Logical Devices

External Files

A unit specifier associated with an external file must be either an integer expression or an asterisk (*). The integer expression must be in the range 0 (zero) to a maximum value of 2,147,483,640. (The predefined parameters FOR_K_PRINT_UNITNO, FOR_K_TYPE_UNITNO, FOR_K_ACCEPT_UNITNO, and FOR_K_READ_UNITNO may not be in that range. For more information, see the Language Reference.)

The following example connects the external file UNDAMP.DAT to unit 10 and writes to it:

OPEN (UNIT = 10, FILE = 'UNDAMP.DAT')
WRITE (10, '(A18,\)') ' Undamped Motion:'

The asterisk (*) unit specifier specifies the keyboard when reading and the screen when writing. The following example uses the asterisk specifier to write to the screen:

WRITE (*, '(1X, A30,\)') ' Write this to the screen.'

Intel Fortran has four units preconnected to external files (devices), as shown in the following table.

The asterisk (*) specifier is the only unit specifier that cannot be reconnected to another file, and attempting to close this unit causes a compile-time error. Units 0, 5, and 6, however, can be connected to any file with the OPEN statement. If you close unit 0, 5, or 6, it is automatically reconnected to its preconnected device the next time an I/O statement attempts to use that unit.

Intel® Fortran does not support buffering to stdout or stdin. All I/O to units * and 6 use line buffering. Therefore, C and Fortran output to stdout should work well as long as the C code is not performing buffering. In the case of buffering, the C code will have to flush the buffers with each write. For more information on stdout and stdin, see Assigning Files to Logical Units.

You can change these preconnected files by doing one of the following:

• Using an OPEN statement to open unit 5, 6, or 0. When you explicitly OPEN a file for unit 5, 6, or 0, the OPEN statement keywords specify the file-related information to be used instead of the preconnected standard I/O file.

• Setting the appropriate environment variable (FORTn) to redirect I/O to an external file.

To redirect input or output from the standard preconnected files at run time, you can set the appropriate environment variable or use the appropriate shell redirection character in a pipe (such as > or <).

When you omit the file name in the OPEN statement or use an implicit OPEN, you can define the environment variable FORTn to specify the file name for a particular unit number n. An exception to this is when the compiler option -fpscomp filesfromcmd (Linux* OS and Mac OS* X) or /fpscomp:filesfromcmd (Windows) is specified.

For example, if you want unit 6 to write to a file instead of standard output, set the environment variable FORT6 to the path and filename to be used before you run the program. If the appropriate environment variable is not defined, a default filename is used, in the form fort.n where n is the logical unit number.

The following example writes to the preconnected unit 6 (the screen), then reconnects unit 6 to an external file and writes to it, and finally reconnects unit 6 to the screen and writes to it:

     REAL a, b
!  Write to the screen (preconnected unit 6).
    WRITE(6, '('' This is unit 6'')')
!  Use the OPEN statement to connect unit 6
!  to an external file named 'COSINES'.
    OPEN (UNIT = 6, FILE = 'COSINES', STATUS = 'NEW')
    DO a = 0.1, 6.3, 0.1
         b = COS (a)
!  Write to the file 'COSINES'.
          WRITE (6, 100) a, b
100        FORMAT (F3.1, F5.2)
    END DO
!  Close it.
    CLOSE (6)
!  Reconnect unit 6 to the screen, by writing to it.
    WRITE(6,' ('' Cosines completed'')')
    END

Internal Files

The unit specifier associated with an internal file is a character string or character array. There are two types of internal files:

• An internal file that is a character variable, character array element, or noncharacter array element has exactly one record, which is the same length as the variable, array element, or noncharacter array element.

• An internal file that is a character array, a character derived type, or a noncharacter array is a sequence of elements, each of which is a record. The order of records is the same as the order of array elements or type elements, and the record length is the length of one array element or the length of the derived-type element.

Follow these rules when using internal files:

• Use only formatted I/O, including I/O formatted with a format specification and list-directed I/O. (List-directed I/O is treated as sequential formatted I/O.) Namelist formatting is not allowed.

• If the character variable is an allocatable array or array part of an allocatable array, the array must be allocated before use as an internal file. If the character variable is a pointer, it must be associated with a target.

• Use only READ and WRITE statements. You cannot use file connection (OPEN, CLOSE), file positioning (REWIND, BACKSPACE), or file inquiry (INQUIRE) statements with internal files.

You can read and write internal files with FORMAT I/O statements or list-directed I/O statements exactly as you can external files. Before an I/O statement is executed, internal files are positioned at the beginning of the variable, before the first record.

With internal files, you can use the formatting capabilities of the I/O system to convert values between external character representations and Fortran internal memory representations. That is, reading from an internal file converts the ASCII representations into numeric, logical, or character representations, and writing to an internal file converts these representations into their ASCII representations.

This feature makes it possible to read a string of characters without knowing its exact format, examine the string, and interpret its contents. It also makes it possible, as in dialog boxes, for the user to enter a string and for your application to interpret it as a number.

If less than an entire record is written to an internal file, the rest of the record is filled with blanks.

In the following example, str and fname specify internal files:

    CHARACTER(10) str
   INTEGER n1, n2, n3
   CHARACTER(14) fname
   INTEGER   i
   str = " 1   2   3"
! List-directed READ sets n1 = 1, n2 = 2, n3 = 3.
   READ(str, *) n1, n2, n3
   i = 4
! Formatted WRITE sets fname = 'FM004.DAT'.
   WRITE (fname, 200) i
200 FORMAT ('FM', I3.3, '.DAT')