatted sequential files

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### 4.9 File format of unformatted sequential files

Unformatted sequential files are stored as logical records using record markers. Each logical record consists of one of more subrecords.

Each subrecord consists of a leading record marker, the data written by the user program, and a trailing record marker. The record markers are four-byte integers by default, and eight-byte integers if the `-fmax-subrecord-length=8` option (which exists for backwards compability only) is in effect.

The representation of the record markers is that of unformatted files given with the `-fconvert` option, the CONVERT specifier in an open statement or the GFORTRAN_CONVERT_UNIT environment variable.

The maximum number of bytes of user data in a subrecord is 2147483639 (2 GiB - 9) for a four-byte record marker. This limit can be lowered with the `-fmax-subrecord-length` option, although this is rarely useful. If the length of a logical record exceeds this limit, the data is distributed among several subrecords.

The absolute of the number stored in the record markers is the number of bytes of user data in the corresponding subrecord. If the leading record marker of a subrecord contains a negative number, another subrecord follows the current one. If the trailing record marker contains a negative number, then there is a preceding subrecord.

In the most simple case, with only one subrecord per logical record, both record markers contain the number of bytes of user data in the record.

The format for unformatted sequential data can be duplicated using unformatted stream, as shown in the example program for an unformatted record containing a single subrecord:

    program main
      use iso_fortran_env, only: int32
      implicit none
      integer(int32) :: i
      real, dimension(10) :: a, b
      call random_number(a)
      open (10,file='test.dat',form='unformatted',access='stream')
      inquire (iolength=i) a
      write (10) i, a, i
      close (10)
      open (10,file='test.dat',form='unformatted')
      read (10) b
      if (all (a == b)) print *,'success!'
    end program main

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