All logical unit numbers are automatically preconnected to specific files.
These files need not exist and will only be opened or created if they are
accessed with READ or WRITE without an explicit OPEN.
By default the specific filename for unit n is
fort.n;
however if the environment variable FORTnn exists its
value is used as the filename.
Note that there are two digits in this variable name, e.g. the variable
controlling unit 1 is FORT01
whereas the default filename is ‘fort.1’ (unless the
prefix has been changed, see the description of module
F90_PRECONN_IO).
A file preconnected in this manner is opened with
ACCESS='SEQUENTIAL'.
If the initial READ or WRITE is an unformatted i/o statement, it
is opened with FORM='UNFORMATTED' otherwise it is opened
with FORM='FORMATTED'.
By default a formatted connection is opened with
BLANK='NULL' and POSITION='REWIND' (see
module F90_PRECONN_IO).
Automatic preconnection applies only to the initial use of a logical unit; once
CLOSEd the unit will not be reconnected automatically but must be
explicitly OPENed.
Note that this facility means that it is possible for a READ or
WRITE statement
with an IOSTAT= clause to receive an i/o error code
associated with the implicit OPEN.