===================================================================
@@ -1172,6 +1172,7 @@
* Data consistency and durability::
* Files opened without an explicit ACTION= specifier::
* File operations on symbolic links::
+* File format of unformatted sequential files::
@end menu
@@ -1402,8 +1403,66 @@
@end itemize
+@node File format of unformatted sequential files
+@section File format of unformatted sequential files
+@cindex file, unformatted sequential
+@cindex unformatted sequential
+@cindex sequential, unformatted
+@cindex record marker
+@cindex subrecord
+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
+@option{-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 @option{-fconvert} option, the @xref{CONVERT specifier}
+on the open statement or the @xref{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 @option{-fmax-subrecord-length} option, altough this is
+rarely useful. If the lenght 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:
+
+@smallexample
+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
+@end smallexample
+
@c ---------------------------------------------------------------------
@c Extensions
@c ---------------------------------------------------------------------
Hello world, seems like we never got around to documenting the structure of our unformatted sequential files. Well, here is a patch for that. Regards Thomas 2017-08-12 Thomas Koenig <tkoenig@gcc.gnu.org> * gfortran.texi: Document format of unformatted sequential files.