NAG Fortran Compiler, Release 7.2: Automatic File Preconnection

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## 2.10 Automatic File Preconnection

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 `FORT`nn 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 `CLOSE`d the unit will not be reconnected automatically but must be explicitly `OPEN`ed.

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`.